Analytica Chimica Acta 842 (2014) 35–41
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The importance of chain length for the polyphosphate enhancement of acidic potassium permanganate chemiluminescence$ Brendan J. Holland a , Jacqui L. Adcock a , Pavel N. Nesterenko b , Anton Peristyy b , Paul G. Stevenson a , Neil W. Barnett a , Xavier A. Conlan a , Paul S. Francis a, * a b
Centre for Chemistry and Biotechnology, School of Life and Environmental Sciences, Deakin University, Geelong, Victoria, Australia Australian Centre for Research on Separation Science, School of Chemistry, University of Tasmania, Hobart, Tasmania, Australia
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
Polyphosphates characterised by titration, NMR and ion chromatography. Different enhancement of permanganate chemiluminescence. There is a minimum polyphosphate chain length required for a large enhancement. No further advantage with much longer chain lengths. Optimum concentration of polyphosphate is dependent on the analyte.
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 18 July 2014
Sodium polyphosphate is commonly used to enhance chemiluminescence reactions with acidic potassium permanganate through a dual enhancement mechanism, but commercially available polyphosphates vary greatly in composition. We have examined the influence of polyphosphate composition and concentration on both the dual enhancement mechanism of chemiluminescence intensity and the stability of the reagent under analytically useful conditions. The average chain length (n) provides a convenient characterisation, but materials with similar values can exhibit markedly different distributions of phosphate oligomers. There is a minimum polyphosphate chain length (6) required for a large enhancement of the emission intensity, but no further advantage was obtained using polyphosphate materials with much longer average chain lengths. Providing there is a sufficient average chain length, the optimum concentration of polyphosphate is dependent on the analyte and in some cases, may be lower than the quantities previously used in routine detection. However, the concentration of polyphosphate should not be lowered in permanganate reagents that have been partially reduced to form high concentrations of the key manganese(III) co-reactant, as this intermediate needs to be stabilised to prevent formation of insoluble manganese(IV). ã 2014 Elsevier B.V. All rights reserved.
Keywords: Chemiluminescence Acidic potassium permanganate Enhancer Sodium polyphosphate Sodium hexametaphosphate
1. Introduction $
Selected papers presented at 21st Annual RACI Research and Development Conference, 11–13th December 2013, Canberra, ACT, Australia. * Corresponding author at: School of Life and Environmental Sciences, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia. Tel.: +61 3 5227 1294; fax: +61 3 5227 1040. E-mail address: [email protected] (P.S. Francis). http://dx.doi.org/10.1016/j.aca.2014.07.012 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.
Acidic potassium permanganate was first used for chemiluminescence detection in 1975 [1] and has subsequently been utilised for an extensive range of analytical applications in pharmaceutical, clinical, forensic, food and agricultural settings [2,3]. The
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characteristic red emission of light from redox reactions with permanganate originates from an electronically excited Mn(II) species, thought to be generated by reduction of the immediate Mn(III) precursor by the radical oxidation intermediates of the analyte [4–6]. Several different compounds have been found to enhance the chemiluminescence, such as low molecular weight aldehydes, sodium thiosulfate and sodium polyphosphates, but their mechanisms are not yet fully understood [2,3]. Polyphosphates (chains of four or more repeating phosphate units) are a popular choice of enhancer due to their innocuous nature and reported 50-fold increases in permanganate chemiluminescence intensity [2,3]. Although the mechanism of enhancement is yet to be completely elucidated, their presence in the reaction mixture induces a shift in the maximum emission wavelength from approximately 734 nm to 689 nm, indicating a strong interaction with the emitting species [6]. A dual mechanism of enhancement has been postulated [6], which includes stabilisation of the Mn(III) precursor with respect to Mn(IV) oxides that would otherwise flocculate under the mildly acidic conditions [7–10], and formation of protective “cage-like” structures around the Mn(II) emitter that inhibit nonradiative relaxation [6]. The shorter chain ortho- and pyro-phosphates also prevent disproportionation of Mn(III), but they are not believed to form protecting structures around the emitter, as they do not enhance the emission to the same extent as polyphosphates, and there is no blue shift in the emission spectrum [6]. Commercially available sodium polyphosphates, including ‘sodium hexametaphosphate’ or Graham’s salt [11], are in fact a complex mixture of mostly linear oligomers with a wide range of chain lengths [12]. These products can be characterised by their average number of phosphate atoms per oligomer, or average chain length (n), typically ranging from 4.5 to 18 [11]. Linear polyphosphates generally exhibit stronger complexing power relative to their cyclic counterparts, arising from the greater net charge at each end of the chain compared to the middle groups [11]. Manganese polyphosphate complexes tend to be more stable as the chain length increases, but in this case the end groups have a lesser influence on complexing power because their contribution to the overall charge of the molecule is reduced [13]. Polyphosphates from a variety of sources have been used to enhance chemiluminescence reactions with permanganate [3], but the influence of the average chain length on emission intensity is not known. It is also unknown why the greatest enhancement requires that the polyphosphates be present in large excess (e.g. 1% m/v) compared to permanganate (1.0 mM) [3,14], though it has been postulated that these conditions favour the formation of the protective cage-like structures and help maintain their structural integrity throughout the reaction [6]. It is also possible that such large excesses are required to ensure sufficient quantities of the oligomers of specific lengths. Herein, we explore a series of commercially available polyphosphate materials as enhancers of permanganate chemiluminescence, examining the influence of polyphosphate structure and concentration on both the enhancement of chemiluminescence intensity and the stability of reagent solutions under analytically useful conditions. 2. Experimental 2.1.
