TITRIMETRIC ANALYSIS OF CHLORIDE

Introduction

The purpose of this experiment is to compare two titrimetric methods for the analysis of chloride in a water-soluble solid. The two methods are:

• a weight titration method using a chemical indicator;
• a volumetric titration method using potentiometric detection.

The most important difference between the methods is how the endpoint is determined. In the first case, the color change of the indicator signifies that the titration is complete, while the second method generates a titration curve from which the endpoint is determined. For the potentiometric method, an automatic titrator will be used to perform the titration, and to obtain the titration curve.

Background

Argentometric Titrations

In order for a titrimetric method to be viable, the titration reaction
(1) must be complete (i.e., Ktitration is large) and
(2) should be rapid.

There are many precipitation reactions that can satisfy the first requirement, but far fewer that satisfy the second. Precipitation reactions of silver salts are usually quite rapid, and so argentometric titrations, which use AgNO3 as the titrant, are the most common precipitation titrations.

Argentometric titrations can be used to analyze samples for the presence of a number of anions that form precipitates with Ag+

Weight Titrations The goal of any titrimetric method is to determine the number of moles of titrant needed to reach the equivalence point of the titration reaction:

titration reaction aA + tT → product(s)

where a and t are the stoichiometric coefficients in the reaction between titrant and analyte. In a titrimetric analysis, solution of titrant is added until the equivalence point of the titration reaction is reached. At the equivalence point, neither analyte nor titrant is present in excess. By definition, the equivalence point is the point during the titration at which the following relationship is true: equivalence point

nA/a = nT/t

where nA is the number of moles of analyte originally present in the sample solution, and nT is the number of moles of titrant that must be added to the sample to reach the equivalence point.
From this last expression, we see that, in order to determine the number of moles of analyte originally present in the sample solution, we must
(a) know the stoichiometry of the reaction, and
(b) determine how many moles of titrant are needed to reach the endpoint. In order to meet this last requirement, we must somehow keep track of the quantity of titrant solution that is added to the sample solution.