Chemical Hazards in the Workplace - ACS Publications - American

10 Monitoring for Airborne Inorganic Acids

Downloaded by UNIV OF ARIZONA on January 7, 2013 | http://pubs.acs.org Publication Date: April 2, 1981 | doi: 10.1021/bk-1981-0149.ch010

M. E. CASSINELLI and D. G. TAYLOR National Institute for Occupational Safety and Health, 4676 Columbia Parkway, Cincinnati, OH 45226

Airborne inorganic acids exist in the industrial environment in the form of both vapors and particulates. This study was undertaken to answer a need for a simple sampling and analytical method for monitoring both vaporous and aerosol acid contaminants quantitatively. Inorganic acids have similar acute toxic properties: corrosive action on the skin, the respiratory tract, and especially the eyes where corneal damage may occur. Severe exposures may cause blindness, pulmonary edema, and even death. The onset of symptoms may be delayed for several hours after exposure. Prolonged exposures to low concentrations produce chronic effects such as tooth erosion, chronic bronchitis, and photosensitization of the skin (1>2,J3). Past sampling methods for acid mists used impingers containing liquids as diverse as the acids being sampled. The analytical methods are equally as varied. (4). The Occupational Safety and Health Administration (OSHA) along with many industrial hygienists have expressed a desire for a non-liquid sampling device which will collect a l l the common inorganic acids (HCl, H3PO4, HBr, HNO3, and H S0i|) and a method for determining these acids collected on a single sampler. Ion chromatography (IC) offers the analytical tool for the determination of each of the inorganic acids in a single sample. The principle of ion chromatography is the separation and measurement of ions in solution using ion exchange resins, background suppression, and conductimetric detection (5.). This overall study of acid mists began with the development of a sampling and analytical method for hydrogen chloride (6,7). Various solid sorbents and filters, both treated and untreated, were evaluated as collection media and for compatibility with ion chromatography. The sorbent of choice was silica gel which had been washed with deionized water to remove inorganic impurities. Recently, packed beds have been used for the collection of 2

This chapter not subject to U.S. copyright. Published 1981 American Chemical Society

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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a e r o s o l s . Lee and Gieseke developed a t h e o r e t i c a l approach to p r e d i c t i n g the c o l l e c t i o n e f f i c i e n c y o f packed beds f o r a e r o s o l p a r t i c l e s ( 8 ) . In another study u s i n g c h a r c o a l tubes, i t was reported t h a t approximately 70$ o f the a i r b o r n e p a r t i c u l a t e mass was deposited on the g l a s s wool plug preceding the primary bed o f c h a r c o a l , w h i l e 17$ was found on the two beds o f c h a r c o a l (9). Thus, we hypothesized that the s i l i c a g e l tube, which i s t r a d i t i o n a l l y a gas and vapor c o l l e c t o r , may have the c a p a b i l i t y o f c o l l e c t i n g a e r o s o l s as w e l l as gases and vapors. Sampling tubes, as used i n the e a r l i e r HC1 study, were packed w i t h 400 mg o f s i l i c a g e l and evaluated u s i n g t e s t atmospheres c o n t a i n i n g i n o r g a n i c a c i d s (6., 7). C o l l e c t i o n p r o p e r t i e s were e x c e l l e n t f o r the vapor-forming a c i d s , but a g r e a t e r c o l l e c t i o n c a p a b i l i t y was r e q u i r e d f o r the a e r o s o l - f o r m i n g a c i d s , phosphoric and s u l f u r i c acids. The l a t e s t work has focused on the development of a s i l i c a gel sampling tube which w i l l c o l l e c t a e r o s o l - f o r m i n g a c i d s as w e l l as the other a c i d vapors. This paper d e s c r i b e s the development o f the improved sampler. S e v e r a l v a r i a b l e s f o r p a r t i c u l a t e c o l l e c t i o n were examined to determine the optimum c o l l e c t i o n tube geometry. Experiments were performed by v a r y i n g the tube diameter which a f f e c t s the sampling v e l o c i t y , the s i l i c a g e l l o a d i n g which determines the l e n g t h o f the sorbent bed, and the mesh s i z e o f the s i l i c a g e l which determines the a i r f l o w p r o p e r t i e s . A l l three v a r i a b l e s a f f e c t the pressure drop across the tube. U l t i m a t e l y , a sampling device was designed which c o l l e c t s both vaporous and p a r t i c u l a t e forms of i n o r g a n i c a c i d s f o r subsequent a n a l y s i s by i o n chromatography. Experimental Apparatus. Atmospheres c o n t a i n i n g the i n o r g a n i c a c i d s were generated i n a system i l l u s t r a t e d by the schematic i n Figure 1. D i l u t e s o l u t i o n s o f mixed a c i d s were n e b u l i z e d w i t h a Retec medical n e b u l i z e r and entered the chamber through a Teflon tube d i r e c t e d downward through the center o f the chamber to w i t h i n approximately 12 inches from the bottom. D i l u t i o n a i r entered the chamber through f o u r p o r t s i n the bottom cover. Flows through the chamber were c o n t r o l l e d w i t h Gilmont flowmeters at 20-30 Lpm f o r the d i l u t i o n a i r and 2-5 Lpm f o r the n e b u l i z e r . A Gast a i r compressor provided the a i r for both n e b u l i z a t i o n and d i l u t i o n a t approximately 25 p s i . The pressure w i t h i n the chamber was balanced a t atmospheric pressure, as i n d i c a t e d w i t h a manometer, through the use of an AADCO vacuum pump. A l l components exposed to the a c i d s were f a b r i c a t e d from p l a s t i c or g l a s s . The top cover o f the chamber contained an exhaust o u t l e t ,



10.

