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UDC 661.731 : 543 : 620.1
IS0
I NT ERN AT1 ON A L O RG AN iZ AT I O N FO R STAN DARD IZATION
IS0 RECOMMENDATION
R 753
ACETIC ACID FOR INDUSTRIAL USE
METHODS OF TEST
1st EDITION
June 1968
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 753, Acetic acid for indusmal use - Methods of test, was drawn
up by Technical Committee ISO/TC 47, Chemistry, the Secretariat of which is held by the Ente
Nazionale Italian0 di Unificazione (UNI).
Work on this question by the Technical Committee began in 1956 and led, in 1962, to the
adoption of a Draft IS0 Recommendation.
L-
In November 1963, this Draft IS0 Recommendation (No. 652) was circulated to all the IS0
Member Bodies for enquiry. It was approved, subject to a few modifications of an editorial nature, by
the following Member Bodies :
Australia Hungary Romania
Austria India Spain
Belgium Israel Switzerland
Chile Italy U.A.R.
Colombia Korea, Rep. of United Kingdom
Czechoslovakia Netherlands U.S.A.
France Poland U.S.S.R.
Yugoslavia
Germany Portugal
Two Member Bodies opposed the approval of the Draft :
Japan
New Zealand
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council,
which decided, in June 1968, to accept it as an IS0 RECOMMENDATION.
L
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ISO/R 753-1968 (E)
CONTENTS
Page
5
1 . Scope .
.......................... 5
2 . Sample
PART I . METHODS OF TEST FOR GENERAL USE
........ 5
3 . Determination of the crystallizing point of glacial acetic acid
6
........
Determination of acetic acid content .
4 .
7
........
Determination of residue on evaporation on a water bath .
5 .
........ 7
Determination of iron content .
6 .
........ 9
.........
7 . Limit test for inorganic chlorides
4
........ 10
.........
8 . Limit test for inorganic sulphates
........ 11
Limit test for heavy metals (includingiron) .
9 .
........ 11
........
Determination of formic acid content
10 .
........ 13
Determination of acetaldehyde content .
11 .
PART II . METHODS OF TEST FOR SPECIAL PURPOSES
Determination of arsenic content . 14
12 .
Determination of water content . 16
13 .
Determination of permanganate index . 17
14 .
............. 18
15 .
Determination of total acetaldehyde content
Determination of total halogen content* . 19
16 .
19
...............
Determination of total sulphur content*
17 .
Determination of dichromate index . 19
18 .
Determination of mercury . 20
19 .
4
Test report . 21
20 .
Not included in this IS0 Recommendation as these determinations are still under study .
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ISO/R 753-1968 (E
IS0 Recommendation R 753 June 1968
ACETIC ACID FOR INDUSTRIAL USE
METHODS OF TEST
1. SCOPE
This IS0 Recommendation describes methods of test for acetic acid for industrial use, and is divided
into two parts, namely :
Part I - Methods of test for general use.
Part II - Methods of test for special purposes.
2. SAMPLE
In all cases,take the sample of acetic acid in the liquid condition. If it is solidified, melt it completely
in a warm room with the temperature not above 30 OC, and thoroughly agitate it by rolling the
container or by other suitable means before sampling.
Take a volume of sample that is sufficient for ail analyses to be carried out so that it is representative
of the bulk.
Place the sample in a clean, dry and air-tight giass stoppered bottle of such a size that it is nearly
filled by the sample.
When it is necessary to seal the container, care should be taken to avoid risk of contaminating the
contents in any way.
PART 1 - METHODS OF TEST FOR GENERAL USE
3. DETERMINATION OF THE CRYSTALLIZING POINT OF GLACIAL ACETIC ACID
3.1 Principle
Determination of the temperature to which the slightly supercooled sample in fluid form
rises during crystallization.
3.2 Applicability
The method is applicable only to 98 to 100 (m/m) acetic acid.
