Chemical Hazards in the Workplace - American Chemical Society

6 Estimation of Airborne Sodium Hydroxide

Downloaded by NORTH CAROLINA STATE UNIV on January 7, 2013 | http://pubs.acs.org Publication Date: April 2, 1981 | doi: 10.1021/bk-1981-0149.ch006

E. REID Wolfson Bioanalytical Unit, Robens Institute of Industrial and Environmental Health and Safety, University of Surrey, Guildford GU2 5XH, United Kingdom

The NIOSH method for determining sodium hydroxide in air (1) entails collection by bubbler into standard hydrochloric acid, and electrometric titration of the remaining acid with standard sodium hydroxide solution. Even with the stipulated long collection time, the method has to rely on establishing a titration difference which is rather small and which would be vitiated by any inadvertent spillage during or after collection. The sensitive method now advocated entails trapping of the alkali aerosol into boric acid, in a quantity which need not be exact, and final colorimetric titration with standard acid. The nature of the two methods is evident from Figure 1. The indicator (2) is present at the outset; hence arrival of traces of alkaline aerosol in the collecting liquid is signalled by a color change. The chosen indicator in conjunction with boric acid as trapping agent has long been in use for determining the ammonia formed in micro-Kjeldahl digestions, more simply than i f standard mineral acid is the trapping agent. This use of boric acid dates back to 1913 (3). The method description now given is followed by a comparison with the NIOSH method, mainly on a simulated basis entailing spiking with alkali rather than on an aerosol-collection basis such as ought to be included in hoped-for verification in other laboratories. Late in the study, a draft (lacking the Figures) of the s t i l l unpublished NIOSH validation by D.V. Sweet (4) was kindly furnished by Dr. G. Choudhary; i t is reassuring in respects such as the air-collection mode and rate (bubbler) in the method now advocated, and is candid about problems with the NIOSH method. Neither the latter nor the present method are specific for NaOH as distinct from alkali in general. Proposed Method Materials and Apparatus. In general the chemicals are of analytical reagent quality. Water is distilled from glass, with no precautions to exclude CO2 thereafter. 0097-6156/81/0149-0087$05.00/0 © 1981 American Chemical Society

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

88

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

the

acid:

sake of

this

Bromcresol ethanol: stock

green, (2)

stable

is

mixed i n d i c a t o r

1% w/v

0.1%,

is

and m e t h y l of

100

ml o f

the b o r i c

to

the

a c i d or

desired

alkali.

for

titration:

the twice

in

96% red

0.01%,

this

3.75

alternative),

w h i c h c a n be made by d i l u t i o n (e.g.

priately

0.50

mM

is

standard

titrations

are

trap for

i n the

Dorset).

It

from e x a c t l y from a

1M

reliable

Dorset,

U.K.). appro-

done w i t h a

aerosol

collection

U . K . by MDA S c i e n t i f i c

after

intact, use

is

successful

indicator

t h a t has

become

11 ml o f

boric

Procedure. indicator

and

standardized

10 m l b u r e t

of

Place

air

through

not

e x c e e d 1.0 L / m i n

at

the

(UK) L t d . l o n g as

our

onto

the

air:

lest

trap.

the

the

i n neither

dis-

soaking

removing

fabric.

12 ml o f

allow of

trap

fabric

A minimum

the

boric-acid/

volume assessment,

i n series

L/min,

s h o u l d be

and a s i n g l e - t r a p

c a s e s h o u l d any

for

occurs

after

well

the

to

but

30

yg of

as

l o n g as

the

initial

overshoot 30

as

3 h if

'Threshold Limit 2 mg/m

tip

water r i n s i n g s

NaOH h a s

tried

color

change

i n an occur.

the

(2-4

color.

3

of

1

into

Use s t a n d a r d 1 ml o f

Collection

concentration

V a l u e / ' T i m e Weighted

Titrate

i n d i c a t e d by a r e d

Calculation: y g o f NaOH.

is which

of

NaOH

Average

is

1

air.

contents ml).

as

to b l u e ,

been c o l l e c t e d . the

should For

b l a n k s h o u l d be done

change from r e d d i s h - p u r p l e

about

viz.