C A S S i N E L L i AND

TAYLOR

Airborne

Inorganic

Acids

probes f o r a hygrometer and f o r a p a r t i c l e s i z e r , and i n i t i a l l y s i x sampling p o r t s but l a t e r m o d i f i e d to i n c l u d e twelve sampling p o r t s . Two curved g l a s s p o r t s entered the s i d e s of the chamber f o r a t t a c h i n g impingers or f i l t e r c a s s e t t e s . Samples were drawn by a 1/10 horsepower M i l l i p o r e vacuum pump at a nominal f l o w r a t e of 0.2 Lpm u s i n g c r i t i c a l o r i f i c e s . The Retec medical n e b u l i z e r d e l i v e r s a e r o s o l s w i t h a volume median diameter (VMD) o f 5.1 ym w i t h a °g o f 2.0 (13)· The c o n c e n t r a t i o n s of a c i d s used to generate the atmospheres were 0.4-0.8 mL o f H 3 P O 4 per l i t e r of s o l u t i o n , 0.3-0.5 mL H 2 S O 4 per l i t e r , 3 - 6 mL H N O 3 / L , and 10-20 mL of 35$ HBr/L. As the a e r o s o l s t r a v e l through the chamber evaporation occurs r e s u l t i n g i n much s m a l l e r p a r t i c l e s . C a l c u l a t i o n s show t h a t complete evaporation would r e s u l t i n approximately 0.4 ym VMD p a r t i c l e s . The p a r t i c l e s i z e was considered to be l e s s than 1 ym. Analyses were done on a Dionex Model 14 Ion Chromatograph ( I C ) , equipped w i t h a Waters WISP 710A autosampler, L i n e a r r e c o r d e r , and i n t e r f a c e d w i t h a Hewlett-Packard 3 3 5 4 Laboratory Automated System. The p r i n c i p a l components o f the IC, shown i n Figure 2, are (A) eluent r e s e r v o i r , (B) pump, (C) i n j e c t i o n v a l v e , (D) separator column, (E) suppressor column, (F) c o n d u c t i v i t y c e l l , and (G) conductance meter w i t h a recorder ( i n t e g r a t o r ) . Reagents and Standards. The eluent used i n the i o n chromatograph was a b u f f e r s o l u t i o n , 0.003 M NaHC03/0.0024 M Na2CÛ3, at a pH o f 10.4. Stock standards i n the c o n c e n t r a t i o n of 1000 yg/mL were prepared i n f i l t e r e d d e i o n i z e d water from the sodium or potassium s a l t s of each of the a c i d s s t u d i e d . From these stock s o l u t i o n s , mixed working standards were prepared i n the eluent s o l u t i o n . These working standards are s t a b l e f o r at l e a s t three days. A l l chemicals used were reagent grade or b e t t e r . Procedures S i l i c a Gel Tubes. Glass tubes were packed w i t h s i l i c a g e l obtained from F i s h e r , Grade 01. To remove i n o r g a n i c i m p u r i t i e s , the s i l i c a g e l was washed i n heated d e i o n i z e d water f o r approximately 3 0 minutes, w i t h o c c a s i o n a l s t i r r i n g , decanted, and r i n s e d f o u r to f i v e times w i t h d e i o n i z e d water. I t was then heated again i n d e i o n i z e d water f o r 15-30 minutes and r i n s e d thoroughly. The s i l i c a g e l was then d r i e d overnight i n a 100°C oven u n t i l f r e e f l o w i n g . I f a blank of the s i l i c a g e l shows any i m p u r i t i e s , p a r t i c u l a r l y s u l f a t e , the washing procedure i s repeated. C o l l e c t i o n tubes (Figure 3) are made from 7-mm O.D./4.8-mm I.D. g l a s s t u b i n g approximately 13 cm i n l e n g t h , packed w i t h


1

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C H E M I C A L HAZARDS IN T H E W O R K P L A C E

EXHAUST

VACUUM PUMP TO HOOD

SAMPLING PORTS IMPINGER, PORTS

SAMPLING PUMP


GENERATION CHAMBER

FLOW STRAIGHTENER

FLOWMETER

SPOILER

DILUTION AIR Figure 1.

LJFLOWMETER

Generation system for acid mists

CONDUCTIVITY CELL

TO WASTE RECORDER CONDUCTIVITY METER

ELUENT RESERVOIR

PUMP

INTEGRATOR

SAMPLE INJECTION VALVE

SEPARATOR COLUMN

Figure 2.