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IW/R 753-1968 (E
I
3.3 Apparatus
3.3.1 Test tube, 150 mm X 25 mm.
3.3.2 Thermometer, of the mercury-in-glass type, graduated for use at 100 mm immersion, certi-
fied for accuracy, and complying with the following requirements :
Length Certificate to
Distance from
show necessary
Thermometer Graduations Diameter
Main scale Bu'b bulb
bottom of
of stem
corrections
*we
to main scale
to readings
"C
"C
mm mm mm mm
-0.5 to not less not less to within
o. 1 10 to 15 5.5 to 7.0 than 3o
about 40.5 than 280 ? 0.05 "C
3.4 Procedure
3.4.1 Fill the dried test tube (3.3.1) with the test sample to a depth of about 100 mm, and insert
the thermometer (3.3.2). Place the test tube in water at 10 to 11 "C so that the portion
occupied by the sample is completely immersed, and allow it to remain without stirring,
until the thermometer indicates about 2 "C below the expected crystallizing point. Then
lift the tube out of the water and stir rapidly with the thermometer to induce the formation
of minute crystals. At the moment crystallization begins, the temperature will rise rapidly
and then remain constant for a few minutes. As soon as the steady temperature is approached,
cease stirring and suspend the thermometer so that its bulb is centrally disposed in the
crystallizing mass. Read to the nearest 0.05 "c the temperature at which the thermo-
meter reading remains constant, apply the thermometer correction and record the corrected
reading as the crystallizing point.
3.4.2 Prevent contamination of the sample with moisture during the test.
3.4.3 If, after cooling and stirring as described above, the temperature rise exceeds 3 "C the
observed crystallizing point is liable to be below the true figure, and the operation should
be repeated with less supercooiing.
3.4.4. If crystallization will not begin after removal of the test tube from the cold water and
vigorous stirring, the thermometer should be withdrawn and touched against some solid
acetic acid previously prepared, then quickly re-inserted in the sample under test and the
stirring resumed.
4. DETERMINATION OF ACETIC ACID CONTENT
4.1 Principle
Titration of acidity with a standard volumetric solution of sodium hydroxide using phenol-
phthalein as indicator, making allowance for any formic acid present.
4.2 Reagents
Distilled water or water of equivalent purity should be used in the test.
4.2.1 Sodium hydroxide, N standard volumetric solution.
4.2.2 Phenolphthalein, 5 g/l ethanolic solution. Dissolve 0.5 g of phenolphthalein in 100 ml of
95 (v/v) ethanol and make faintly pink by the addition of dilute sodium hydroxide
solution.
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lSO/R 753-1968 (E)
4.3 Apparatus
Ordinary laboratory apparatus and
4.3.1 weighing pipette, capacity 10 mi.
4.4 Procedure
4.4.1 Transfer, by means of the weighing pipette (4.3.1), an accurately weighed quantity of the
test sample, equivalent to 2 to 3 g of glacial acid, to a 250 ml conical flask containing about
50 ml of recently boiled and cooled water. Suitable quantities based on various nominal
strengths of the acid are given in the following Table :
Nominal strength
Mass of test sample to be taken
acetic acid
I
98 to 100
80
60 4.0
40 6.0
4.4.2 Add 0.5 ml of phenolphthalein solution (4.2.2) and titrate with sodium hydroxide solution
(4.2.1).
4.5 Expression of results
60X V
Acetic acid content (CH, COOH) per cent by mass = - 1.3 A
M
where
V is the volume in millilitres, of N sodium hydroxide solution (4.2.1) used,
M
is the mass, in grammes, of the test portion,
A is the formic acid content, per cent by mass, determined by the method described
in section 10.
5. DETERMINATION OF RESIDUE ON EVAPORATION ON A WATER BATH
Use the method described in IS0 Recommendation R 759, Method for the determination of residue
on evaporation on a water bath.
6. DETERMINATION OF IRON CONTENT
6.1 Principle
Conversion of any iron present in the sample into the sulphate by evaporation to dryness of the
specimen with sulphuric acid, and colorimetric determination of the iron using 2.2 Lbipyridyl.
NOTE. - Although this method specifies the use of a spectrophotometer or photometer, ic is permissible to
employ, as an alternative procedure, a visual method comparing the test solution with a series of standard
matching solutions (see clause 6.5 S).
6.2 Reagents
Distilled water or water of equivalent purity should be used in the test.
6.2.1 Sulphuric acid, d = 1.84, diluted 1 + 6 by volume.
6.2.2 Nitric acid, d = 1.4, diluted 1 + 3 by volume.
6.2.3 Urea solution. Dissolve 100 g of urea in 100 ml of water.
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ISO/R 753-1968 (E
6.2.4 Hydroxylammonium chloride, 1 O0 g/l solution.
6.2.5 Ammonium acetate, 500 g/l solution.
6.2.6 2.2 'bipyridyl, 5 g/l hydrochloric acid solution. Dissolve 0.5 g of 2.2 'bipyridyl in 100 ml of
N hydrochloric acid solution.
