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54
Aaalyst, January, 1975, Vol. 100, $19.54-62
Analytical Methods Committee
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REPORT PREPARED BY THE METALLIC IMPURITIES I N ORGANIC MATTER SUB-COMMITTEE
The Determination of Small Amounts of Arsenic in Organic Matter The Analytical Methods Committee has received and approved for publication t h e following Report from its Metallic Impurities in Organic Matter Sub-committee.
Report The constitution of the Metallic Impurities in Organic Matter Sub-committee responsible for the preparation of this Report was: Dr. L. E. Coles (Chairman), Mr. J. R. Bishop (resigned May, 1973), Mr. W. Cassidy, Dr. J. C. Gage (resigned May, 1972), Dr. R. A. Hoodless, Mr. E. E. J. King, Mr. D. A. Lambie, Dr. H. Liebmann (resigned September, 1972), Dr. R. F. Milton, Mr. W. L. Sheppard and Mr. C. A. Watson, with Mr. P. W. Shallis as Secretary. In 1960, the Analytical Methods Comniittee recommended1 the use of the Gutzeit method for the determination of arsenic and a specified molybdenum-blue method for the accurate determination of arsenic in organic matter in the range 1.5 to 15 pg. Subsequently, another colorimetric method involving the use of the reagent silver diethyldithiocarbamate has been proposed and has been published by the British Standards Institution.2 The Metallic Impurities in Organic Matter Sub-Committee has, at the request of the Analytical Methods Committee, carried out a comparison of its recommended molybdenumblue method with the silver diethyldithiocarbamate method, and the results of this work, which included an investigation of the volatility of arsenic during wet oxidation, are given in this report. Although the Sub-committee found that its recommended molybdenum-blue method was, of the two, the method to be preferred for determining small amounts of arsenic in organic matter, this method was itself considered to have disadvantages in respect of the time required to carry out the determination and the high degree of analytical expertise needed. In consequence, at a late stage in the work described, the Sub-committee investigated an alternative molybdenum-blue procedure in which the arsenic is first separated by distillation as the bromide in a special This method was found to offer considerable advantages over the originally recommended molybdenum-blue method,l which it is now proposed it should replace. The Sub-committee also considered the possibility of determining arsenic by atomicabsorption spectroscopy, but in view of the problems associated with low signal t o noise ratios in the flame, it was decided not to pursue this at present. Introduction The presence of arsenic in food is regarded as purely adventitious. It is, however, used in various forms as an additive within prescribed limits to animal feeds for the purpose of improving growth, health and feed conversion of livestock, especially pigs and poultry, although the animals may retain arsenic in their livers. Arsenic can be found on apples that have been sprayed with the insecticide lead arsenate and in shellfish, in which it appears t o occur naturally. The Arsenic in Food Regulations 1959 impose, in general, a maximum limit of 1 p.p.m. of arsenic, with lower limits for certain beverages and higher limits for some specified foods. Experimental Comparison of Molybdenum-blue and Silver Diethyldithiocarbamate Methods Before the direct comparison of the two methods was carried out, the recovery of arsenic after wet oxidation was investigated. Collaborative comparison of the methods1s2 was then made by determining by both methods known amounts of arsenic (a) from pure solution and (b) from dried liver, sucrose and animal feedingstuff after wet oxidation. Additional
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ANALYTICAL METHODS COMMITTEE
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work by the silver diethyldithiocarbamate method alone was carried out on the recovery of arsenic added as an organic arsenical to dried liver, sucrose and animal feedingstuff and on the recovery of arsenic added to sucrose in the presence of possible interfering metals. Details of this work and the results are given below. Recovery of arsenic after wet oxidation One member of the Sub-committee measured arsenic radiochemically before and after wet oxidation in order to establish whether any loss of arsenic due to volatilisation could be expected. In separate experiments, about 30 pg of arsenic containing arsenic-74 tracer were added to 5 g of sucrose and 5 g of animal feedingstuff. Each mixture was oxidised with sulphuric and nitric acids5 in the apparatus recommended by Gorsuch,6 which allows the liquid driven off during the oxidation to be collected. The distillate was collected as two fractions: the first from the preliminary boiling with 20 ml of nitric acid, during which the volume was reduced to 10 ml, and the second from the remainder that was distilled during the boiling with sulphuric and nitric acids. The distribution of the arsenic, measured radiochemically, between the fractions and the residue (digest) is shown in Table I. The slight deficiency in recovery was probably due to absorption on the glass vessel or mechanical loss owing to difficulty in recovering arsenic from the flask and that part of the receiver - condenser assembly into which it may have been splashed. These experiments show that arsenic, as an inorganic compound added to organic materials, is not lost by volatilisation during the recommended wet-oxidation procedure with sulphuric and nitric acids, provided that oxidising conditions are maintained throughout the digestion.
TABLE I RECOVERY OF Sample Sucrose (1) .. Sucrose (2) .. Feedingstuff ..
-4RSENIC AFTER WET OXIDATION O F ORGANIC MATTER
..
..
..
Arsenic added/pg 33-0 33-0 33.4
Arsenic recoveredlpg 32.7 32.6 32.8
Arsenic in first fractionlpg 0 0.02 0.06
Arsenic in second fractionlpg 0.03
0.04 0.15
Recovery of arsenic f r o m pure solution Solutions containing 15 pg of arsenic and no interfering substances or organic matter were subjected to wet oxidation with sulphuric and nitric acids, and the arsenic was determined in the digests by both methods. The results, which indicate a higher mean recovery of arsenic by the silver diethyldithiocarbamate method, are shown in Table 11. The standard deviations given in this and subsequent tables were calculated from the equation s.d. =
J
(x, - Zl)2 (n, - 1)
+ (x2-
+
(n2
4
2
+ (x, -
- 1)
+
(n,
Z3)2
+ ... -
- 1) *
TABLEI1 RECOVERY OF
ARSENIC BY BOTH METHODS FROM PURE SOLUTION In each test, 15 pg of arsenic were present.
Molybdenum-blue method r
Silver diethyldithiocarbamate method 1
Laboratory A
Arsenic foundlpg 12.9, 13.2, 12.9
Recovery, per cent. 86.0, 88.0, 86.0
B C D
14.6, 16.25, 14-0, 15.0, 13.75, 15-5 13.4, 13.0 13.3, 13.8, 12.5
97.3, 108, 93.3, 100, 91.7, 103 89.3, 86.7 88.7, 92.0, 83.3
E
15.8, 14.8, 15.3
105, 98.7, 102
14.1
94.0
Mean .. .. Standard deviation
0-70
Arsenic found/pg Recovery, per cent. 13-4, 13.0, 13.0, 89.3, 86.7, 86.7, 17.0, 16-6, 16.4 113, 111, 109 14-75. 15.25, 14.6, 98.3, 102, 97-3, 15.0. 15.5, 14-75 100, 103, 98.3 14.8, 14.8, 15.0 98.7, 98.7, 100 14.8, 14-8, 98.7, 98.7, 15.0, 14-5,15.4 100, 96.7, 103 13.75 91.7 14.9 1.12
99.3
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TABLEI11
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RECOVERY OF
ARSENIC BY THE SILVER DIETHYLDITHIOCARBAMATE METHOD
Arsenic Arsenic Laboratory blank/ p g €ound/pg From 5 g of sucrose containing 15 pg of added amenic-A 0.1 12.9, 12.4, 12-2 B 0.1 12.8, 11.6, 13.4 C 0.3 15.6, 17.5, 15.6 D 0 14.8, 14.8, 15.0 Mean Standard deviation
Arsenic recovered/ pg
Recovery, per cent.
