Quantitative Analysis of Soda Ash by Double-Indicator Titration

INTRODUCTION

In a volumetric analysis, the analyte is made to react by titration method with a measurable volume of a standard solution of known concentration [1]. When the reaction is complete, the amount of the desired substance is calculated from the amount of standard reagent used.

For every volumetric analysis, the starting point is the standard solution that will be used to titrate the sample [2]. For neutralization titrations, the standard solutions are always prepared from strong acids and strong bases as primary standards since they cause the most definite change in pH at equivalence point [3]. Weak acids and weak bases are not suitable for this because a sharp end point cannot be obtained. Sodium hydroxide is widely used for titration of acids; while hydrochloric acid is for bases.

Acids are usually standardized against weighed quantities of sodium carbonate according to the following consecutive reactions [3]:

Reaction (1) corresponds to the conversion of carbonate ion to bicarbonate ion that occurs at about pH 8.3; while reaction (2) is the conversion of the bicarbonate ion to carbonic

acid that can be detected at about pH 3.8 [3]. For

the standardization process, these two reactions

take place to reach the end point. From the

gathered data, the actual molarity of the standard

solution can be computed.

With the standard solution, the sample with the

sought-for substance is titrated. The percentage

composition of the sample can be determined by

employing the concentration and volume of the

standard solution used in titration and relating it

to a specific constituent by stoichiometry.

Determination of the components of a solution

containing sodium carbonate, sodium hydrogen

carbonate and sodium hydroxide demonstrates

how neutralization titrations can be used to

analyze mixtures [3]. Only two of these three

constituents can exist in significant amount in

any solution because reaction will remove the

third [3]. Two-titration or double...