6.2.7 Standard iron solution (10 pg Felml). Dissolve 0.7022 g of pure iron (II) ammonium
sulphate hexahydrate (FeSO, .(",)*SO, .6H,O) in 50 ml of sulphuric acid solution (6.2.1)
and dilute to 1000 ml with water. Dilute 100 ml of the solution thus obtained to 1000 ml
with water.
6.3 Apparatus
Ordinary laboratory apparatus, and
6.3.1 Spectrophotometer or photometer.
6.4 Calibration charts
6.4.1 Place in 100 ml one-mark volumetric flasks the following quantities of standard iron solution
(6.2.7) :
O ~ 2.0 ~ 4.0 ~ 7.0 ~- 10.0 - 15.0 and 20.0 ml.
To each add 20 ml of nitric acid solution (6.2.2), 2 ml of urea solution (6.2.3) and 2 ml of
hydroxylammonium chloride solution (6.2.4). Mix and allow to stand for 2 minutes. Then
30 ml of ammonium acetate solution (6.2.5) and 5 ml of 2.2'bipyridyl solution
add
(6.2.6). Dilute to the mark with water.
6.4.2 Measure the optical densities of the solutions in the spectrophotometer or photometer
(6.3.1), determining the optical density at a wave length between 5 10 and 520 nm.
6.4.3 Draw a graph plotting optical densities as a function of the quantities of iron (in micro-
grammes) in 100 ml of the solutions.
6.5 Procedure
6.5.1 Weigh 100 g of the test sample in a platinum basin of capacity about 150 ml and evaporate
to dryness on a water bath under a hood having a good draught. Allow to cool and add 10 ml
of sulphuric acid solution (6.2.1). Evaporate, first on a water bath and finally on a sand
bath, until white fumes are just evolved.
6.5.2 Allow to cool, add a few drops of nitric acid solution (6.2.2), and re-evaporate until white
fumes just cease to be evolved. If tarry products remain, add a few further drops of nitric
acid solution (6.2.2) and again evaporate on the sand bath.
6.5.3 Take up the residue with 20 ml of nitric acid solution (6.2.2) warming to assist solution of
salts. Transfer the solution quantitatively to a 100 ml one-mark volumetric flask rinsing the
platinum basin. Add 2 ml of urea solution (6.2.3), stir and add 2 ml of hydroxylammonium
chloride solution (6.2.4), mix and allow to stand for 2 minutes. Then add 30 ml of
ammonium acetate solution (6.2.5) and 5 ml of 2,2'-bipyridiyl solution (6.2.6), and dilute
to the mark with water.
6.5.4 Measure the optical density of the solution in the spectrophotometer lor photometer (6.3.1)
at a wave length between 510 and 520 nm using a cell with the same optical path length as
those used in the preparation of the calibration chart and, by reference to the calibration
chart prepared as indicated in paragraph 6.4, read the iron content (in microgrammes per
100 mi) corresponding to this optical density.
6.5.5 As an alternative to measurement of optical density using a spectrophotometer or photo-
meter, the test solution prepared as in clause 6.5.3 may be compared visually with a series of
standard matching solutions prepared under similar conditions, and its iron content (in
microgrammes per 100 mi) deduced.
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ISO/R 753-1968 (E
6.6 Expression of results
Express the iron content of the sample in parts per million, by mass, calculated by dividing by
100 the iron content determined according to clause 6.5.4 or clause 6.5.5.
7. LIMIT TEST FOR INORGANIC CHLORIDES
This method is applicable when the chloride content, expressed as C 1, is not greater than 0.05 and
not less than 0.0005 O/'. If the chloride content lies outside that range, the mass of test portion taken
0.05
(7.4.1) should be reduced or increased and an appropriate adjustment made to the expression - ml
X
in clause 7.4.4.
7.1 Principle
Comparison of the turbidity, obtained by the addition of silver nitrate to a solution prepared
from the test sample in presence of nitric acid, with that similarly obtained from a chloride
solution of known concentration.
7.2 Reagents
Distilled water or water of equivalent purity should be used in the test. All reagents and filter
paper should be chloride free.
