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 (As_{2}O_{3}) 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.
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