SCAN-N 30:85

Accepted April 1985

White and green liquors

Total, active and effective alkali

Potentiometric titration

2.2 Active alkali (of white and green liquors):

The total concentration of alkaline constituents, except
0 Introduction
carbonates, as determined by titration of a sample of
This SCAN-test Method has been developed for use in the liquor with strong acid according to the procedure
laboratories employing electrometric titration for the specified in this Method.
analysis of white and green liquors. The results
obtained by the method described in this Method do Note - In practice, active alkali is considered to be
not differ significantly from those obtained by the the sum of the concentrations of hydroxyl and
method described in SCAN-N 2 (the traditional hydrosulphide ions, including hydroxyl ions formed
titration using barium chloride precipitation, the by hydrolysis. of sulphides. See also the Annex.
formaldehyde reaction and visual end point detection).
2.3 Effective alkali (of white and green liquors):
1 Scope and field of application The concentration of strongly alkaline constituents
determined by titration of a sample of the liquor with
This SCAN-test Method specifies an electrometric
strong acid to the first inflexion point according to the
titration procedure for the determination of total, active
procedure specified in this Method.
and effective alkali in normal white and green liquors
obtained and used in the kraft or sulphate pulping
Note - In practice this is considered to be the con-
process. This Method may not be used for the analysis
centration of hydroxyl ions, including those formed
of liquors such as oxidized white liquors which contain
from sulphides by hydrolysis. See also the Annex.
significant amounts of polysulphides. The definitions
given below do not apply to such liquors. This Method
The terms sulphidity and degree of causticizing are
is not intended for the determination of particular ionic
explained in the Annex.
species, such as sulphides or carbo- nates.

3 Principle
2 Definitions
A sample of the liquor is titrated with hydrochloric acid
2.1 Total alkali (of white and green liquors):
of known concentration. The pH value (or a suitable
The total concentration of alkaline constituents deter-
function of the pH value) of the reaction mixture and
mined by titration of a sample of the liquor with strong
the volume of hydrochloric acid are recorded
acid to the third inflexion point according to the
continuously and from the recorded data the
procedure specified in this Method.
consumption of acid at the inflexion points is
determined.
Note -See also the Annex.
SCAN-N 30:85
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From the amounts of acid required to reach the three now agree to within 0,2 pH unit with the value
inflexion points, the effective, active and total alkali of assigned to the buffer. Failure to do so indicates a
the sample are calculated. faulty electrode.