Finally

to

be f o r

below the

(TLV-TWA),

color

a

draw

rather v i o l e n t .

C o l l e c t i o n s h o u l d b e f o r a t l e a s t 1 h , o r s a y 10 times as l o n g needed

in

any

W i t h a pump w h i c h c a n b e o f

b u b b l i n g be

exercise,

as

(Wimborne,

advisable.

approximately

a second

aerosol-collection

may h a v e

con-

U.S.-made

recommended

a r a t e w h i c h c a n s u i t a b l y be 0 . 8

method-checking, clean

is

and m u s t

is

polyethylene,

i n substantially

adsorbed acid

s o l u t i o n i n the

p e r s o n a l l y worn t y p e ,

as

provided that

volume o f

with

acid

suitable

acid.

c a n be used r e p e a t e d l y

remains

methanol

performed,

type.

A suitable

perser

the

i n method t e s t i n g ,

p o r t a b l e Midget Impinger (Model P M I - A ) , made o f supplied

be

sulfuric

made up f r o m NaOH p e l l e t s

against

Colorimetric

spiking

of

exceptionally

m o l a r i t y w o u l d be a

BDH C h e m i c a l s L t d . , P o o l e , used f o r

M: this

electrometrically ventional

as

12 ml

Only

s u i f u r i c a c i d as f u r n i s h e d by a n ampule o b t a i n e d source

each

reddish-purple color

acid of

S t a n d a r d NaOH,

for

The w o r k i n g s o l u t i o n ,

acid.

(hydrochloric

commercial

preferred

[A s t r o n g e r m e t h y l

prepare.]

of

acid

2% i f

i s made by a d d i n g

need a d j u s t m e n t a d d i n g drops

(or

red,

1:2.

d i f f i c u l t to

indefinitely,

per

strength

capacity).

by

Standard


of

mix i n a volume r a t i o

solution

which

is

very high trapping

3.75

a

titration

with alkali

the to

vessel,

standard correct

and add

a c i d back any

color. mM s u l f u r i c a c i d

to

inadvertent

corresponds


to

6.

REID

Airborne

Sodium

V a l i d a t i o n by S p i k i n g In comparisons different

amounts

an E r l e n m e y e r immaterial


sealed

to

or

into

with

0 . 0 7 5 M HC1 i n a or

the

titrations

essentially

tion

added by means o f

and t h e t i t r a n t

microburet

( Agla ;

mounted

as

sealed

f

so

tube,

n i t r o g e n was

as

Wellcome F o u n d a t i o n ,

f

to

was

discharge,

just

above

via fine-bore

polypropylene

conducted

similar

m a g n e t i c s t i r r i n g was

via

performed

a

tubing;

then

NIOSH

NIOSH

descrip-

syringe-type Kent,

solution

w e l l as

in

w h i c h was

i n the

tubing into

as

acid

i n the

Beckenham,

the

approach,

boric

t u b e (130 x 25 mm;

method w e r e done by pH m e t e r , (J_),

present

round-bottomed)

Electrometric

f

Collection

NaOH w e r e s p i k e d i n t o

flat-bottomed

Parafilm .

f

NIOSH a p p r o a c h

standard

flask,

89

Simulate Aerosol

of t h e

of

whether

with

Hydroxide

U.K.),

i n the

a stream

the

nearof

vessel,

and

hand-swirling.

C o m p a r a t i v e r e s u l t s . F i g u r e 2 i n c o n j u n c t i o n with i t s legend documents the e x c e l l e n t a c c u r a c y and r e p r o d u c i b i l i t y o b t a i n e d range,

even w e l l below the

used with

t h e NIOSH method

titration

difference.

tracted

f r o m 0.461

Illustrative electrometric approach the

to

simple

approach

with

as

colorimetric not worthy

with

3(b),

trapping

method. that

liquid

is

that

pH 4 . 9 0

pH 10,

least

40

by Ma and Z u a z a g a

the

(and,

overshoot

of

3(a)

acid

titrant,

the at

shows

acid

be

(0.425 ml

sub-

representative

put

an

into

the

NaOH) i n

namely

4.57,

(2,),

namely

is

boric

the

electrometric the

trap,

lack

of

any of

proposed

i n good a c c o r d these

had a l r e a d y

of

acid

high capacity

4.52;

color,

NIOSH

checking

the

However, as

and h e n c e

red

the

Electrometric

i n showing the

mg o f

acid

a

performed with

alkaline bluish color

conversely,

to

accurate

performed w i t h

2.

informative

The i n i t i a l pH v a l u e ,

reported

stated

Figure

boric

an

o n l y 0 . 0 3 6 ml

illustration. the

of

a wide s p i k i n g

t h a t have

mg s p i k e .