7.2.1 Nitric acid, approximately 5 N solution.
7.2.2 Standard chloride solution (0.1 mg Cllml). Dilute 28.2 ml of 0.1 N hydrochloric acid
solution to 1000 ml with water.
7.2.3 silver nitrate, 50 g/l Solution.
7.3 Apparatus
Ordinary laboratory apparatus.
1.4 Procedure
7.4.1 Weigh 50 5 0.5 g of the test sample, transfer to a 250 ml one-mark volumetric flask, dilute to
the mark with water and mix.
7.4.2 If the solution is not clear, pass it through a filter paper. This should remove turbidity due to
aluminium. If any turbidity remains in the filtrate due to contamination with wax, remove
it by shaking with a suitable solvent, for example, light petroleum.
7.4.3 To prepare the chloride solution of known concentration,add to a 100 ml Nessler cylinder
1.0 ml of the standard chloride solution (7.2.2), dilute to the mark with water, add 2 ml of
nitric acid solution (7.2.1) and mix.
7.4.4 For a sample required to contain not more than x of chloride,expressed as C 1, transfer to
O O5
a 100 ml Nessler cylinder an aliquot,- ml, of the solution prepared from the test sample
X
(7.4.1), dilute to the mark with water, add 2 ml of nitric acid solution (7.2.1), and mix.
7.4.5 Add to each Nessler cylinder 1 ml of silver nitrate solution (7.2.3) and mix. Allow the
cylinders to stand in the dark for 5 minutes then compare the turbidity produced by the
test sample with that produced by the chloride solution of known concentration.
7.5 Expression of results
A sample required to contain not more than x of C 1 does so if the turbidity produced from
its solution (7.4.4) is equal to or less than that produced from the chloride solution of known
concentration (7.4.3).
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IW/R 753-1968 (E
8. LIMIT TEST FOR INORGANIC SULPHATES
This method is applicable when the sulphate content,expressed as SO,,is not greater than 0.1 and
not less than 0.001 'k. If the sulphate content lies outside that range the mass of test portion taken
o. 1
(8.4.1) should be reduced or increased and an appropriate adjustment made to the expression - ml
X
in clause 8.4.4.
8.1 Principle
Comparison of the turbidity, obtained by the addition of barium chloride to a solution pre-
pared from the sample in presence of hydrochloric acid, with that similarly obtained from a
sulphate solution of known concentration.
8.2 Reagents
Distilled water or water of equivalent purity should be used in the test.
8.2.1 Sodium carbonate, N solution.
8.2.2 Hydrochloric acid, N solution.
8.2.3 Barium chloride, BaCl, .2H,O, 100 g/l solution.
8.2.4 Standard sulphate solution (0.1 mg SO,/ml). Dilute 20.8 ml of 0.1 N standard volumetric
solution of sulphuric acid to 1000 ml with water and mix thoroughly.
8.3 Apparatus
Ordinary laboratory apparatus.
8.4 Procedure
8.4.1 Weigh 100 f 1 g of the test sample, add 0.2 ml of sodium carbonate solution (8.2.1) and
evaporate to dryness in an evaporating basin on a boiling water bath. Dissolve the residue in
water containing 1 ml of hydrochloric acid solution (8.2.2), transfer to a 250 ml one-mark
volumetric flask, dilute to the mark with water, and mix.
8.4.2 If the solution is not clear, pass it through a filter paper. This should remove turbidity due to
aluminium. If any turbidity remains in the filtrate due to contamination with wax, remove it
by shaking with a suitable solvent, for example, light petroleum.
8.4.3 To prepare the sulphate solution of known concentration, add to a 100 ml Nessler cylinder
4.0 ml of the standard sulphate solution (8.2.4), dilute to the mark with water, add 2 ml of
hydrochloric acid solution (8.2.2) and mix.
8.4.4 For a sample required to contain not more than x of SO,, transfer to a 100 ml Nessler
O1
cylinder an aliquot,- ml, of the solution prepared from the test sample (8.4.1). Dilute to
X
the mark with water, add 2 ml of hydrochloric acid solution (8.2.2) and mix.
8.4.5 Add to each Nessler cylinder 2 ml of barium chloride solution (8.2.3) and mix. Ailow the
cylinders to stand for 5 minutes, mix again, and compare the turbidity produced by the test
sample with that produced by the sulphate solution of known concentration.
