Determination of sodium chloride in sodium hypochlorite | Autotitrator COM-A19 - JM Science
HIRANUMA APPLICATION DATA Automatic Titrator Data No. H6 Apr. 5,2019
SODA PULP INDUSTRY Determination of sodium chloride in sodium hypochlorite

1. Abstract

Sodium hypochlorite is produced by having sodium hydroxide absorb chlorine gas. Produced sodium hypochlorite contains residual alkali (sodium hydroxide, sodium carbonate) and sodium chloride generated by the degradation of hypochlorous acid. The residual alkali is determined by neutralization titration, and sodium chloride is determined by precipitation titration.

This report introduces an example of the determination for sodium chloride in the sodium hypochlorite with potentiometric titration as the following procedure: the titration for the total chlorine component (sodium hypochlorite + sodium chloride) is performed first. After that, sodium hypochlorite (available chlorine) is titrated by another method. Finally, sodium chloride is determined by the subtraction of sodium hypochlorite from the total chloride component.

The total sodium chloride (total chloride component) is determined according to the following formula (1) and (2): add hydrogen peroxide into a sample to degrade sodium hypochlorite, and generate sodium chloride as described in formula (1). Titrate with nitric acid standard solution until the pH goes down to 2 ~ 3. After the solution is acidified, determine the total sodium chloride with silver nitrate standard solution by precipitation titration as formula (2).

NaClO + H₂O₂ → NaCl + H₂O + O₂  ・・・(1)
NaCl + AgNO₃ → AgCl + NaNO₃  ・・・(2)

Regarding the determination of available chlorine, free iodine generated by the addition of potassium iodide as described in formula (3) is determined with sodium thiosulfate standard solution by redox titration as shown in formula (4).

NaClO + 2KI ± H₂O → I₂ + NaCl + 2KOH  ・・・(3)
I₂ + 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆  ・・・(4)

2. Configuration of instruments and Reagents

(1) Configuration of instruments
  Main unit : Hiranuma Automatic Titrator COM series
  Option : One buret, one buret head
  Electrodes : [For total sodium chloride]
Glass electrode GE-101B
Silver combination electrode AGR-811Z (Double junction type)
[For available chlorine]
Platinum electrode PT-301
Reference electrode RE-201Z
(2) Reagents    
  [For total sodium chloride]    
  Titrant : 0.2 mol/L Silver nitrate standard solution
0.1 mol/L Nitric acid standard solution
  Additive : 3 % Hydrogen peroxide solution
  [For available chlorine]    
  Titrant : 0.2 mol/L Sodium thiosulfate standard solution
  Additive : 10 % Potassium iodide solution
2 mol/L Acetic acid solution

3. Measurement procedure

(1) Measurement for total sodium chloride
(i) Dispense 0.5 mL of sample into a 100 mL beaker with micropipette.
(ii) Add 50 mL of DI water.
(iii) Add 5 mL of 3 % hydrogen peroxide solution and degrade sodium hypochlorite.
(iv) Immerse the electrodes (GE-101B/AGR-811Z) into sample solution.
  Titrate with 0.1 mol/L nitric acid standard solution and adjust the pH to 2 ~ 3.
(v) Titrate with 0.2 mol/L silver nitrate standard solution.
(2) Measurement for available chlorine
(i) Dispense 0.5 mL of sample into a 100 mL beaker with micropipette.
(ii) Add 50 mL of DI water.
(iii) Add 1 mL of 10 % potassium iodide solution.
(iv) Add 10 mL of 2 mol/L acetic acid solution.
(v) Immerse electrodes (PT-301/RE-201Z) and titrate with 0.2 mol/L sodium thiosulfate standard solution.