4 Apparatus 7 Procedure
Automatic titration equipment, including glass and Operate the titration equipment as instructed by the
reference electrodes, that records the titration curve manufacturer. Use the hydrochloric acid (5.1) as the
(pH against titrant consumed) or any function of the titrant.
titration curve, such as the first derivate, so that the
inflexion points of the titration curve can be Note - Hydrogen sulphide will be formed during
determined. The reference electrode should preferably the titration. Since hydrogen sulphide is a toxic
be of the double junction type. Check the precision of gas, the titration should be performed under a
the equipment as instructed in Section 6. hood or other measures should be taken to protect
the operator.
Note - For the analysis of green liquors a titration
equipment that automatically evaluates the Choose the sample volume (v ml) so that about half the
inflexion points is recommended. burette capacity (or equivalent volume-measuring
device) is used. Normally a sample volume of 20 % of
the burette capacity is suitable.
5 Reagents
With the aid of a calibrated pipette or equivalent
5.1 Hydrochloric acid, 1mol/l, analytical grade. The device, transfer the chosen volume of sample to the
actual concentration should be known to the nearest titration vessel. The sample volume should be known
0,005 mol/l. with a precision of at least 1 per cent. Dilute the sample
with distilled water to a suitable volume. Start the
5.2 Buffer solutions of known pH values near 4 and titration and note the volume of acid consumed as
9. Suitable buffer solutions are commercially available. follows:
They can also be prepared in the laboratory as follows:
Buffer solution, pH 4,01: In a 1000 ml volumetric flask Volume consumed at the first inflexion point: a ml
dissolve 10,12 g of potassium hydrogen phthalate, Total volume consumed at the second
KHC8 H 4O 4 , in distilled water and dilute to the mark. inflexion point: b ml
There is normally no need to dry the salt. The solution Total volume consumed at the third
is stable for 2 months. inflexion point: c ml
Buffer solution, pH 9,18: In a 1000 ml volumetric
flask dissolve 3,80 g of sodium tetraborate decahydrate Note - Normally the three inflexion points appear
(borax), Na 2 B4O7 ⋅10 H 2 O , in distilled water from at the following pH values:
which carbon dioxide has been expelled by boiling. 1st inflexion point - close to 11
Dilute to the mark. The solution is stable for 6 weeks. 2nd inflexion point - between 8 and 9
The solution will absorb carbon dioxide when in 3rd inflexion point - close to 4
contact with ambient air. Therefore keep the solution in
a stoppered bottle and do not leave the bottle open
more than is absolutely necessary. 8 Calculation and report
The results may be given in units of substance
6 Calibration and check of pH meter concentration (moles per litre) or in the conventional
way as <<grams of sodium hydroxide per litre>>.
Operate the pH meter in accordance with the To obtain the results in moles per litre calculate as
manufacturer's instructions. Wash the glass and follows:
reference electrodes with distilled water; allow the
water to drain from the electrodes, but do not wipe Effective alkali = a⋅m/v
them. Active alkali = (2a − 2b + c) ⋅ m / v
Fill a sample cup with the first buffer solution and Total alkali = c⋅m/v
immerse the electrodes. Adjust the meter so that it
indicates the known pH of the buffer solution. If the where
reading slowly but continuously increases or decreases, m is the concentration of the hydrochloric acid in.
this indicates faulty electrodes. moles per litre,
Wash the electrodes with distilled water as before v is the volume of sample taken, in millilitres.
and immerse them in the second standard buffer
solution. Do not readjust the meter. The reading should
 SCAN-N 30:85
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If the final results are required in <<grams of sodium

hydroxide per litre>>, multiply the above results by 40
(the relative molecular mass of NaOH). For calculation
of sulphidity, and degree of causticizing, see the
Annex.
Report the results to the nearest 0,01 mol/litre or to
the nearest 1g/l.
The report shall include reference to this SCAN-test
Method and the following particulars:
(a) date and place of testing,
(b) identification mark of the sample tested,
(c) the results,
(d) any departure from the standard procedure and
any other circumstances that may have affected the test
results.

9 Additional information
Comparison with SCAN-N 2 - A number of different
white (69) and green (15) liquors were analysed according
to the procedure in this Method and that in SCAN-N 2.
There was no significant difference bet- ween the two
sets of results at the 99% confidence level.
SCAN-N 30:85
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Annex