31
P NMR
Solution 31P NMR was performed by dissolution of polyphosphates (approximately 70 mg) in 0.6 mL deionised water and 0.1 mL deuterium oxide (for signal lock). Phosphorous NMR measurements were recorded on a Bruker AVANCE 500 spectrometer (Bruker, Karlsruhe, Germany) operating at 202 MHz, with an acquisition time of 1.63 s. Temperature was regulated at 25 C, and 64 scans were
collected. Chemical shifts were referenced to H3PO4 in acetone. The well-established Eq. (1) was used to calculate the average chain length (n) of each polyphosphate sample [15], where PP1 refers to terminal (end group) phosphates, PP2 and PP3 to second and third position phosphates, and PPn to the inner phosphates: n¼
ðPP1 þ PP2 þ PP3 þ PPnÞ 2 PP1
(1)
2.2. Titration A modified end-group titration without hydrolysis [16] was performed using a Metrohm 907 Titrando (Metrohm, FL, USA) and controlled by Tiamo 2.3 software. Dilute hydrochloric acid solution (2 M) was prepared from analytical grade concentrated hydrochloric acid (32% w/w, Chem Supply, Gilman, SA, Australia). Sodium hydroxide (0.1 M) was prepared from sodium hydroxide pellets (Ajax Finechem, Taren Point, NSW, Australia) and standardised against analytical reagent grade potassium hydrogen phthalate (Fisher Scientific, Loughborough, Leicester, UK). Polyphosphate samples were prepared by complete dissolution in deionised water (2% m/v). The average chain length was calculated using the procedure detailed by Griffith [16]. 2.3. Flow injection analysis (FIA) The analytes were injected (70 mL) on a simple two-line FIA manifold with an automated six-port valve (Valco Instruments, Houston, TX, USA) into a deionised water carrier stream, which merged at a T-piece with the acidic potassium permanganate reagent prior to entering a coiled-tubing detection flow-cell comprising 0.8 mm i.d. PTFE tubing (DKSH, Caboolture, Queensland, Australia). The carrier and reagent lines were propelled through 0.8 mm i.d. PTFE tubing (DKSH) at 4 mL min1 using a peristaltic pump (Gilson Minipuls 3, John Morris Scientific, Balwyn, Victoria, Australia) with bridged PVC tubing (DKSH). The flow-cell was mounted flush against the window of a photomultiplier tube (Electron Tubes model 9828SB; ETP, NSW, Australia) encased in a light tight housing and powered by a stable power supply at 950 V. Chemiluminescence intensities were documented with a chart recorder (YEW type3066, Yokogawa Hokushin Electric, Tokyo, Japan), and the peak heights were measured manually. 2.4. Ion chromatography Chromatographic analysis was carried out using an ICS 3000 chromatography system (Dionex, Thermo Scientific, Sunnyvale, CA, USA). The system was equipped with an autosampler, eluent generation system, anion suppressor and electrochemical conductivity detector. Chromeleon software (version 7.1, Dionex) was used for data acquisition and instrument control. Separation was carried out using a 250 4.0 mm ID column IonPac AS-19 (Dionex), with a 200 mL injection volume and 1 mL min1 flow rate. A 60 min potassium hydroxide gradient was used, starting at 40 mM and increasing linearly to 100 mM over the first 10 min, then held at 100 mM for the remaining 50 min. The column was re-equilibrated for 15 min prior to subsequent analyses. Samples were prepared at 1000 ppm and standards at 200 ppm in a 40 mM potassium hydroxide solution, and analysed in duplicate. 2.5. Spectrophotometric monitoring A Cary 300 Bio UV–vis spectrophotometer (Varian, Mulgrave, Victoria, Australia) was used to measure the absorption of permanganate solutions in a 10 mm quartz cuvette. Sodium thiosulfate was added to the potassium permanganate solution
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Table 1 Comparison of chemiluminescence intensity for the reaction of morphine (1 106 M) and bisphenol A (1 105 M) with acidic potassium permanganate containing phosphate materials of different chain length. Average chain length was classified from supplier specified values and data from titration and 31P NMR experiments. Relative chemiluminescence intensitya
Short chain phosphates
Orthophosphate Pyrophosphate Tripolyphophate Trimetaphosphate (cyclic)
Morphine
Bisphenol A
0.01 0.01 0.02 0.00
0.00 0.00 0.05 0.05
Relative chemiluminescence intensitya
Polyphosphates with average chain length (n) Specified 4 4 7 – – b
16 16 14–18 – 28 30
Titration
31
3.7 4.0 6.7 10.5 10.9 14.4 10.0 11.1 16.1 15.8 22.7 25.7
4.0 4.4 7.2 11.4 12.0 13.9 11.0 11.9 15.0 16.3 25.1 28.4
P NMR
Morphine
Bisphenol A
0.54 0.90 1.45 1.11 1.00 1.46 1.28 1.18 1.14 1.02 1.02 1.22
0.29 0.81 0.90 0.85 1.00 1.24 0.95 0.90 1.00 0.95 1.00 1.00
a Each reagent contained 1 103 M potassium permanganate and 0.1% m/v phosphate material, and was adjusted to pH 2.5 using sulfuric acid. Intensities are relative to those obtained using sodium polyphosphate (+80 mesh) from Sigma– Aldrich. b ‘Crosslinked’ material.
Fig. 1. Representative (a) alkalinity and weak acid titration curve and (b) 31P NMR spectrum used to compare the average chain length of each polyphosphate sample in aqueous solution to the supplier specified values. Example shown is sodium polyphosphate crystals (+80 mesh) sample sourced from Sigma–Aldrich (n = 12).
containing polyphosphates, immediately prior to recording the first spectrum, and the mixture remained untouched until the final spectrum was recorded. 2.6. Sequential injection analysis (SIA) A milliGAT pump (Global FIA, Fox Island, WA, USA) was used for sequential aspiration of permanganate reagent and analyte standards with a ten-port multi-position valve (model C25Z, Valco, SGE, Melbourne, Victoria, Australia) and chemiluminescence detector. The detector comprised a single-inlet serpentine flow cell (Global FIA) mounted flush against the window of a photomultiplier tube (Electron Tubes model 9828SB; ETP) encased in a light tight housing and powered by a stable power supply at 900 V. A data acquisition board (LabJack U12, National Instruments, Victoria, Australia) and LabVIEW software (version 8.0, National Instruments) were used to control the pump and valve. Output from the photomultiplier tube was recorded using the e-corder 410 data acquisition system (eDAQ, Denistone East, NSW, Australia). This automated SIA system was programmed to repeatedly aspirate 150 mL of reagent (valve position 1; 167 mL s1), 1500 mL of the analyte standard (position 2; 167 mL s1) and 1000 mL of deionised water (position 3, 100 mL s1), with a 60 min pause after every 5th cycle.
