Determination of Fe (II) using KMnO4 with oxalic acid as primary standard.

Estimation of Cu⁺² from the Hypo, as K2CrO7 primary standard by Iodometric method

Aim: to estimate the Cu⁺² from the Hypo, as K₂Cr₂O₇ primary standard by iodometric method

Principle: Copper is estimated iodometrically, in this titration iodine is liberated which is titrated against sodium thiosulphate solution using starch as indicator.

 

Apparatus: Conical Flask, standard flask, Burette, Pipette, etc….

Chemicals: potassium dichromate, sodium thiosulphate, starch solution, HCl, ammonia solution,      

                  Potassium thiocyanate, etc…

Part-I: Preparation of Standard potassium dichromate solution:

Weigh accurately above 1.25gms. of K₂Cr₂O₇and transfer the substance in a 250 ml standard flask and add 100ml. distilled water and dissolve the alt and hen make the solution up to the mark with distilled water, shake well for uniform concentration, Calculate the molarity of K2Cr2O7

Observation:

·         Weight of the Empty Bottle (W1)=

·         Weight of the Bottle + compound (W2)=

·         Weight of the Bottle after transfer of salt (W3)=

·         Weight of the K₂Cr₂O₇ (W4) = W2-W3=

Molarity o f the K₂Cr₂O₇Solution (M1) =

Part: II- Standardization of sodium thiosulphate solution by Standard K₂Cr₂O₇ solution:

Take a clean burette and rinse it with hypo solution, fill the burette with the given hypo solution, support it with a burette clamp. Take 40 ml of water in a 250 ml conical flask. To that add 20 ml of potassium iodide (KI) solution (10%) and 1 g of sodium bicarbonate (NaHCO3). Add 6 ml of concentrated hydrochloric acid (HCl) to the above solution and finally add 20 ml of standard K2Cr2O7 solution using a pipette. Shake the flask and keep it covered with watch glass in dark for 3–5 minutes.

Titrate the liberated iodine with Na2S2O3 solution taken in burette. When the colour of the solution fades to straw yellow, add 2ml. of starch solution, the colour of the solution turns to deep blue. And continue the titration till the colour of the solution changes from deep blue to bluish green. Repeat the titration three times.

Observation:

S.No	Volume of K2Cr2O7 in ml	Burette readings		Volume of Hypo
		Initial	Final

Molarity of K₂Cr₂O₇solution is calculated using the formula: =

K2Cr2O7	Hypo
Molarity of K2Cr2O7 solution(M1)= Volume of K2Cr2O7 solution ( V1) = Moles of K2Cr2O7 solution (n1)     =	Molarity of Hypo   solution (M2) = Volume of Hypo solution  ( V2) =      Moles of Hypo Solution (n2)      =

    Molarity of K₂Cr₂O₇solution M2 =

Part: III- Estimation of Copper (II) ion using standard Na₂S₂O₃ solution

Fill the burette with the given hypo solution, support it with a burette clamp. Pipette out 20 ml of solution in a 250 ml conical flask and neutralized the solution by dropwise addition of ammonium hydroxide (NH4OH) solution (1:1) until a blue precipitate appears. Dissolve the precipitate in glacial acetic acid (CH3COOH) adding about 0.5 ml in excess. Dilute the solution to about 80 ml and add 20 ml of potassium iodide solution (10%). Keep the flask covered with watch glass in a dark and cool place for about 5 minutes and titrate the liberated iodine with standard sodium thiosulphate solution form burette. When the colour of the solution fades to a light yellow, add 2ml of starch followed by 20 ml (or 2gms.) of ammonium thiocyanate (NH4SCN) or potassium thiocyanate solution (10%). Titrate the solution till the blue colour discharges and a white or flesh white residue is left in the flask. Repeat the titration three times.

Observation:

S.No	Volume of CuSO4 in ml	Burette readings		Volume of Hypo rundown
		Initial	Final

Molarity of copper sulphate solution is calculated using the formula  =

Hypo	CuSO4
Molarity of Hypo   solution (M2) = Volume of Hypo solution  ( V2) = Moles of Hypo Solution (n2)   =	Molarity of   CuSO4  solution M3 = Volume of   CuSO4   solution V3 = Moles of Hypo Solution (n3)   =

Molarity of copper sulphate solution M3 = 

Amount of CuSO₄ present in 100 ml of the given solution =

Amount of Cu⁺² ion present in 100 ml of the given solution = 

Report:

Ø  Molarity o f the K₂Cr₂O₇Solution (M1) =

Ø  Molarity of Hypo solution M2 =

Ø  Molarity of copper sulphate solution M3

Ø  Amount of CuSO₄ present in 100 ml of the given solution =

Ø  Amount of Cu⁺² ion present in 100 ml of the given solution = 

Precautions:   

v  Hypo solution is always taken in the burettw in iodometric titrations

v  Sufficient amount of KI solution is to be added

v  The indicator, starch, should be added just before the end point

v  NH₄SCN solution should be added near the end point to displace the adsorbed iodine rand to get a sharp end point.

Regent preparation:

v  0.1M Hypo solution:

v  0.1M CuSO4 solution:

v  10% KI Solution:

20% NH4 SCN or 20%KSCN: