Environmental Letters

ISSN: 0013-9300 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/lesa17

Measurement of SO2 in Air Using Coated Piezoelectric Crystal Detectors K. H. Karmarkar , L. M. Webber & G. G. Guilbault To cite this article: K. H. Karmarkar , L. M. Webber & G. G. Guilbault (1975) Measurement of SO2 in Air Using Coated Piezoelectric Crystal Detectors, Environmental Letters, 8:4, 345-352, DOI: 10.1080/00139307509437444 To link to this article: http://dx.doi.org/10.1080/00139307509437444

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Date: 06 November 2015, At: 11:45

ENVIRONMENTAL LETTERS, 8(4), 345-352

(1975)

MEASUREMENT O F SO2 IN AIR USING COATED PIEZOELECTRIC CRYSTAL DETECTORS KEY WORDS:

P i e z o e l e c t r i c c r y s t a l s , quadrol, sulfur dioxide, hydrophobic m e m b r a n e f i l t e r

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K. H. K a r m a r k a r , L.

M. Webber, and G. G. Guilbault

Department of C h e m i s t r y University of New O r l e a n s New O r l e a n s , Louisiana 70122 ABSTRACT

U s e of hydrophobic m e m b r a n e f i l t e r s is shown t o effectively reduce the i n t e r f e r i n g m o i s t u r e l e v e l i n the detection of SO2 i n air at the p a r t s p e r billion range.

Quadrol i s used a s a sensitive coating o n

piezoelectric c r y s t a l d e t e c t o r s i n a flow system.

INTRODUCTION Coated piezoelectric q u a r t z c r y s t a l s a r e c u r r e n t l y being developed as selective and sensitive air pollution s e n s o r s .

The principle of detection

by a piezoelectric q u a r t z c r y s t a l is that the frequency of vibration of a n o s c i l l a t i n g c r y s t a l d e c r e a s e s by the adsorption of a foreign m a t e r i a l on i t s surface.

A gaseous pollutant is seIectively absorbed by a coating on the

c r y s t a l s u r f a c e , thereby i n c r e a s i n g the weight o n the c r y s t a l and d e c r e a s i n g the frequency of vibration. amount of gas absorbed.

The d e c r e a s e i n frequency is a m e a s u r e of the

i 2 King and K a r a s e k and T i e r n a y have used these

345 Copyright 0 1975 by hlsrcel Dekker, Inc. All Rights Reserved. Neither this work nor any.psrt msy be reproduced or transmitted in any form or by any means, electronic or mechanical. including photocopying, microfilming. and recording, or by any information storage and retrieval system, without permission in writing from the publisher.

U K W , WEBBER, k?TD GUILSAULT

c r y s t a l s a s selective and sensitive g. c, detectors. Freund

3

Janghorbani and

have described the use of t h e s e c r y s t a l s as digital s e n s o r s for

sulfur compounds commonly found i n pulp mill effluents, and F r e c h e t t e 4

and F a s c h i n g have proposed t h e i r use i n a closed s y s t e m f o r SO detection. 2

K a r m a r k a r and Guilbault, 5'

with t h e i r new sensitive c e l l design and

coatings, have detected SO2, NO2 and NH i n the ppb range. 3

I t was

reported e a r l i e r 4 - 6 that a t m o s p h e r i c m o i s t u r e physically condenses on the c r y s t a l s u r f a c e thereby giving a s t r o n g response.

Under t h e s e

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c i r c u m s t a n c e s , i t becomes difficult to differentiate between m o i s t u r e response and a pollutant response. In this p a p e r we d e s c r i b e a method f o r the detection of SO2 i n

air.

The quadrol coating, which proved sensitive f o r SO i n d r y gases, 2

is shown to 'be useful f o r the sensitive detection of SO2 i n the p r e s e n c e of moisture.

EXPERIMENTAL Apparatus

Fig. 1 shows the experimental s e t up using apiezoelectric c r y s t a l detector i n a flow system.

The c e l l design is based o n a design d e s

-

cribed by the authors e a r l i e r 5 and is the m o s t sensitive one f o r u s e i n a flow system.

The c r y s t a l s used i n this study are 9 MHz AT cut q u a r t z

c r y s t a l s with s i l v e r plated m e t a l electrodes on both s i d e s and w e r e obtained f r o m J a n C r y s t a l s , Florida.

