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BACKGROUND
Sodium thiosulphate solution is standardized against potassium dichromate in presence of hydrochloric acid and potassium iodide. Potassium dichromate oxidizes the iodide ion in acidic medium to equivalent amount of iodine. The iodine formed in the reaction oxidizes sodium thiosulphate giving sodium tetrathionate ion and the end point is detected by starch solution.
Hence, based on the above theory our aim is to prepare and standardize sodium thiosulphate using potassium dichromate and potassium iodide in presence of hydrochloric acid and starch solution as indicator.1
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REQUIREMENTS
Apparatus: Cleaned and calibrated burette
Erlenmeyer flask
Chemicals: Sodium thiosulphate
Potassium dichromate
Potassium iodide
Hydrochloric acid
Starch solution
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PROCEDURE
Preparation of sodium thiosulphate solution
Dissolve 25 g of sodium thiosulphate in CO2 free water and make the volume upto 1000 ml. Keep the solution aside and filter to remove any cloudieness, if appears.
Preparation of starch solution
Add one gram of starch to few ml of water, prepare slurry and add gradually to 100 ml of boiling water till a translucent solution will be obtained.
Standardization of sodium thiosulphate
Dissolve 0.125 g of accurately weighed potassium dichromate in 25 ml of water present in a 250 ml erlenmeyer flask. Add 10 ml of hydrochloric acid and 2 g of potassium iodide, stopper, shake and keep in dark for 15 min. Add 100 ml of water to the above mixture and titrate with sodium thiosulphate using starch as the indicator. Near end point the color will be changed from dark blue to bottle green. Each ml of 0.1 M sodium thiosulphate is equivalent to 0.04904 g of K2Cr2O7.
Tabulation of standardization
|
Sl. No. |
Weight of Pot. dichromate taken (ml) |
Burette reading of Sod. thiosulphate (ml) |
Volume of Sod. thiosulphate consumed (ml) |
|
|
IR |
FR |
|||
| 1. | ||||
| 2. | ||||
| 3. | ||||
Normality of sodium thiosulphate = (Weight of Pot.dichromate)/(Volume of sod.thiosulfate consumed)× 0.04904
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CONCLUSION
From the above experiment it was evident that sodium thiosulpahte can be effectively standardized by using potassium dichromate and iodide with hydrochloric acid and starch as the visual indicator. After performing the calculations, strength of the prepared sodium thiosuplhate solution was found to be……..N.
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REFERENCES
1. Kasture AV, Wadodkar SG, Gokhale SB. Practical Pharmaceutical Chemistry-I. Nirali Prakashan; 1993:56.