SUPPRESSOR COLUMN

Schematic of ion chromatograph


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700 mg o f 20-40 mesh s i l i c a g e l i n the f r o n t s e c t i o n and 200 mg i n the backup s e c t i o n . Polyurethane foam plugs a r e used between the s e c t i o n s and a t the end. The f r o n t s e c t i o n of s i l i c a g e l i s held i n place w i t h a g l a s s wool plug. Sampling i s done a t a nominal flow o f 0.2 Lpm. A c t u a l flow r a t e s averaged 0.17 Lpm w i t h the sampler i n l i n e . A n a l y t i c a l . Samples were desorbed by p l a c i n g the f r o n t s e c t i o n o f s i l i c a g e l w i t h the g l a s s wool plug and the backup s e c t i o n i n separate 15-mL graduated c e n t r i f u g e tubes, adding approximately 6 mL o f e l u e n t , and heating i n a 100°C waterbath f o r 10 minutes. Upon c o o l i n g the samples were brought to 10-mL volume w i t h e l u e n t , covered w i t h P a r a f i l m , and shaken thoroughly. The samples were then f i l t e r e d through a 12-mL p l a s t i c syringe f i t t e d w i t h an A c r o d i s c in-line f i l t e r i n t o a second s y r i n g e f o r manual i n j e c t i o n or i n t o an autosampler v i a l . Samples and standards are i n j e c t e d i n t o the IC i n 100-uL a l i q u o t s . The i o n s a r e separated by t h e i r v a r y i n g affinités for the ion exchange r e s i n i n the anion separator column, the o p p o s i t e l y charged i o n s are s t r i p p e d away by the suppressor column l e a v i n g only the separated anions and water t o be detected by the c o n d u c t i v i t y c e l l . Samples are q u a n t i t a t e d by comparison w i t h a c a l i b r a t i o n curve. Since i o n s a r e i d e n t i f i e d only by r e t e n t i o n times, i n t e r f e r e n c e s may be d i f f i c u l t to i d e n t i f y . T y p i c a l i n s t r u m e n t a l o p e r a t i n g c o n d i t i o n s a r e l i s t e d i n Table I . Table I IC Conditions Eluent: Flow Rate: Columns :

Conductivity s e t t i n g : I n j e c t i o n volume:

0.003 M NaHCOo/0.0024 M N a C 0 138 mL/hr (30% pump c a p a c i t y ) 3 X 150 mm Anion precolumn 3 X 500 mm Anion Separator 6 X 250 mm Anion Suppressor 10 imho/cm 100 yL 2

3

A c o n d u c t i v i t y meter s e t t i n g o f 10 ymhos i s a p p r o p r i a t e for most samples. I f bromide or n i t r a t e a t two times the OSHA standard l e v e l s or c h l o r i d e a r e being analyzed, the l e s s s e n s i t i v e s e t t i n g o f 30 ymho/cm may be used. R e s u l t s and D i s c u s s i o n A n a l y t i c a l Method. Ion chromatography o f f e r s a v i a b l e a n a l y t i c a l t o o l f o r the determination o f each o f the i n o r g a n i c a c i d s i n a s i n g l e sample. IC i s a r e l a t i v e l y new technique


C H E M I C A L HAZARDS IN

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THE

WORKPLACE

developed by Small, Stevens, and Bauman (6>). Both anions and c a t i o n s may be analyzed by t h i s technique; however, i t s most s i g n i f i c a n t c o n t r i b u t i o n i s i n the determination of anions, since i t separates each of the i o n i c species from the others (e.g. NO2 , NO3 ,SC>3 , SO4 ). With proper s e l e c t i o n of e l u e n t , f l o w r a t e , and conductance s e t t i n g , the i o n s present i n the sample are separated and measured w i t h e s s e n t i a l l y no interference. A l l samples and eluents must be f i l t e r e d to prevent o b s t r u c t i o n of flow through the system, e s p e c i a l l y at i n l e t s to the columns. For manual i n j e c t i o n 1-2 mL of sample are r e q u i r e d to assure good r i n s e of the i n j e c t i o n loop. For automatic i n j e c t i o n , as w i t h the WISP autosampler, the amount of sample r e q u i r e d i s r e l a t i v e l y l e s s s i n c e only the volume i n j e c t e d i s a c t u a l l y used. Factors governing peak s e p a r a t i o n and e l u t i o n time are the eluent s t r e n g t h , the flow r a t e , and the l e n g t h of the separator column. Buffered eluent s o l u t i o n s are g e n e r a l l y used to maintain a constant pH, thereby keeping constant the e l u t i o n order of m u l t i v a l e n t i o n s . E l u t i o n times vary d i r e c t l y w i t h the eluent c o n c e n t r a t i o n and i n v e r s e l y w i t h the flow r a t e (JK)). The eluent used f o r the a c i d s work, 0.003 M NaHCO /0.0024 M Na C0 , pumped at a flow r a t e of 138 mL/hr (30$ of pump c a p a c i t y ) gave good s e p a r a t i o n of the anions s t u d i e d . With prolonged use ("one yr) the separator column begins to l o s e r e s o l u t i o n . This i s s i g n i f i c a n t i n the s e p a r a t i o n of bromide and n i t r a t e ions s i n c e they are c l o s e l y e l u t i n g species. C a l i b r a t i o n curves were prepared by a n a l y z i n g mixed standards i n the c o n c e n t r a t i o n range of 1 to 20 Pg/mL, and p l o t t i n g peak height vs. c o n c e n t r a t i o n . Figure 4 i l l u s t r a t e s t y p i c a l c a l i b r a t i o n curves obtained from mixed standards at a s e n s i t i v i t y s e t t i n g of 10 ymhos. V a r i a t i o n of p o i n t s was approximately ±5$, mainly owing to the f a c t t h a t the c o n d u c t i v i t y c e l l i s s e n s i t i v e to temperature changes. The measurement o f peak height i s recommended over peak area f o r IC c a l c u l a t i o n s (JK) ). Based on a 4-hour sample taken at a nominal f l o w r a t e of 0.2 Lpm (48 L) at a i r c o n c e n t r a t i o n s from 0.2 to 2 times the OSHA standard, the a c i d mist samples would be expected to c o n t a i n : 10-100 yg of H2SO4 and H3PO4, 100-960 yg HBr, and 50-500 Pg HNO3. At the 10 vmho conductance s e t t i n g , the c a l i b r a t i o n curves are l i n e a r to 20 ug/mL. Since 10 mL i s the d i l u t i o n volume, l a r g e r samples of HBr and HNO3, would r e q u i r e f u r t h e r d i l u t i o n or the use of a l e s s s e n s i t i v e conductance s e t t i n g to f a l l i n the l i n e a r range of the method. Under the instrumental c o n d i t i o n s s t a t e d i n the experimental s e c t i o n , the presence of 0.3 yg/mL of each of the a c i d s can be detected w i t h a p r e c i s i o n of 10$ r e l a t i v e


