Procedure for preparation and standardization 0.1N Sodium Thiosulfate

1. OBJECTIVE
To lay down the procedure for preparation and standardization of 0.1 N Sodium Thiosulfate

2. SCOPE
This procedure is applicable to Quality Control Department

3. RESPONSIBILITY
• Officer QC
• Manager QC

4. SAFETY INSTRUCTIONS
• Proper handling the apparatus.
• Proper handling the chemicals
• Always wear appropriate personal protective equipment when handling this chemical.

5. PROCEDURE
5.1 PREPARATION
5.1.1 5.1.1 Calculate the molecular weight of sodium thiosulfate pentahydrate, which is Na2S2O3•5H2O. The atomic weights of sodium, sulfur, oxygen, and hydrogen are 22.99, 32.06, 16.00, and 1.01 g/mol, respectively. Therefore, the molecular weight of sodium thiosulfate pentahydrate is:
      2 x 22.99 + 2 x 32.06 + 3 x 16.00 + 10 x 1.01 = 248.18 g/mol
5.1.2 Weigh out 24.818 grams of sodium thiosulfate pentahydrate on a calibrated balance. This will give you 0.1 mole of sodium thiosulfate pentahydrate.
5.1.3 Dissolve the sodium thiosulfate pentahydrate in distilled water in a volumetric flask of 1 liter.
5.1.4 Stir until the sodium thiosulfate pentahydrate is completely dissolved.
5.1.5 Add distilled water to the mark of 1 liter, and stir again to ensure homogeneity

5.2 STANDARDIZATION
5.2.1 Accurately weigh about 210 mg of primary standard potassium dichromate, previously pulverized and dried at 120 for 4 hours.
5.2.2 Dissolve in 100 mL of water in a glass-stoppered, 500-mL flask. Swirl to dissolve the solid, remove the stopper, and quickly add 3 g of potassium iodide, 2 g of sodium bicarbonate, and 5 mL of hydrochloric acid.
5.2.3 Insert the stopper gently in the flask, swirl to mix, and allow to stand in the dark for exactly 10 minutes. Rinse the stopper and the inner walls of the flask with water, and titrate the liberated iodine with the sodium thiosulfate solution until the solution is yellowish green in color. Add 3 mL of starch TS, and continue the titration until the blue color is discharged.
5.2.4 Perform a blank determination.
5.2.5 And calculate the normality by the following formula:

         mg K₂Cr₂O₇
N= ——————————————————
         49.04 x ml Na₂S₂O₃

6. RELATED DOCUMENTS
N/A

7. DISTRIBUTION INDEX
Original Copy will be retained by Quality Assurance Department.

8. ABBREVIATIONS
Q.C Quality Control Department
TS Test Solution
Gm Gram
ml Milliliter

9. REVISION HISTORY