A low frequency OX t r a n s i s t o r

oscillator, built f r o m a n o s c i l l a t o r kit, supplied by International C r y s t a l Co., Oklahoma, was used i n this study.

The oscillator is powered by

a Heathkit i -30 V d. c. v a r i a b l e power supply model iP-28. voltage was kept constant a t 9 volts.

346

The applied

A Systron-Donner model 8050 with a

ATMOSPHERIC SO2 MEAS-

Oscillator

Frequency

Meter

Power s u p p l y

‘ ? - & k -- .d

Filter

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t v FIG. i Experimental Setup with a P i e z o e l e c t r i c C r y s t a l Detector

range of 0-30 MHz and a resolution of 0. frequency meter.

Hz was used a s a digital

Nitrogen was used as a carrier gas a t a r a t e of 15 mllmin.

Hydrophobic Membrane F i l t e r :

A hydrophobic Acropor f i l t e r , Type

ANH-450 ( P o r e s i z e 0 . 4 5 pm) was supplied by Gelman Instrument Company, Ann A r b o r , Michigan, as a f r e e sample.

A single f i l t e r

l a y e r of the above s a m p l e was placed i n the flow s y s t e m between the injection p o r t and the empty column with the help of a swagelok union. Reagents Quadrol (obtained f r o m Applied Science Lab. a coating.

, Inc.

) was used as

SOz, NOZ, NO, CO, COz, NH3, and propane gas w e r e

obtained f r o m Matheson Co.

, Inc. , in l e c t u r e

bottles.

Benzene, c h l o r o -

f o r m , carbon disulfide, dimethylsulfoxide, and ethanol w e r e used f r o m Technical g r a d e bottles.

347

m,WEBBEX,

AND GUILBAULT

Method of Application of Coating:

A solution of quadrol in c h l o r o f o r m .

is applied over the e n t i r e s u r f a c e of the electrode, on both s i d e s , with a tiny brush.

Chloroform evaporates quickly leaving the quadrol

coating o n the crystal.

The amount of coating in this study corresponded

t o a d e c r e a s e of 1 1 , 0 0 0 H z i n the basic frequency (9MHz)of the crystal. Syringe Dilution Method:

A syringe dilution method described e a r l i e r ” I !

was used to obtain high dilutions of t e s t g a s e s with l a b o r a t o r y air.

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E i t h e r a 5 ml o r a 10 ml sample m i x t u r e is injected t o obtain response.

RESULTS AND DISCUSSION T h e Gelman hydrophobic f i l t e r proved to be v e r y useful f o r detecting SO2 in air i n the p r e s e n c e of moisture.

A single l a y e r of

f i l t e r allows the c a r r i e r gas and other gases to p a s s through v e r y e a s i l y but the p a s s a g e of m o i s t u r e is greatly reduced,

The l a b air response,

which is mainly due to m o i s t u r e , is reduced f r o m 300 Hz to 70 Hz by using this filter.

Responses f o r 5 ml o r 10 ml l a b air a r e obtained

by s e v e r a l injections which r e m a i n constant under n o r m a l conditions. The c-onstant l a b air signal is subtracted f r o m the total signal f o r various dilutions of the t e s t gas, and the t r u e signal f o r the t e s t gas is obtained.

O u r e a r l i e r study5 had shown that SO2 a s low a s 1 ppb could be detected with a quadrol coating using d r y nitrogen both a s a c a r r i e r gas

as well a s a diluant f o r SO2 s a m p l e mixtures.

I t is nw observed that

the quadrol coating is sensitive f o r SO2 even i n the p r e s e n c e of a t m o s p h e r i c m o i s t u r e using a f i l t e r t o minimize the effect of moisture.

Figs.

2 and 3 show calibration plots f o r 5 ml and 10 ml s a m p l e s i z e s of SO and l a b air mixtures, respectively.

348

2

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ATMOSPHERIC SO2 MEASURPIENT

so2 w m . FIG. 2 P l o t of Frequency Change A F (AF = A F total-

mLab Air)

f o r SO2 i n L a b A i r Sample Used: 5 ml Mixture (AFLab Air = 70 Hz)

T h e observed lower l i m i t of detection of SO2 i n the p r e s e n c e of m o i s t u r e f o r a 5 m l s a m p l e is 0. i pprn which is not a s low as the i ppb l i m i t observed i n d r y nitrogen.

T h i s apparent ioss i n sensitivity i n the

p r e s e n c e of m o i s t u r e is believed to be due to the fact that SO2 r e a c t s with a t m o s p h e r i c m o i s t u r e t o f o r m H2S03.

The f i l t e r used does not

allow H SO to p a s s t h r 0 u g h . A ~a r e s u l t , the real concentration of SO 2 2 3 is appreciably lower than 0. 1 ppm. In addition to this, the small amount of m o i s t u r e which p a s s e s through the f i l t e r m a y occupy

some

of the active s i t e s of quadrol s u r f a c e . thereby reducing the chance f o r SO2 absorption.

A 10 ml sample however, m a k e s i t possible t o detect

SO as low a s 10 ppb i n the p r e s e n c e of moisture. 2

T h e response is

l i n e a r f r o m 0. i p p m t o 40 pprn SO2 for 5 ml s a m p l e s , and f r o m

349

URMARUR, WEBBER, AND GUILBAULT

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2401

FIG. 3 P l o t of Frequency Change AF, (AF = Mtotal- AFLab Air) f o r SO2 i n L a b A i r Sample Used: 10 ml Mixture

(aLab Air = 150 Hz)

10 ppb to 10 p p m SO2 f o r 10 ml samples.

The r e s p o n s e f o r SO2 is

c o m p l e t e l y r e v e r s i b l e even f o r high concentrations.

Since we

observed that triethanolamine gives a v e r y l a r g e response with m o i s t u r e , only quadrol was used i n the p r e s e n t studies. Listed in Table 1 a r e the i n t e r f e r e n c e s f r o m o t h e r g a s e s and v a p o r s i n the detection of sulfur dioxide.

T h e i n t e r f e r i n g concentra-

tions are those which produce a r e s p o n s e roughly equivalent to that f o r 0. 1 p p m sulfur dioxide f o r a 5 ml sample injection.

It is c l e a r

f r o m T a b l e 1 that v a r i o u s g a s e s did not produce a g r e a t e r i n t e r f e r e n c e i n the p r e s e n c e of m o i s t u r e than in d r y nitrogen.

Nitrogen dioxide

is the m a j o r i n t e r f e r e n c e which shows a n i r r e v e r s i b l e absorption a t

high concentrations.

The i n t e r f e r e n c e by high concentrations of organic

solvents is due t o t h e i r dissolving of the coating. 350

Gas

co =O2

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NH3

Interfering Conc. PPm

Interfering Conc. PPm

Gas

100,000

Chloroform

200

100,000

Benzene

400

EtOH

550

10,000

1

It is observed that the use of two l a y e r s of the above hydrophobic f i l t e r f u r t h e r reduces the L a b air (moisture) r e s p o n s e to only 35 Hz without affecting SO2 response.

T h e study of finding the optimum number

of f i l t e r l a y e r s a s well as the optimum f i l t e r p o r e s i z e i n o r d e r t o

completely s u p p r e s s m o i s t u r e r e s p o n s e is i n p r o g r e s s .

ACKNOWLEDGEMENT The a u t h o r s gratefully acknowledge the financial a s s i s t a n c e of the U. S. A r m y R e s e a r c h Office, G r a n t No.

ARO DAHC 7 4 G0119.

The f r e e s a m p l e supply of hydrophobic m e m b r a n e f i l t e r s , Type ANH450 f r o m the Gelman Instrument Co., A nn A r b o r , Michigan, is a l s o

gratefully acknowledged.

351

KARMARKAR, WEBBER, AND CUILBAULT

REFERENCES

1.

W. H. King, J r . , Apal. Chem. , 2 6 , 1735 (1964).

2.

F. W. Karasek and J. M. Tiernay, J. Chromatog.,

3.

M. Janghorbani and H. Freund. Anal. C h e m . ,

4.

M. W. Frechette and J. L. Fasching, Environ. Sci. Technol.,

45,

E, 31

(1974).

325 (1973).

1135 (1973). 5.

K . H. Karmarkar and G. G. Guilbault, Anal. C h i m Acta,

4i9 (1974).

K. H. Karmarkar and G. G . Guilbault, Anal. C h i m A c t a . 7 5 , 111 (1975).

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6.

E,

Received Accepted

352

12/11/74 1/2/75

1,