12.8, 12.3, P2.1 12.7, 11.5, 13.3 15.3, 17.2, 15.3 14.8, 14.8, 15.0 13-9 0.74
85.3, 82.0, 80.7 P -A - I-I., 76.7, 88.7 102, 115, 102 98.7, 98.7, 100 92.6
F r o m 5 g of feedingstuff containing 15 p g of added arsenic-B 0.9 14.3, 14.6, 15-7, 12-8, 14.4 C 0.8 13.2, 12.7, 13.8 F 0.9 15.2, 13.2 Mean Standard deviation
13.4, 13.7, 14.8, 11.9, 13.5 12.4, 11-9, 13-0 14.3, 12.3 13.1 0.99
89.3, 91.3, 98.7, 79.3, 90.0 82.7, 79-3, 86.7 95.3, 82.0
F r o m 5 g of dried liver powder containing 15 pg of added arsenic12.5, 12-4, 12.0 A 2-6 13.7, 13.5, 13.9 B 3.1 13.0, 10.1, 11.8 C 2.0 10.5, 10.5, 11.0 D 2.6 3.0 14.2 iMean Standard deviation
9.9, 9.8, 9.4 10.6, 10.4, 10.8 11.0, 8-1, 9.8 7.9, 7.9, 8.4 11.2 9.6 1.15
66.0, 65.3, 62.7 70.7, 69-3, 72.0 73-3, 54.0, 65-3 52.7, 52.7, 56.0 74'7 64.2
87.5
Recovery of arsenic from organic matter Samples of sucrose, an animal feedingstuff and dried liver powder, to each of which 15 pg of arsenic were added as a standard solution of an inorganic arsenic compound, were wet oxidised with sulphuric and nitric acids. The arsenic content was determined by both methods, and the results are shown in Tables I11 and IV. From the results in these tables it can be seen that recoveries of arsenic from sucrose and the feedingstuff were satisfactory by both methods and that recoveries of arsenic from the dried liver powder were satisfactory by the molybdenum-blue method and rather low by the silver diethyldithiocarbamate method. RECOVERY OF Laboratory
Arsenic blank/ pg
TABLE IV MOLYBDENUM-BLUE METHOD
ARSENIC BY THE
Arsenic found/ pg
Arsenic recovered/ pg
Recovery, per cent.
F r o m 5 g of sucrose containing 16 pg of added arsenic13.0, 12.4, 12-3 A 0.27 13.5, 13.0, 11-0 B 0.1 11.1, 12.0 C 1.0 12.8, 13.3 D 0.5 Mean Standard deviation
12-7, 12.1, 12.0 13.4, 12.9, 10-9 10.1, 11.0 12.3, 12.8 12.0 0.79
86.7, 80.7, 80.0 89.3, 86.0, 71-3 67.3, 73-3 82.0, 84.3 80.4
F r o m 5 g of feedingstuff containing 15 p g of added arsenicB 2-6 15.0, 14.8, 14.8, 15.0 D 2.1 15.0, 16.1, 15-3 Mean Standard deviation
12-4, 12.2, 12.3, 12.4 12.9, 14.0, 13.2 12.8 0-37
82.6. 81.3, 82.0, 82.6 86.0, 93.3, 88.0 86.1
12.5, 10.7, 11.6 14.4, 14-4, 9.6 14.6 11.7, 14.4, 11.7 12.6 1.91
83.3, 71.3, 77.3 96.0, 96.0, 64.0 97.4 78.0,96.0, 78-0 83-1
From 5 g of dried powder containing 15 p g of added arsenic-
B C D E Mean Standard deviation
3.2 2.9 1.0 4.6
15.8, 14-0, 14.9 17.3, 17.3, 12.6 15-6 16.3, 19.0, 16.3
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57
SMALL AMOUNTS OF ARSENIC IN ORGANIC MATTER
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Recovery of arsenic added as an organic arsenical All of the experimental work described above had been carried out on the recovery of arsenic added as a known volume of a standard solution of inorganic arsenic. It was decided, therefore, to ensure that arsenic in an organic form could be determined satisfactorily. Samples of sucrose, animal feed and dried liver to which 15 pg of arsenic, as 4-ureido-l-phenylarsonic acid (carbarsone), had been added, were wet oxidised with sulphuric and nitric acids, and the arsenic contents were determined by the silver diethyldithiocarbamate method. The results are shown in Table V and indicate that arsenic in organic combination can, after wet oxidation, be determined satisfactorily, with recovery levels similar to those obtained for the addition of inorganic arsenic.
TABLE V RECOVERY OF Sample
Sucrose . . Sucrose . . Feedingstuff Feedingstuff Dried liver Dried liver
.. .. .. .. .. ..
.. .. .. .. .. ..
ORGANIC ARSENIC FROM ORGANIC MATTER
Arsenic added as carbarsone/pg 15 16 15 15 15 15
Arsenic recoveredig 13.7 13.95 12-5 12.1 11.2 9.9
Recovery, per cent. 91-3 93.0 83.3 80.7 74.7 66.0
Interfe~ingszcbstances The dried liver powder used in the collaborative work was known to contain high levels of certain trace metals, which could have been responsible for the rather low recoveries reported. In view of this, a collaborative investigation was carried out in which laboratories added to sucrose known amounts of arsenic and certain other metals in order to establish whether or not these metals had any effect on the subsequent determination of arsenic. After wet oxidation of the organic matter with sulphuric and nitric acids, the arsenic contents were determined by the silver diethyldithiocarbamate method. The results are shown in Table VI and indicate that the presence of lead, calcium, iron, manganese and chromium, at the levels used, do not reduce the recovery of arsenic by the silver diethyldithiocarbamate method.
TABLE VI RECOVERY OF
ARSENIC FROM SUCROSE IN THE PRESENCE OF OTHER METALS
A 5-g sample of sucrose containing 16 pg of added arsenic was used in each test. Laboratory A 13
C D F
Metal Lead Calcium Iron Manganese Chromium
Level of addition, per cent.
Arsenic recovered/pg* 0.01 11.6, 12.7, 12-5 1-0 13.1, 13-9, 11.0 0.4 16.2 0.1 15.0, 15.3, 15.3 0.12 14.0, 13.7, 13.9 * After subtraction of blank.
Recovery, per cent. 77.3, 84.7, 83.3 87.3, 92.7, 73.3 101.3 100.0, 102.0, 102.0 93.3, 91.3, 92.7
Discussion The silver diethyldithiocarbamate method was found to be quicker to apply and to require less analytical expertise than did the molybdenum-blue method. For the determination of arsenic in pure solutions and in organic materials, such as sucrose and animal feedingstuff, that are easy to digest with sulphuric and nitric acids, both methods gave satisfactory results. However, for the determination of arsenic in dried liver powder, a material that is difficult to oxidise with sulphuric and nitric acids, satisfactory results were obtained by the molybdenum-blue method. The Sub-committee is of the opinion that severe conditions of wet oxidation requiring the use of comparatively large volumes of acids can have a profound effect on the recovery of arsenic by the silver diethyldithiocarbamate method. However, in an attempt to obtain more information on the reasons for the failure
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ANALYTICAL METHODS COMMITTEE
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to recover all of the arsenic by the silver diethyldithiocarbamate method, a radiochemical investigation has been arranged, the results of which the Sub-committee hopes to be able to report later. In the view of members of the Sub-committee, the silver diethyldithiocarbamate method suffers certain other disadvantages. Wide variations between batches of the reagent were noticed and, in the collaborative work undertaken by the Sub-committee, all members used reagent from a selected satisfactory batch. Batches of the reagent giving absorbance readings between 0.02 and 0-05unit per microgram of arsenic have been found; variations of this type have not been found for the molybdenum-blue method over a period of 13 years. One laboratory examined the precisions of the two methods, without pre-separation, on a sample of hydrochloric acid containing 10 pg of added arsenic. The standard deviation of the results by the molybdenum-blue method was 0.2 pg, whereas for the silver diethyldithiocarbamate method it was 0-4 pg. For samples requiring a separation procedure, better results would still be expected by the molybdenum-blue method than by the silver diethyldithiocarbamate method. Another problem with the silver diethyldithiocarbamate method results from the unknown composition of the arsenic complex, which not only varies in colour intensity from batch to batch of the reagent, but can also have a wavelength of maximum absorption in the range from about 520 to 550 nm. The composition of the reagent itself can vary between having a small excess of silver and a small excess of diethyldithiocarbamate. The reagent containing a small excess of silver yields a complex that has a higher sensitivity to arsenic and absorbs at a longer wavelength than does the complex yielded by the reagent containing a small excess of diethyldithiocarbamate. Highest sensitivity of the reagent to arsenic is, however, accompanied by high sensitivity to antimony and germanium ; the reaction with antimony may have up to 60 per cent. of the sensitivity of that with arsenic on a mass basis. On the other hand, the form of the reagent that gives lowest sensitivity to arsenic may have a sensitivity to antimony as low as 1 per cent. of that to arsenic. These findings are similar to those of Dubois et a,?.’; they do not, however, accord with the findings of Bode and Hachmann.s Difficulties can also be encountered in the silver diethyldithiocarbamate method owing to the presence of substances that interfere with the formation of the complex or inhibit at much lower levels of arsenic the evolution of arsine. Martins and W h i t n a ~ k working ,~ than the Sub-committee, found the reagent to be unreliable for determining arsenic in drinking water owing to the presence of trace amounts (2-500 p.p.b.1 of chromium, molybdenum and vanadium, which suppressed the formation of the arsenic complex, and copper, which caused high results. Early in the Sub-Committee’s work, two members, using different batches of reagent, found an apparent loss of arsenic in the presence of between 2 and 70 mg of nitrate, as nitric acid. This loss could be particularly serious when determining arsenic in wet digests from organic matter, which frequently contain small amounts of nitrate that might be difficult to remove without loss of arsenic. Although the collaborative comparison of the two methods showed that the molybdenumblue method gave satisfactory results under all the conditions examined, whereas the silver diethyldithiocarbamate method did not, the general levels of recovery of arsenic by the molybdenum-blue method were inferior to those reported by the Sub-committee in 1960.l In view of this finding and the fact that the method is time consuming and requires considerable analytical expertise in order to apply it successfully, the Sub-committee decided that, before making a final recommendation, it would investigate an alternative molybdenumblue procedure based on Hoffman and Rowsome’s modification3 of Magnuson and Watson’s method.* Bromide Distillation Method It has been shown3t4that a wet digest containing arsenic can be distilled with a quantititative transfer of arsenic, as the bromide, to the distillate. After a carefully controlled wet oxidation with sulphuric and nitric acids, the digest is distilled in a special apparatus (Figs. 1 and 2) in the presence of potassium bromide. Arsenic in the distillate is determined spectrophotometrically as the molybdenum-blue complex. The method is far quicker to use than the currently recommended molybdenum-blue procedure,l the colour complex formed is stable and reproducible, frequent calibration is not required and readily available repro-
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JanzLary, 1975 SMALL AMOUNTS OF ARSENIC I N ORGANIC MATTER 59 ducible reagents are used. It is also virtually free from interferences, except from large amounts of phosphate and tin. The method was collaboratively investigated on samples of an animal feedingstuff and dried liver powder. The results, which are given in Table VII, indicate a generally higher level of recovery of arsenic than is shown in Tables I11 and IV. In addition, two laboratories checked the recovery of arsenic by the method in the absence of organic matter; results of 100.0 and 101.0 per cent. were obtained.
mm 0
R 50
I 00
I50
G
200
250
Fig. 2. Distillation apparatus
for arsenic (details of trap head). Fig. 1. Distillation apparatus for arsenic (complete assembly).
In amounts over 500 pg, tin interferes in the method by causing turbidity in the final solution after development of the molybdenum-blue colour, giving rise to erroneous absorbance measurements. This interference can be avoided by a simple modification involving addition of aqueous cupferron followed by extraction with chloroform before the distillation. The modification was tested on samples of an animal feedingstuff and the results, which show that there is little difference in the recovery of arsenic in the presence or absence of tin, are shown in Table VIII. Finally, one laboratory carried out recovery experiments on arsenic added as carbarsone to organic matter; the results are shown in Table IX. Conclusions The Sub-committee recommends that the molybdenum-blue procedure applied after separation of arsenic by distillation, full details of which are given in the Appendix, should replace the previously recommended methodl for the determination of small amounts of
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60
A.Izdyd, VOZ. 100
TABLE VII RECOVERY OF Arsenic blank/pg
Laboratory
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F r o m 6 g of feedingstuflA 0.06 B
0.9
C
0.1 1.3
ARSENIC BY BROMIDE DISTILLATION METHOD
Arsenic added/pg
Arsenic found 'pg
Arsenic recovered/ pg
Recovery, per cent.
10 20 10 20 10 20 10 20
9.4 18.2 10.1, 9.7, 10.9 18.9, 19.2, 17.5 12.0, 12.0, 11.2 22.5, 20.0, 19.5 12.0 22.0, 20.3
9.34 18.14 9.2, 8.8, 10.0 18.0, 18.3, 16-6 11.9, 11.9, 11.1 22.4, 19.9, 19.4 10.7 20.7, 19.0
93.4 90.7 92.0, 88.0, 100.0 90.0, 91.5, 83.0 119,119,111 112, 99.5, 97.0 107 103.5, 96.0 99.5
10
10.8, 12.6, 12.2 16.8, 16.6, 16.5 20.9
8.0, 9.7, 9.4 14.0, 13.8, 13.7 18.4
80-0, 97.0, 94.0 93.3, 92.0, 91.3 92.0 91.4
Mean
F r o m 5 g of dried liver powderA 2.8 B 2.8 D 2.5 Mean
15
20
arsenic in organic matter. The newly recommended method is at least as precise and sensitive as the previous method and is also easier and quicker to carry out. Only tin and phosphate interfere; interference from both tin and phosphate can be avoided by simple modifications, details of which are given in the Appendix.
TABLEVIII RECOVERY OF ARSENIC
FROM ANIMAL FEEDINGSTUFF BY THE BROMIDE DISTILLATION METHOD I N T H E PRESENCE O F T I N
Laboratory A
Arsenic added/pg 10 20
B
10 20
Tin addedlmg Arsenic recovered*/pg 1-25 9.05 1.25 17.95 0.50 8-7 0.50 18.4 * After subtraction of blank.
Recovery, per cent. 90.6 89-75 87.0 92.0
From the results obtained in the collaborative comparison, the Sub-committee considers that the silver diethyldithiocarbamate method2 cannot be recommended for the determination of small amounts of arsenic in organic matter generally. This method could, however, find use for the routine determination of arsenic in materials that do not require prolonged or rigorous treatment in order to obtain the arsenic in solution and also as a more precise alternative to the Gutzeit screening test. In view of incomplete recovery of arsenic from dried liver by the silver diethyldithiocarbamate procedure, the Sub-committee intends to have a radiotracer investigation carried out to see if some explanation can be found.
