c. will equal 0.500 gram of arsenic.
A solution of starch similar to that used in the iodide-copper assay
will be required. Use 2 c.c. for each assay. Variations in the quantity
of starch used do not interfere; but the solution must be freshly
prepared, as after seven or eight days it becomes useless.
To standardise the iodine solution, weigh up 0.3 gram of white arsenic;
dissolve in caustic soda; neutralise; after acidulating, add 2 grams of
bicarbonate of soda and 2 c.c. of the starch solution, and dilute to 200
c.c. with cold water. Fill a burette having a glass stop-cock with the
iodine solution, and run it into the solution of arsenic, rapidly at
first, and then more cautiously, till a final drop produces a blue
colour throughout the solution. Calculate the standard in the usual way.
White arsenic contains 75.76 per cent. of arsenic.
The following experiments show the effect of variation in the conditions
of the titration:--
Make a solution of arsenic by dissolving 6.60 grams of white arsenic in
100 c.c. of sodic hydrate solution; render slightly acid with
hydrochloric acid; add 10 grains of bicarbonate of soda, and dilute to 1
litre: 100 c.c. will contain 0.50 gram of arsenic.
~Effect of Varying Temperature.~--The reaction goes on very quickly in
the cold, and, since there is no occasion for heating, all titrations
should therefore be carried out cold.
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