Background

The purpose of this experiment is four-fold:

o improve proficiency with quantitative handling of materials and laboratory measurements, particularly those related to gravimetry (quantitative and selective measurement of mass).

o provide an introduction to the fundamental concept of analytical separations science (quantitative physical separation of an analyte from other analytes and/or interfering substances)

o provide one (of three) proficiency test(s) designed to certify each student as qualified to perform instrument and reagent validation operations required by Good Laboratory Practice guidelines, and to conduct more complex analytical studies

o produce a quantity of solid material whose chloride content is accurately known, based on the results of the gravimetric chloride assay. This material will be used as a reference "standard" for a subsequent analytical study (Experiment B.2).

 

Synopsis of the Analytical Problem

The sample to be analyzed for chloride content is a solid crystalline powder containing a mixture of chloride salts (e.g., KCl, NaCl) as well as some inert (unreactive and non-interfering) material. The objective is to determine the chloride composition (percent) of the sample by first dissolving an accurately weighed portion of the sample, and then separating the chloride from other ions by reacting it selectively with silver ion to form insoluble silver chloride. The silver chloride precipitate is collected quantitatively and subsequently purified and weighed. The mass of chloride contained in the silver chloride precipitate should be identical to that originally contained in the weighed sample mixture; thus, the weight percent chloride in the original sample can be calculated (to four significant figures).

Significance of the Chloride Assay

The sample assayed for chloride content is homogeneous, and the quantity provided is more than sufficient for the gravimetric procedure. The remaining substance will be used as a reference "standard" for a subsequent study (Experiment B.2). Be sure to label and retain the remaining material in a closed container.

Procedure