Example Analysis of Brass

As an illustration of the use of electrode potentials, consider the classical method of analysis of copper in brass, which involves dissolving the weighed sample in nitric acid to obtain Cu +(aq), adjusting the pH to a weahly acidic level, allowing the Cu + to react completely with excess potassium iodide to form iodine and the poorly soluble Cul, and then titrating the iodine with sodium thiosulfate solution that has been standardized against pure copper by the same procedure  

It follows from reactions 15.11 and 15.20 that copper(II) ion can oxidize iodide to iodine, and from reactions 15.11 and 15.21 that thiosulfate can reduce I2 back to 1 while forming the tetrathionate ion (Section 3.4). Because the relevant AE° values are several hundred millivolts, the equilibrium constants are large, and these reactions go to essential completion. 

We must, however, ensure that there is no contaminant such as free Fe in the solution before the potassium iodide is added otherwise, more 1 will be oxidized than there is Cu in the sample (reaction 15.12). The effective concentration of Fe + can be reduced to negligible levels by adding sodium fluoride to complex it. (The divalent copper ion is little affected.) 

Prom the E° of half-reaction 15.22, it would seem that the nitrate ion present from the dissolution of the brass should also oxidize iodide ion. This E° value, however, refers to standard conditions, which implies 1 mol H+, whereas we have adjusted the pH to near neutrality. Suppose the pH is adjusted to 7.0, that is, [H+] = 1 x 10 mol while [N02 ] and [NOs ] retain their standard-state values of unity the corresponding EMF for the half-reaction 15.22 is then 

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Manipulation and Use of Electrode Potentials 15.2.1 Example Analysis of Brass... 

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