8.5 Expression of results
A sample required to contain not more than x of SO, does so if the turbidity produced from
its solution (8.4.4) is equal to or less than that produced from the sulphate solution of known
concentration (8.4.3).
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ISO/R 753-1968 (I
9. LIMIT TEST FOR HEAVY METALS (INCLUDING IRON)
9.1 Principle
Conversion of heavy metals, such as lead, copper, and iron, to their sulphides in ammoniacal
solution, and comparison of the colour produced with that given by a standard lead solution
treated with sodium sulphide in the same way.
NOTE. - The method detects only the heavy metals present in noncomplex form and is not specific for any
one heavy metal.
9.2 Reagents
Distilled water or water of equivalent purity should be used in the test.
9.2.1 Aqueous ammonia, d = 0.88.
9.2.2 Sodium sulphide, 1 O0 g/l solution.
9.2.3 Standard lead solution (10 pg Pb/ml), freshly prepared. Dissolve 0.0160 g of lead nitrate in
water and make up to 1000 ml.
9.3 Apparatus
Ordinary laboratory apparatus.
9.4 Procedure
9.4.1 Pipette 25 ml of the test sample into a 250 ml one-mark volumetric flask. Dilute to the
mark with water and mix well.
9.4.2 Transfer a 10 ml aliquot to a Nessler cylinder. Add aqueous ammonia (9.2.1) until the
solution is alkaline to litmus paper, and dilute to 50 ml with water. Add 0.1 ml (two
drops) of sodium sulphide solution (9.2.2) and mix well.
9.4.3 Preparation of agreed standard matching solution. To 20 ml of water contained in a second
Nessler cylinder add an agreed volume of standard lead solution (9.2.3) and 1 ml of aqueous
ammonia (9.2.1). Dilute to 50 ml with water and mix well. Add 0.1 ml (two drops) of
sodium sulphide solution (9.2.2) and again mix well.
9.4.4 Compare the darkening of the test solution (9.4.2) with that of the standard matching
solution (9.4.3).
9.5 Expression of results
Report the darkening produced in the test solution as greater than, equal to, or less than that of
the agreed standard matching solution, mentioning the lead content of the latter.
10. DETERMINATION OF FORMIC ACID CONTENT
10. ï Principle
Oxidation of the formic acid in the sample to carbon dioxide by mercuric chloride according
to the reaction.
2 HgC1, + HCOOH + Hg,Cl, + CO, + 2HC1
Iodometric determination of the mercurous chloride produced.
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ISO/R 753-1968 (E
10.2 Reagents
Distilled water or water of equivalent purity should be used in the test.
10.2.1 Sodium hydroxide, N solution.
10.2.2 Hydrochloric acid, 2 N solution.
10.2.3 Mercuric chloride, 50 g/l filtered solution.
10.2.4 Potassium iodide, crystals.
10.2.5 Zodine, O. 1 N standard volumetric solution.
10.2.6 Sodium thiosulphate, O. 1 N standard volumetric solution.
10.2.7 Starch, 10 g/l solution, freshly prepared.
10.3 Appi~atus
Ordinary laboratory apparatus, and
10.3.1 Conical flask with ground-glass neck, capacity 250 ml.
10.3.2 Reflux condenser, highly efficient type, with ground glass joint to fit the flask (10.3.1).
10.3.3 Microburette, 5 ml graduated in 0.05 ml.
10.3.4 Weighing pipette, capacity 1 O ml.
10.4 Procedure
10.4.1 Weigh by means of the weighing pipette (10.3.4) 5.0ml of the test sample and transfer to the
flask (10.3.11, add 30 mi of sodium hydroxide solution (10.2.1) followed by 10 ml of
hydrochloric acid solution (10.2.2). Add 40 ml of mercuric chloride solution (10.2.3),
connect the reflux condenser (10.3.2) to the flask, and heat for 2 hours on a boiling water
bath.
10.4.2 Cool, add 5 g of potassium iodide (10.2.4) dissolved in 10 ml of water (îhe mercuric iodide
formed redissolves) and run in, by means of a pipette, 5.0 ml of the iodine solution (10.2.5).
Shake until the mercurous chloride dissolves and, from the microburette (10.3.3), back-
titrate with the sodium thiosulphate solution (10.2.6) adding 0.5 ml of starch solution
(10.2.7) just before the end point is reached.
