disscuss the principal involved in the volumetric estimation of cu++?

Answer 1

Lancenigo di Villorba (TV), Italy

ELECTROCHEMICAL FUNDAMENTS
As you know, copper ions are related to a Transition's Metal.
This relation is based on an electrochemical trendy obeying to an electromotive force leading this phenomenon.
W. H. Nernst collected the "Electrochemical potential at Standard Conditions" that is he dipped a metallic copper rod in a 1 M copper ion's aqueous solution : this system is a "First Electrode Type" and it may form a Voltaic Pile when it is connected to a S.H.E. Electrode. The latter is formed by a platinum's sponge dipped in a 1 M hydrochloric acid's aqueous solution receiving a gas-stream of hydrogen bubbles flowing at 1 atm.
Nernst measured a "E.M.F." like 0.34 V ; he assigned 0.00 V to S.H.E.'s system, hence the Copper's Electrode receive the Standard Electrical Potential as E° = +0.34 V, meaning that Copper's Electrode is MORE NOBLE than S.H.E.'s ones.
You may change this experiment if you dipp the metallic copper rod in a 1 M POTASSIUM IODIDE's aqueous solution instead copper ion's one. You may measure an "E.M.F." like 0.19 V ; Nernst assigned 0.00 V to S.H.E.'s Electrode, hence the NEW COPPER's ELECTRODE have a Standard Electrical Potential as E° = -0.19 V, meaning the NEW Electrode results LEAST NOBLE than S.H.E.'s one.

LAB's PROCEDURES
The copper ion's analyte is diluted to an exact volume while you control its pH which WILL BE LOWER THAN Precipitation's Value = 5.5.
You manipule several spoons of POTASSIUM IODIDE which undergo correct WEIGHT DOSING. This known amount is added to the greenish coloured liquid.
An appropriate mixing-time later, you will see a change in liquid's hue toward a brownish one.
What does reaction happen?

2 Cu++(aq) + 4 KI(aq) ----> 2 CuI(s) + I2(aq) + 4 K+(aq)

The brownish hue owing IODINE's formation (e.g. dark purple coloured) while at the bottom precipitate white-creamish bodies as COPPER's IODIDE.
This is a reaction OBEYING to the Titrimetric Determination's Requests :
-) Fast Reaction ;
-) Complete Reaction ;
-) Stiochiometric Relationship known.
The Second Point involve the Electrochemical Potential overwritten : starting from the "van't Hoff-Gibb-Nernst's Isothermal Relationship" on the Equilibria's Constant, execute the following calculation

Keq = EXP(0 - DeltaG° / (R * T°)) =
= EXP(n * F * DeltaE° / (R * T°)) =
= EXP(2 * 96,500 *(0.86 - 0.55) / (8.31 * 298.16)) = 2.6E+10

confirming the COMPLETE's NATURE.

Adding some drops of an aqueous starch's solution, the chemist will use the iodine formed by dosing an appropriate reactive, e.g. sodium thiosulphate

I2(aq) + 2 Na2S2O3(aq) <---> 2 NaI(aq) + Na2S4O6(aq)

The starch suffices to darkens the solution until iodine is available to the determination, that is starch is the most common MARKER about the determination on iodine.

I hope this helps you.

Answer 2

Volumetric Estimation