2

3

2

3


10.

Airborne


Inorganic

Acids

143

• 13 cm 4.8mm 7mm

INLETGLASS

SILICA GEL


Figure 3.

POLYURETHANE FOAM

Silica gel collection tube

I40i

120 Γ

ΙΟ pg / ml

15

20

Figure 4. Calibration curves for inor ganic acids: Ο HBr, (A) HN0 , (0) H SO,„ (O) H P0f 3

2

3


t


144

standard d e v i a t i o n , and 0.5 yg/mL a t 5$ R S D . The RSD f o r each of the a c i d s a t a c o n c e n t r a t i o n o f 0.5 yg/mL (n=5) was determined

to be:

H 3 P O 4 - 3.4%,

HBr - 2.9$, HNOo

-

3.5$, and H2SO4 - 2.9$.

Table I I l i s t s the OSHA permissable exposure l i m i t s ( Vl_), the sample range a t 0.2 - 2 times the OSHA standard l e v e l , and the a n a l y t i c a l range o f the method. Table I I OSHA Standard (mg/m ) 1 10 5 1 7(O

Acid

3


H3PO4

HBr HNO HaSOit HC1

a

Sample Range (pg/sample) 10-100 100-960 50-500 10-100 20-200

A n a l y t i c a l Range (yg/mL) 0.5-20 0.5-20 0.5-20 0.5-20 0.2-20

a

Based on 15-L sample owing to c e i l i n g c o n c e n t r a t i o n o f 7 mg/m ( & ) . 3

A n a l y t i c a l Recovery. Mean r e c o v e r i e s from spiked s i l i c a gel samples i n d i c a t e d t h a t the d e s o r p t i o n e f f i c i e n c i e s f o r each o f the a c i d s s t u d i e d i s e s s e n t i a l l y complete. Table I I I l i s t s the mean r e c o v e r i e s and p r e c i s i o n f o r each o f the a c i d s .

Acid H3PO4

HBr HNOo H2SO4

Table I I I A n a l y t i c a l Recovery and P r e c i s i o n Precision Ν Mean Recovery ($) ( $ RSD) 18 99.1 2.9 8 102.0 5.6 6 102.0 1.8 12 97.6 2.8

This complete d e s o r p t i o n i s owing t o the sorbent nature o f the s i l i c a g e l . An aqueous s o l u t i o n w i l l f i l l the pores o f the s i l i c a g e l to such an extent that the s i l i c a g e l occupies l e s s than 1$ o f the space. When 10-mL o f s o l u t i o n are added to a 1-mL volume o f s i l i c a g e l i n a graduated c e n t r i f u g e tube, the r e s u l t i n g volume i n t h e tube i s l e s s than 10.1 mL. Thus the s i l i c a g e l occupies l e s s than 1$ o f the t o t a l volume. This f r e e movement o f l i q u i d through t h e s i l i c a g e l a l l o w s f o r complete s o l u t i o n o f the absorbed substances. Aerosol C o l l e c t i o n I n A Sorbent Tube. A e r o s o l s i n the atmosphere are drawn i n t o the sampling tube by a vacuum a p p l i e d to the tube a t a constant flow r a t e . Very l a r g e


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p a r t i c l e s w i l l f o l l o w the a i r stream l i n e s and enter the sampler. The incoming p a r t i c l e may impact or i n t e r c e p t on t h e g l a s s wool p l u g , or i t may penetrate the g l a s s wool by f o l l o w i n g the a i r stream t o the sorbent bed. Each granule o f sorbent i n the packed bed o f f e r s an opportunity f o r c a p t u r i n g the a e r o s o l p a r t i c l e by impaction, d i r e c t i n t e r c e p t i o n or d i f f u s i o n . The p o s s i b i l i t i e s f o r capture may be increased by lengthening the sorbent bed and/or by using a smaller mesh sorbent, thereby reducing the v o i d space w i t h i n the packed bed. These a l t e r a t i o n s , however, a r e l i m i t e d by the i n c r e a s i n g pressure drop across the tube which may completely impede the a i r flow. O p t i m i z a t i o n o f C o l l e c t i o n Tube Geometry. A s o l i d sorbent tube, a t r a d i t i o n a l gas and vapor c o l l e c t i o n device, has been adapted to c o l l e c t p a r t i c l e s as w e l l . The p h y s i c a l processes of c o l l e c t i n g a e r o s o l s r e q u i r e d m o d i f i c a t i o n o f the sorbent tube to e f f i c i e n t l y r e t a i n the p a r t i c u l a t e s as w e l l as the vapors. S e v e r a l f a c t o r s p l a y a r o l e i n t h e c o l l e c t i o n o f an a e r o s o l by a packed bed: the a e r o s o l p a r t i c l e s i z e , the i n l e t v e l o c i t y , the sorbent mesh s i z e , and the l e n g t h o f the sorbent bed (1_2). I n i t i a l l y , s i l i c a g e l c o l l e c t i o n tubes developed f o r the HCl method mentioned above (7mm 0.D./4.8 mm I.D. g l a s s tube packed w i t h 20-40 mesh s i l i c a , 400 mg f r o n t and 200 mg backup) were evaluated i n t e s t atmospheres c o n t a i n i n g the i n o r g a n i c a c i d m i s t s : HCl, H3PO4, HNO3, and H S0i|. H y d r o c h l o r i c a c i d was i n c l u d e d here f o r a comparison w i t h r e s u l t s o f previous work where hydrogen c h l o r i d e gas was used i n sample generation. C o l l e c t i o n o f the vapor-forming a c i d s was e x c e l l e n t , 100$ f o r both HCl and HNO3 (n=6); however, the two aerosol-forming a c i d s were not completely c o l l e c t e d on the g l a s s wool plug and f r o n t s e c t i o n o f s i l i c a g e l . Independent determinations o f generated a c i d concentrations were not made i n a l l o f the experiments. I n i t i a l l y impingers were used as the independent sampling method t o c o l l e c t both the vapors and a e r o s o l s . However, they were found t o be l e s s than 60$ e f f i c i e n t . Consequently, i n t h e o p t i m i z a t i o n o f the sampling tube f o r p a r t i c l e c o l l e c t i o n , a measure o f c o l l e c t i o n e f f e c t i v e n e s s was c a l c u l a t e d using q u a n t i t i e s found on the backup s e c t i o n o f the sampling tube. The mass o f sample c o l l e c t e d on the f r o n t s e c t i o n (and t h e i n i t i a l g l a s s wool plug) d i v i d e d by that mass p l u s the mass on the backup s e c t i o n i s defined as the c o l l e c t i o n r a t i o . 2

C o l l e c t i o n ($) = mass found on f r o n t s e c t i o n and g l a s s wool Ratio t o t a l mass found This c o l l e c t i o n r a t i o was used t o i n d i c a t e b e t t e r


χ


146


performance. At very high c o l l e c t i o n r a t i o s ( >90$) t h i s number i s assumed to be e q u i v a l e n t to the c o l l e c t i o n e f f i c i e n c y . I n other experiments a d i r e c t comparison was made between the s i l i c a g e l c o l l e c t i o n and the r e s u l t s o f f i l t e r c o l l e c t i o n f o r H 3 P O 4 and H 2 S O 4 . The mean c o l l e c t i o n r a t i o (n=6) o f the 400-mg s i l i c a g e l tube f o r H 3 P O 4 was 94.8$ w i t h a r e l a t i v e standard d e v i a t i o n (RSD) o f 4.8$, and 86.4$ f o r H 2 S O 4 w i t h an RSD o f 4.6$. The bar graphs i n Figures 5 and 6 i l l u s t r a t e the e f f e c t on the c o l l e c t i o n r a t i o o f the sampler made by v a r y i n g the s i l i c a g e l mesh s i z e , tube diameters and r e s u l t a n t i n l e t v e l o c i t i e s , and the length o f the sorbent bed. The c o l l e c t i o n r a t i o s given are the mean o f r e s u l t s from s e v e r a l sample generations w i t h i n the a i r c o n c e n t r a t i o n range o f 1-2 mg/nw. Mesh S i z e . An i n t e r e s t i n g point noted by the data i n Figure 5 i s that the s i l i c a g e l w i t h the l a r g e r mesh s i z e , 20-40, was a b e t t e r c o l l e c t o r o f the a c i d mists than the smaller 35-60 mesh. This may be explained by a combination o f the absorbing p r o p e r t i e s o f the s i l i c a g e l and the f a c t t h a t the p a r t i c l e s are l i q u i d . I t i s speculated that the l a r g e r granules may b e t t e r absorb and hold the l i q u i d d r o p l e t s o f the a e r o s o l once they are encountered. Sampling V e l o c i t y . The data i l l u s t r a t e d i n Figure 6 f o r 20-40 mesh s i l i c a g e l i n d i c a t e decreasing r e t e n t i o n o f the a e r o s o l w i t h decreasing v e l o c i t i e s i n the sampling tube. The sampling v e l o c i t y i s the flow r a t e through the tube d i v i d e d by the c r o s s - s e c t i o n a l area o f the tube. Higher v e l o c i t i e s should improve the p a r t i c l e c o l l e c t i o n by impaction. The f i r s t group o f data i n F i g u r e 6 shows c o l l e c t i o n r a t i o s o f three d i f f e r e n t sampling v e l o c i t i e s . The c o l l e c t i o n of

