296	QUANTITATIVE AGRICULTURAL ANALYSIS
the emulsion to the form of water in oil. The continuous film
of oil will then injure foliage with which it conies in contact.
Arsenic Sprays.—Since many insecticidal compounds rely
upon arsenic for their effective killing qualities it is important
that the student become familiar with a few of the large number
of available arsenic compounds. The more important ones are
as follows: Arsenic trioxide, As_20_3 (also called "arsenic" or
"white arsenic^77); arsenic pentoxide, AssOs; arsenic acid, H_3As0_4;
metarsenic acid, HAs0_3; potassium arsenite, K_3As0_3; potassium
arsenate, K_3As0_4; Paris green, Cu(C_2H_30_2)_2-Cu3(AsO,'02; lead
arsenate, Pb_3(As04)_2; and calcium arsenate, Ca_3(As0_4)_2.
PARIS GREEN
This is seen from the formula given above to be an aceto-
arsenite of copper. Theoretically the compound contains 58.55
per .cent of As_20_3, 31.39 per cent of CuO and 10.06 per cent of
(CH_3CO)_20, the respective anhydrides. However, this does not
usually represent the actual composition, either of the solid Paris
green or of a solution in which it is suspended. Copper arsenate,
or even free arsenic or arsenous acids may be present.
Total Arsenic.—The determination of total arsenic in Paris
green is based upon the volatility of arsenic trichloride. Cuprous
chloride is added to the hydrochloric acid solution in which the
sample has been dissolved, and this reduces any pentavalent
arsenic to the more volatile, trivalent form. The distillate
containing arsenous chloride and hydrochloric acid is absorbed
in cold water. The acid is neutralized, sodium bicarbonate
is added in excess and the arsenic is titrated with standard
iodine solution. The arsenite is oxidized by free iodine as
follows:
Na_3As0_3 + I_2 + H_20 -» Na_3As0_4 + 2HL
A neutral solution is maintained by the excess of sodium
bicarbonate, which neutralizes hydriodic acid as fast as the
latter is formed, thus preventing reversal of the reaction.
The apparatus for total arsenic determination is shown in
Fig. 61. A is a 250-cc distilling flask fitted with a 50-cc drop-
ping funnel; 5, C, and D are flasks holding 500, 1000 and 100
cc respectively. B and C are surrounded by cracked ice and