[Doi 10.1002_jctb.5000670102] D. Flint -- A Method for the Determination of Small Concentrations of So3 in the Presence of Larger Concentrations of So2

8/12/2019 [Doi 10.1002_jctb.5000670102] D. Flint -- A Method for the Determination of Small Concentrations of So3 in the P

1/4

a ,FLINT-DETERMINATION OF SO,

1

I

I

10 100 I000

-

CHLORI0E.p.p.m.

FXG.

.

Calibrariori c t i r ve

be obtained by thermostatic control of the temperature of the

solutions, and by the use of a slide-wire potentiometer of greater

accuracy than the titrimeter, b ut i t was felt that such precautions

would limit the application of th e m ethod. As the resistance

of

the system is.not high, a simple slide-wire potentiometer could

also be employed instead of the electronic titrimeter.

In o rder to avoid systematic error, t he instrument should be

checked when carrying out a determination on a solution of

unknown chloride content by measuring concurrently and at

the same temperature, the e.m.f. of two standard ch loride

solutions of high a nd low concentration

1000

mg. per litre and

I mg. per lit re are suggested). Since the calibration is linear

these two fixed points serve to define the curve. It is not necessary

to know the actual millivolts corresponding to the bridge

reading in this

case.

Interference by other ions

T he following substances interfere m ore or less seriously with

the

determination of chloride. (I) Free ammonia, and other

materials able t o dissolve silver chloride by complex formation

j

(2)

strong reducing agents able to reduce silver chloride to silver j

(3)

anions

whose silver salts are less soluble

th n

silver chloride,

such

as bromide, iodide, thiocyanate, cyanide, etc. None

of

these substances is likely to be pre sent i n boiler water, for which

thismetho d is primariily intended. Sulphates, nitrates, sulphites,

and phosphates do not interfere, and free acid u p to a p~ of 3

is

also

without

effect.

In

alkaline solution (pn

>

.0) interf eren e may be expected

from carbonate and sulphide. Th e effect

of

the former may be

eliminated by the addition of a drop or two of nitric acid, and

of the latter by acidification, followed by boiling. It is recom-

mended that

a l l

water having a pn of 7 or more should be

acidified

with a drop or two of nitric acid before measurement.

Research Laboratories of The British Aluminium Co. Ltd.,

Chalfont Park, Ge rrards

Cross,

Bucks.

Reccivcd

Scptcmbcr

22, I947

-rghtlisb,J.,~r~FPrsdrySoc.1942aJ),389 .

. . .

A METHOD FOR

THE

DETERMINATION OF

SMALLCONCENTRATIONSOFSO

IN

THE

PRESENCE OF LARGER CONCENTRATIONS

By D.

FLINT

The sources of errors likely to arise in the estimation of minute

quantities of SOs n gases containing much larger quantities

of

SO,

are discussed with particular reference to boiler

flue gases.

A

laboratory method is described whereby these difficulties in esti -

mation m a y

be

eliminated,

as

demonstrated by the results obtained

on a synthetic gas mixture of known SO, and

SO,

content.

Th e estimation of SO, in

gases

containing SO, is of consider-

able importance in a num ber of technical fields, including those

of sulphuric acid manufacture, sulphide ore smelung and

in

the

combu stion of solid, liquid and gaseous fuels.

In

the combustion

of fuels, the concentration of SO3 n the flue gas may be only a few

parts per

million

whilst that of

SO,

may reach 0.3 by volume j

it is for th e estimation of concentrations of this order that the

present method has been devised. It has been developed as part

of a programme

of

esearch into boiler deposits and corrosion

instigated by the Boiler Availability Committee.*

Th e sources of error likely to arise in estimating SO2and SO,

in boiler flue gases are fully discussed later.

As

a

first step

in

developing a suitable technique for use on flue

gases

i t WBS

considered essential to evolve a laboratory method capable of

complete and accurate analysis of synthetic gas mixtures contain-

ing known amounts of SO2and SO,.

Sourcesof errorand heir avoidance

In published methods1I2 for the estimation of SO2and

SO,

in flue gases, it is the author's opinion that

not

ll of he diffi-

culties which may arise during the sampling, absorption and

analysis of the gases have received sufficient anention.

