Cobalt 2+ can be analysed by treatment with a known excess of thiocyanate in the presence of pyridine according to the equation: Co2+ + 4C5H5N + 2SCN- -------> Co(C5H5N)4(SCN)2 (s)
The precipitate is filtered off, and the SCN- content of the filtrate is determined by Volhard titration.
25ml of unknown solution was treated with 3ml of pyridine and 25ml of 0.1028M KSCN in a 250ml volumetric flask.
The solution was diluted to the mark, mixed and filtered.
After the first few ml's of the filtrate were discarded, 50ml of the filtrate was acidified with HNO3 and treated with 5ml aliquot of 0.1055M AgNO3
After addition of ferroin indicator the excess silver ion required 3.76ml of 0.1028M KSCN to reach the volhard end point.
In the first reaction:
Co2+ + 4 C5H5H + 2 SCN- --> Co(C5H5H)(SCN)2
Co2+ is the limiting reagent. You added in excess SCN- to precipitate it.
Now, to that solution with excess SCN-, you take 50 mL out of the 250 mL to precipitate it with Ag+.
Ag+ + SCN- --> AgSCN
Now that Ag+ becomes the excess reagent. To that excess Ag+, you back-titrated it with SCN- to determine how much excess Ag+ you have.
(we're actually calculating backwards)
In the last reaction: Ag+ ...
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