Quantities and units for the characterization of white and

green liquors

When the smelt from the recovery furnace in a

normal sulphate pulp mill is dissolved in water, the (x+y) OH − + (x+y) H + → (x+y) H 2O
major anionic species formed are OH − , HS− and
2−
CO3 . The quantity <<effective alkali>>is thus a measure of
The major cationic species are Na + and K + . The the total concentration of hydroxyl ions, regardless of
liquor also contains a number of minor constituents, such whether these are formed by dissolving sodium (or
as SO 24− , SO32− , S2 O 32− , Cl− and many others as traces. potassium) hydroxide or by hydrolysis of sulphide ions.
The quantity <<active alkali>>is the calculated sum
Oxidized liquors may also contain significant amounts of
polysulphides, i.e. ions of the general composition of the concentrations of OH − and HS− .
The quantity <<total alkali>>is determined from a
Sn S2− or HSn S− .
titration of all alkaline species, including HCO3− ions
The analytical procedures described in this Standard
and in SCAN-N 2 are based on the assumption that only formed from CO32− . Minor concentrations of alkaline
three anionic specis OH − , HS− and CO32− are present in constituents other than the main components are also
significant amounts. The presence of other anionic included. Any sulphite ions present are included in the
species is thus disregarded. If the concentrations of other values for total and active alkali.
species, such as polysulphides and others forming weak From acid consumption at the three inflexion points
acids, rise to a significant level, the above assumption it is possible to calculate the concentrations of the three
becomes invalid and the properties calculated from the ionic species OH − , HS− and CO32− present at the
titration procedure lose their meaning. beginning of the titration:
When the titration curve (pH against acid
consumption) is recorded, three inflexion points are
a ⋅ m = v ⎡OH − ⎤
obtained. These are considered to be the end points of ⎣ ⎦
the reactions:
(
⎣ ⎦ ⎣ )
b ⋅ m = v ⎡ OH − ⎤ + ⎡ CO32− ⎤
⎦
x OH − + y S2− + (x+y) H + → x H 2O + y HS− c ⋅ m = v ( ⎡ OH ⎤ + 2 ⎡ CO ⎤ + ⎡ HS ⎤ )
− 2− −
⎣ ⎦ ⎣ ⎦ ⎣
3 ⎦
which gives
z CO32− + z H + → z HCO3−
⎡ OH − ⎤ = a ⋅ m / v
y HS− + z HCO3− + (y+z) H + → ⎣ ⎦
→ y H 2S (g) + z CO 2 (g) +z H 2O ⎡ HS− ⎤ = ( a − 2b + c ) ⋅ m / v
⎣ ⎦
where ⎡ CO3 ⎤ = ( b − a ) ⋅ m / v
2 −
x is the amount of hydroxyl ions ⎣ ⎦
y is the amount of sulphide ions
z is the amount of carbonate ions The brackets denote amounts of substance
in the original sample. concentrations of ionic species. The other symbols are
explained in sections 7 and 8.
In reality no S2− ions are present in white and green The concentrations of HS− and CO32− are obtained
liquors because of hydrolysis according to the reaction as the differences between experimentally determined
numbers. If the difference is small compared with these
S2− + H 2O → HS− + OH − numbers, it is markedly influenced by experimental
errors. The results for HS− and CO32− , when calculated
This reaction takes place when the smelt is dissolved in
water. More correctly, the first reaction in the titration as indicated, therefore often have a poor precision. This
should be borne in mind when such results are used.
should be written as
 SCAN-N 30:85
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Three derived properties are more or less commonly

used to describe white and green liquors:

2 ⎡ HS− ⎤
Sulphidity = ⎣ ⎦
⎡ OH − ⎤ + ⎡ HS− ⎤
⎣ ⎦ ⎣ ⎦
⎡OH − ⎤ − ⎡ HS− ⎤
Degree of causticizing = ⎣ ⎦ ⎣ ⎦
⎡ OH ⎤ − ⎡ HS ⎤ + 2 ⎡ CO32− ⎤
− −
⎣ ⎦ ⎣ ⎦ ⎣ ⎦
⎡ HS− ⎤
Degree of reduction = ⎣ ⎦
[ total sulphur ]
These properties are dimensionless quotients and they
are normally expressed as percentages.
To calculate the derived properties sulphidity and
degree of causticizing the following

2 ( a − 2b + c )
Sulphidity =
2a − 2b + c

Degree of causticizing =
2b − c 2b − c
=
4b − 2a − c ( 2b − c ) + 2 ( b − a )

Note - The rather complicated definition of the

degree of causticizing is explained by the old
definition:

Degree of causticizing =
[ NaOH ]
[ NaOH ] + ⎡⎣Na 2CO3 ⎤⎦
which was based on the assumption that white or
green liquors are mixtures of dissolved NaOH ,
Na 2S and Na 2CO3 .

More precise estimates of these quantities may be

obtained using values for the sulphide and carbonate
concentrations that have been determined separately.

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