(u) Sunrpling of firre guses.-It h as b een s h ~ w n ~ ~hat

the

presence of a few parts per million

of

sulphuric acid i n fluegases

c n raise the dewpoint very much above that corresponding to

th e water vapour content. For example, a gas containing 10%

water vaFour and 0.005 sulphuric acid by volume has a dew-

point of

150'.

Thus if the sampling tube of the analytical

apparatus in use is n ot maintained at a tem perature well above

the dewpoint of the gas undergoing test, then condensation c n

Occur,

leading to a result which, as a measure

of

the

SO,

content

of the

gas,

may either be too low

or

too high. Th e result will be

too low if sulphuric acid conden sesout in he sampling ube and

the precaution of washing out this condensate and adding

it to the absorbing solution is omitted. Th e recovery of the

condensate may lead to a high result if SO2

has

been oxidized at

th e liquid/gas interface leading to an increase

of

the SO3conten

of the condensate.

In addition tooxides of sulphur, flue

gases

areknown o

contain

metallic sulphates in a very line state of subdivision and it is

essen t i a l that these should be filtered out before the hot gases

enter the absorbing solution. T h e presence of metallic sulphata

in the absorbing solution would obviously lead to a high

value

for the SO, and i t is known'

also

that th e presence of metalli

ions in aqueous solution can materially increase the

oxidation

of

SO1.

It

is

obvious that t he filter incorporated

in

he

sampling

linemust also bekept at a temperature above th e

d e y i n t ,

bu

below th e temperature

of

decompositionof unstable s phates.

(b) Absorption of rhe

gas.-One

of

the major sources

of ctro

in the estimation

of

SO,

in

the presence of SOB ribes

from

he

rapidity wt which dissolved SO, oxidizes in uninhibited

aqueous solutions, particularly at elevated temperatutes.

Th

reaction, which is known to b e a chain profess, is inhibited by

The Boiler Availability Committee includes representativesof t h

Central EIectriaty Board, The Water

Tube

Boilcrmakcr~Asmaatio

EIccuic y Undertakings, The

Fuel

Rescarch Board and

T h e

Bfirish

carluullsrtloaRacpIch~on.

OF SO,


2/4

J.S.C.1, 67,

Jurouy,

I @

th e presence of certain organic compounds which are preferen-

t i d y oxidized. Alyea and B ackstromG found that isoproryl,

sec.-butyl an d benzy l alcohols were a ll active inhibitors of the

oxidation of dissolved SOz. isoPropy1 alcohol was selected in

the method under discussion for the following reasons.

I

It is

completely miscible with water, a nd t he proportions

used,

80

isopropyl alcohol and

20

water, give a constant

boilingmiaUte. T he use of a constant boiling mixture ensures

the presence

of

isopropyl alcohol

in

any condensate in the

absorption train following the passage of hot gas through th e

absorbing solutions.

2. It s inhibiting action extends over a longer period than tha t

of benzyl alcohol.

3.

It retains its powers of inhibition even when hot gas is

passed into th e absorbing solution.

Th e design of the first absorption vessel is of importance since

the gas must not come into contact with surfaces below its

dewpoint. T h e conventional type of gas washing bottle with its

inlet tube immersed in the absorbing solution is therefore

unsuitable, and a U-shaped bubbler was employed, so designed

that the absorbing solution lapped back into a heated zone of

the inlet arm of th e bubbler.

Attention has also been given to the question of complete

absorption of

SO,

in aqu eous solvents. Doolcy and GoodeveO

have shown that when a gas containing

SO

vapour is bubbled

through

a

liquid having a high partial pressure of water vapour,

absorption of the SO, is incomplete on account of the formation

of small particles of sulphuric acid mist which

are

very d ifficult

to remove completely using liquid reagents. The se authors sug-

gest that th e mist panicles should be allowed to increase in size by

the condensation of water on their surface and by coalescence

before attempting their removal from the gas. I n the present

technique the particles are filtered out

by

passing the gas through

a G.4 sintered glass iilter, the washings from which are added

to th e absorbing solution at the end of the sampling period.

The absorption of the SO, remaining in the gas after the

complete removal of the SO, presents

no

difficulty an d can be

effected by hydrogen peroxide solution.

