Purity determination of sodium hydroxide | Autotitrator COM-A19 - JM Science
HIRANUMA APPLICATION DATA Automatic Titrator Data No. H1 Oct. 7,2022
SODA PULP INDUSTRY Purity determination of sodium hydroxide

1. Abstract

Sodium hydroxide is one of the most elemental reagent in chemical industrial reagents. Its production amount is large and it is used in a broad range of fields. Sodium hydroxide absorbs carbonate gas and water in atmosphere because of its properties, the purity of sodium hydroxide is gradually reduced. Japanese Industrial Standard (JIS) K8576 defines determination method for purity of sodium hydroxide (titration with indicator). Representative impure substance in sodium hydroxide includes sodium carbonate.

This report introduces an example for the quantitative determination of sodium hydroxide purity and sodium carbonate by successive and fractional potentiometric titration.

First, sodium carbonate is precipitated as barium carbonate by adding barium chloride before the titration (formula (1)). Secondly, most sodium hydroxide is titrated with highly concentrated hydrochloric acid standard solution. After that, residual sodium hydroxide is titrated with low-concentrated hydrochloric acid standard solution. It enhances the accuracy of quantitative determination for sodium hydroxide. Barium carbonate generated on pretreatment is continuously titrated with low-concentrated hydrochloric acid standard solution. The titrated volume for carbonate is doubled compared with the procedure without adding barium chloride, so it is especially available for the small amount determination of carbonate.

Na₂CO₃ + BaCl₂ → BaCO₃↓ + 2NaCl (Pretreatment) ・・・(1)
NaOH + HCl → NaCl + H₂O ・・・(2)
BaCO₃ + 2 HCl → BaCl₂ + CO₂ + H₂O (Titration) ・・・(3)

 

The reaction between sodium hydroxide and hydrochloric acid is described in formula (2). Formula (3) shows the reaction between barium carbonate and hydrochloric acid. The pHs reach 9 and 4 on the reaction (2) and (3) at each end point.

 

2. Configuration of instruments and Reagents

(1) Configuration
  Main unit : Hiranuma Automatic Titrator COM series
  Option : One buret
  Electrode : Glass electrode GE-101B
    : Reference electrode RE-201Z
*Instead of above electrodes, the following combination electrodes are usable.
・GR-501BZ (Fixed sleeve type)
・GR-511BZ (Flexible sleeve type)
(2) Reagents    
  Titrant : 1.0 mol/L Hydrochloric acid standard solution
0.1 mol/L Hydrochloric acid standard solution
  Additive : 10 % Barium chloride solution

3. Measurement procedure

(1) Take about 1.5 g of sample into 200 mL beaker and accurately weigh it.
(2) Add 50 mL of DI water which does not contain carbon dioxide, and dissolve the sample.
(3) Add 10mL of 10 % barium chloride solution. Flow nitrogen gas to the headspace on sample solution with 300 mL/min flow rate.
(4) Immerse the electrodes with keeping nitrogen gas flowing, and start titration. 1.0 mol/L hydrochloric acid standard solution is dispensed first (35 mL here)*, and secondly 0.1 mol/L hydrochloric acid standard solution is titrated. *Addition amount of 1.0 mol/L hydrochloric acid is equivalent to 90 % of sodium hydroxide in sample solution.

4. Measurement conditions and results

Examples of titration conditions

(1) Dispense 1.0 mol/L hydrochloric acid

Cndt No. 1  
Method Disp  
Buret No. 1  
S-Timer 5 sec
Disp.Vol. 35 mL

 

(2) Titration of sodium hydroxide

Cndt No 2
Method Auto
Buret No. 2
Amp No. 1
D. Unit pH
S-Timer 5 sec
C.P. mL 0 mL
T Timer 0 sec
D.P. mL 0 mL
End Sens 1000
Over mL 0 mL
Max .volume 40 mL
ConstantNo. 2
Size 1.5002 g
Blank 35 mL
Molarity 0.1 mol/L
Factor 1.004
K 40
L 1.001
Unit %
Formula
(D*F*M+B*L)*K/(S*10)
Digits 3
Auto input Parameter
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

*Enter the dispensing volume and factor of 1.0 mol/L hydrochloric acid standard solution in B (Blank) and L (coefficient 2), respectively.

(3) Titration of sodium carbonate (sodium carbonate)

Cndt No. 3
Method Auto
Buret No. 2
Amp No. 1
D. Unit pH
S-Timer 0 sec
C.P. mL 0 mL
T Timer 0 sec
D.P. mL 0 mL
End Sens 500
Over mL 0.2 mL
Max. volume 2 mL
ConstantNo. 3
Size 1.5002 g
Blank 0 mL
Molarity 0.1 mol/L
Factor 1.004
K 53
L 0
Unit %
Formula
(D-B)*K*F*M/(S*10)
Digits 3
Auto input Parameter
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

 

Measurement results

Measurement results of sodium hydroxide

Number of
measurement
Size
(g)
Titrant
volume(mL)
Concentration
(%)
1 1.5396 22.998 97.023
2 1.5002 13.590 97.052
3 1.5110 15.970 96.991
Statistic
calculation
Average 97.02 %
Standard deviation 0.031 %
Coefficient of variation 0.03 %

 

Measurement results of sodium carbonate

Number of
measurement
Size
(g)
Titrant
volume(mL)
Concentration
(%)
1 1.5396 1.236 0.429
2 1.5002 1.152 0.409
3 1.5110 1.085 0.382
Statistic
calculation
Average 0.407 %
Standard deviation 0.024 %
Coefficient of variation 5.80 %

Examples of titration curves

5. Note

Atmosphere environment
 Atmosphere contains about 0.03 % carbon dioxide. When carbon dioxide is absorbed into sample solution during titration for sodium hydroxide, the titrant volume consumed for sodium hydroxide is decreased, while it is increased on the titration for sodium carbonate. Flowing nitrogen gas to the headspace on sample solution serves as an effective measure against this problem. Using air passed through soda lime absorption column instead of nitrogen gas also has a positive effect.

Keywords:Sodium hydroxide, Sodium carbonate, Neutralization titration, Barium carbonate, JIS K 8576

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

Hiranuma aquacounter autotitrator com-a19Soda pulp industry

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