Morphine was sourced from GlaxoSmithKline (Port Fairy, Victoria, Australia). Stock solutions of morphine (1 103 M) were prepared in acidified deionised water and sonicated to aid dissolution. Bisphenol A (1 103 M) was prepared in methanol and diluted in deionised water as required. The phosphate samples obtained for comparison were sodium dihydrogen orthophosphate from Univar (Ingleburn, NSW, Australia); tetrasodium pyrophosphate from AnalaR (VWR International, Murrarie, Queensland, Australia); sodium hexametaphosphate, polyphosphate sodium salt (+200 mesh), sodium polyphosphate crystals (+80 mesh), trisodium trimetaphosphate, tri-polyphosphate, sodium hexametaphosphate (65–70% P2O5 basis) and sodium phosphate glass from Sigma–Aldrich; sodium hexametaphosphate P60 and P68 from Fibrisol (Heatherton, Victoria, Australia); Budit 3H, 4H, 6H, 7H and 8H instantised sodium polyphosphate, Budit 7N coarse powder sodium polyphosphate and FFB 779 (Budit 9N replacement) sodium polyphosphate powder from BassTech International (Fort Lea, NJ, USA). The FIA permanganate reagents were prepared by dissolving the potassium permanganate in water (1 103 M), adding the sodium polyphosphate and adjusting to pH 2.5 using sulfuric acid. The ‘enhanced’ permanganate reagents [17] for UV–vis analysis were prepared in the same manner, except that the initial potassium permanganate concentration was 1.9 103 M. Sodium thiosulfate was added as required to a final concentration of 1 103 M, using an appropriate volume of a 0.1 M stock solution.
2.7. Chemicals and reagents 3. Results and discussion Potassium permanganate (AR Grade) and methanol (AR Grade) were obtained from Chem Supply (Gilman, SA, Australia). Sulfuric acid (98%) was supplied by Merck (Kilsyth, Victoria, Australia). Deuterium oxide was supplied by Cambridge Isotope Laboratories (Tewksbury, MA, USA). Sodium thiosulfate (98%) and bisphenol A were purchased from Sigma–Aldrich (Castle Hill, NSW, Australia).
3.1. Polyphosphate chain length and its influence on chemiluminescence intensity Polyphosphate salts are useful in various industrial applications due to their sequestering and dispersing properties [11], such as in
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Fig. 2. Photographs of chemiluminescence from the reaction of morphine (1 103 M) with the acidic potassium permanganate reagent containing (a) orthophosphate, (b) polyphosphate n = 4, and (c) polyphosphate n = 12, in a coiled-tubing flow-cell. The reactant solutions were continuously merged at a flow rate of 3.5 mL min1 per line. A 92 s exposure time was used for each photograph.
the processing of foods [15], and thus, they are commercially available in numerous grades from many suppliers. In this study, 16 phosphate materials were obtained from five suppliers, which covered a wide range of specified chain lengths and grades. Each sample was initially characterised according to average chain length (n), using both modified end group titration (Fig. 1a) and 31P nuclear magnetic resonance spectroscopy (Fig. 1b). The average chain length ranged from 3.7 to 28.4, with results of the two methods in good agreement (Table 1). The enhancement in chemiluminescence intensity given by each individual phosphate material to the reaction of acidic potassium permanganate with morphine and bisphenol A was compared using FIA (Table 1), under previously optimised reagent conditions [18]. For both analytes, the linear short-chain ortho-, pyro- and tri-phosphates (chain lengths of 1, 2 and 3, respectively) and the cyclic trimetaphosphate gave only minor enhancements in intensity. In comparison, polyphosphates with an average chain length of greater than or equal to 7 all provided a strong enhancement. The degrees of enhancement provided by various phosphate materials were corroborated by visual examination of the emission of light when continuously merging a high concentration of morphine (1 103 M) with the permanganate reagent in a flow-cell (Fig. 2). The two samples with a nominal average chain length of four gave a degree of enhancement that varied by a factor of ca. 2.5 under otherwise identical conditions (Table 1). This difference could not be adequately explained by the end-point titration and NMR data, which both indicated a similar average chain length. Anion chromatography was, therefore, used to investigate the distribution of phosphate oligomers in these materials [15,19,20]. These results are illustrated in Fig. 3 and reveal important differences between the ‘n = 4’ polyphosphates. The sample that provided greater chemiluminescence intensity exhibited a wider distribution of phosphate species (Fig. 3b), and therefore, a greater proportion of oligomers with n > 6. Chromatograms of polyphosphate samples with greater n values exhibited an increasingly wide distribution of polyphosphate species (e.g. see n = 7 in Fig. 3c). Although there is a minimum chain length required for significant enhancement of chemiluminescence intensity, the presence of much longer chain polyphosphates in the n = 30 material did not result in a notable advantage compared to the n = 7 material. 3.2. Polyphosphate concentration The differences in the enhancement between the n = 4 samples may also explain why a large excess of polyphosphates is required for the greatest enhancement in chemiluminescence intensity. Given the number of different oligomers within the polyphosphate mixtures, the excess may be necessary to provide sufficient concentration of those that are most important for stabilising the Mn(III) precursor and the Mn(II) emitter. To study the concentration dependence of enhancement by polyphosphates, the permanganate chemiluminescence intensities with morphine and
Fig. 3. Ion chromatograms for polyphosphate samples specified as (a) n = 4, (b) n = 4, and (c) n = 7. The peaks corresponding to phosphate chains of 1, 2 and 3 have retention times of 1.8, 3.1 and 5.2 min, respectively and are labelled 1, 2 and 3 on each chromatogram.
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Table 2 Change in average polyphosphate chain length monitored over 48 h using 31P NMR under neutral and acidic (reagent) conditions, for an intermediate and a long chain polyphosphate. Average chain length (n)
Fig. 4. The influence of polyphosphate concentration (log scale) on chemiluminescence intensity for the polyphosphate samples: n = 4, red; n = 11, black; n = 14, purple; and n = 30, green, with (a) morphine and (b) bisphenol A, using flow injection analysis methodology. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bisphenol A were examined using polyphosphate concentrations between 0.0001% and 1% (m/v) (Fig. 4). For polyphosphate materials with n = 11, 14 and 30, optima were achieved at 0.01% (m/v) and 0.1% (m/v) polyphosphate for the detection of morphine and bisphenol A, respectively. However, for the n = 4 polyphosphate, the concentration that provided the greatest intensity was 5- to 10-fold larger. These experiments showed that for n 7, the optimum polyphosphate concentration was more dependent on the analyte than the source of the enhancer, and for some analytes it may be considerably lower than the quantities that have been used in previous publications. However, for polyphosphates containing a large proportion of short chains (e.g., n = 4), greater concentrations of the enhancer are required to provide sufficient longer chain polyphosphates for the largest enhancement in chemiluminescence intensity. Nonetheless, even at higher concentrations, the n = 4 sample did not provide the same increase in chemiluminescence intensity as the polyphosphate materials with longer average chain length. 3.3. Stability of the polyphosphate enhancer The stability of acidic potassium permanganate solutions containing sodium polyphosphate is limited by both degradation of the oxidant and the pH dependent hydrolysis of the enhancer [21,22]. Hydrolytic degradation of linear polyphosphates (leading to random scission along the chains forming shorter linear chains and ring structures) is most rapid under basic and mildly acidic conditions [12,23]. The reduction in the average chain length of
t=0
t=3h
t = 24 h
t = 48 h
Neutral
12.0 28.4
12.0 30.2
12.2 28.4
12.2 28.3
1.4% 0.4%
Acidic (pH 2.5)
12.0 29.0
11.6 27.9
11.4 26.7
11.4 24.6
4.9% 14.9%
Total change
polyphosphates over time was monitored with 31P NMR, for two samples (specified n = 12 and 30) prepared in aqueous media at pH 2.5 (as per the conventional chemiluminescence reagent solution) and at neutral pH (Table 2). Over 48 h, the samples were more stable at neutral pH, with the average chain length changing by less than 1.5% for each sample, consistent with previous studies [12]. At pH 2.5, the shorter chain polyphosphate underwent a 5% change and the longer chain material showed a 15% reduction. Typically permanganate chemiluminescence reagents are prepared daily; nonetheless, the shorter average chain length polyphosphate would be a better choice for the greatest stability given that both polyphosphates are above the minimum average chain length required for the enhancing effect (Table 1). The stability of permanganate in the chemiluminescence reagent containing dissolved polyphosphates was monitored over 48 h by assessing the changes in visible absorbance (Fig. 5). Both a conventional form of the reagent (0.5 mM KMnO4, 1% m/v sodium polyphosphate (n = 12.0 by 31P NMR), adjusted to pH 2.5) and a recently reported ‘enhanced’ form of the reagent, in which a large portion of the oxidant is initially reduced to Mn(III) [17], were examined. This enhanced form of the reagent reacts at a much faster rate with some analytes, increasing the peak emission intensities by up to two-orders of magnitude [17]. However, it is heavily reliant on the presence of polyphosphates for stability, to prevent the conversion of Mn(III) to Mn(IV) and Mn(II). To monitor the influence of polyphosphates on reagent stability, it was prepared as previously described [17] using a higher initial concentration of permanganate (1.9 mM), which was partially reduced to Mn(III) by sodium thiosulfate (1 mM), immediately prior to measuring the structured absorbance bands of permanganate (lmax = 525 nm). Although suitable for the traditional permanganate chemiluminescence reagent (Fig. 5a), a sodium polyphosphate concentration of 0.01% (m/v) was not sufficient to stabilise the enhanced reagent form, which was observed as an immediate loss of the characteristic purple colour of permanganate and generation of a brown suspension of Mn(IV). A sodium polyphosphate concentration of 0.1% (m/v) was sufficient to initially stabilise the enhanced reagent upon the addition of thiosulfate, but after 90 min a significant increase in the absorption at 425 nm was observed (Fig. 5b), attributable to the formation of Mn(IV) [24], irrespective of the chain length of the polyphosphate. In agreement with previous work [17], the enhanced reagent containing 1% (m/v) sodium polyphosphate was reasonably stable over 48 h, with a only a small decrease in the absorbance of the band at 525 nm and increase at 425 nm (Fig. 5c). The reagent stability was further assessed by monitoring chemiluminescence intensity from the reaction of the acidic potassium permanganate (with thiosulfate) and morphine over time, using sequential injection methodology. A set of five replicate morphine injections was performed each hour over 48 h for a short (n = 4), intermediate (n = 12) and long (n = 30) chain polyphosphate sample, each at a concentration of 1% (m/v). The chemiluminescence intensity for the reagent containing the shorter chain (n = 4)
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4. Conclusions Although there was a minimum polyphosphate chain length (6) required for significant enhancement of permanganate chemiluminescence, no significant advantage was obtained using polyphosphate materials with much longer average chain lengths. The average chain length provided a convenient characterisation of polyphosphates, but materials with similar n values can exhibit markedly different distributions of individual oligomers. This was most evident for the two n = 4 polyphosphates that resulted in significantly different chemiluminescent intensities. Longer chain polyphosphates degraded faster than their medium chain counterparts under the permanganate reagent conditions (pH 2.5), but at the concentrations used to enhance this reagent a decrease in chemiluminescence intensity was not observed over 48 h. Providing there was sufficient average chain length, the optimum concentration of polyphosphate in the permanganate reagent was dependent on the target analyte and may be lower than the quantities listed in previous reports. However, the concentration of polyphosphate cannot be lowered in permanganate reagents that have been partially reduced to generate high concomitant levels of Mn(III) due to the greater need to stabilise this key intermediate and prevent the formation of insoluble Mn(IV). Acknowledgements The authors thank Erwin Schmidt (BU Performance Materials, Budenheim, Germany), David Holgate and Michelle Rathbone (Bisley & Company, Sydney, Australia), Michael Wainwright (Fibrisol Service, Australia) for generously supplying polyphosphate materials and Gail Dyson and Daniel Gunzelmann (Deakin University) for their assistance with the phosphorus NMR experiments. BJH acknowledges receipt of an Australian Postgraduate Award and PSF acknowledges the receipt of an Australian Research Council Future Fellowship. References
Fig. 5. Typical absorption spectra collected while monitoring stability of (a) conventional acidic potassium permanganate reagent (5.0 104 M KMnO4, pH 2.5) containing 0.01% (m/v) sodium polyphosphate, measured every 90 min over 48 h, (b) ‘enhanced’ acidic potassium permanganate reagent (1.9 103 M KMnO4, pH 2.5, 1.0 103 M Na2S2O3) containing 0.1% (m/v) sodium polyphosphate, measured every 1.5 min over 90 min, and (c) ‘enhanced’ acidic potassium permanganate reagent containing 1% (m/v) sodium polyphosphate, measured every 90 min over 48 h. In each case, the first spectrum is in bold and every 15 recorded spectrum is shown.
polyphosphate was found to decrease by 11% over 48 h (RSD of 49 sets of 5 replicates was 4%), whereas the intensities for the intermediate (n = 12) and long chain (n = 30) polyphosphates increased by 14% and 3% (RSD of 49 sets of 5 replicates was 7% and 11%), respectively.
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The importance of chain length for the polyphosphate enhancement of acidic potassium permanganate chemiluminescence$ Brendan J. Holland a , Jacqui L. Adcock a , Pavel N. Nesterenko b , Anton Peristyy b , Paul G. Stevenson a , Neil W. Barnett a , Xavier A. Conlan a , Paul S. Francis a, * a b
Centre for Chemistry and Biotechnology, School of Life and Environmental Sciences, Deakin University, Geelong, Victoria, Australia Australian Centre for Research on Separation Science, School of Chemistry, University of Tasmania, Hobart, Tasmania, Australia
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
Polyphosphates characterised by titration, NMR and ion chromatography. Different enhancement of permanganate chemiluminescence. There is a minimum polyphosphate chain length required for a large enhancement. No further advantage with much longer chain lengths. Optimum concentration of polyphosphate is dependent on the analyte.
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 18 July 2014
Sodium polyphosphate is commonly used to enhance chemiluminescence reactions with acidic potassium permanganate through a dual enhancement mechanism, but commercially available polyphosphates vary greatly in composition. We have examined the influence of polyphosphate composition and concentration on both the dual enhancement mechanism of chemiluminescence intensity and the stability of the reagent under analytically useful conditions. The average chain length (n) provides a convenient characterisation, but materials with similar values can exhibit markedly different distributions of phosphate oligomers. There is a minimum polyphosphate chain length (6) required for a large enhancement of the emission intensity, but no further advantage was obtained using polyphosphate materials with much longer average chain lengths. Providing there is a sufficient average chain length, the optimum concentration of polyphosphate is dependent on the analyte and in some cases, may be lower than the quantities previously used in routine detection. However, the concentration of polyphosphate should not be lowered in permanganate reagents that have been partially reduced to form high concentrations of the key manganese(III) co-reactant, as this intermediate needs to be stabilised to prevent formation of insoluble manganese(IV). ã 2014 Elsevier B.V. All rights reserved.
Keywords: Chemiluminescence Acidic potassium permanganate Enhancer Sodium polyphosphate Sodium hexametaphosphate
1. Introduction $
Selected papers presented at 21st Annual RACI Research and Development Conference, 11–13th December 2013, Canberra, ACT, Australia. * Corresponding author at: School of Life and Environmental Sciences, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia. Tel.: +61 3 5227 1294; fax: +61 3 5227 1040. E-mail address: [email protected] (P.S. Francis). http://dx.doi.org/10.1016/j.aca.2014.07.012 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.
Acidic potassium permanganate was first used for chemiluminescence detection in 1975 [1] and has subsequently been utilised for an extensive range of analytical applications in pharmaceutical, clinical, forensic, food and agricultural settings [2,3]. The
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characteristic red emission of light from redox reactions with permanganate originates from an electronically excited Mn(II) species, thought to be generated by reduction of the immediate Mn(III) precursor by the radical oxidation intermediates of the analyte [4–6]. Several different compounds have been found to enhance the chemiluminescence, such as low molecular weight aldehydes, sodium thiosulfate and sodium polyphosphates, but their mechanisms are not yet fully understood [2,3]. Polyphosphates (chains of four or more repeating phosphate units) are a popular choice of enhancer due to their innocuous nature and reported 50-fold increases in permanganate chemiluminescence intensity [2,3]. Although the mechanism of enhancement is yet to be completely elucidated, their presence in the reaction mixture induces a shift in the maximum emission wavelength from approximately 734 nm to 689 nm, indicating a strong interaction with the emitting species [6]. A dual mechanism of enhancement has been postulated [6], which includes stabilisation of the Mn(III) precursor with respect to Mn(IV) oxides that would otherwise flocculate under the mildly acidic conditions [7–10], and formation of protective “cage-like” structures around the Mn(II) emitter that inhibit nonradiative relaxation [6]. The shorter chain ortho- and pyro-phosphates also prevent disproportionation of Mn(III), but they are not believed to form protecting structures around the emitter, as they do not enhance the emission to the same extent as polyphosphates, and there is no blue shift in the emission spectrum [6]. Commercially available sodium polyphosphates, including ‘sodium hexametaphosphate’ or Graham’s salt [11], are in fact a complex mixture of mostly linear oligomers with a wide range of chain lengths [12]. These products can be characterised by their average number of phosphate atoms per oligomer, or average chain length (n), typically ranging from 4.5 to 18 [11]. Linear polyphosphates generally exhibit stronger complexing power relative to their cyclic counterparts, arising from the greater net charge at each end of the chain compared to the middle groups [11]. Manganese polyphosphate complexes tend to be more stable as the chain length increases, but in this case the end groups have a lesser influence on complexing power because their contribution to the overall charge of the molecule is reduced [13]. Polyphosphates from a variety of sources have been used to enhance chemiluminescence reactions with permanganate [3], but the influence of the average chain length on emission intensity is not known. It is also unknown why the greatest enhancement requires that the polyphosphates be present in large excess (e.g. 1% m/v) compared to permanganate (1.0 mM) [3,14], though it has been postulated that these conditions favour the formation of the protective cage-like structures and help maintain their structural integrity throughout the reaction [6]. It is also possible that such large excesses are required to ensure sufficient quantities of the oligomers of specific lengths. Herein, we explore a series of commercially available polyphosphate materials as enhancers of permanganate chemiluminescence, examining the influence of polyphosphate structure and concentration on both the enhancement of chemiluminescence intensity and the stability of reagent solutions under analytically useful conditions. 2. Experimental 2.1.
