302

QUANTITATIVE AGRICULTURAL ANALYSIS

titration very slowly, using starch solution near the end point. Calculate
the per cent of total arsenic oxide, AsaOs.

Determination of Water-soluble Arsenic Oxide.—The agents needed are
starch indicator, standard arsenous oxide solution and a standard iodine
solution. These are prepared as directed on page 297.

To 2 gm of the original sample, if a powder, or 4 gm if a paste, in a 1000-
cc volumetric flask, add nearly 1000 cc of recently boiled water which has
been cooled to exactly 32°. Stopper the flask and place in a water bath
kept at 32° by means of a thermostat. Digest for 24 hours, shaking hourly
for eight hours during this period. Dilute to the mark, mix and filter
through a dry filter, rejecting the first 25 cc of nitrate. Transfer 250 or
500 cc of the clear filtrate to an Erlenmeyer flask, add 3 cc of concentrated
sulphuric acid and evaporate on a hot plate.. When the volume reaches
about 100 cc add 1 gm of potassium iodide and continue the boiling until
the volume is about 40 cc. Cool, dilute to about 200 cc, remove the excess
iodine with twentieth-normal sodium thiosulphate, avoiding the use of
starch solution, and proceed as directed on page 298 for the determination of
arsenic in Paris green, beginning with "nearly neutralize with sodium
hydroxide............."

Calculate and report as per cent of water-soluble arsenic oxide, As206.

Determination of Total Arsenous Oxide.—Prepare the starch indicator,
standard arsenous oxide and standard iodine solution as directed in the
determination of Paris green on page 297. Prepare also:

(a) Dilute Sulphuric Acid Solution.—Dilute 15 cc of concentrated sul-
phuric acid with 85 cc of water.

(6) Sodium Hydroxide Solution.—Dissolve 25 gm of sodium hydroxide
in 50 cc of water.

Weigh 0.25 gm of the powdered sample, transfer to a 200-cc Erlenmeyer
flask, add 100 ce of dilute sulphuric acid (a), and boil for 30 minutes. Cool,
transfer to a 200-cc volumetric flask, dilute to the mark, shake thoroughly
and filter through a dry filter. Nearly neutralize 100 cc of the filtrate with
sodium hydroxide (6), using a few drops of phenolphthalein as indicator.
If the neutral point is passed, make acid again with dilute sulphuric acid.
Continue as directed in the determination of total arsenic in Paris green,
page 299, beginning with the neutralization by sodium bicarbonate.
Calculate the per cent of total arsenous oxide in the sample.

CALCIUM ARSENATE

This is one of the newer insecticides. It is somewhat similar
to arsenate of lead but, in its present form, it is not recommended
for use on the more sensitive foliage, such as that of the stone
fruits, because of the large amount of water-soluble arsenic it
often contains, this causing considerable damage to foliage.