Question

In Iodometric Analysis of Copper in Copper Oxide, why did I not need to accurately weigh out the KI used in the standard and unknown analysis?

Answer 1

The basic reaction in the determination of copper using the iodometric method is represented by the equation:

2Cu²⁺ + 4I^- <===> 2CuI(s) + I₂

This is a rapid, quantitative reaction in slightly acidic solutions, if there is a large excess of iodide ion present and if the copper is in the form of a simple ion rather than a complex one. The iodine that is liberated can be titrated in the usual manner with standard thiosulfate solution. The reaction involving cupric ion and iodide takes place quantitatively since the cuprous ion formed as result of the reduction is removed from the solution as a precipitate of cuprous iodide.

Iodometric methods depend on the following equilibrium:

I₂ + I^-<===> I₃^-

Since the solubility of I₂ in water is quite low, the formation of the tri-iodide ion, I₃^-, allows us to obtain useful concentrations of I₂ in aqueous solutions.

The amount of iodine liberated in the reaction between iodide ion and an oxidizing agent is a measure of the quantity of oxidizing agent originally present in the solution. The amount of standard sodium thiosulfate solution required to titrate the liberated iodine is then equivalent to the amount of oxidizing agent. Iodometric methods can be used for the quantitative determination of strong oxidizing agents such as potassium dichromate, permanganate, hydrogen peroxide, cupric ion and oxygen.