Sample Preparation for Titrations

Let us now consider a specific example for each type of titration  

Precipitation titration Let us suppose our task is to determine to 0.1% the iodide content of an approximately 0.01 M I solution. We choose for this a precipitation titration with silver ions. According to the tables, the Agl produced in the reaction has a solubility product of about 10 . We then calculate the 3p value  

According to the instructions in the last section, however, for an accuracy of 0.1% in the end result jSp need only be 10 . In order to keep the amount of I occluded in the precipitate during the start of the titration small, we dilute our test solution 1 10 with distilled water. Now our starting concentration is 10 M, corresponding to a j3p value of  

This value still assures a sharp end point jump in the titration curve. This same example for a chloride rather than iodide analysis would be far less favorable, since the solubility product of AgCl is only 10 . With a starting chloride ion concentration of 10 M, Pp is only 10 . This value would allow a final accuracy of 0.1% only under the most highly favorable conditions (extremely careful titer determination, using a standard having the same matrix as the sample solution and standardized analytical procedures). Here any diluting of the sample would smear out the end point. 

Complexometric Titration Suppose we are going to titrate Mg ions with EGTA. From tabular complications we find that the Mg — EGTA complex formation constant is Kcf 10 III order that the 3c value not fall below 10, then according to j3c = Acf Gqs the starting concentration of the Mg solution should be no lower than 0.1 M. If this concentration is substituted in the equation for b, then an interfering ion term of 10 M must be attained if b is to remain 10 . Only if these conditions are fulfilled can we expect an accuracy of 0.1%. 

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