4. Measurement conditions and results

Examples of titration conditions

(1) Measurement for total sodium chloride
(i) Titration with 0.1 mol/L nitric acid standard solution

Cndt No 1
Method Set
Buret No. 1
Amp No. 1
D. Unit pH
S-Timer 5 sec
C.P. mL 0 mL
Direction
T Timer 0 sec
D.P. mL 0 mL
End Point 3 pH
Over mL 0 mL
Max Vol. 20 mL
Constant No. 1
Size 0.5 mL
Blank 0 mL
Molarity 0.1 mol/L
Factor 1.004
K 0
L 0
Unit mL
Formula D
Decimal Places 3
Auto In Pram.
None
Mode No. 4
Pre Int 0 sec
Del K 9
Del Sens 0 mV
Int Time 3 sec
Int Sens 3 mV
Brt Speed 2
Pulse 40

 

(ii) Titration with 0.2 mol/L silver nitrate standard solution

Cndt No 2
Method Auto
Buret No. 2
Amp No. 2
D. Unit mV
S-Timer 5 sec
C.P. mL 0 mL
T Timer 0 sec
D.P. mL 0 mL
End Sens 100
Over mL 0.2 mL
Max Vol. 20 mL
Constant No. 2
Size 0.5 mL
Blank 0 mL
Molarity 0.2 mol/L
Factor 1
K 58.44
L 0
Unit %
Formula (D-B)*K*F*M/(S*10)
Decimal Places 4
Auto In Pram.
None
Mode No. 8
Pre Int 0 sec
Del K 5
Del Sens 0 mV
Int Time 5 sec
Int Sens 3 mV
Brt Speed 2
Pulse 40

 

(2) Measurement for available chlorine (Calculation of sodium chloride concentration)
(i)Titration with 0.2 mol/L sodium thiosulfate standard solution

Cndt No. 3
Method Auto
Buret No. 1
Amp No. 2
D. Unit mV
S-Timer 5 sec
C.P. pH 0 mL
T Timer 0 sec
D.P. mL 0 mL
End Sens 200
Over mL 0.3 mL
Max Vol. 20 mL
ConstantNo. 3
Size 0.5 mL
Blank 0 mL
Molarity 0.2 mol/L
Factor 1.002
K 35.45
L 0
Unit %
Formula (D-B)*K*F*M/(S*10)
Decimal Places 4
Auto In Pram.
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

 

(ii) Calculation of sodium chloride concentration

Cndt No. 4
Method Calc
ConstantNo. 4
Size 0.5 mL
Blank 0 mL
Molarity 0.2 mol/L
Factor 0
K 14.8 *1
L 0.8243 *2
Unit %
Formula K-CA*L
Decimal Places 4
Auto In Pram.
None

 *1 Input the result of total sodium chloride.

*2 Conversion coefficient from available chlorine to sodium chloride (58.44(NaCl)/70.9(Cl2))

Measurement results

(1) Results of total sodium chloride

Number of
measurement
Size
(mL)
Titrant
Volume(mL)
Total sodium chloride
Concentration (%)
1 0.5 6.329 14.795
2 0.5 6.324 14.783
3 0.5 6.339 14.818
Statistic
calculation
Avg. 14.80 %
SD 0.02 %
RSD 0.12 %

 

(2) Results of available chlorine (Calculated results of sodium chloride concentration)

Number of
measurement
Size
(mL)
Titrant
volume (mL)
Available chlorine
Concentration (%)
Sodium chloride
Concentration (%)
1 0.5 10.174 14.456 2.884
2 0.5 10.223 14.525 2.827
3 0.5 10.276 14.601 2.765
Statistic
calculation
Average 14.52 % 2.83 %
Standard deviation 0.07 % 0.06 %
Coefficient of variation 0.50 % 2.11 %

 Examples of titration curves

Measurement for total sodium chloride

Measurement for available chlorine

 

5. Note

1) Measurement for total sodium chloride
Residual sodium hypochlorite could cause measurement error. Addition of excess hydrogen peroxide is required to degrade sodium hypochlorite completely.
2) Measurement for available chlorine
The following tips can improve measurement accuracy.
i) Weigh a sample size quickly because available chlorine is unstable.
ii) Titrate free iodine immediately because the iodine generated by addition of potassium iodide is easy to volatilize. Addition of plenty potassium iodide is required because the volatilization of iodine depends on the concentration of added potassium iodide.

Keywords: Sodium hypochlorite, Sodium chloride, Residual alkali, Available chlorine, Precipitation titration

*Some measurement would not be possible depending on optional configuration of system.

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