TABLE I:X RECOVERY OF
Sucrose . . Sucrose . Feedingstuff Feedingstuff Dried liver Dried liver
.
ORGANIC ARSENIC FROM ORGANIC MATTER B Y T H E BROMIDE DISTILLATION METHOD
Sample
.. .. .. .. .. ..
..
.. .. .. .. ..
Arsenic added as carbarsonelpg Arsenic recovered*/pg 15 13.0 15 12.0 15 13.0 15 12.45 15 13-8 15 14.0 * After subtraction of blank.
Recovery, per cent. 86.6 80.0 86.6 83.0 92.0 93.3
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January, 1975
SMALL AMOUNTS O F ARSENIC I N ORGANIC MATTER
61
APPENDIX Recommended Method for the Determination of Small Amounts of Arsenic in Organic Matter Principle of Method After destruction of the organic matter by wet oxidation, the solution is transferred to the distillation apparatus and the arsenic is distilled off as arsenic(II1) bromide. The arsenic is then determined spectrophotometrically as a molybdenum-blue compound. If tin is present, it must be removed after wet oxidation by extraction with cupferron.1 Range For arsenic contents from 1 to 25 pg in the sample taken. Apparatus Details of the apparatus are shown in Figs. 1 and 2 (see NOTE). Reagents Sulphuric acid. For foodstuffs analysis. Nitric acid. For foodstuffs analysis. Potassium bromide solution. A 30 per cent. m/V solution in water. Ammonium molybdate solzdion. A 1per cent. m/V solution in 10 per cent. V / V sulphuric acid. Hydrazine sulphate solution. A 0.05 per cent. m/V solution in water. Standard arsenic solution. Dissolve 4.17 g of analytical-reagent grade sodium arsenate, Na,HAs0,.7H20, in water and dilute the solution to 1 1 (solution A). Dilute 10.0ml of solution A to 1 1 with water (solution B). Prepare solution B freshly as required. 1 ml of solution = 10.0 pg of arsenic. Ammonia solution, sp. gr. 0438. Hydrochloric acid, 1 M. The following additional reagents are required if tin is present. Chloroform. Cupferron solution. A 5 per cent. m/V solution in water. Procedure Reagent Blank Carry out a blank test by the entire procedure, using the exact amounts of reagents used in the test, omitting only the sample. Destruction of Organic Matter Wet oxidise a known mass, not exceeding 5 g of dry matter, of the sample with 5 ml of concentrated sulphuric acid and a suitable amount of concentrated nitric acid. Adjust the volume to 5 ml with sulphuric acid. Transfer the wet digest to the flask A (Fig. l), using three 5-ml portions of distilled water to wash out the digestion flask, heat the contents of the flask, without assembling the apparatus, until fuming and then allow to cool. Procedure in the Presence of Tin Transfer the residue from the wet oxidation to a 100-ml separating funnel and dilute with water to 50 ml. Cool and add 2 ml of cupferron solution and 10 ml of chloroform. Shake vigorously for 2 min. Allow the layers to separate, run off the chloroform layer immediately and discard. Extract the aqueous layer with 10 ml of chloroform and discard the chloroform. Transfer the aqueous layer to the distillation flask, A, evaporate to fumes and then allow to cool. Procedure for the Separation of Arsenic Assemble the apparatus with both TIand T, shut, but without condenser C in position. Introduce two or three drops of water into the capillary, B, through the open top, T, and 5 ml of water into the flask, A, through the tap funnel, G. Introduce 3 ml of 30 per cent. m/V potassium bromide solution into the tap-funnel, G, ready for addition to flask A.
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ANALYTICAL. METHODS COMMITTEE
Heat flask A gently and, when the condensing vapour front reaches the splash plate, D, pipette 3 ml of water into trap H through the open top, T, avoiding the side-arm, F. Fit the condenser in position, open tap TI, force the potassium bromide solution into flask A by means of the rubber teat, R, followed by 1 ml of water and then remove the bung and teat. Close tap TI, continue the heating and ensure that the vapours follow their correct path past plate D and through trap H by shielding the arm, B, with asbestos-paper or string. When white fumes appear at the top of flask A, open the tap, TI,of funnel G and remove the source of heat. Dismantle the apparatus and run the distillate in trap H through tap T, into a 25-ml calibrated flask. Wash trap H and the top part of the apparatus with three 2-ml portions of water and add the washings to the contents of the flask. There is a positive interference if phosphate is present in the digest after wet oxidation to the extent that 0.1 g of phosphate gives a colour in the final determination after distillation equivalent to 1 p g of arsenic. This interference can be overcome by double-distillation as follows. Return the distillate from trap H to flask A after the residue has been removed, with the three 2-ml portions of washing water, add 5 rnl of concentrated sulphuric acid, evaporate to fumes and then allow to cool. Proceed as before starting at “Assemble . . .”.
Determination of Arsenic Make the contents of the flask just alkaline by adding concentrated ammonia solution, using one drop of phenolphthalein solution as indicator, and then add 3 ml of 1 M hydrochloric acid. Add 2 ml of ammonium molybdate solution, 2 ml of hydrazine solution and make the volume up to 25 ml with water. Heat the flask for 10-15 min in a boiling water bath, and allow to cool for 15 min. Measure the absorbance at 840 nm in a 20-mm cuvette with distilled water in the reference cell. Preparation of Calibration Graph Prepare a set of standards by pipetting aliquots of the standard arsenic solution corresponding to 0, 5, 10, 15, 20 and 25 pg of arsenic into a series of 25-ml calibrated flasks. Add 3 ml of 1 M hydrochloric acid, 2 ml of ammonium molybdate solution, 2 ml of hydrazine solution and dilute to 25 ml with water. Heat each flask for 10-15 min in a boiling water bath and allow to cool for 15 min. Measure the absorbance at 840nm in 20-mm cuvettes with distilled water in the reference cell. NOTEIt is essential that, before using the apparatus, concentrated hydrochloric acid should be refluxed in i t for several hours in order to remove any traces of tin, which would interfere. The original apparatus, as described by Chaney and Magnuson,ll has been slightly modified by introducing a coil or zig-zag instead of a small fractionating column. Experience in the use of this apparatus by members of the Sub-committee has shown that the coil, as a part of trap H, is less convenient than an ordinary zig-zag.
References Analytical Methods Committee, Analyst, 1960, 85, 629. British Standards Institution, B.S. 4404 : 1968, Hoffman, L., and Rowsome, M., Analyst, 1960, 85, 151. Magnuson, H. J., and Watson, E. B., Ind. Engng Chem. Analyt. Edn, 1944, 16, 339. Analytical Methods Committee, Analyst, 1960, 85, 643. Gorsuch, T. T., Analyst, 1959, 84, 147. Dubois, L., Tachman, T., Baker, C. J., Zdrojewski, A., and Monkman, J. L., Microclzim. Acta, 1969, 185. 8. Bode, H., and Hachmann, K., 2.Analyt. Clzem., 1968, 171, 383. 9. Martins, H. M., and Whitnack, G. C., Science, N . Y , 1971, 171, 383. 10. Bartlet, J . C., Wood, M., and Chapman, R. A., Analyt. Chem., 1952, 24, 1821. 11. Chaney, A. L., and Magnuson, H. J., Ind. Engng Chem. Analyt. Edn, 1940, 12, 691. 1. 2. 3. 4. 6. 6. 7.
