Std Na2s2o3.docx

Title: Preparation and Standardization of 0.1N Sodium Thiosulfate. General: Na2S2O3.5H2O is a secondary standard chemical which is used in a standard form to determine the concentration of available chlorine in hypochlorite solution. Therefore, firstly, it is prepared of about approximate concentration and then standardized for accurate concentration. Principle: An aliquot of the Na2S2O3 solution is reacted with the iodine, liberated by the reaction between K2Cr2O7 and KI under acidic conditions. Apparatus: Burette 50ml, volumetric flask 100ml, beaker 500ml, pipettes 1ml, 10ml, 20ml, Erlenmeyer flask 100ml. Reagents: 1. D. water. 2. Standard 0.1N K2Cr2O7 solution (prepared by dissolving it 4.903 gm in a L). 3. 0.1N Na2S2O3 (prepared by dissolving its 24.8 g in one Liter). 4. 12% (w/w) KI solution. 5. 10% H2SO4 solution. 6. 1% starch solution. Procedure: 1. Fill the burette with 0.1N standard solution of Na2S2O3. 2. In a conical flask, take accurately 10ml of 0.1N K2Cr2O7 solution. Add 8.5ml of 10% KI solution and then add 5ml of 10% Sulfuric acid solution. 3. Keep the solution in dark place for 5-10 min; as a result it will become dark greenish brown. 4. Titrate it with Na2S2O3 solution such that its color become greenish yellow. Add about 1ml starch indicator in it, as a result it will become dark blue. Titrate it further with Na2S2O3 solution until it gives sea green color which is the end point. 5. Record the volume of Na2S2O3 solution and carry out the titration in duplicate. 6. With the help of given formula calculate the actual concentration of Na2S2O3 solution.

Calculation: NORMALITY OF Na2S2O3 SOLUTION = N (K2Cr2O7) x V ml (K2Cr2O7) V ml (Na2S2O3) Note: 0.1 M = 0.1 N of Na2S2O3 Solution.

Result: CONCENTRATION OF Na2S2O3 SOLUTION = ---------- M or N. Whereas, Na2S2O3 KI H2SO4 K2Cr2O7

= = = =

Sodium Thiosulfate (pent hydrated) Potassium Iodide Sulfuric Acid Potassium Dichromate

Shelf life and Standardization Interval: 1. Shelf life of dilute Na2S2O3 solution is 30 days, after that it should be discarded and fresh solution should be prepared and standardized. 2. Standardization of dilute Na2S2O3 solution should be carried out after every 5 days (max.) if not used. In case of daily usage, it should be standardize before use, each day. Precaution: 1. Before starting work with chemicals, must go through the M.S.D.S. of all the chemicals and follow all the instruction strictly. 2. Wear gloves, goggles, mask, and apron before use of harmful chemicals and use all the chemicals in fume hood. 3. In case of any damage or harm, stop the working immediately, follow the safety instruction given in M.S.D.S. and inform to supervisor. Responsibilities: