HIRANUMA APPLICATION DATA

Automatic Titrator

Data No.

H5Oct. 7,
2022

Determination of residual alkali insodium hypochlorite

SODA PULP
INDUSTRY
1. Abstract

Sodium hypochlorite is produced by having sodium hydroxide absorb chlorine gas. Produced sodium
hypochlorite contains sodium chloride generated by the degradation of hypochlorous acid.
This report introduces an example for the determination of residual alkali (sodium hydroxide, sodiumcarbonate) in the sodium hypochlorite with potentiometric titration. The procedure is described as follows: Addhydrogen peroxide to sodium hypochlorite to decompose the sodium hypochlorite (reaction (1)). Run
neutralization titration with hydrochloric acid standard solution (reaction (2) ~ (4)). The pH reaches at around 8 on
the reaction of (2) and (3). In the case of sodium carbonate, the sodium hydrogen carbonate generated on reaction(3) is continuously titrated with hydrochloride acid, and pH reaches to around 4 at the end point on the reaction(4). The reaction from (2) to (4) and related titration curve are illustrated in figure 1.
Titration volume for sodium hydroxide can be calculated by subtracting titration volume (b mL) of 2nd end
point titration from titration volume (a+b mL) of 1st end point.
The concentration of sodium carbonate can be determined by titration volume of 2nd end point. Therefore
sodium hydroxide and sodium carbonate can be determined separately with successive titration. Inflection point ataround pH 11.5 is expected as the end point for sodium hydroxide, but the inflection point is unclear. Therefore it
is not identified as the end point on this titration.

NaClO +NaOH+
Na2CO3 +NaHCO3 +H2O2HClHClHCl→→→→

NaCl + H2O + O2…………
NaCl+ H2 O………………
NaHCO3 + NaCl………………
NaCl + CO2 + H2O …………

(1)
(2)
(3)
(4)

pHNaOH
Na2CO3Titration volume (mL)

NaHCO3Figure 1 Neutralization titration curve for sodium hydroxide and sodium carbonate with hydrochloric acid

2. Configuration of instruments and reagents(1) Configuration of instruments
Main unit: Hiranuma Automatic Titrator COM series

Electrodes: Glass electrodeGE-101B: Reference electrodeRE-201Z*Instead of above electrode, the following electrodes are useable.
・GR-501B (Glass reference combination electrode)…Fixed sleeve type・GR-511B (Glass reference combination electrode)…Moveable sleeve type<Remark> Combination with PT-301 allows to measure available chlorine insodium hypochlorite..
(2) Reagents
Titrant: 0.2 mol/L Hydrochloric acid standard solution

Additive solution: 1 mL of 30 % Hydrogen peroxide

3. Measurement procedure(1) Dispense 2 mL of sample and weigh accurately.
(2) Add 30 mL of DI water.
(3) Add 1 mL of 30 % hydrogen peroxide to decompose the sodium hypochlorite.
Please note that it reacts vigorously in this procedure.
(4) Immerse the electrodes and titrate with 0.2 mol/L hydrochloric acid standard solution.

4. Measurement conditions and results
Examples of titration conditions(1) Measurement condition of sodium chloride and sodium carbonate
Cndt No.
MethodBuret No.
Amp No.
D. Unit
S-Timer
C.P. mLT Timer
D.P. mLEnd Sens
Over mL
Max.Vol.

1Auto
11pH
60
000
700
020sec
mL
sec
mL
mL
mL

ConstantNo.
1
Size0Blank0Molarity
0.2
Factor
1.004
K40.0L0Unit%
Formula(VA-VB)*K*F*M/(S×10)
Decimal Places
4Auto In Pram.

gmL
mol/L

None

Mode No.
Pre Int
Del K
Del Sens
Int Time
Int Sens
Brt SpeedPulse

5050332400.05

sec
mV
sec
mV

mL

(2) Measurement condition of sodium carbonate
Cndt No.
MethodBuret No.
Amp No.
D. Unit
S-Timer
C.P. pH
T Timer
D.P. pH
End Sens
Over mL
Max.Vol.

2
Auto1
1
pH
0
9.0
0
6.0
500
0.3
20

sec
pHSecpH
mL
mL

Constant No.
2Size0Blank
0Molarity
0.2
Factor
1.004K
105.99
L0Unit%Formula(D-B)*K*F*M/(S×10)
Decimal Places
4Auto In Pram.

gmLmol/L

Mode No.
Pre Int
Del K
Del Sens
Int Time
Int Sens
Brt SpeedPulse

505033240
0.05

sec
mV
sec
mV

mL

None

<NOTE> Connect condition of (1) and (2) for the titration.

Measurement results
Number ofMeasurement

Size(g)

1st End point
Titrant Volume(mL)

SodiumHydroxide (%)

2nd End point
Titrant Volume (mL)

SodiumCarbonate (%)

123

2.54562.52882.5144

6.770
6.723
6.673

1.61491.6005
1.58351.600 %
0.016 %
0.98 %

1.652
1.684
1.716

1.3812
1.41731.45251.417 %
0.036 %
2.52 %

Avg.
SDRSD

Avg.
SDRSD

Example of titration curve

5. NoteSince the residual sodium hypochlorite could cause measurement errors, the excess hydrogen peroxideshould be added to decompose sodium hypochlorite completely.
Keywords: Sodium hypochlorite, Residual alkali, Sodium hydroxide, Sodium carbonate, Neutralization titration*Some measurement would not be possible depending on optional configuration of system.