FLINT-DETERMINATION OF

SO,

3

(c) Analysis

of

the absorbing soliriions.-Many me thods have

been used for the analysis of a soluiion containing absorbed

SO, aud SO,. When the quantity of SO is very small compared

with that of SO2differential titration methods are likely

to

giv

rise to fairly large errors. T he positive method of removing he

SO, completely from solution by sweeping it ou t with oxygen-

free

nitrogen'

has

much to commend

it

under such conditions

Th is m ethod leaves

a l l

the SO, in one absorbing solution while

the SO, is carried into the absorbing solution containing the

remainder of the SOz.

E.pcrimentp1

technique

I Preparation of

known

gas mixture

(see

Figure

I).-Oxygcn

and carbon dioxide from cylinders pass through the capillary

flow mete rs A and B respectively and mix in

the

vessel

E. This

gas

mixture acts as a carrier for water vapour and

SO

generate

from dilute sulphuric acid of known normality. T h e burette G

and spiral H measure and con trol the rate of flow of acid throug

jets I and K nd, in conjunction with a by-pass fiom E, sustai

a constant flow of acid into t he quartz vaporizing

tubeL

which

projects into a n electrically heated Pyrex ube J Th e temperatur

gradient along the Pyrex tube is adjusted to give a maximum

temperature of 6co a t a point three quarters down the tube L

and then a fall to a constant temperature of

500

for the remainde

of its length.

SO,

from a syphon and nitrogen from a cylinder pass throug

flow meter s

C

and D respectively, mix in vessel

F

and are heate

to 500

in

the Pyrex tube M. They join the first gas stream a

th e base of the Pyrex spiral N maintained a t po , where

mixin

is completed. The

gas

mixture normally used had the compo

sition: 70% Nz 5% CO,, 10% H20, and

5

0 o whic

small

additions of SO? and/or SO3 were made as required

Only a proportion of the mixed gas was drawn for analysis

the remainder going to waste.

11. The absorption train (see Figure z).-A portion of the syn

thetic gas mixture is aspirated from the T-piece A

through

connecting device

B

ealed with powdered glass ground to pass

300 B.S. sieve. Leakage from the atmosphere through this s e

was found to be negligible at th

low pressure difference

obtainin

(less

than

3 mm.

Hg). T h

sampling tube, seal B and

G.

sintered glass filter C a re heate

to 250 to prevent the condensa

tion of sulphuric acid. (Althoug

this filter is unnecessary for

th

analysis of the synthetic ga

prepared

in

the laboratory,

was inserted to simulate plan

conditions where it is

essen t i

for the removal of

solids

in th

flue

gas.

T h e

g a s

t hen a t e r

the U-shaped absorption vess

D which contains 40

ml.

of

iro

propyl alcohol and 10 ml.

d i s

tilled water. Some SOpand SO

are absorbed and th e

gas

lavin

the solution is saturated wit

isopropyl alcohol vapour. T h

SO, remaining in t he gas

is

no

present as suiphuric sad vapou

or

mist.

T h e gas then

passes

nto

bott

E which contains 160 ml. o

iryrop yl alcohol and 40

ml.

o

dlstilled water. This a b s o

~ O I C

t h cSO lt ld SO

Md

t

frec space

above

the *

A a


3/4

FLINT-DETERMINATION OF

SOs

solution in bottle E s sufficient

to permit th e growth of sulphuric

acid mist panicles to such a size

that they are filtered out on the

G.4 shtered

glass

filter

F.

All

the

SO,

and some of the

SO2

have now been removed from th e

gas stream. Bottle

H

s by-passed

through stop cock G during the

samrling

stage, the gas passing

into bottle I containing 400

ml.

of10 20 vol. hydrogen perox ide

solution in which the remaining

SO s absorbed and oxidized.

A

similar bottle J with zco

rnl.

of hydrogen Deroxide sclution

TOYM I L

serves

as ache& on the efficiency

of bottle I.

Bodes

K

containing concen-

trated sulphuricacid),L empty),

and M concentrated potxsium 'O

*'IYL

hydroxide solution) with t he dry-

ing tower

N

(fused calcium

chloride and Sofnolite) and cold

trap

0

(solid carbon dioxide)

remove

CO,,

H,O and iso-

propyl alcohol vapour which

remains in the gas

stream

after passage through the solutions.

Th is leaves onlyN, nd 0 f the original gas mixture, and these

are

metered by the capillary flow meter P nd manometer Q n

order to ascertain the total quantity of the prepared gas mixture

withdrawn

for

analysis. These (N2 03 inally pass throug h

catch flask R to a water suction pump.