31
P NMR
Solution 31P NMR was performed by dissolution of polyphosphates (approximately 70 mg) in 0.6 mL deionised water and 0.1 mL deuterium oxide (for signal lock). Phosphorous NMR measurements were recorded on a Bruker AVANCE 500 spectrometer (Bruker, Karlsruhe, Germany) operating at 202 MHz, with an acquisition time of 1.63 s. Temperature was regulated at 25 C, and 64 scans were
collected. Chemical shifts were referenced to H3PO4 in acetone. The well-established Eq. (1) was used to calculate the average chain length (n) of each polyphosphate sample [15], where PP1 refers to terminal (end group) phosphates, PP2 and PP3 to second and third position phosphates, and PPn to the inner phosphates: n¼
ðPP1 þ PP2 þ PP3 þ PPnÞ 2 PP1
(1)
2.2. Titration A modified end-group titration without hydrolysis [16] was performed using a Metrohm 907 Titrando (Metrohm, FL, USA) and controlled by Tiamo 2.3 software. Dilute hydrochloric acid solution (2 M) was prepared from analytical grade concentrated hydrochloric acid (32% w/w, Chem Supply, Gilman, SA, Australia). Sodium hydroxide (0.1 M) was prepared from sodium hydroxide pellets (Ajax Finechem, Taren Point, NSW, Australia) and standardised against analytical reagent grade potassium hydrogen phthalate (Fisher Scientific, Loughborough, Leicester, UK). Polyphosphate samples were prepared by complete dissolution in deionised water (2% m/v). The average chain length was calculated using the procedure detailed by Griffith [16]. 2.3. Flow injection analysis (FIA) The analytes were injected (70 mL) on a simple two-line FIA manifold with an automated six-port valve (Valco Instruments, Houston, TX, USA) into a deionised water carrier stream, which merged at a T-piece with the acidic potassium permanganate reagent prior to entering a coiled-tubing detection flow-cell comprising 0.8 mm i.d. PTFE tubing (DKSH, Caboolture, Queensland, Australia). The carrier and reagent lines were propelled through 0.8 mm i.d. PTFE tubing (DKSH) at 4 mL min1 using a peristaltic pump (Gilson Minipuls 3, John Morris Scientific, Balwyn, Victoria, Australia) with bridged PVC tubing (DKSH). The flow-cell was mounted flush against the window of a photomultiplier tube (Electron Tubes model 9828SB; ETP, NSW, Australia) encased in a light tight housing and powered by a stable power supply at 950 V. Chemiluminescence intensities were documented with a chart recorder (YEW type3066, Yokogawa Hokushin Electric, Tokyo, Japan), and the peak heights were measured manually. 2.4. Ion chromatography Chromatographic analysis was carried out using an ICS 3000 chromatography system (Dionex, Thermo Scientific, Sunnyvale, CA, USA). The system was equipped with an autosampler, eluent generation system, anion suppressor and electrochemical conductivity detector. Chromeleon software (version 7.1, Dionex) was used for data acquisition and instrument control. Separation was carried out using a 250 4.0 mm ID column IonPac AS-19 (Dionex), with a 200 mL injection volume and 1 mL min1 flow rate. A 60 min potassium hydroxide gradient was used, starting at 40 mM and increasing linearly to 100 mM over the first 10 min, then held at 100 mM for the remaining 50 min. The column was re-equilibrated for 15 min prior to subsequent analyses. Samples were prepared at 1000 ppm and standards at 200 ppm in a 40 mM potassium hydroxide solution, and analysed in duplicate. 2.5. Spectrophotometric monitoring A Cary 300 Bio UV–vis spectrophotometer (Varian, Mulgrave, Victoria, Australia) was used to measure the absorption of permanganate solutions in a 10 mm quartz cuvette. Sodium thiosulfate was added to the potassium permanganate solution
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Table 1 Comparison of chemiluminescence intensity for the reaction of morphine (1 106 M) and bisphenol A (1 105 M) with acidic potassium permanganate containing phosphate materials of different chain length. Average chain length was classified from supplier specified values and data from titration and 31P NMR experiments. Relative chemiluminescence intensitya
Short chain phosphates
Orthophosphate Pyrophosphate Tripolyphophate Trimetaphosphate (cyclic)
Morphine
Bisphenol A
0.01 0.01 0.02 0.00
0.00 0.00 0.05 0.05
Relative chemiluminescence intensitya
Polyphosphates with average chain length (n) Specified 4 4 7 – – b
16 16 14–18 – 28 30
Titration
31
3.7 4.0 6.7 10.5 10.9 14.4 10.0 11.1 16.1 15.8 22.7 25.7
4.0 4.4 7.2 11.4 12.0 13.9 11.0 11.9 15.0 16.3 25.1 28.4
P NMR
Morphine
Bisphenol A
0.54 0.90 1.45 1.11 1.00 1.46 1.28 1.18 1.14 1.02 1.02 1.22
0.29 0.81 0.90 0.85 1.00 1.24 0.95 0.90 1.00 0.95 1.00 1.00
a Each reagent contained 1 103 M potassium permanganate and 0.1% m/v phosphate material, and was adjusted to pH 2.5 using sulfuric acid. Intensities are relative to those obtained using sodium polyphosphate (+80 mesh) from Sigma– Aldrich. b ‘Crosslinked’ material.
Fig. 1. Representative (a) alkalinity and weak acid titration curve and (b) 31P NMR spectrum used to compare the average chain length of each polyphosphate sample in aqueous solution to the supplier specified values. Example shown is sodium polyphosphate crystals (+80 mesh) sample sourced from Sigma–Aldrich (n = 12).
containing polyphosphates, immediately prior to recording the first spectrum, and the mixture remained untouched until the final spectrum was recorded. 2.6. Sequential injection analysis (SIA) A milliGAT pump (Global FIA, Fox Island, WA, USA) was used for sequential aspiration of permanganate reagent and analyte standards with a ten-port multi-position valve (model C25Z, Valco, SGE, Melbourne, Victoria, Australia) and chemiluminescence detector. The detector comprised a single-inlet serpentine flow cell (Global FIA) mounted flush against the window of a photomultiplier tube (Electron Tubes model 9828SB; ETP) encased in a light tight housing and powered by a stable power supply at 900 V. A data acquisition board (LabJack U12, National Instruments, Victoria, Australia) and LabVIEW software (version 8.0, National Instruments) were used to control the pump and valve. Output from the photomultiplier tube was recorded using the e-corder 410 data acquisition system (eDAQ, Denistone East, NSW, Australia). This automated SIA system was programmed to repeatedly aspirate 150 mL of reagent (valve position 1; 167 mL s1), 1500 mL of the analyte standard (position 2; 167 mL s1) and 1000 mL of deionised water (position 3, 100 mL s1), with a 60 min pause after every 5th cycle.
