SCAN-N 36:98 - PFI

SCAN-N 36:98
Accepted 1998
Black liquors and sulphite spent liquor
Sulphite, sulphate and thiosulphate ion concentrations
0 Introduction
This SCAN-test Method has been developed for the
determination of sulphur compounds concentrations in
black liquors and sulphite spent liquor by laboratories
employing ion chromatography.
The Method is not intended for the determination of
hydrogen sulphide ion concentration in black liquor.
Work within SCAN-test has shown that in that case
results are obtained much more easily and more rapidly
by potentiometric titration of hydrogen sulphide ions
with silver nitrate solution, cf. SCAN-N 31:94.
1 Scope
This Method describes a procedure for the determination
of sulphite, sulphate and thiosulphate ion concentrations
in black liquors obtained and used in the kraft pulping
process, or in the sulphite spent liquor obtained and used
in the sulphite pulping process.
The Method is applicable to liquors having a dry matter
content below 20 %.
Note – If the dry matter content of the liquor exceeds
20 %, the liquor must be diluted with oxygen-free
water (5.0) to a dry matter content of about 20 %.
The procedure can, however, result in the underestimation
of sulphite due to unintentional oxidation
during the dilution.
2 Reference
SCAN-N 22 Black liquors – Dry matter content and
fibre content
3 Definitions
For the purpose of this Method, the following definitions
apply:
3.1 Sulphite ion concentration − The concentration
of ions present in the liquor.
2-
SO 3
3.2 Sulphate ion concentration − The concentration
of ions present in the liquor.
2-
SO 4
3.3 Thiosulphate ion concentration − The concentration
of ions present in the liquor.
4 Principle
2-
SO 2 3
The procedure is based on ion chromatography (IC). After
dilution of the liquor in oxygen-free deionized water,
isopropanol is added in order to stabilize the oxidizable
hydrogen sulphite, thiosulphate and sulphide ions.
5 Reagents
All chemicals must be of analytical grade. Use oxygenfree
water.
Note 1 – Oxygen-free water can be prepared from
distilled water either by boiling the water for
15 min or by displacing the oxygen with nitrogen
or argon gas. Protect from oxygen during storage.
5.1 Isopropanol, CHO 3 8 , pro analysi.
5.2 Oxygen-free water, containing isopropanol
1 per cent by volume.

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5.3 Concentrated standard sulphite solution, c(SO 3 ) =
1000 mg/l. Dissolve (1,574 ± 0,002) g Na2SO, 3 previously
dried for 2 h at 105 °C, in 100 ml isopropanol
(5.1) and dilute with water (5.2) to the mark in
a 1000 ml volumetric flask.
Note 2 – Protect powdered Na2SO 3 from oxidation
during storage.
Note 3 – The solution should be used within one
week.
5.4 Standard sulphite solution, c(SO 3 ) = 20 mg/l.
Transfer 20 ml of the concentrated standard
solution (5.3) to a 1000 ml volumetric flask and fill
to the mark with water (5.2).
Check whether the standard sulphite solution
contains any sulphate ions and, if it does, determine
the sulphate ion concentration and calculate the real
sulphite ion concentration in the standard sulphite
solution.
5.5 Concentrated standard sulphate solution, c(SO 4 ) =
1000 mg/l. Dissolve (1,479 ± 0,002) g absolutely
dry Na2SO 4 in a 1000 ml volumetric flask and fill
to the mark with water (5.2).
Note 4 – The solution is stable for at least one
month.
5.6 Standard sulphate solution, c(SO 4 ) = 20 mg/l.
Transfer 20 ml of the concentrated standard
solution (5.5) to a 1000 ml volumetric flask and fill
to the mark with water (5.2).
5.7 Concentrated standard thiosulphate solution,
2-
c(S2O 3 ) = 1000 mg/l. Dissolve (1,410 ± 0,002) g
absolute dry Na2SO 2 3 in a 1000 ml volumetric
flask and fill to the mark with water (5.2).
Note 5 – The solution is stable for one week.
5.8 Standard thiosulphate solution, c(S2O 3 ) =
20 mg/l. Transfer 20 ml of the concentrated
standard solution (5.7) to a 1000 ml volumetric
flask and fill to the mark with water (5.2).
Note 6 – The concentrations of the standard
solutions can be adjusted as required.
5.9 Eluent solution, for ion chromatography. The composition
of this solution is highly dependent on the
type of ion chromatography column used.
Therefore, follow the recommendations given by
the IC column supplier.
2-
2-
2-
2-
2-
Note 7 – Commercially available eluent solutions
may be used.
6 Apparatus
Ordinary laboratory equipment and the following:
6.1 Syringe, calibrated, volume between 20 μl and
1000 μl.
6.2 Serum flasks, with inert membrane stopper,
volume 5 ml.
6.3 Syringe, volume 5 ml.
6.4 Balance, with a resolution of 0,1 mg.
6.5 Ion chromatograph with a fractionating column,
suitable for the determination of sulphite, sulphate
and thiosulphate, and with a conductivity detector.
Note – UV (ultraviolet radiation), wavelength
200 nm, combined with conductivity, i.e. column-
UV-conductivity, can also be used.
7 Sampling and sample pre-treatment
Black liquor and sulphite spent liquor are sensitive to
oxidation by air. Prevent oxidation by keeping the
sample bottles filled to the rim and tightly closed (12.1).
Keep the bottles cold (between 4 °C and 8 °C). The
sulphur compounds distribution does not change for at
least two weeks if the liquor is stored in this way.
Determine the dry matter content of the original
sample according to SCAN-N 22.
All samples for the determination of sulphur
compounds have to be diluted. If the dry matter content
exceeds 20 %, follow the instructions given in 7.1, and
if the dry matter content is lower than 20 %, follow the
instructions given in 7.2.
7.1 Dilution at dry matter content >20 %
Heat the sample in a water bath at 90 °C for 20 min,
keeping the bottle closed during the heating period.
Mix the sample carefully with a glass rod, especially
sediment on the bottom of the sample container must be
stirred and homogenized with the rest of the sample.
Place a 100 ml volumetric flask on the balance. Tare
the balance.
Note 2 – It may be convenient to weigh the black
liquor, if a plug is used. Tare the balance with the
volumetric flask and the plug.
Weigh carefully between 2 g and 10 g (m) of the homogenized
sample into the flask to the nearest 0,1 mg and
fill, as soon as possible, to the mark (V) with water (5.2).