H3PO4 at

25,

15,

and

10

cm/s

was

97,

95,

and

88$

respectively. The c o l l e c t i o n o f H S0i| was 89, 86, and 76$ r e s p e c t i v e l y f o r the same v e l o c i t i e s . The second group o f data shows H 3 P O 4 c o l l e c t i o n a t 100$ and 88$ f o r v e l o c i t i e s o f 15 and 10 cm/s, and H S0i| c o l l e c t i o n a t 98$ and 94$ a t these v e l o c i t i e s . These data i n d i c a t e t h a t higher v e l o c i t i e s r e s u l t i n b e t t e r sample c o l l e c t i o n . 2

2

Bed Length. While the f i r s t group o f data i n Figure 6 i n d i c a t e s that a higher sampling v e l o c i t y r e s u l t s i n b e t t e r c o l l e c t i o n , the c o l l e c t i o n r a t i o s are not a t the optimum. A means o f f u r t h e r o p t i m i z i n g the c o l l e c t i o n r a t i o s i s t o i n c r e a s e the l e n g t h o f the sorbent bed. The f i r s t group o f data was derived from s i l i c a g e l tubes w i t h a bed l e n g t h o f 3.5 cm. At t h i s bed l e n g t h , the best c o l l e c t i o n r a t i o s r e s u l t e d from a sampling v e l o c i t y o f 25 cm/s, which corresponds to a 6-mm O.D. tube w i t h a 300-mg s i l i c a g e l


10.


Airborne

Inorganic

Acids

100

2 en

οζ

KEY:

85 80


A,

4cm BED 500mg SG 7mm OD U5cm/s)

Figure 5.

20-40 MESH

III

90

75 O

35-60 MESH

4.8cm BED 600 mg 7mm 0D(l5cm/s)

Effects of silica gel mesh size on collection ratio

H P0 H S0 3

4

2

4

H P0 3

4

^94

Γ

' ψ(Δ /

oc

ν,

ο

iι H S0 H P0 H S0 2

4

y,

α 0 3.5 cm BED LENGTH VEL. DIA. 25 cm/s 6mm 300mgSG 15cm/s 7mm 400mq I0cm/s 8mm 600mg

Figure 6.

147

3

4

2

4

%

4.0cm

4.8cm

5.7cm

500m g 800mg

600mg

700mg

Collection of phosphoric acid and sulfuric acid on 20^40 mesh silica gel varying collection tube diameter and bed length

American Chemical Society Library 1155 16th St. N. w . Washington, C. 20036 In Chemical Hazards in the D. Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


148


l o a d i n g . When the bed l e n g t h was i n c r e a s e d t o 4 cm w i t h 400-mg s i l i c a g e l l o a d i n g , the pressure drop across the tube increased to 6.3 inches o f water and v i r t u a l l y impeded flow through the tube. When the bed l e n g t h o f the 7-mm O.D. tube w i t h corresponding i n l e t v e l o c i t y o f 15 cm/s was i n c r e a s e d t o 4-cm by a 500-mg l o a d i n g , t h e c o l l e c t i o n r a t i o s i n c r e a s e d t o 100$ and 98$ f o r phosphoric and s u l f u r i c a c i d s r e s p e c t i v e l y , as shown i n the second group o f data i n F i g u r e 6. To achieve a 4-cm bed l e n g t h i n the 8-mm O.D. tube, 800 mg o f s i l i c a g e l was r e q u i r e d . Larger l o a d i n g s t o i n c r e a s e bed l e n g t h seemed i m p r a c t i c a l . The t h i r d and f o u r t h groups o f data i n Figure 6 i l l u s t r a t e the use o f longer bed lengths i n the 7-mm O.D. tube to achieve 100$ c o l l e c t i o n r a t i o s . Consequently, t h e recommended c o l l e c t i o n tube geometry i s a g l a s s tube, 7-mm O.D./4.8-mm I.D. packed w i t h 20-40 mesh washed s i l i c a g e l , 700 mg i n the primary s e c t i o n and 200 mg i n the backup s e c t i o n . Breakthrough Study. The s i l i c a g e l sampling tube has been optimized f o r the c o l l e c t i o n o f a e r o s o l - f o r m i n g a c i d m i s t s as w e l l as vapor-forming a c i d s . Now i t s c a p a b i l i t y f o r c o l l e c t i n g a t l e a s t a 4-hour a i r sample must be determined. In the absence o f a d i r e c t method o f monitoring breakthrough f o r each o f the i n o r g a n i c a c i d s , breakthrough was determined by a n a l y s i s o f the backup s e c t i o n s o f the samplers. Twelve c o l l e c t i o n tubes were placed i n the generation system. An atmosphere o f mixed a c i d m i s t s was generated a t approximately two times the OSHA permissable exposure l i m i t s , and samples were c o l l e c t e d over a four-hour p e r i o d . The f i r s t sample was removed a t 1 hour 15 minutes. T h e r e a f t e r samples were removed i n 15-minute increments. A f t e r the 4-hour p e r i o d , breakthrough had not o c c u r r e d . Therefore, t h e recommended s i l i c a g e l sampling tube w i l l c o l l e c t a t l e a s t a 4-hour sample a t a f l o w r a t e o f 0.2 Lpm. Sample S t a b i l i t y . The s t a b i l i t y o f i n o r g a n i c a c i d samples on the s i l i c a g e l c o l l e c t i o n tubes was determined by s t o r i n g samples f o r a period o f 10 days. Twelve 3-hour a c i d mist samples were generated a t an a i r c o n c e n t r a t i o n e q u i v a l e n t t o Table IV S t a b i l i t y Study

