Cyanide ions can be determined by indirect complexometry. An excess of Ni2+ is added to complex all CN- ions as well as the excess is the titrated par EDTA:

4CN- + Ni2+ --> Ni(CN)4 2- kf=10^30

Ni2+ + Y4- ---> NiY2- kf = 10^18

As the excess of nickle is titrated by EDTA(y4-), the comples with cyanide does not react. if 12.75mL of an unknown CN- solution is treated with 25mL of a 0.0140 M Ni2+ solution, the excess of Ni2+ needs10.21mL of 0.012M EDTA afterward. Determine the molarity of the cyanide solution.