The Titration.—The standard solution of uranium acetate is made by dissolving 34.1 grams of the salt (with the help of 25 c.c. of acetic acid) in water; and diluting to 1 litre. The water and acid are added a little at a time, and warmed till solution is effected; then cooled, and diluted to the required volume: 100 c.c. will equal 0.50 gram of arsenic.
The sodic acetate and acetic acid solution is made by dissolving 100 grams of sodic acetate in 500 c.c. of acetic acid, and diluting with water to 1 litre. Five c.c. are used for each assay.
The solution of potassic ferrocyanide used as indicator is made by dissolving 10 grams of the salt in 100 c.c. of water.
To standardise the solution of uranium acetate, weigh up a quantity of white arsenic (As2O3) which shall be about equivalent to the arsenic contained in the assay (0.1 or 0.2 gram); transfer to a flask, and dissolve in 10 c.c. of nitric acid with the aid of heat. Evaporate to a small bulk (taking care to avoid the presence of hydrochloric acid); dilute with water; add a small piece of litmus paper; render faintly alkaline with ammonia; then add 5 c.c. of the sodic acetate mixture; dilute to 150 c.c.; and heat to boiling.
Fill an ordinary burette with the uranium acetate solution, and run into the assay a quantity known to be insufficient. Again heat for a minute or two. Arrange a series of drops of the solution of ferrocyanide of potassium on a porcelain slab, and, with the help of a glass rod, bring a drop of the assay solution in contact with one of these. If no colour is produced, run in the uranium acetate, 1 c.c. at a time, testing after each addition, till a brown colour is developed. It is best to overdo the assay, and to count back. It is not necessary to filter off a portion of the assay before testing with the "ferrocyanide," since the precipitate (uranic arsenate) has no effect.
The following experiments show the effect of variation in the conditions of titration. Make a solution of arsenic acid by dissolving 4.95 grams of arsenious acid (As2O3) in a covered beaker with 35 c.c. of nitric acid; evaporate down to 7 or 8 c.c.; and dilute with water to 1 litre: 100 c.c. will contain 0.375 gram of arsenic. Use 20 c.c. for each experiment.
Effect of Varying Temperature.—It is generally recommended to titrate the boiling solution, since it is possible that the precipitation is only complete on boiling. Low results are obtained in a cold solution, the apparent excess of uranium acetate striking a colour at once; on boiling, however, it ceases to do so; consequently, the solution should always be boiled directly before testing.
In four experiments made in the way described, but with 20 c.c. of a solution of arsenic acid stronger than that given (100 c.c. = 0.5 gram As), the results at varying temperatures were:—
| Temperature | 15° C. | 30° C. | 70° C. | 100° C. |
| "Uranium" required | 18.0 c.c. | 18.5 c.c. | 18.5 c.c. | 18.7 c.c. |
Effect of Varying Bulk.—These experiments were like those last mentioned, but were titrated boiling, and the volume was varied:—