10.4.3 Carry out a blank test using the same volumes of reagents, omitting the test portion.
10.5 Expression of results
0.23 (V, - V,)
Formic acid content, (HCOOH),
(mlm) =
M
where
is the volume,in millilitres, of 0.1 N sodium thiosulphate solution used in clause
V,
10.4.3.
is the volume, in millilitres, of 0.1 N sodium thiosulphate solution used in clause
V,
10.4.2
M
is the mass, in grammes, of test portion.
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ISO/R 753-1968 (E
11. DETERMINATION OF ACETALDEHYDE CONTENT
1 1.1 Principle
Conversion of the aldehyde present in the sample quantitatively into an addition product, by
reaction with a measured quantity of solution of sodium hydrogen sulphite according to the
reaction.
H
I
CH3CH0 + NaHS03 + CH3 - C - OH
I
SO3 Na
Iodometric determination of the excess of sodium hydrogen sulphite.
NOTE. - This method will only determine acetaldehyde monomer. For total acetaldehyde content including
polymers, see Part II, section 15.
11.2 Reagents
Distilled water or water of equivalent purity should be used in the test.
c
11.2.1 Sodium hydrogen sulphite solution freshly prepared. Dissolve 16.6 g of sodium metabi-
sulphite (Na,S,O,) in water and dilute to 1000 ml.
11.2.2 Sodium thiosulphate, 0.02 M standard volumetric solution.
i 1.2.3 Zodine, 0.02 N standard volumetric solution.
11.2.4 Starch, 10 g/1 solution, freshly prepared.
11.3 Apparatus
Ordinary laboratory apparatus, and
11.3.1 Two conical flasks, ground-glass stoppered, capacity 250 mi.
11.3.2 Microburette, 5 ml graduated in 0.02 ml divisions.
11.3.3 Weighing pipette, capacity 20 ml.
L
11.4 Procedure
11.4.1 Weigh 10 ml of the test sample by means of the weighing pipette (1 1.3.3) and transfer to a
50 ml one-mark volumetric flask containing 10 ml of water. Add 5.0 ml of sodium hydrogen
sulphite solution (1 1.2.1) from the microburette (1 1.3.2), dilute to 50 ml with water, mix
well and allow to stand for 30 minutes.
i 1.4.2 In a second 50 ml one-mark volumetric flask, prepare a blank solution by diluting 5 .O ml of
sodium hydrogen sulphite solution (11.2.1) to 50 ml with water. Mix well and allow to
stand for 30 minutes.
11.4.3 At the same time pipette 50.0 ml of iodine solution (11.2.3) into each of two 250 ml
conical flasks (11.3.1) and place these in an ice-water bath.
11.4.4 At the end of the 30 minute period, pipette 20.0 mi of the analysis solution (I 1.4.1) into
one of these flasks, and 20.0 mi of the blank solution (1 1.4.2) into the other. Titrate the
two solutions with sodium thiosulphate solution (1 1.2.2), adding 0.5 ml of starch solution
(1 1.2.4) just before the end point is reached.
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ISO/R 753-1968 (E)
I 1.5 Expnession of results
0.110 (V, - V,)
Ace taidehyde content , (CH CHO), 'k (m/m) =
M
where
is the volume, in millilitres, of 0.02 M sodium thiosulphate solution used for the
VI
analysis solution,
is the volume, in millilitres, of 0.02 M sodium thiosulphate solution used for the
V,
blank solution,
M is the mass, in grammes, of test portion.
PART II - METHODS OF TEST FOR SPECIAL PURPOSES
In cases where the acetic acid is required for special purposes, for example, pharmaceutical, the fol-
lowing additional determinations may be required.
12. DETERMINATION OF ARSENIC CONTENT
I 2.1 Principle
Reduction of the arsenic in the sample to arsenic trihydride which, in contact with a mercuric
bromide paper, gives a coloured stain which varies from yellow to orange or brown according to
the quantity of arsenic present. Colorimetric determination by a comparison with a series of
stains, obtained under the same conditions, from solutions containing known quantities of
arsenic.
12.2 Reagents
Distilled water or water of equivalent purity should be used in the test. The reagents used should
be free from arsenic.
12.2.1 Lead acetate cotton wool pellets. Soak pellets of absorbent cotton wool of a diameter of
5 to 6 mm in a 50 g/l neutral solution of lead acetate, then drain and lightly press.