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Analytical Methods Committee
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REPORT PREPARED BY THE METALLIC IMPURITIES I N ORGANIC MATTER SUB-COMMITTEE
The Determination of Small Amounts of Arsenic in Organic Matter The Analytical Methods Committee has received and approved for publication t h e following Report from its Metallic Impurities in Organic Matter Sub-committee.
Report The constitution of the Metallic Impurities in Organic Matter Sub-committee responsible for the preparation of this Report was: Dr. L. E. Coles (Chairman), Mr. J. R. Bishop (resigned May, 1973), Mr. W. Cassidy, Dr. J. C. Gage (resigned May, 1972), Dr. R. A. Hoodless, Mr. E. E. J. King, Mr. D. A. Lambie, Dr. H. Liebmann (resigned September, 1972), Dr. R. F. Milton, Mr. W. L. Sheppard and Mr. C. A. Watson, with Mr. P. W. Shallis as Secretary. In 1960, the Analytical Methods Comniittee recommended1 the use of the Gutzeit method for the determination of arsenic and a specified molybdenum-blue method for the accurate determination of arsenic in organic matter in the range 1.5 to 15 pg. Subsequently, another colorimetric method involving the use of the reagent silver diethyldithiocarbamate has been proposed and has been published by the British Standards Institution.2 The Metallic Impurities in Organic Matter Sub-Committee has, at the request of the Analytical Methods Committee, carried out a comparison of its recommended molybdenumblue method with the silver diethyldithiocarbamate method, and the results of this work, which included an investigation of the volatility of arsenic during wet oxidation, are given in this report. Although the Sub-committee found that its recommended molybdenum-blue method was, of the two, the method to be preferred for determining small amounts of arsenic in organic matter, this method was itself considered to have disadvantages in respect of the time required to carry out the determination and the high degree of analytical expertise needed. In consequence, at a late stage in the work described, the Sub-committee investigated an alternative molybdenum-blue procedure in which the arsenic is first separated by distillation as the bromide in a special This method was found to offer considerable advantages over the originally recommended molybdenum-blue method,l which it is now proposed it should replace. The Sub-committee also considered the possibility of determining arsenic by atomicabsorption spectroscopy, but in view of the problems associated with low signal t o noise ratios in the flame, it was decided not to pursue this at present. Introduction The presence of arsenic in food is regarded as purely adventitious. It is, however, used in various forms as an additive within prescribed limits to animal feeds for the purpose of improving growth, health and feed conversion of livestock, especially pigs and poultry, although the animals may retain arsenic in their livers. Arsenic can be found on apples that have been sprayed with the insecticide lead arsenate and in shellfish, in which it appears t o occur naturally. The Arsenic in Food Regulations 1959 impose, in general, a maximum limit of 1 p.p.m. of arsenic, with lower limits for certain beverages and higher limits for some specified foods. Experimental Comparison of Molybdenum-blue and Silver Diethyldithiocarbamate Methods Before the direct comparison of the two methods was carried out, the recovery of arsenic after wet oxidation was investigated. Collaborative comparison of the methods1s2 was then made by determining by both methods known amounts of arsenic (a) from pure solution and (b) from dried liver, sucrose and animal feedingstuff after wet oxidation. Additional
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work by the silver diethyldithiocarbamate method alone was carried out on the recovery of arsenic added as an organic arsenical to dried liver, sucrose and animal feedingstuff and on the recovery of arsenic added to sucrose in the presence of possible interfering metals. Details of this work and the results are given below. Recovery of arsenic after wet oxidation One member of the Sub-committee measured arsenic radiochemically before and after wet oxidation in order to establish whether any loss of arsenic due to volatilisation could be expected. In separate experiments, about 30 pg of arsenic containing arsenic-74 tracer were added to 5 g of sucrose and 5 g of animal feedingstuff. Each mixture was oxidised with sulphuric and nitric acids5 in the apparatus recommended by Gorsuch,6 which allows the liquid driven off during the oxidation to be collected. The distillate was collected as two fractions: the first from the preliminary boiling with 20 ml of nitric acid, during which the volume was reduced to 10 ml, and the second from the remainder that was distilled during the boiling with sulphuric and nitric acids. The distribution of the arsenic, measured radiochemically, between the fractions and the residue (digest) is shown in Table I. The slight deficiency in recovery was probably due to absorption on the glass vessel or mechanical loss owing to difficulty in recovering arsenic from the flask and that part of the receiver - condenser assembly into which it may have been splashed. These experiments show that arsenic, as an inorganic compound added to organic materials, is not lost by volatilisation during the recommended wet-oxidation procedure with sulphuric and nitric acids, provided that oxidising conditions are maintained throughout the digestion.
TABLE I RECOVERY OF Sample Sucrose (1) .. Sucrose (2) .. Feedingstuff ..
-4RSENIC AFTER WET OXIDATION O F ORGANIC MATTER
..
..
..
Arsenic added/pg 33-0 33-0 33.4
Arsenic recoveredlpg 32.7 32.6 32.8
Arsenic in first fractionlpg 0 0.02 0.06
Arsenic in second fractionlpg 0.03
0.04 0.15
Recovery of arsenic f r o m pure solution Solutions containing 15 pg of arsenic and no interfering substances or organic matter were subjected to wet oxidation with sulphuric and nitric acids, and the arsenic was determined in the digests by both methods. The results, which indicate a higher mean recovery of arsenic by the silver diethyldithiocarbamate method, are shown in Table 11. The standard deviations given in this and subsequent tables were calculated from the equation s.d. =
J
(x, - Zl)2 (n, - 1)
+ (x2-
+
(n2
4
2
+ (x, -
- 1)
+
(n,
Z3)2
+ ... -
- 1) *
TABLEI1 RECOVERY OF
ARSENIC BY BOTH METHODS FROM PURE SOLUTION In each test, 15 pg of arsenic were present.
Molybdenum-blue method r
Silver diethyldithiocarbamate method 1
Laboratory A
Arsenic foundlpg 12.9, 13.2, 12.9
Recovery, per cent. 86.0, 88.0, 86.0
B C D
14.6, 16.25, 14-0, 15.0, 13.75, 15-5 13.4, 13.0 13.3, 13.8, 12.5
97.3, 108, 93.3, 100, 91.7, 103 89.3, 86.7 88.7, 92.0, 83.3
E
15.8, 14.8, 15.3
105, 98.7, 102
14.1
94.0
Mean .. .. Standard deviation
0-70
Arsenic found/pg Recovery, per cent. 13-4, 13.0, 13.0, 89.3, 86.7, 86.7, 17.0, 16-6, 16.4 113, 111, 109 14-75. 15.25, 14.6, 98.3, 102, 97-3, 15.0. 15.5, 14-75 100, 103, 98.3 14.8, 14.8, 15.0 98.7, 98.7, 100 14.8, 14-8, 98.7, 98.7, 15.0, 14-5,15.4 100, 96.7, 103 13.75 91.7 14.9 1.12
99.3
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TABLEI11
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RECOVERY OF
ARSENIC BY THE SILVER DIETHYLDITHIOCARBAMATE METHOD
Arsenic Arsenic Laboratory blank/ p g €ound/pg From 5 g of sucrose containing 15 pg of added amenic-A 0.1 12.9, 12.4, 12-2 B 0.1 12.8, 11.6, 13.4 C 0.3 15.6, 17.5, 15.6 D 0 14.8, 14.8, 15.0 Mean Standard deviation
Arsenic recovered/ pg
Recovery, per cent.
12.8, 12.3, P2.1 12.7, 11.5, 13.3 15.3, 17.2, 15.3 14.8, 14.8, 15.0 13-9 0.74
85.3, 82.0, 80.7 P -A - I-I., 76.7, 88.7 102, 115, 102 98.7, 98.7, 100 92.6
F r o m 5 g of feedingstuff containing 15 p g of added arsenic-B 0.9 14.3, 14.6, 15-7, 12-8, 14.4 C 0.8 13.2, 12.7, 13.8 F 0.9 15.2, 13.2 Mean Standard deviation
13.4, 13.7, 14.8, 11.9, 13.5 12.4, 11-9, 13-0 14.3, 12.3 13.1 0.99
89.3, 91.3, 98.7, 79.3, 90.0 82.7, 79-3, 86.7 95.3, 82.0
F r o m 5 g of dried liver powder containing 15 pg of added arsenic12.5, 12-4, 12.0 A 2-6 13.7, 13.5, 13.9 B 3.1 13.0, 10.1, 11.8 C 2.0 10.5, 10.5, 11.0 D 2.6 3.0 14.2 iMean Standard deviation
9.9, 9.8, 9.4 10.6, 10.4, 10.8 11.0, 8-1, 9.8 7.9, 7.9, 8.4 11.2 9.6 1.15
66.0, 65.3, 62.7 70.7, 69-3, 72.0 73-3, 54.0, 65-3 52.7, 52.7, 56.0 74'7 64.2
87.5
Recovery of arsenic from organic matter Samples of sucrose, an animal feedingstuff and dried liver powder, to each of which 15 pg of arsenic were added as a standard solution of an inorganic arsenic compound, were wet oxidised with sulphuric and nitric acids. The arsenic content was determined by both methods, and the results are shown in Tables I11 and IV. From the results in these tables it can be seen that recoveries of arsenic from sucrose and the feedingstuff were satisfactory by both methods and that recoveries of arsenic from the dried liver powder were satisfactory by the molybdenum-blue method and rather low by the silver diethyldithiocarbamate method. RECOVERY OF Laboratory
Arsenic blank/ pg
TABLE IV MOLYBDENUM-BLUE METHOD
ARSENIC BY THE
Arsenic found/ pg
Arsenic recovered/ pg
Recovery, per cent.
F r o m 5 g of sucrose containing 16 pg of added arsenic13.0, 12.4, 12-3 A 0.27 13.5, 13.0, 11-0 B 0.1 11.1, 12.0 C 1.0 12.8, 13.3 D 0.5 Mean Standard deviation
12-7, 12.1, 12.0 13.4, 12.9, 10-9 10.1, 11.0 12.3, 12.8 12.0 0.79
86.7, 80.7, 80.0 89.3, 86.0, 71-3 67.3, 73-3 82.0, 84.3 80.4
F r o m 5 g of feedingstuff containing 15 p g of added arsenicB 2-6 15.0, 14.8, 14.8, 15.0 D 2.1 15.0, 16.1, 15-3 Mean Standard deviation
12-4, 12.2, 12.3, 12.4 12.9, 14.0, 13.2 12.8 0-37
82.6. 81.3, 82.0, 82.6 86.0, 93.3, 88.0 86.1
12.5, 10.7, 11.6 14.4, 14-4, 9.6 14.6 11.7, 14.4, 11.7 12.6 1.91
83.3, 71.3, 77.3 96.0, 96.0, 64.0 97.4 78.0,96.0, 78-0 83-1
From 5 g of dried powder containing 15 p g of added arsenic-
B C D E Mean Standard deviation
3.2 2.9 1.0 4.6
15.8, 14-0, 14.9 17.3, 17.3, 12.6 15-6 16.3, 19.0, 16.3
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57
SMALL AMOUNTS OF ARSENIC IN ORGANIC MATTER
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Recovery of arsenic added as an organic arsenical All of the experimental work described above had been carried out on the recovery of arsenic added as a known volume of a standard solution of inorganic arsenic. It was decided, therefore, to ensure that arsenic in an organic form could be determined satisfactorily. Samples of sucrose, animal feed and dried liver to which 15 pg of arsenic, as 4-ureido-l-phenylarsonic acid (carbarsone), had been added, were wet oxidised with sulphuric and nitric acids, and the arsenic contents were determined by the silver diethyldithiocarbamate method. The results are shown in Table V and indicate that arsenic in organic combination can, after wet oxidation, be determined satisfactorily, with recovery levels similar to those obtained for the addition of inorganic arsenic.
TABLE V RECOVERY OF Sample
Sucrose . . Sucrose . . Feedingstuff Feedingstuff Dried liver Dried liver
.. .. .. .. .. ..
.. .. .. .. .. ..
ORGANIC ARSENIC FROM ORGANIC MATTER
Arsenic added as carbarsone/pg 15 16 15 15 15 15
Arsenic recoveredig 13.7 13.95 12-5 12.1 11.2 9.9
Recovery, per cent. 91-3 93.0 83.3 80.7 74.7 66.0
Interfe~ingszcbstances The dried liver powder used in the collaborative work was known to contain high levels of certain trace metals, which could have been responsible for the rather low recoveries reported. In view of this, a collaborative investigation was carried out in which laboratories added to sucrose known amounts of arsenic and certain other metals in order to establish whether or not these metals had any effect on the subsequent determination of arsenic. After wet oxidation of the organic matter with sulphuric and nitric acids, the arsenic contents were determined by the silver diethyldithiocarbamate method. The results are shown in Table VI and indicate that the presence of lead, calcium, iron, manganese and chromium, at the levels used, do not reduce the recovery of arsenic by the silver diethyldithiocarbamate method.
TABLE VI RECOVERY OF
ARSENIC FROM SUCROSE IN THE PRESENCE OF OTHER METALS
A 5-g sample of sucrose containing 16 pg of added arsenic was used in each test. Laboratory A 13
C D F
Metal Lead Calcium Iron Manganese Chromium
Level of addition, per cent.
Arsenic recovered/pg* 0.01 11.6, 12.7, 12-5 1-0 13.1, 13-9, 11.0 0.4 16.2 0.1 15.0, 15.3, 15.3 0.12 14.0, 13.7, 13.9 * After subtraction of blank.