Morphine was sourced from GlaxoSmithKline (Port Fairy, Victoria, Australia). Stock solutions of morphine (1 103 M) were prepared in acidified deionised water and sonicated to aid dissolution. Bisphenol A (1 103 M) was prepared in methanol and diluted in deionised water as required. The phosphate samples obtained for comparison were sodium dihydrogen orthophosphate from Univar (Ingleburn, NSW, Australia); tetrasodium pyrophosphate from AnalaR (VWR International, Murrarie, Queensland, Australia); sodium hexametaphosphate, polyphosphate sodium salt (+200 mesh), sodium polyphosphate crystals (+80 mesh), trisodium trimetaphosphate, tri-polyphosphate, sodium hexametaphosphate (65–70% P2O5 basis) and sodium phosphate glass from Sigma–Aldrich; sodium hexametaphosphate P60 and P68 from Fibrisol (Heatherton, Victoria, Australia); Budit 3H, 4H, 6H, 7H and 8H instantised sodium polyphosphate, Budit 7N coarse powder sodium polyphosphate and FFB 779 (Budit 9N replacement) sodium polyphosphate powder from BassTech International (Fort Lea, NJ, USA). The FIA permanganate reagents were prepared by dissolving the potassium permanganate in water (1 103 M), adding the sodium polyphosphate and adjusting to pH 2.5 using sulfuric acid. The ‘enhanced’ permanganate reagents [17] for UV–vis analysis were prepared in the same manner, except that the initial potassium permanganate concentration was 1.9 103 M. Sodium thiosulfate was added as required to a final concentration of 1 103 M, using an appropriate volume of a 0.1 M stock solution.
2.7. Chemicals and reagents 3. Results and discussion Potassium permanganate (AR Grade) and methanol (AR Grade) were obtained from Chem Supply (Gilman, SA, Australia). Sulfuric acid (98%) was supplied by Merck (Kilsyth, Victoria, Australia). Deuterium oxide was supplied by Cambridge Isotope Laboratories (Tewksbury, MA, USA). Sodium thiosulfate (98%) and bisphenol A were purchased from Sigma–Aldrich (Castle Hill, NSW, Australia).
3.1. Polyphosphate chain length and its influence on chemiluminescence intensity Polyphosphate salts are useful in various industrial applications due to their sequestering and dispersing properties [11], such as in
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Fig. 2. Photographs of chemiluminescence from the reaction of morphine (1 103 M) with the acidic potassium permanganate reagent containing (a) orthophosphate, (b) polyphosphate n = 4, and (c) polyphosphate n = 12, in a coiled-tubing flow-cell. The reactant solutions were continuously merged at a flow rate of 3.5 mL min1 per line. A 92 s exposure time was used for each photograph.
the processing of foods [15], and thus, they are commercially available in numerous grades from many suppliers. In this study, 16 phosphate materials were obtained from five suppliers, which covered a wide range of specified chain lengths and grades. Each sample was initially characterised according to average chain length (n), using both modified end group titration (Fig. 1a) and 31P nuclear magnetic resonance spectroscopy (Fig. 1b). The average chain length ranged from 3.7 to 28.4, with results of the two methods in good agreement (Table 1). The enhancement in chemiluminescence intensity given by each individual phosphate material to the reaction of acidic potassium permanganate with morphine and bisphenol A was compared using FIA (Table 1), under previously optimised reagent conditions [18]. For both analytes, the linear short-chain ortho-, pyro- and tri-phosphates (chain lengths of 1, 2 and 3, respectively) and the cyclic trimetaphosphate gave only minor enhancements in intensity. In comparison, polyphosphates with an average chain length of greater than or equal to 7 all provided a strong enhancement. The degrees of enhancement provided by various phosphate materials were corroborated by visual examination of the emission of light when continuously merging a high concentration of morphine (1 103 M) with the permanganate reagent in a flow-cell (Fig. 2). The two samples with a nominal average chain length of four gave a degree of enhancement that varied by a factor of ca. 2.5 under otherwise identical conditions (Table 1). This difference could not be adequately explained by the end-point titration and NMR data, which both indicated a similar average chain length. Anion chromatography was, therefore, used to investigate the distribution of phosphate oligomers in these materials [15,19,20]. These results are illustrated in Fig. 3 and reveal important differences between the ‘n = 4’ polyphosphates. The sample that provided greater chemiluminescence intensity exhibited a wider distribution of phosphate species (Fig. 3b), and therefore, a greater proportion of oligomers with n > 6. Chromatograms of polyphosphate samples with greater n values exhibited an increasingly wide distribution of polyphosphate species (e.g. see n = 7 in Fig. 3c). Although there is a minimum chain length required for significant enhancement of chemiluminescence intensity, the presence of much longer chain polyphosphates in the n = 30 material did not result in a notable advantage compared to the n = 7 material. 3.2. Polyphosphate concentration The differences in the enhancement between the n = 4 samples may also explain why a large excess of polyphosphates is required for the greatest enhancement in chemiluminescence intensity. Given the number of different oligomers within the polyphosphate mixtures, the excess may be necessary to provide sufficient concentration of those that are most important for stabilising the Mn(III) precursor and the Mn(II) emitter. To study the concentration dependence of enhancement by polyphosphates, the permanganate chemiluminescence intensities with morphine and
Fig. 3. Ion chromatograms for polyphosphate samples specified as (a) n = 4, (b) n = 4, and (c) n = 7. The peaks corresponding to phosphate chains of 1, 2 and 3 have retention times of 1.8, 3.1 and 5.2 min, respectively and are labelled 1, 2 and 3 on each chromatogram.
