ESTIMATION OF COPPER (IODOMETRY)

Aim: To estimate the amount of Copper present in the given solution using a standard solution of Potassium Dichromate and Hypo as the link solution.

Apparatus: 100 ml standard flask, Funnel, Burette, Iodometric flask, Conical Flask, Pipette, Simple balance with weights

Chemicals Required: K2Cr2O7, Hypo (Na2S2O3), KI, Conc.HCl, dil. Acetic acid, NaHCO3, H2SO4, CuSO4, Ammonia solution, Starch & distilled water.

Principle: Any cupric salt in neutral medium when treated with Potassium Iodide forms a white precipitate of cuprous iodide and iodine is set free quantitatively. The liberated Iodine is treated against Hypo using starch as the indicator.

Chemical reactions:

CuSO4 + 2KI -> CuI2 + K2 SO4

2CuI2 -> Cu2I2 + I2

I2 + 2Na2S2O3 + O2 -> Na2S2O6 + 2NaI

2CuSO4 = I2 = Na2S2O3

Procedure:

1. Preparation of standard potassium dichromate: Weigh out accurately the given pure crystalline sample of potassium dichromate and transfer into 100 ml standard (volumetric) flask provided with a funnel. Dissolve the dichromate in a small quantity of distilled water, and make upto the mark. The contents in the flask are shaken well for uniform concentration. Calculate the normality of potassium dichromate.

2. Standardization of sodium thiosulphate: Rinse the burette and fill it up with hypo solution without any air bubbles. Note the burette reading. Take about 20 ml of 10%KI solution in a clean conical flask and add 2 grams of sodium bicarbonate followed by 5 ml of concentrate HCl gently rotate the flask for mixing the liquids. Rinse the pipette with a little of potassium dichromate solution and then transfer 20 ml of the same to the conical flask. Shake it well, stopper it, and keep it in dark place for 5 minutes. Titrate the liberate iodine by running down hypo from the burette with constant stirring. When the solution attains a pale yellow colour add 2 ml of freshly prepared starch solution. The colour changes to blue. Continue the titration drop-wise till the colour changes from blue to light green indicating the end point. Repeat the titration for concurrent values.

3. Estimation of copper: Make up the given solution of copper to 100 ml with distilled water and shake well for uniform concentration. Pipette out 20 ml of this solution into a clean conical flask. Add few drops of ammonia when bluish white precipitate is obtained. Redissolve the precipitate in dilute acetic acid. Now add 10 ml of 5% KI, when iodine is liberated giving a brown colour. Titrate this solution against standard hypo solution till light yellow colour is obtained. Now add 2 ml of starch solution and continue the titration till blue colour changes to creamy white, which is the end point. Repeat the titration for

concurrent values and calculate the amount of copper.

Preparation of Standard solution:

W1 = Weight of bottle + substance = ____________ gms

W2 = Weight of bottle = ____________ gms

Weight of substance = (W1-W2) = ____________ gms.

 Normality of the solution (K2Cr2O7) =((W1-W2) X 10)/Equivalent Weight =

                               =( (W1-W2) X 10)/49

N1 = Normality of Potassium dichromate =

V1 = volume of Potassium dichromate = 20 ml

N2 = Normality of Hypo =

V2 = Volume of Hypo =

N1 V1 = N2 V2

N2 = (N1 V₁)/V2=

N2 = Normality of Hypo =

N3= Normality of Copper solution =

V3 = volume of Copper Solution = 20 ml

N4 = Normality of Hypo =

V4 = Volume of Hypo =

N3V3 = N4V4

Normality of Copper solution = N3= (N4V₄)/V3

Amount of Copper present in the whole of the given solution (100 ml) = (N3 X 63.54)/10 =

Result: Amount of Copper present in the whole of the given solution (100 ml) = _______ gm.