HNO3

Day Analyzed Day 1

Ν 6

HBr Cone. P r e c i s i o n 3.8 10.6

Cone. P r e c i s i o n 10.1 3.0

H?S0u Cone. P r e c i s i o n 1.0 2.8

Day 10

6

4.1

11.8

1.2

7.4

2.1

Average c o n c e n t r a t i o n i n mg/m3 and the percent r e l a t i v e standard d e v i a t i o n .


10

10.

CASSiNELLi

A N D TAYLOR

Airborne

Inorganic

Acids

149


the OSHA standard f o r each o f the a c i d s . S i x samples were desorbed on day 1, and s i x were s t o r e d a t room temperature f o r 10 days. The data l i s t e d i n Table IV i n d i c a t e t h a t the samples are s t a b l e . Sampling and A n a l y t i c a l P r e c i s i o n and Accuracy. The accuracy o f r e s u l t s obtained from a sampling and a n a l y t i c a l method are determined by comparison w i t h an independent method. The a e r o s o l s o f phosphoric and s u l f u r i c a c i d s were c o l l e c t e d on mixed c e l l u l o s e e s t e r f i l t e r s (J_4) and analyzed by i o n chromatography. Table V shows the c o l l e c t i o n e f f i c i e n c y o f the 7-mm 0.D./700-mg s i l i c a g e l c o l l e c t i o n tubes w i t h respect t o the r e s u l t s obtained from f i l t e r samples, and the p r e c i s i o n obtained from c a l c u l a t i o n o f the pooled r e l a t i v e standard d e v i a t i o n . Table V Sampling and A n a l y t i c a l P r e c i s i o n and C o l l e c t i o n E f f i c i e n c y Based on R e s u l t s o f F i l t e r Samples

Acid H3PO4 H S0i| 2

N_ 18 25

Collection Efficiency($) 88 90

Precision* (RSD) 10.5 8.7

* Pooled value f o r three c o n c e n t r a t i o n l e v e l s . Independent sampling methods f o r the vapor-forming a c i d s i n v o l v e d the use o f impingers or bubblers. E a r l y experiments i n t h i s study found that the e f f i c i e n c i e s o f impingers, c o n t a i n i n g the b u f f e r s o l u t i o n used as eluent i n the a n a l y t i c a l method (0.003 M NaHC0 /0.0024 M N a C 0 a t a pH o f 10.4), averaged approximately 60$. Even when three impingers or bubblers were c o l l e c t e d i n s e r i e s , s i g n i f i c a n t amounts o f a c i d were found i n the t h i r d impinger. For t h i s reason coupled w i t h the v a r i a t i o n found i n d u p l i c a t e samples, the use o f impingers was d i s c o n t i n u e d as an independent sampling media. S i l i c a g e l was found to be a b e t t e r c o l l e c t o r o f the vapor-forming a c i d s than were impingers or bubblers. Silica gel samplers c o l l e c t e d an average o f 60$ more HC1 than p a r a l l e l impinger samples, and 35$ more HN0 . Although the s i l i c a g e l tubes used here v a r i e d i n s i z e and l o a d i n g , a l l the HC1 and HN0 vapors were c o l l e c t e d on the primary s e c t i o n . Since the c o l l e c t i o n r a t i o s were 100$ and the samples are s t a b l e f o r a t l e a s t 10 days, i t i s concluded t h a t s i l i c a g e l samplers are s u p e r i o r t o impingers f o r the c o l l e c t i o n o f a c i d vapors. The s i l i c a g e l c o l l e c t i o n tubes a r e being f i e l d t e s t e d i n an e l e c t r o p l a t i n g o p e r a t i o n f o r H S0i}, HN0 , and HC1, and i n metal p r e p a r a t i o n f o r p a i n t i n g f o r H P0ij. 3