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Table 2 Change in average polyphosphate chain length monitored over 48 h using 31P NMR under neutral and acidic (reagent) conditions, for an intermediate and a long chain polyphosphate. Average chain length (n)
Fig. 4. The influence of polyphosphate concentration (log scale) on chemiluminescence intensity for the polyphosphate samples: n = 4, red; n = 11, black; n = 14, purple; and n = 30, green, with (a) morphine and (b) bisphenol A, using flow injection analysis methodology. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bisphenol A were examined using polyphosphate concentrations between 0.0001% and 1% (m/v) (Fig. 4). For polyphosphate materials with n = 11, 14 and 30, optima were achieved at 0.01% (m/v) and 0.1% (m/v) polyphosphate for the detection of morphine and bisphenol A, respectively. However, for the n = 4 polyphosphate, the concentration that provided the greatest intensity was 5- to 10-fold larger. These experiments showed that for n 7, the optimum polyphosphate concentration was more dependent on the analyte than the source of the enhancer, and for some analytes it may be considerably lower than the quantities that have been used in previous publications. However, for polyphosphates containing a large proportion of short chains (e.g., n = 4), greater concentrations of the enhancer are required to provide sufficient longer chain polyphosphates for the largest enhancement in chemiluminescence intensity. Nonetheless, even at higher concentrations, the n = 4 sample did not provide the same increase in chemiluminescence intensity as the polyphosphate materials with longer average chain length. 3.3. Stability of the polyphosphate enhancer The stability of acidic potassium permanganate solutions containing sodium polyphosphate is limited by both degradation of the oxidant and the pH dependent hydrolysis of the enhancer [21,22]. Hydrolytic degradation of linear polyphosphates (leading to random scission along the chains forming shorter linear chains and ring structures) is most rapid under basic and mildly acidic conditions [12,23]. The reduction in the average chain length of
t=0
t=3h
t = 24 h
t = 48 h
Neutral
12.0 28.4
12.0 30.2
12.2 28.4
12.2 28.3
1.4% 0.4%
Acidic (pH 2.5)
12.0 29.0
11.6 27.9
11.4 26.7
11.4 24.6
4.9% 14.9%
Total change
polyphosphates over time was monitored with 31P NMR, for two samples (specified n = 12 and 30) prepared in aqueous media at pH 2.5 (as per the conventional chemiluminescence reagent solution) and at neutral pH (Table 2). Over 48 h, the samples were more stable at neutral pH, with the average chain length changing by less than 1.5% for each sample, consistent with previous studies [12]. At pH 2.5, the shorter chain polyphosphate underwent a 5% change and the longer chain material showed a 15% reduction. Typically permanganate chemiluminescence reagents are prepared daily; nonetheless, the shorter average chain length polyphosphate would be a better choice for the greatest stability given that both polyphosphates are above the minimum average chain length required for the enhancing effect (Table 1). The stability of permanganate in the chemiluminescence reagent containing dissolved polyphosphates was monitored over 48 h by assessing the changes in visible absorbance (Fig. 5). Both a conventional form of the reagent (0.5 mM KMnO4, 1% m/v sodium polyphosphate (n = 12.0 by 31P NMR), adjusted to pH 2.5) and a recently reported ‘enhanced’ form of the reagent, in which a large portion of the oxidant is initially reduced to Mn(III) [17], were examined. This enhanced form of the reagent reacts at a much faster rate with some analytes, increasing the peak emission intensities by up to two-orders of magnitude [17]. However, it is heavily reliant on the presence of polyphosphates for stability, to prevent the conversion of Mn(III) to Mn(IV) and Mn(II). To monitor the influence of polyphosphates on reagent stability, it was prepared as previously described [17] using a higher initial concentration of permanganate (1.9 mM), which was partially reduced to Mn(III) by sodium thiosulfate (1 mM), immediately prior to measuring the structured absorbance bands of permanganate (lmax = 525 nm). Although suitable for the traditional permanganate chemiluminescence reagent (Fig. 5a), a sodium polyphosphate concentration of 0.01% (m/v) was not sufficient to stabilise the enhanced reagent form, which was observed as an immediate loss of the characteristic purple colour of permanganate and generation of a brown suspension of Mn(IV). A sodium polyphosphate concentration of 0.1% (m/v) was sufficient to initially stabilise the enhanced reagent upon the addition of thiosulfate, but after 90 min a significant increase in the absorption at 425 nm was observed (Fig. 5b), attributable to the formation of Mn(IV) [24], irrespective of the chain length of the polyphosphate. In agreement with previous work [17], the enhanced reagent containing 1% (m/v) sodium polyphosphate was reasonably stable over 48 h, with a only a small decrease in the absorbance of the band at 525 nm and increase at 425 nm (Fig. 5c). The reagent stability was further assessed by monitoring chemiluminescence intensity from the reaction of the acidic potassium permanganate (with thiosulfate) and morphine over time, using sequential injection methodology. A set of five replicate morphine injections was performed each hour over 48 h for a short (n = 4), intermediate (n = 12) and long (n = 30) chain polyphosphate sample, each at a concentration of 1% (m/v). The chemiluminescence intensity for the reagent containing the shorter chain (n = 4)
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4. Conclusions Although there was a minimum polyphosphate chain length (6) required for significant enhancement of permanganate chemiluminescence, no significant advantage was obtained using polyphosphate materials with much longer average chain lengths. The average chain length provided a convenient characterisation of polyphosphates, but materials with similar n values can exhibit markedly different distributions of individual oligomers. This was most evident for the two n = 4 polyphosphates that resulted in significantly different chemiluminescent intensities. Longer chain polyphosphates degraded faster than their medium chain counterparts under the permanganate reagent conditions (pH 2.5), but at the concentrations used to enhance this reagent a decrease in chemiluminescence intensity was not observed over 48 h. Providing there was sufficient average chain length, the optimum concentration of polyphosphate in the permanganate reagent was dependent on the target analyte and may be lower than the quantities listed in previous reports. However, the concentration of polyphosphate cannot be lowered in permanganate reagents that have been partially reduced to generate high concomitant levels of Mn(III) due to the greater need to stabilise this key intermediate and prevent the formation of insoluble Mn(IV). Acknowledgements The authors thank Erwin Schmidt (BU Performance Materials, Budenheim, Germany), David Holgate and Michelle Rathbone (Bisley & Company, Sydney, Australia), Michael Wainwright (Fibrisol Service, Australia) for generously supplying polyphosphate materials and Gail Dyson and Daniel Gunzelmann (Deakin University) for their assistance with the phosphorus NMR experiments. BJH acknowledges receipt of an Australian Postgraduate Award and PSF acknowledges the receipt of an Australian Research Council Future Fellowship. References
Fig. 5. Typical absorption spectra collected while monitoring stability of (a) conventional acidic potassium permanganate reagent (5.0 104 M KMnO4, pH 2.5) containing 0.01% (m/v) sodium polyphosphate, measured every 90 min over 48 h, (b) ‘enhanced’ acidic potassium permanganate reagent (1.9 103 M KMnO4, pH 2.5, 1.0 103 M Na2S2O3) containing 0.1% (m/v) sodium polyphosphate, measured every 1.5 min over 90 min, and (c) ‘enhanced’ acidic potassium permanganate reagent containing 1% (m/v) sodium polyphosphate, measured every 90 min over 48 h. In each case, the first spectrum is in bold and every 15 recorded spectrum is shown.
polyphosphate was found to decrease by 11% over 48 h (RSD of 49 sets of 5 replicates was 4%), whereas the intensities for the intermediate (n = 12) and long chain (n = 30) polyphosphates increased by 14% and 3% (RSD of 49 sets of 5 replicates was 7% and 11%), respectively.
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