The concentration of cyanide ion, CN - , in a copper plating bath can be determi

Question

The concentration of cyanide ion, CN-, in a copper plating bath can be determined by complexometric titration with Ag+. A 5.00 mL sample from the bath is treated with 100 mL water, made basic with 5 mL of 20% NaOH solution, and 5 mL of 10% KI indicator added. The sample is then titrated with 0.1012 M AgNO3 solution, forming a soluble complex Ag(CN)2-: Ag+(aq) + 2 CN-(aq) -> Ag(CN)2-(aq). When all the cyanide is complexed to the Ag+, the next drops of AgNO3 cause a yellow precipitate of AgI to form at the end point. It requires 27.36 mL of AgNO3 to reach the endpoint. Calculate the concentration of cyanide as molarity of sodium cyanide, NaCN. (The NaCN concentration in such baths typically lies in the range of 1 to 2 M).

Explanation / Answer

moles of AgNO3 used = molarity x volume

                                    = 0.1012 M x 27.36 ml

                                    = 2.77 mmol

moles CN- present = 2 x 2.77 mmol

                               = 5.54 mmol

molarity of CN- in solution = 5.54 mmol/5 ml

                                           = 1.11 M

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