Determination of residual alkali in sodium hypochlorite | Autotitrator COM-A19 - JM Science
HIRANUMA APPLICATION DATA Automatic Titrator Data No. H5 Oct. 7,2022
SODA PULP INDUSTRY Determination of residual alkali in sodium hypochlorite

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, sodium carbonate) in the sodium hypochlorite with potentiometric titration. The procedure is described as follows: Add hydrogen 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 at around 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 + H₂O₂ → NaCl + H₂O + O₂  ・・・(1)
NaOH + HCl → NaCl + H₂O  ・・・(2)
Na₂CO₃ + HCl → NaHCO₃ + NaCl  ・・・(3)
NaHCO₃ + HCl → NaCl + CO₂ + H₂O  ・・・(4)

Figure 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 electrode GE-101B
Reference electrode RE-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
Combination with PT-301 allows to measure available chlorine in sodium 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 1
Method Auto
Buret No. 1
Amp No. 1
D. Unit pH
S-Timer 60 sec
C.P. mL 0 mL
T Timer 0 sec
D.P. mL 0 mL
End Sens 700
Over mL 0 mL
Max Vol. 20 mL
Constant No. 1
Size 0 g
Blank 0 mL
Molarity 0.2 mol/L
Factor 1.004
K 40
L 0
Unit %
Formula
(VA-VB)*K*F*M/(S×10)
Decimal Places 4
Auto input Param.
None
Mode No. 5
Pre Int 0 sec
Del K 5
Del Sens 0 mV
Int Time 3 sec
Int Sens 3 mV
Brt Speed 2
Pulse 40
0.05 mL

 (2) Measurement condition of sodium carbonate

Cndt No. 2
Method Auto
Buret No. 1
Amp No. 1
D. Unit pH
S-Timer 0 sec
C.P. pH 9 pH
T Timer 0 sec
D.P. pH 6 pH
End Sens 500
Over mL 0.3 mL
Max Vol. 20 mL
ConstantNo. 2
Size 0 g
Blank 0 mL
Molarity 0.2 mol/L
Factor 1.004
K 105.99
L 0
Unit %
Formula
(D-B)*K*F*M/(S×10)
Decimal Places 4
Auto input Param.
None
Mode No. 5
Pre Int 0 sec
Del K 5
Del Sens 0 mV
Int time 3 sec
Int Sens 3 mV
Brt Speed 2
Pulse 40
0.05 mL

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

Measurement results

Sodium Hydroxide

Number of
measurement
Size
(g)
1st End point
Titrant Volume(mL)
Sodium
Hydroxide (%)
1 2.5456 6.770 1.6149
2 2.5288 6.723 1.6005
3 2.5144 6.673 1.5835
Statistic
calculation
Avg. 1.600 %
SD 0.016 %
RSD 0.98 %

 

Sodium Carbonate

Number of
measurement
Size
(g)
2nd End point
Titrant volume (mL)
Sodium
Carbonate (%)
1 2.5456 1.652 1.3812
2 2.5288 1.684 1.4173
3 2.5144 1.716 1.4525
Statistic
calculation
Average 1.417 %
Standard deviation 0.036 %
Coefficient of variation 2.52 %


Examples of titration curves

5. Note

Since the residual sodium hypochlorite could cause measurement errors, the excess hydrogen peroxide should 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.

Hiranuma aquacounter autotitrator com-a19Soda pulp industry

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