ANSWER:
Volhard method is an indirect titration method, usually use to determine chloride ions.
(1)
Silver chloride is very insoluble in water. Addition of AgNO3 to a solution containing chloride ions results in formation of a finely divided white precipitate AgCl. This reaction provides a basis for determination of chloride ions.
(2)
Ag+ reacts with KSCN to form the precipitate AgSCN, decreasing the KSCN concentration and shifting the equilibrium to the left.
(3)
Conc. HNO3 is added to sodium extract before adding silver nitrate for determining chloride ions, to decompose any NaCN or Na2S if present. If conc. HNO3 is not added to sodium extract before adding silver nitrate, NaCN and Na2S will react with AgNO3 and interfere with the test for chloride ions.
When Nitric acid is not added: -
NaCN + AgNO3 → AgCN + NaNO3
When Nitric acid is aaded:-
NaCN + HNO3 → HCN + NaNO3
(4)
The swirling is done vigorously to make mixing of the reagents as complete as possible, and also to get a precise end point.
(5)
First consider the reaction,
Ag2cr04 Ag2Cr04 --> 2Ag+ + Cr042
Ksp [Ag+]^2 [Cr042-]
Where,
Ksp = 2.4 x 10-12
[Cr042-] = 0.1
2.4 x 10-12= [Ag+]^2 [0.1] [Ag+] 4.9 x 10-6 M
now consider AgCl
AgCl -> Ag+ + Cl-
Ksp = [Ag+] [CI-]
Where,
Ksp = 1.8 x 10-10
use [Ag+] = 4.9 x 10-6
we get
1.8 x 10-10 = 4.9 x 10-6 x [CI-]
[CI-] = 3.674 x 10-5
the concentration of Cl- when Ag2Cr04 begins to precipitate is 3.674 x 10-5 M