SCAN-N 36:98
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Put a stirring magnet into the flask and close well with the
plug. Place the flask on a magnetic stirrer and stir until the
sample is totally homogenized. The sample is now ready
to be analysed following the instructions given in 7.2.
7.2 Pre-treatment at dry matter content 20 %
Calculate the concentrations of the different sulphur
compounds in the liquor, in milligrams per kilogram,
from the equation:
X
s
a⋅C⋅D⋅V
=
b⋅
m
[5]

SCAN-N 36:98
Page 4
where
Xs
a
C
D
V
b
m
is the concentration of the sulphur compound in
the liquor as received, in milligrams per kilogram;
is the peak area or peak height of the sample;
is the concentration of the standard solution in
milligrams per litre;
is the dilution factor of the liquor sample solution
(D = 1 if there is no dilution);
is the volume in millilitres, by dilution of the
original liquor (here 100 ml);
is the peak area or peak height of the standard
solution;
is the weight of the original liquor, in gram
If the results are to be expressed in milligrams per
kilogram dry substance, calculate the sulphur compound
concentration from the equation:
X s ⋅100
X = [6]
f
where
X is the concentration of the sulphur compound in the
liquor, in milligrams per kilogram dry substance;
f is the dry matter content of the original liquor, in
per cent;
100 is the numerical factor which brings the result
into dry substance.
For the sulphite ion concentration, the sulphate ion
concentration and the thiosulphate ion concentration,
calculate the mean values of at least two parallel
determinations. The results of the parallel determinations
should not deviate by more than 5 % from their
mean.
Note – The sulphite ion concentration in black liquors
and the thiosulphate ion concentration in sulphite
spent liquors are often very low and difficult to
determine. The coefficient of variation (CV) is also
higher.
10 Report
The test report shall include reference to this SCAN-test
Method and the following particulars:
11 Precision
11.1 Repeatability. One laboratory analysed three
different spent liquors. Five parallel determinations were
made in each case. The results were as follows:
Sample 2-
SO 3
Black
liquor
Sulphite
spent liquor
mean,
mmol/l
Sample 2-
SO 3
Conc waste
liquor
CV*,
%
2-
SO 4
mean,
mmol/l
CV*,
%
2-
SO 2 3
mean,
mmol/l
CV*,
%
2,4 2,2 48 0,5 16 0,5
100 0,5 120 0,7 8,9 0,8
mean,
mmol/kg
CV*,
%
2-
SO 4
mean,
mmol/kg
CV*,
%
2-
SO 2 3
mean,
mmol/kg
CV*,
%
18 3,0 310 0,3 94 1,6
11.2 Reproducibility. Three spent liquors were
analysed in six laboratories. The results were as follows:
Sample 2-
SO 3
Black
liquor
Sulphite
spent liquor
mean,
mmol/l
Sample 2-
SO 3
Conc waste
liquor
CV*,
%
2-
SO 4
mean,
mmol/l
CV*,
%
2-
SO 2 3
mean,
mmol/l
CV*,
%
4 17 39 3,4 14 6,2
190 5,8 74 5,3 20 22
mean,
mmol/kg
CV*,
%
2-
SO 4
mean,
mmol/kg
CV*,
%
2-
SO 2 3
mean,
mmol/kg
CV*,
%
60 26 270 1,9 100 7,5
* CV is the coefficient of variation, i.e. the standard
deviation divided by the mean value.
Note – The contents of thiosulphate in sulphite
spent liquor and sulphite in concentrated waste
liquor are so low that they cannot be reliably
determined by this technique.
(a)
(b)
(c)
(d)
(e)
date and place of testing;
identification of the sample tested;
information about whether the sample is diluted
before analysis;
the results to two significant figures;
any departure from this Method and any other
circumstances that may have affected the result.
12 Literature
12.1 Douek, M., Sullivan, J. and Ing, J. Effect of
sample-related sources of error on analysis of pulp and
paper mill process liquids by ion chromatography: Part 1 –
Determination of sulphur species. TAPPI Pulping Conf.,
Orlando 1991, Book 2, 911-923

SCAN-N 36:98
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SCAN-test Methods are issued and recommended by
KCL, PFI and STFI-Packforsk for the pulp, paper and
board industries in Finland, Norway and Sweden.
Distribution: Secretariat, Scandinavian Pulp, Paper
and Board Testing Committee, Box 5604,
SE-114 86 Stockholm, Sweden.