300 QUANTITATIVE AGRICULTURAL ANALYSIS
Determination of Water-soluble Arsenic.—Weigh 2 gm (if a paste, use
4 gm) of the sample on a counterpoised glass or scoop, brush into a 1000-cc
volumetric flask, and add nearly 1000 cc of recently boiled distilled water
which has been cooled to exactly 32°. Stopper the flask and immerse in a
water bath which is kept at 32° (± 1°) by means of a thermostat. Digest
for 24 hours, shaking hourly for the first eight hours of this period. Dilute
to the mark, mix and filter through a dry filter, reject, the first 25 cc and
collect exactly 250 cc in a volumetric flask. Rinse into a 1000-cc flask or
beaker and titrate with the standard iodine solution that was used for total
arsenic. Calculate the amount of water-soluble arsenic as arsenous oxide.
LEAD ARSENATE
Of the internal poisons for insects, lead arsenate is used most
extensively. Lead arsenate was recommended as an insecticide
in 1892 and it was first used against tent caterpillars. It is
made by treating disodium arsenate with either lead nitrate or
lead acetate. Lead arsenate made from lead nitrate contains
about 31.5 per cent of combined arsenic pentoxide while that
made from lead acetate contains about 25.5 per cent.
Lead arsenate (" neutral") is gradually replacing Paris green
as a spray because of its low degree of solubility, it being a safer
spray on this account. The arsenic becomes more soluble if the
lead arsenate solution is prepared with water containing sodium
sulphate or sodium chloride. Solutions containing only 0.1 per
cent of the former or 0.05 per cent of the latter will dissolve an
appreciable amount of arsenic from lead arsenate. Spraying
tests have shown that 10 grains of sodium chloride per gallon,
when used with lead arsenate in the spray fluid, produced injury
and 40 grains per gallon injured about half of the foliage. It is
therefore important to avoid ordinary mineral water and salt
water in preparing the spray.
Lead arsenate has the advantage over Paris green in that it
sticks to the foliage well when applied as a spray.
Determination of Moisture: (a) In Powder.—Weigh a porcelain crucible
without cover, then add about 2 gm of sample and reweigh. Dry to con-
stant weight at 105° to 110° and report the loss of weight as moisture.
(6) In Paste.—In a weighed dish dry 50 gm for one hour at 105° to 110°.
Cool and reweigh. Calculate the moisture thus obtained as p. Grind the
partly dried sample to a fine powder, mix well and transfer a small portion
to a sample bottle. Weigh 2 gm of this into a crucible and dry again for