Aim: To determine the strength of given copper sulphate solution by titrating against N/40 sodium thiosulphate (hypo) solution. Principle: The strength of copper sulphate is determine by iodometric method. When KI is added to a solution of copper sulphate a white cuprous iodide, Cu 2I2 is precipited and an equivalent amount of iodine is librated. The free iodine is titrated with standard solution of hypo, using starch an indicator. As soon as all the librated iodine has been reduced to iodide (NaI), the blue colour of iodostarch complex will disappear and the colour of precipitate in conical flask will be white that is of cuprous iodide. This indicates the endpoint. The titration reactions take place in the neutral medium hence any mineral acid present in the solution should be neutralized by adding sodium carbonate solution and excess of sodium carbonate must be neutralized by adding dil. Acid. This should be done before b efore staring the titration. Reagents: Copper sulphate solution, Standard N/40 sodium thiosulphate solution, starch Solution, KI crystals Apparatus: Pipettes, burette, conical flask.
Procedure: Clean the burette and fill with hypo solution and note the initial reading. Pipette out 10 ml of copper co pper sulphate solution in a clean conical flask. Now add 0.5 gm solid KI, mix well and cover the mouth of conical flask by watch glass or filter paper and allow the mixture to stand for 2-5 minutes in the dark. The solution becomes brown in colour due to librated iodine. Now titrate the librated iodine with the hypo solution added from burette. The brown colour of iodine fades slowly and when only a very faint yellow colour (light straw colour) remains, adds 1 ml of starch solution. This immediately forms a deep blue iodo-starch complex. Now, add further hypo solution drop by drop, shaking well in whirling motion and titrate till the blue colour just disappears. If the colour does not return within 10 seconds, this will indicate end point. Note this burette reading. Repeat the titration until two concordant readings are obtained.
Observation: Burette: N/40 sodium thiosulphate (hypo) solution Pipette: 10 ml of copper sulphate solution Indicator: Starch solution (freshly prepared) Colour change: Blue to colourless Observation table: S. No.
Reaction: The chemical reactions taking place are: 2CuSO4 + 4KI → Cu2I2↓ + 2K2SO4 + I2 white ppt. 2Na2S2O3 + I2 →
Na2S4O6 + 2NaI Sodium tetrathionate
Calculation: Volume X normality of unknown copper sulphate solution= Volume X normality of hypo solution 10 X N1 = N2 X V2 (Burette reading) N1= 0.025 X Burette reading 10 Strength = Normality of copper sulphate X Eq. weight of copper sulphate =..................................gm/lit Result: The strength of given unknown copper sulphate is .....................gm/liter.