111. Anabsk-When sampling is completed the suction is

stopped, and the solution from the U-tube

D

and th e washings

from th e mist filter F

are

added to the bottle E. This bottle is

then connected to a stream of nitrogen which has been freed

from t ra ce of oxygen by passing it over copper tu rn ings a t 850-

900'. The suction is restarted and the solution in bottle E is

acidified with

5

ml. of concentrated hydrochloric acid. Th e

SO?dissolvedin the solution is carried over by the nitrogen into

bottles I and J. Bottle H ontaining a very dilute iodine solution

coloured with starch, is brought in to line eriodically by operating

the three-way stop-cock o indicate wl en a l l the SO2has been

transferred. This usually takes from 60 o

go

minutes.

Table I

% SO, added SO,

found :

SOa

found

A

oxidation*

0 . 2 2 0.224


4/4

J*S-CS

67,

WV

Wa

CLARK A N D

SPITTLE-HYDROGEN SULPHIDE. 5

separately and typical results are shown n Table 11. Th e propor-

tions of th e major constituents of th e

gas

mixture, the sampling

rate and the sampling time were as before.

Th e complete absence of SO, in the sodium hydroxide solu-

tions (except for a trace in test I) indicates that it is possible,

with the absorption train, to remove a l l the SO, from a gas

stream.

Quantities varying from

5

to

20%

of the total SO3.

were found on the acid mist filter.

The percentage of SO,

recovered was withim

=t

of that calculated to be i n the gas

stream and this once ag ain was cons idered satisfactory.

(c) The third and final stage in checkiug the method in the

laboratory was the analysis of a gas mixture containing both

SO,

and

SOp

This stage, however, could only reveal errors

greater than the inherent errors of the separate determinations

of the SO and SO Le., greater than f 5 . Tests showed

that the difference between the calculated and estimated results

was

still within th e limits of

=t

5 .

For

example, 225 itres ofgas mixture was prepared containing

0.2633 SO and O - O I ~ S SOp An analysis of a 41 itre

sample

of

h is gave

0.2722% SO

nd

0.0153

SO .

conclusiom

It

has been found possible, under laboratory conditions,

to estimate small concentrations of SO3in the presence of much

larger

concentrations of SO2.

The essential features of

the

method are as follows

:

I.

An

80

isopropyl alcohol/20% w ater mixture is employed.

With this, negligible oxidation of

SO2

occurs while the gases

a r t

b e i i g

sampled and absorbed.

2. By first acidifying the absorbing solution with hydrochloric

acid an d then passing oxygen-free nitrogen through th e liquid,

dissolved

SO

is removed completely without oxidation.

This

SOz sabsorbed in hydrogen peroxide.

3.

Sulphuric acid mist is completely removed by the use of

a sintered glass filter

in

conjunction with the absorbing solutions

of isopropyl alcohol an d water.

4. T h e design of t he first absorption vessel coupled with

the heating of th e surfaces over which the gas passes precludes

the possibility of errors arising from the condensation of sul-

phuric acid before entering the absorbing solution.

Th e laboratory method

as

described here has been in use in

the

B.C.U.R.A. laboratories for some considerab le time and

has been used

in

studies of t he oxidation of SOo in

combustion

T he technique described has been slightly modified to make

it suitable for use

in

industry and experiments are now proceeding

on

he determination of

SO3

content in the flue gas of a power

stationboiler. The results of this in vestigation will be published

induecourse. .

Acknowlcdgemcnt~ .

The author is indebted to the Boiler Availability Committee

for perm ission to present this paper and to Dr.

I).

H. Bangham

and

Mr.

A. Dooley for their keen interest and assistance with

the publication of the paper. .

13,Grosvenor Gardens,

Th e British

Coal

Utilisation

Research Association,

London , S.W.1

Rcccived June25,

1947

I Johnstone, H. F., Univ. Illinois

Bull.

UB,gt

Gillham.E.W.

F.,

J.S.C.1. 1946.65.370

a

Johnstone, H. ., Wdv.

Illin&

Documents

[Doi 10.1002_jctb.5000670102] D. Flint -- A Method for the Determination of Small Concentrations of So3 in the Presence of Larger Concentrations of So2