The Titration.—The standard solution of barium chloride is made by dissolving 76.25 grams of the crystallized salt (BaCl2.2H2O) in distilled water, and diluting to 1 litre. 100 c.c. will equal 1 gram of sulphur. As indicator, use dilute sulphuric acid. The strength of the solution may be checked by the titration of 5 grams of ferrous sulphate (oxidized with permanganate of potassium or a few drops of nitric acid), which should require 57.5 c.c. of the barium chloride solution; or any pure sulphate of known composition can be used; anhydrous salts should be preferred.
Fill an ordinary 100 c.c. burette with the solution of barium chloride. The evaporating dish containing the assay solution is placed on a round burner (as shown in fig. 65), and the solution is kept steadily boiling. An ordinary Bunsen-burner flame will cause bumping, and should not be used. Run in the standard solution in quantity known to be insufficient; then withdraw a portion of about 2 c.c., with a pipette, and filter through a fine filter-paper into a test tube. Run in another 0.5 c.c. of the standard solution, and withdraw and filter into a test tube another portion of 2 c.c.; and continue this operation until half-a-dozen or more portions have been drawn off. The test tubes should be arranged in order in a stand resting on a piece of paper, so that each test tube representing 0.5 c.c. of the standard baric chloride may have its value recorded beneath it (fig. 66). Add to each test tube 3 drops of dilute sulphuric acid; that which shows the first appearance of a precipitate marks the point at which the titration is complete. Suppose, for example, that the test tube marked 48.5 c.c. shows no precipitate, while that at 49.0 c.c. shows one, it is evident that the finishing point lies between these readings. With a little practice, one can judge from the appearance of the precipitate in the 49 c.c. tube, whether 1/4 c.c. should be deducted or not.
It is better to add dilute sulphuric acid, and to watch for the appearance of a precipitate in the test tube, than to add baric chloride and to look for its non-appearance; besides, baric chloride is much less likely to be present in a test tube as impurity than sulphates are. In this way the chance of error from what are termed "accidental causes" is diminished.
The following experiments show the effect of variation in the conditions of titration:—
Make a standard solution of sulphuric acid by diluting 43.65 grams of sulphuric acid (sp. g. 1.6165) to 1 litre: 100 c.c. will contain 1 gram of sulphur. An equivalent solution may be made by dissolving 100.62 grams of sodium sulphate crystals (Na2SO4.10H2O), or 86.88 grams of ferrous sulphate (FeSO4.7H2O), in water (oxidising the latter), and diluting to 1 litre.
The order in which these experiments are given is that in which they were made in an investigation into the conditions under which the titration could most accurately be effected.
Effect of Hydrochloric and Nitric Acids.—The titrations were performed in the manner already described, but sodic acetate and acetic acid were absent. Twenty c.c. of the standard solution of sulphuric acid were used.
| Hydrochloric acid present | 0.0 | c.c. | 1.0 | c.c. | 2.0 | c.c. | 5.0 | c.c. |
| "Baric chloride" required | 20.0 | " | 20.0 | " | 19.7 | " | 12.5 | " |
| Nitric acid present | 0.0 | c.c. | 1.0 | c.c. | 2.0 | c.c. | 5.0 | c.c. |
| "Baric chloride" required | 20.0 | " | 19.5 | " | 18.0 | " | 10.0 | " |