12.2.2 Lead acetate paper. Immerse strips of filter paper 8 mm X 50 mm in size in a 10 g/l neutral
aolution of lead acetate, drain, remove surplus liquid by pressing them lightly between two or
three sheets of filter paper.
J
12.2.3 Sulphuric acid, d = 1.84.
12.2.4 Sodium chlonde acid solution. Mix one volume of sulphuric acid (12.2.3) with four volumes
of water. Then dissolve 100 g of pure sodium chloride in 1000 ml of the approximately
300 g/l sulphuric acid thus obtained.
12.2.5 Iron (III) - ammonium sulphate, acid solution. Dissolve in water 84 g of iron (III) - am-
monium sulphate (",),SO, .Fe,(SO,), .24 H,O, add 10 ml of the sodium chloride solution
(12.2.4), and make up to 1000 ml with water.
12.2.6 Stannous chloiide, acid solution. Dissolve 22.6 g of stannous chloride, SnCl,. 2H,O, in
water. Add 56 ml of the sodium chloride solution (12.2.4) and make up to 1000 ml with
water. Keep the solution in a bottle of yellow glass in the presence of a few pieces of pure
tin.
12.2.7 Pure granulated zinc, in pieces of 4 to 5 mm diameter. The zinc weighed for the experiment
should be washed at the time of use with the sodium chionde solution (12.2.4) and then
with water.
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ISO/R 753-1968 (E
12.2.8 Sensitized mercuric bromide paper. Immerse sheets of consistent, fine grained filter paper
for 1 hour in a 50 g/l ethanolic solution of mercuric bromide. Dry the well drained and
pressed paper, preferably in dust-free air, or more rapidly in an oven at 90 OC. Cut from the
dried sheets, strips exactly 3 mm X 120 mm. Keep these in a dark blue, ground glass
stoppered, container.
12.2.9 Standard arsenic solution. Dissolve 0.132 g of very pure arsenious oxide in 20 ml of 350 g/l
sodium hydroxide solution. Dilute with a little boiled and cooled water, and then add
slowly 10 ml of sulphuric acid (12.2.3), make the solution up to 1000 ml with boiled and
cooled water. 1 ml of this solution contains 0.1 mg of arsenic. Take 10 mi (equivalent to
1 mg of arsenic) and make up to 1000 ml again with boiled and cooled water. 1 ml of the
resulting solution contains 1 .O pg of arsenic.
12.2.10 Hydrogen peroxide, 100 g/l solution (30 volumes).
12.3 Apparatus
Ordinary laboratory apparatus, and
12.3.1 Assembly (see Fig. 1) consisting of
12.3.1.1 Conicalflask, 100 ml of borosilicate glass, to which are connected, by means of ground
joints, the following components in series.
12.3.1.2 Tube for removal of hydrogen sulphide, with an internal diameter of 12 mm and a
height of 70 mm above a bulb of 20 mm diameter. In the bottom of the bulb is placed
a thin layer of dry glass wool, mixed, to the extent of one third, with lead acetate
cotton wool pellets (12.2.1), followed' by another light layer of glass wool, and lastly,
strips of damp lead acetate paper (12.2.2).
12.3.1.3 Glass tube, with an internal diameter of 3 mm and 120 mm in length above a slight
constriction at the bottom end. in this tube is placed a sensitized mercuric bromide
paper (12.2.8).
12.3.2 Conical flask, capacity 200 ml, with ground glass stopper.
12.3.3 Graduated pipette, 10 ml capacity, graduated in 0.1 ml divisions.
12.4 Procedure
12.4.1 Preparation of a standard colorimetric series of stains on filter paper. Prepare a standard
colorimetric series of stains corresponding respectively to 0.001,0.002,0.004,0.006,0.008
and 0.010 mg of arsenic according to the method given in clauses 12.4.1.1 and 12.4.1.2
below.
12.4.1.1 Into the flask (12.3.1.1), pipette 1 ml of standard arsenic solution (12.2.9) using the
graduated pipette (12.3.3) 30 ml of acid sodium chloride solution (12.2.4), 10 ml of
iron (III) - ammonium sulphate solution (12.2.5) and 20 ml of stannous chloride
solution (12.2.6).
Heat to boiling, cool immediately to
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