Figure

for

of

(at

was

0.49

laboriously

m a r k e d i n f l e x i o n e v e n up to the

interests

titrations

a l k a l i of

shown i n F i g u r e

the

Titrations.

titration

obtain values

is

titration

i n the

The l a t t e r

ml)

over

high spike quantities

with

authors

been a t t a i n e d

relevant

to

at

any

pH 4 . 2 6 ) .

A t t e m p t e d V a l i d a t i o n by A e r o s o l G e n e r a t i o n and

Collection

The c o m p a r i s o n s w e r e b a s e d o n i n t r o d u c i n g , and f i n a l l y e s t i m a t i n g , amounts o f NaOH w h i c h w e r e n o t a s c e r t a i n a b l e a t the t i m e b u t w h i c h w e r e c o m p a r a b l e f o r t h e two methods i n a g i v e n d u a l t e s t where f i n a l t i t r a t i o n showed how much h a d b e e n c o l l e c t e d . Apparatus.

Generation of

achieved

by a p p l y i n g a g e n t l e

(as

for

used

reservoir, tal

glass

the

exit

spraying

the

containing nitrogen

chromatograms) w i t h

o u t f l o w from the

chamber the

aerosols stream of

(approx.

jet

ml o f

ingress

an

atomizer

20% NaOH i n

being directed

1 L) w i t h f r e e

a i r - d i l u t e d o u t f l o w was

50

NaOH was

to

into of

p u l l e d through

the

a horizon-

air.

From

a single


trap

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

PRINCIPLES OF METHODOLOGY FOR 'CAUSTIC MIST' NIOSH METHOD Standard in

HC1

an exact

amount

NEW METHOD QUANTITY OF

Boric

ACID

PUT

+ indicator Initially reddish-purpJs\

INTO


TRAP

+ NaOH: Hatching

ELECTROMETRIC T I T R A T I O N S (2) in a c l o s e d system with standard NaOH — then s u b t r a c t

Figure 1.

VÉ =500 yg ' NaOH, = TLV i f present in 250 L o f air

acid,

m ή

TITRATION back to r e d d i s h purple with standard a c i d gives NaOH amount

Principles of the two methods for estimating airborne sodium hydroxide

0.8

1.6

'NIOSH' BACK-TITRATION VALUE (mg) for NaOH SPIKED INTO HC1 (x)

Figure 2. NaOH values by the two methods, following spiking of the amounts shown parenthetically into boric acid or HCl. The plotting of the replicate points for the boric acid method exaggerates the variability: the coefficient of variation was only 0.2% for the 0.98-mg spikes and 0.5% for the 0.49-mg spikes (6 and 4 obser vations, respectively). Boric value = (1.054 X NIOSH HCl value) + 0.0024; r = 0.999.



6.

REID

Airborne

io

Sodium

Hydroxide

91

r

2 Titrant

4 (0.122 M NaOH),

6 ml

Figure 3. Electrometric titration curves showing the characteristic shapes: (a) ΝI OSH-type method: titration of 7.5 mL of HCl (originally 7.5mM but one-fifth neutralized for a spiking experiment) with 0.489M NaOH, by a syringe-type microburet (Agla); the intersection of the diagonals was taken as the neutralization point, (b) Titration of 12 mL of 1% boric acid with 0.122M NaOH; note the wide buffer ing range and consequent high capacity for trapping NaOH (40 mg could be coped with).


