A Text-book of Assaying: For the Use of Those Connected with Mines Part 58

[Ill.u.s.tration: FIG. 65.]

Fill an ordinary 100 c.c. burette with the solution of barium chloride.

The evaporating dish containing the a.s.say 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 b.u.mping, and should not be used. Run in the standard solution in quant.i.ty 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 t.i.tration 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.

[Ill.u.s.tration: FIG. 66.]

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 t.i.tration:--

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 (Na_{2}SO_{4}.10H_{2}O), or 86.88 grams of ferrous sulphate (FeSO_{4}.7H_{2}O), 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 t.i.tration could most accurately be effected.

~Effect of Hydrochloric and Nitric Acids.~--The t.i.trations 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 "

These show clearly the interference of free mineral acids, although very dilute hydrochloric acid (1 c.c. in 500 of water) has no effect.

~Effect of Acetic and Citric Acids.~--A similar series of experiments with these acids gave the following results:--

Acetic acid present 0.0 c.c. 5.0 c.c. 50.0 c.c. 100.0 c.c.

"Baric chloride" required 20.0 " 20.0 " 20.0 " 20.0 "

Citric acid present 0 gram 1 gram 5 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c.

These acids do not interfere.

~Effect of Sodic Acetate and Acetic Acid.~--In each of these experiments 5 c.c. of acetic acid was present.

Sodium acetate added 0 gram 1 gram 10 grams 50 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c. 20.0 c.c.

As sodic acetate and acetic acid did not interfere, it became desirable to make some experiments on the _finishing point_. The first object sought for was the smallest amount of the standard baric chloride in 500 c.c. of water, required to give an indication when tested in the manner already described.

Baric Chloride Conditions of a.s.say Solution. required.

Water only 0.05 c.c.

With 10 grams of sodium acetate and 5 c.c. of acetic acid 0.05 "

The same with 5 grams of nitre 0.10 "

Like the last, but with 5 grams of salt instead of nitre 0.10 "

These show that as small an amount of baric chloride solution as is equal to only 0.000002 gram of sulphur in the 2 c.c. of solution tested yields a decided precipitate on the addition of 3 drops of sulphuric acid.

To determine whether the same finishing point is obtained on testing the filtered portions in the test tubes with baric chloride as is obtained on testing with sulphuric acid, a t.i.tration was made with 20 c.c. of standard solution of sulphuric acid, together with the usual quant.i.ties of sodic acetate and acetic acid; and two lots of 2 c.c. each were filtered into two sets of test tubes after each addition of the standard baric chloride. To one series 3 drops of baric chloride solution were added, and to the other 3 drops of sulphuric acid. The results were--

With Dilute With Baric "Baric Chloride" added. Sulphuric Acid. Chloride Solution.

19.5 c.c. Clear Cloudy 19.75 " Clear Cloudy 20.0 " Finished Finished 20.25 " Cloudy Clear 20.5 " Cloudy Clear

The two methods of testing give the same result. But this balance is disturbed in the presence of much nitre, the indications with baric chloride being disturbed by an opalescence for some c.c. beyond the finishing point. In solutions containing free hydrochloric or nitric acid, a precipitate is obtained with either baric chloride or sulphuric acid.

~Effect of Varying Sulphur.~--In these and the subsequent experiments the t.i.trations were performed in the presence of 10 grams of sodic acetate and 10 c.c. of acetic acid in the manner already described.

Standard sulphuric acid used 5.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Baric chloride"

required 5.0 " 10.0 " 20.0 " 50.0 " 100.0 "

~Effect of Varying Temperature.~--With 5 c.c. of standard sulphuric acid t.i.trated at 15 C., 5 c.c. of baric chloride were required; but with larger quant.i.ties the results were altogether unsatisfactory when t.i.trated cold.

~Effect of Varying Bulk.~--

Bulk 100.0 c.c. 200.0 c.c. 500.0 c.c. 1000.0 c.c.

"Baric chloride"

required 20.0 " 20.0 " 20.0 " 20.5 "

Considerable variation in bulk has no effect, but 500 c.c. is the most convenient volume to work with. It is well to occasionally replace the water boiled off during t.i.tration.

~Effect of Foreign Salts.~--In all these experiments 20 c.c. of "sulphuric acid" were used, and the t.i.tration was performed in the ordinary way.

_Sodic chloride_ added 0 gram 5 grams 10 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 19.7 c.c.

_Ammonic chloride_ added 0 gram 5 grams 10 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 19.5 c.c.

_Calcic chloride_ added 0 gram 1 gram 2 grams 5 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 19.2 c.c. 19.0 c.c.

_Zinc chloride_ added 0 gram 1 gram 3 grams 5 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c. 20.0 c.c.

_Ferrous chloride_ added 0 gram 1 gram 3 grams 5 grams "Baric chloride" required 20.0 c.c. 19.7 c.c. 19.5 c.c. 19.0 c.c.

_Ferric chloride_ added 0 gram 1 gram 3 grams 5 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c. 20.0 c.c.

_Copper chloride_ added 0 gram 1 gram 3 grams 5 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c. 20.0 c.c.

_Pota.s.sic Nitrate_ added 0 gram 1 gram 5 grams 10 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c. 19.0 c.c.

_Pota.s.sic Nitrite_ added 0 gram 1 gram 5 grams "Baric chloride" required 20.0 c.c. 20.0 c.c. 20.0 c.c.

_Sodic phosphate_ added 0 gram 1 gram "Baric chloride" required 20.0 c.c. 22.5 c.c.

_Sodic a.r.s.enate_ added 0 gram 1 gram "Baric chloride" required 20.0 c.c. 20.5 c.c.