2

3

3

3

2

3

3



150

CHEMICAL HAZARDS IN THE WORKPLACE

Summary This work has demonstrated that sorbent tubes are viable samplers for inorganic acid mists existing as vapors and aerosols. A silica gel sampling tube was developed which will collect at least a 4-hour sample of inorganic acid at a nominal flow rate of 0.2 Lpm. The optimum sampler geometry was determined to be a 7-mm O.D./4.8-mm I.D. glass tube packed with 20-40 mesh washed silica gel, 700 mg in the primary section and 200 mg in the backup. The Lee and Gieseke model (8) for predicting aerosol collection on packed beds assumes that the volume fraction, or solidity, of the packed bed approaches 5/8, and applies to the collection by the packed bed only. Since the silica gel collection tube has a volume fraction of one-half or less, and since the greater percentage of the aerosol is collected on the initial glass wool plug, the model is not applicable to our sampler design. The analytical method by which a l l the inorganic acids may be analyzed in a single sample is ion chromatography. Using the stated instrumental conditions the analytical range is 0.5-20 yg/mL for H3PO4, HBr, HNO3, and H S0i|, and 0.2-20 yg/mL for HCl (30 ymho conductance). The sample range is 10-100 yg (0.5-2 mg/m3) for H3PO4 and H S0ij, 20-200 yg (0.14-14 mg/m3) for HCl, 50-500 yg (1-10 mg/m3) for HNO3, and 100-960 yg (2-20 mg/m3) for HBr. 2

2

Acknowledgement The comments and suggestions of Drs. Peter M. Eller, Paul A. Baron, and Jerome P. Smith concerning aerosol generation and behavior are gratefully acknowledged. Disclaimer Mention of a company name or product does not constitute endorsement by NIOSH. Abstract Airborne inorganic acids exist in the workplace environment as both vapors and particulates. To monitor for the common inorganic acids, a single, non-liquid sampling device to collect both vaporous and aerosol contaminants quantitatively, and an analytical method to determine these acids in a single sample was desired. Ion chromatography offers the analytical tool for the determination of inorganic acids in a single sample. In



10. CASSiNELLi AND TAYLOR

Airborne Inorganic Acids

151

previous work on hydrogen chloride-hydrogen bromide mixtures, various sample collection media, filters and solid sorbents treated and untreated, were evaluated. The sorbent of choice was silica gel which had been washed with deionized water. In the present study, collection tubes packed with 400 mg of washed silica gel were evaluated in test atmospheres containing five inorganic acids: HC1, H3PO4, HBr, HNO3, and H2SO4. To determine the optimum collection tube geometry, experiments were conducted varying (1) the tube diameter which affects the inlet capture velocity, (2) the silica gel loading which determines the length of the sorbent bed, and (3) the mesh size of the silica gel which determines the air flow properties and the pressure drop of the collection tube. Ultimately, a single sampling device was designed which collected both the vaporous and particulate forms of inorganic acids with subsequent analysis by ion chromatography. Literature Cited 1. "Chemical Safety Data Sheets"; Manufacturing Chemists Association: Washington, D. C., 1963. 2. Sax, Ν. I. "Dangerous Properties of Industrial Materials", 3rd ed.; Van Nostrand Reinhold Co., New York, 1968. 3. "International Labour Office: Encyclopaedia of Occupational Health and Safety"; McGraw-Hill Book Co., New York, 1971. 4. Taylor, D. G., Ed. "NIOSH Manual of Analytical Methods" Vol. 4, DHEW (NIOSH) Publication No. 79-141, 1979. 5. Small, H.; Steven, T. S.; Bauman, W. C. Anal. Chem., 1975, 47, "No. 11", 1801-9. 6. Cassinelli, M. E.; Eller, P. M. "Ion Chromatographic Determination of Hydrogen Chloride"; Paper No. 150, American Industrial Hygienist Conference, Chicago, 1979. 7. Hydrogen Chloride, P&CAM Method No. 310, "NIOSH Manual of Analytical Methods", 2nd ed., Vol. 5; DHEW (NIOSH) Pub. No. 79-141; 1979. 8. Lee, K. W.; Gieseke, J. A. Environmental Science and Technology, 1979, 13, 466-470. 9. Ortiz, L. W.; Fairchild, C. I. "Aerosol Research and Development Related to Health Hazard Analysis", LASL Report LA-6539-PR, USERDA Contract W-7405-Eng. 36, 1976.


152

CHEMICAL HAZARDS IN THE WORKPLACE

10. "Ion Chromatography Training Course"; Dionex Corp: Sunnyvale, CA, 1978; 35-7. 11. "Federal Register", 1971, Vol. 36, No. 157, Part 1910. 12. Dennis R., Ed. "Handbook on Aerosols"; Technical Information Center, ERDA, 1976. 13· Raabe, O.G. "The Generation of Aerosols of Fine Particles", Inhalation Carcinogenesis; Academic Press, New York, 1976, 74.


14. "NIOSH Manual of Sampling Data Sheets"; DHEW (NIOSH) Pub. No. 77-159, 1977, 33-1, S174-1. RECEIVED October

14, 1980.


Chemical Hazards in the Workplace - ACS Publications - American

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