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

92 w h i c h was T-piece tested

alternated

to

s i d e by s i d e .

pump t h a t range

0.5-

1.0


jet

within

f r o m 50

250

with the

the

single-trap

early

conventional the

difficulty

was

starting

color

the

sintered blue

d e s i g n was

prone

It

Accordingly, not be

the

the net

spiking

complete,

as

to

mg;

as the

NIOSH v a l u e

agreement, powerful

amounts

became

glass

not

of

with

the

30-60 m i n . available, sintered

reduced

at

level

strength high

i n the

was

study,

c o u l d have

of

Nevercould distinct to

be

an

inserted

just

generally tests

least

done

1 mg

boric-acid

represents

validation

as

if

the

at

any

system.

appeared

were

collecting 1.4

trapping

offer

aerosol

acid/indicator

was

of

to

approach

change

o f NaOH c o l l e c t e d

t h e mean r a t i o

the

immediate

three-fold.

Whilst collections

tests

the

mixture.

least

i n the

by an

generation

a color

how-

constancy

a c i d method:

too

above

for

(facilita-

p o s s i b l y from

NIOSH b y - d i f f e r e n c e

satisfactory

generator

serially

Sometimes,

boric

the

test,

results

with a

(5).

acid/indicator

known l a t e that

every

alkali,

that

ranged

dual

traps

evidenced on o c c a s i o n

d e s i r a b i l i t y of

finding

although

of

absence of

i n 6 pairs

aerosol

Concluding

sensitive

traces

containing boric

NIOSH h a d shown t h e

done

elsewhere

a satisfactory

shown by t h e

trap

Accordingly,

was

experiments.

pump, t h e

below 0.5

testing

each

i n maintaining pre-test

soon r e a l i z e d

strength

additional the

as

collected

collection

i n t h e NIOSH m e t h o d was

before

to

w i t h the

the

smooth f u n c t i o n i n g o f

respective

was

each

NIOSH

p o s i t i o n i n g of

could imperil

made o f

described

volume i n the

increased

from the

the

the

the for

a c i d method was

h period of

entailed

tried,

release

was

satisfactory

theless

as

for

a

be

exhaust in

by a r o t a m e t e r

volumes

that

to

an

flow rate

t h e M i d g e t I m p i n g e r became

encountered

to

n o r m a l l y used

hope o f

3-6

connections)

on a d d i n g the b o r i c

Results. acid

air

drift

that

before

dispersing unit,

color

the

against

b u b b l e r s were

ever, of

boric

The t o t a l

system

frit:

b u b b l e r was

s y s t e m was

total

exactly

i n the

s w i t c h i n g between

trials,

test, or through

two methods

but with s u i t a b l e

during

t e d by c o n e - a n d - s o c k e t In

i n the

Notwithstanding apparent taken

two m e t h o d s :

each

NaOH s u f f i c i e n t l y f a s t

change

improvised system

a p r e c a u t i o n was

unit

ascertained

15 m i n .

L.

during

a l l o w e d the

adjustable

attainable, color

attainable to

as

but

D e l i v e r y of the

that

The f i n a l

L/min

m e t h o d was b a r e l y atomizer

traps

gave a c o n s t a n t

trap outflow.

this

with a partner

two p a r a l l e l

at

(4).

value

passable

s u c h as

a

more

allowed.

Comments

The p u t a t i v e b o r i c a c i d m e t h o d , w h i c h h o p e f u l l y w i l l b e t r i e d i n l a b o r a t o r i e s p r o p e r l y equipped f o r aerosol generation, offers n o t a b l e a d v a n t a g e s o v e r t h e e s t a b l i s h e d NIOSH m e t h o d . (1) T h e c o l l e c t e d s o d i u m h y d r o x i d e i s d e t e r m i n e d by d i r e c t titrat i o n , r a t h e r t h a n by e s t a b l i s h i n g a d i f f e r e n c e , t y p i c a l l y s m a l l , b e t w e e n two t i t r a t i o n s . Accordingly, (a) t h e t r a p p i n g a c i d n e e d n o t b e o f e x a c t s t r e n g t h and d i s p e n s e d e x a c t l y , n o r a s s i g n e d a


REID

6.

