Standardisation of Sodium Thiosulphate

1. Principle

Sodium thiosulphate is readily obtainable in a state of high purity, but there is always some uncertainty as to the exact water content. The substance is therefore unsuitable as a primary standard. Among the variables affecting the stability of thiosulphate solutions are the pH, the presence of micro-organisms and impurities, the concentration of the solution and the presence of atmospheric oxygen. The reaction between iodine and thiosulphate is described by the equation:




2. Reagents

2.1 Potassium Iodide, AR – free from iodate

2.2 Sulphuric Acid (H2SO4), 98%, AR
OR
2.3 Sulphuric Acid, 6 N: Cautiously add 167 mL of H2SO4 to 800 mL of reagent grade water. Make up to 1 L with reagent grade water.

2.4 Potassium bi-iodate (KH(IO3)2), AR

2.5 Standard potassium bi-iodate, 0.021 M: Dissolve 0.8124 g KH(IO3)2 in reagent grade water and dilute to 1 L.

2.6 Sodium Thiosulphate pentahydrate (Na2S2O3.5H2O), AR

2.7 Sodium Hydroxide (NaOH), AR

2.8 Sodium hydroxide, 6 N: with caution dissolve 240 g NaOH in 1 L of reagent grade water in a glass beaker. Prepare this reagent in a fume cupboard.

2.9 Chloroform (CHCl3), AR

2.10 Standard sodium thiosulphate solution, ~0.025 N: Dissolve 6.205 g Na2S2O3.5H2O in reagent grade water. Add 1.5 mL 6 N NaOH or 0.4 g solid NaOH and dilute to 1 L. Add 2 drops of CHCl3.

2.11 Vitex indicator, laboratory grade



3. Procedure

Dissolve approximately 2 g KI in an Erlenmeyer flask with 100 to 150 mL DI water. Add 1 mL 6 N H2SO4 or a few drops of H2SO4 and 20.0 mL Bi-iodate solution. Dilute to 200 mL and titrate liberated iodine with thiosulphate titrant, adding Vitex indicator toward the end of the titration, when a pale straw colour is reached. The titration is complete when the blue colour is discharged. Record information on the work book. Carry our standardization in triplicate. Calculate normality of Na2S2O3 as per Section 5.


4. Frequency of Standardisation

Restandardise every 6 months.


5. Calculation:




6. Reference

APHA 20th Edition. 4500 – 0 C. pp. 4-131.