Recovery, per cent. 77.3, 84.7, 83.3 87.3, 92.7, 73.3 101.3 100.0, 102.0, 102.0 93.3, 91.3, 92.7
Discussion The silver diethyldithiocarbamate method was found to be quicker to apply and to require less analytical expertise than did the molybdenum-blue method. For the determination of arsenic in pure solutions and in organic materials, such as sucrose and animal feedingstuff, that are easy to digest with sulphuric and nitric acids, both methods gave satisfactory results. However, for the determination of arsenic in dried liver powder, a material that is difficult to oxidise with sulphuric and nitric acids, satisfactory results were obtained by the molybdenum-blue method. The Sub-committee is of the opinion that severe conditions of wet oxidation requiring the use of comparatively large volumes of acids can have a profound effect on the recovery of arsenic by the silver diethyldithiocarbamate method. However, in an attempt to obtain more information on the reasons for the failure
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to recover all of the arsenic by the silver diethyldithiocarbamate method, a radiochemical investigation has been arranged, the results of which the Sub-committee hopes to be able to report later. In the view of members of the Sub-committee, the silver diethyldithiocarbamate method suffers certain other disadvantages. Wide variations between batches of the reagent were noticed and, in the collaborative work undertaken by the Sub-committee, all members used reagent from a selected satisfactory batch. Batches of the reagent giving absorbance readings between 0.02 and 0-05unit per microgram of arsenic have been found; variations of this type have not been found for the molybdenum-blue method over a period of 13 years. One laboratory examined the precisions of the two methods, without pre-separation, on a sample of hydrochloric acid containing 10 pg of added arsenic. The standard deviation of the results by the molybdenum-blue method was 0.2 pg, whereas for the silver diethyldithiocarbamate method it was 0-4 pg. For samples requiring a separation procedure, better results would still be expected by the molybdenum-blue method than by the silver diethyldithiocarbamate method. Another problem with the silver diethyldithiocarbamate method results from the unknown composition of the arsenic complex, which not only varies in colour intensity from batch to batch of the reagent, but can also have a wavelength of maximum absorption in the range from about 520 to 550 nm. The composition of the reagent itself can vary between having a small excess of silver and a small excess of diethyldithiocarbamate. The reagent containing a small excess of silver yields a complex that has a higher sensitivity to arsenic and absorbs at a longer wavelength than does the complex yielded by the reagent containing a small excess of diethyldithiocarbamate. Highest sensitivity of the reagent to arsenic is, however, accompanied by high sensitivity to antimony and germanium ; the reaction with antimony may have up to 60 per cent. of the sensitivity of that with arsenic on a mass basis. On the other hand, the form of the reagent that gives lowest sensitivity to arsenic may have a sensitivity to antimony as low as 1 per cent. of that to arsenic. These findings are similar to those of Dubois et a,?.’; they do not, however, accord with the findings of Bode and Hachmann.s Difficulties can also be encountered in the silver diethyldithiocarbamate method owing to the presence of substances that interfere with the formation of the complex or inhibit at much lower levels of arsenic the evolution of arsine. Martins and W h i t n a ~ k working ,~ than the Sub-committee, found the reagent to be unreliable for determining arsenic in drinking water owing to the presence of trace amounts (2-500 p.p.b.1 of chromium, molybdenum and vanadium, which suppressed the formation of the arsenic complex, and copper, which caused high results. Early in the Sub-Committee’s work, two members, using different batches of reagent, found an apparent loss of arsenic in the presence of between 2 and 70 mg of nitrate, as nitric acid. This loss could be particularly serious when determining arsenic in wet digests from organic matter, which frequently contain small amounts of nitrate that might be difficult to remove without loss of arsenic. Although the collaborative comparison of the two methods showed that the molybdenumblue method gave satisfactory results under all the conditions examined, whereas the silver diethyldithiocarbamate method did not, the general levels of recovery of arsenic by the molybdenum-blue method were inferior to those reported by the Sub-committee in 1960.l In view of this finding and the fact that the method is time consuming and requires considerable analytical expertise in order to apply it successfully, the Sub-committee decided that, before making a final recommendation, it would investigate an alternative molybdenumblue procedure based on Hoffman and Rowsome’s modification3 of Magnuson and Watson’s method.* Bromide Distillation Method It has been shown3t4that a wet digest containing arsenic can be distilled with a quantititative transfer of arsenic, as the bromide, to the distillate. After a carefully controlled wet oxidation with sulphuric and nitric acids, the digest is distilled in a special apparatus (Figs. 1 and 2) in the presence of potassium bromide. Arsenic in the distillate is determined spectrophotometrically as the molybdenum-blue complex. The method is far quicker to use than the currently recommended molybdenum-blue procedure,l the colour complex formed is stable and reproducible, frequent calibration is not required and readily available repro-
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JanzLary, 1975 SMALL AMOUNTS OF ARSENIC I N ORGANIC MATTER 59 ducible reagents are used. It is also virtually free from interferences, except from large amounts of phosphate and tin. The method was collaboratively investigated on samples of an animal feedingstuff and dried liver powder. The results, which are given in Table VII, indicate a generally higher level of recovery of arsenic than is shown in Tables I11 and IV. In addition, two laboratories checked the recovery of arsenic by the method in the absence of organic matter; results of 100.0 and 101.0 per cent. were obtained.
mm 0
R 50
I 00
I50
G
200
250
Fig. 2. Distillation apparatus
for arsenic (details of trap head). Fig. 1. Distillation apparatus for arsenic (complete assembly).
In amounts over 500 pg, tin interferes in the method by causing turbidity in the final solution after development of the molybdenum-blue colour, giving rise to erroneous absorbance measurements. This interference can be avoided by a simple modification involving addition of aqueous cupferron followed by extraction with chloroform before the distillation. The modification was tested on samples of an animal feedingstuff and the results, which show that there is little difference in the recovery of arsenic in the presence or absence of tin, are shown in Table VIII. Finally, one laboratory carried out recovery experiments on arsenic added as carbarsone to organic matter; the results are shown in Table IX. Conclusions The Sub-committee recommends that the molybdenum-blue procedure applied after separation of arsenic by distillation, full details of which are given in the Appendix, should replace the previously recommended methodl for the determination of small amounts of
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A.Izdyd, VOZ. 100
TABLE VII RECOVERY OF Arsenic blank/pg
Laboratory
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F r o m 6 g of feedingstuflA 0.06 B
0.9
C
0.1 1.3
ARSENIC BY BROMIDE DISTILLATION METHOD
Arsenic added/pg
Arsenic found 'pg
Arsenic recovered/ pg
Recovery, per cent.
10 20 10 20 10 20 10 20
9.4 18.2 10.1, 9.7, 10.9 18.9, 19.2, 17.5 12.0, 12.0, 11.2 22.5, 20.0, 19.5 12.0 22.0, 20.3
9.34 18.14 9.2, 8.8, 10.0 18.0, 18.3, 16-6 11.9, 11.9, 11.1 22.4, 19.9, 19.4 10.7 20.7, 19.0
93.4 90.7 92.0, 88.0, 100.0 90.0, 91.5, 83.0 119,119,111 112, 99.5, 97.0 107 103.5, 96.0 99.5
10
10.8, 12.6, 12.2 16.8, 16.6, 16.5 20.9
8.0, 9.7, 9.4 14.0, 13.8, 13.7 18.4
80-0, 97.0, 94.0 93.3, 92.0, 91.3 92.0 91.4
Mean
F r o m 5 g of dried liver powderA 2.8 B 2.8 D 2.5 Mean
15
20
arsenic in organic matter. The newly recommended method is at least as precise and sensitive as the previous method and is also easier and quicker to carry out. Only tin and phosphate interfere; interference from both tin and phosphate can be avoided by simple modifications, details of which are given in the Appendix.
TABLEVIII RECOVERY OF ARSENIC
FROM ANIMAL FEEDINGSTUFF BY THE BROMIDE DISTILLATION METHOD I N T H E PRESENCE O F T I N
Laboratory A
Arsenic added/pg 10 20
B
10 20
Tin addedlmg Arsenic recovered*/pg 1-25 9.05 1.25 17.95 0.50 8-7 0.50 18.4 * After subtraction of blank.
Recovery, per cent. 90.6 89-75 87.0 92.0
From the results obtained in the collaborative comparison, the Sub-committee considers that the silver diethyldithiocarbamate method2 cannot be recommended for the determination of small amounts of arsenic in organic matter generally. This method could, however, find use for the routine determination of arsenic in materials that do not require prolonged or rigorous treatment in order to obtain the arsenic in solution and also as a more precise alternative to the Gutzeit screening test. In view of incomplete recovery of arsenic from dried liver by the silver diethyldithiocarbamate procedure, the Sub-committee intends to have a radiotracer investigation carried out to see if some explanation can be found.