Airborne

Sodium

a limited shelf

life

indefinitely

at

room t e m p e r a t u r e ,

and does

encourage

during the

not

(2)

(the

c o l l e c t i o n or

bubbler

to

the

than a n a l y t i c a l l y

(3)

small,

(4)

the

The f i n a l If,

acid

as

transfer

vessel

so

good

little

titration buret

specificity use

is

50

is

2%,

not

merely

exercise

the

as

standard

a c i d are

air

trapping acid regrettable

sample

is

to

can

during

manifest

1%.

the

It

were

is

from

rather

be

1 h.

by a c o l o r

change

trap

i n question,

contents

for

I n common w i t h

for

with

s t r i k i n g that

alkali

a

as

distinct

the

air

hours

the

boric

lacking

without

NIOSH m e t h o d ,

f r o m NaOH;

i n fact hitherto

been f o r

the

estima-

distillates.

NIOSH m e t h o d d e s c r i p t i o n s prepare

up p a r t i c u l a r

if,

the

merely

color,

spill

electrometric.

a c i d has

ammonia i n K j e l d a h l

setting

stable

small

simple, being colorimetric

i n color.

boric

obviated

any

L collected

air

than

through

One f e a t u r e o f elaborate

is

collection.

rather at

of

(b)

of

is

that

as

NaOH i n t h e

any c h a n g e

customary

mixture

shown by i n s p e c t i o n f o r

improbably, trapping capacity

producing

to

final

sample

c o u l d be s e t

ting

is

of

NaOH c a n be p a s s e d the

acid/indicator

m i c r o b i a l growth);

the

as

actual

conventional

93

disastrous.

The p r e s e n c e

during

boric

titration

The s e n s i t i v i t y

relatively


Hydroxide

methods

may h a v e

to

i n general

a primary standard. for

occasional

This

needs

is

an

disincentive

can

be

be v e r i f i e d , b o u g h t - i n ampules

of

trustworthy.

Summary For

estimating

proposed. rather

than s t a n d a r d

finally rather a

NaOH a e r o s o l

The t r a p p i n g l i q u i d there

is

acid

by-difference estimate.

the

NIOSH m e t h o d , notable

advantageous borne has tary

NaOH i s

field

manifested

well

with

tests

that

entailed

amount

titration

titration

as

with standard

t h e method i s ,

only for

alkali

during

p o w e r f u l to

actual

rather

acid,

alkali

to g i v e

i n common w i t h t h a n NaOH,

it

and s i m p l i c i t y , and w o u l d since

the

presence

c o l l e c t i o n by a c o l o r aerosol

of

be

air-

change.

tests.

c o l l e c t i o n need

e q u i p p e d w i t h an a e r o s o l give

is

mixture,

i n t h e NIOSH m e t h o d ;

with standard

Although

especially

method

acid/indicator

t h e NIOSH method i n s p i k i n g

i n a laboratory

sufficiently

a boric

sensitivity

use

agreed

repeated with

specific

precision,

for

a direct-titration

i n exact

a colorimetric

t h a n an e l e c t r o m e t r i c

offers

is

It

Complemento

be

generator

a meaningful t i t r a t i o n

difference

t h e NIOSH m e t h o d .

Acknowledgements E s p e c i a l t h a n k s a r e due to M r s . L i n d a L . B a s a r a b f o r meticulous h e l p . D r . J . P . L e p p a r d and D r . I . R . T e n c h made u s e f u l suggestions, and M r . T . M c D o n a l d d e l i v e r e d t h e p a p e r a t t h e a c t u a l ACS S y m p o s i u m .


CHEMICAL HAZARDS IN THE WORKPLACE


94

Literature Cited 1. NIOSH Manual of Analytical Methods, 2nd edn., Vol. 1, U.S. Department of Health, Education and Welfare (NIOSH), Cincinnati, 1977, #241. 2. Ma, T.S.; Zuazaga, G.; Ind. Eng. Chem. (Anal.), 1942, 14, 280282. 3. Winkler, L.W.Z.; Agnew. Chem., 1913, 26, 231-232. 4. Sweet, D.V.W.; Haartz, J.C.; Hawkins, M.S. "Determination of Sodium Hydroxide Aerosol in Industrial Hygiene Samples", unpublished work cited in ref. 1: personal communication (draft Report) by D.V. Sweet via G. Choudhary. 5. Perry, R.; Young, R.J. "Air Pollution Analysis", 1977, Chapman and Hall, London, 506 pp. (vide p. 265). RECEIVED October

14, 1980.


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