TABLE I:X RECOVERY OF
Sucrose . . Sucrose . Feedingstuff Feedingstuff Dried liver Dried liver
.
ORGANIC ARSENIC FROM ORGANIC MATTER B Y T H E BROMIDE DISTILLATION METHOD
Sample
.. .. .. .. .. ..
..
.. .. .. .. ..
Arsenic added as carbarsonelpg Arsenic recovered*/pg 15 13.0 15 12.0 15 13.0 15 12.45 15 13-8 15 14.0 * After subtraction of blank.
Recovery, per cent. 86.6 80.0 86.6 83.0 92.0 93.3
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SMALL AMOUNTS O F ARSENIC I N ORGANIC MATTER
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APPENDIX Recommended Method for the Determination of Small Amounts of Arsenic in Organic Matter Principle of Method After destruction of the organic matter by wet oxidation, the solution is transferred to the distillation apparatus and the arsenic is distilled off as arsenic(II1) bromide. The arsenic is then determined spectrophotometrically as a molybdenum-blue compound. If tin is present, it must be removed after wet oxidation by extraction with cupferron.1 Range For arsenic contents from 1 to 25 pg in the sample taken. Apparatus Details of the apparatus are shown in Figs. 1 and 2 (see NOTE). Reagents Sulphuric acid. For foodstuffs analysis. Nitric acid. For foodstuffs analysis. Potassium bromide solution. A 30 per cent. m/V solution in water. Ammonium molybdate solzdion. A 1per cent. m/V solution in 10 per cent. V / V sulphuric acid. Hydrazine sulphate solution. A 0.05 per cent. m/V solution in water. Standard arsenic solution. Dissolve 4.17 g of analytical-reagent grade sodium arsenate, Na,HAs0,.7H20, in water and dilute the solution to 1 1 (solution A). Dilute 10.0ml of solution A to 1 1 with water (solution B). Prepare solution B freshly as required. 1 ml of solution = 10.0 pg of arsenic. Ammonia solution, sp. gr. 0438. Hydrochloric acid, 1 M. The following additional reagents are required if tin is present. Chloroform. Cupferron solution. A 5 per cent. m/V solution in water. Procedure Reagent Blank Carry out a blank test by the entire procedure, using the exact amounts of reagents used in the test, omitting only the sample. Destruction of Organic Matter Wet oxidise a known mass, not exceeding 5 g of dry matter, of the sample with 5 ml of concentrated sulphuric acid and a suitable amount of concentrated nitric acid. Adjust the volume to 5 ml with sulphuric acid. Transfer the wet digest to the flask A (Fig. l), using three 5-ml portions of distilled water to wash out the digestion flask, heat the contents of the flask, without assembling the apparatus, until fuming and then allow to cool. Procedure in the Presence of Tin Transfer the residue from the wet oxidation to a 100-ml separating funnel and dilute with water to 50 ml. Cool and add 2 ml of cupferron solution and 10 ml of chloroform. Shake vigorously for 2 min. Allow the layers to separate, run off the chloroform layer immediately and discard. Extract the aqueous layer with 10 ml of chloroform and discard the chloroform. Transfer the aqueous layer to the distillation flask, A, evaporate to fumes and then allow to cool. Procedure for the Separation of Arsenic Assemble the apparatus with both TIand T, shut, but without condenser C in position. Introduce two or three drops of water into the capillary, B, through the open top, T, and 5 ml of water into the flask, A, through the tap funnel, G. Introduce 3 ml of 30 per cent. m/V potassium bromide solution into the tap-funnel, G, ready for addition to flask A.
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ANALYTICAL. METHODS COMMITTEE
Heat flask A gently and, when the condensing vapour front reaches the splash plate, D, pipette 3 ml of water into trap H through the open top, T, avoiding the side-arm, F. Fit the condenser in position, open tap TI, force the potassium bromide solution into flask A by means of the rubber teat, R, followed by 1 ml of water and then remove the bung and teat. Close tap TI, continue the heating and ensure that the vapours follow their correct path past plate D and through trap H by shielding the arm, B, with asbestos-paper or string. When white fumes appear at the top of flask A, open the tap, TI,of funnel G and remove the source of heat. Dismantle the apparatus and run the distillate in trap H through tap T, into a 25-ml calibrated flask. Wash trap H and the top part of the apparatus with three 2-ml portions of water and add the washings to the contents of the flask. There is a positive interference if phosphate is present in the digest after wet oxidation to the extent that 0.1 g of phosphate gives a colour in the final determination after distillation equivalent to 1 p g of arsenic. This interference can be overcome by double-distillation as follows. Return the distillate from trap H to flask A after the residue has been removed, with the three 2-ml portions of washing water, add 5 rnl of concentrated sulphuric acid, evaporate to fumes and then allow to cool. Proceed as before starting at “Assemble . . .”.
Determination of Arsenic Make the contents of the flask just alkaline by adding concentrated ammonia solution, using one drop of phenolphthalein solution as indicator, and then add 3 ml of 1 M hydrochloric acid. Add 2 ml of ammonium molybdate solution, 2 ml of hydrazine solution and make the volume up to 25 ml with water. Heat the flask for 10-15 min in a boiling water bath, and allow to cool for 15 min. Measure the absorbance at 840 nm in a 20-mm cuvette with distilled water in the reference cell. Preparation of Calibration Graph Prepare a set of standards by pipetting aliquots of the standard arsenic solution corresponding to 0, 5, 10, 15, 20 and 25 pg of arsenic into a series of 25-ml calibrated flasks. Add 3 ml of 1 M hydrochloric acid, 2 ml of ammonium molybdate solution, 2 ml of hydrazine solution and dilute to 25 ml with water. Heat each flask for 10-15 min in a boiling water bath and allow to cool for 15 min. Measure the absorbance at 840nm in 20-mm cuvettes with distilled water in the reference cell. NOTEIt is essential that, before using the apparatus, concentrated hydrochloric acid should be refluxed in i t for several hours in order to remove any traces of tin, which would interfere. The original apparatus, as described by Chaney and Magnuson,ll has been slightly modified by introducing a coil or zig-zag instead of a small fractionating column. Experience in the use of this apparatus by members of the Sub-committee has shown that the coil, as a part of trap H, is less convenient than an ordinary zig-zag.
References Analytical Methods Committee, Analyst, 1960, 85, 629. British Standards Institution, B.S. 4404 : 1968, Hoffman, L., and Rowsome, M., Analyst, 1960, 85, 151. Magnuson, H. J., and Watson, E. B., Ind. Engng Chem. Analyt. Edn, 1944, 16, 339. Analytical Methods Committee, Analyst, 1960, 85, 643. Gorsuch, T. T., Analyst, 1959, 84, 147. Dubois, L., Tachman, T., Baker, C. J., Zdrojewski, A., and Monkman, J. L., Microclzim. Acta, 1969, 185. 8. Bode, H., and Hachmann, K., 2.Analyt. Clzem., 1968, 171, 383. 9. Martins, H. M., and Whitnack, G. C., Science, N . Y , 1971, 171, 383. 10. Bartlet, J . C., Wood, M., and Chapman, R. A., Analyt. Chem., 1952, 24, 1821. 11. Chaney, A. L., and Magnuson, H. J., Ind. Engng Chem. Analyt. Edn, 1940, 12, 691. 1. 2. 3. 4. 6. 6. 7.
