298 QUANTITATIVE AGRICULTURAL ANALYSIS
Calculate the theoretical weight of Paris green that would be equivalent
to 250- cc of the standard iodine solution. Weigh out this amount and wash
it and about 5 gm of cuprous chloride into the 250 distilling flask with 100
cc of concentrated hydrochloric acid. Distill until the volume in the distill-
ing flask is reduced to about 40 cc, then add 50 cc of concentrated hydro-
chloric acid by means of the dropping funnel. Continue this process of
addition of acid and distillation until 200 cc of distillate has been obtained.
Wash down the condenser and all connecting tubes, allowing the rinsings to
run into the flasks. Transfer the contents of the receiving flasks to a 1000-
cc volumetric flask, rinsing the former well, dilute to the mark and mix
thoroughly. Measure 100 cc of this solution into a 1000-cc Erlenmeyer
flask, add phenolphthalein and nearly neutralize with a concentrated sodium
hydroxide solution. The solution should be kept cold. Add 10 gm of
sodium bicarbonate and titrate the arsenic with standard iodine solution,
using starch as indicator. Calculate the per cent of total arsenic, as both
element and arsenous oxide.
The official method for the determination of arsenic, which has
just been described, is in some respects less desirable than the
method which was formerly official.1 One of the principal diffi-
culties of the older method is the formation of a yellow colloidal
solution of arsenous iodide when potassium iodide and hydro-
chloric acid are added to reduce the arsenic solution. This color
makes impossible the exact removal of iodine by sodium thio-
sulphate but if the analysis is performed carefully as described
below, this difficulty will disappear.
Determination of Total Arsenic and of Copper in Paris Green without
Distillation.—To 2 gm of Paris green in a 250-cc flask add about 100 cc
of a 2-per cent solution of sodium hydroxide. Boil until all the green
compound has been decomposed and only red cuprous oxide remains.
Cool, filter into a 250-cc volumetric flask, washing the paper well, dilute
to the mark and mix well. Reserve the cuprous oxide on the filter for the
copper determination.
Measure two or three portions of 50 cc each of the solution into 250-cc
flasks and concentrate by boiling to about half the original volume. Cool to
60°, add 10 cc of concentrated hydrochloric acid and 1 gm of potassium
iodide. Mix and allow to stand for 10 minutes. From a burette carefully
add sodium thiosulphate solution until the iodine is all reduced. Starch
should not be added but care should be exercised in reaching the end point.
If a persistent yellow color (see above) develops at this point, use starch
solution on a test plate as an outside indicator, touching drops of the titrated
solution to the starch. If the end point has been passed, add iodine solution
until the iodine-starch reaction is barely produced. Allow to stand for 5
1 U. S. Dept. ofAgr., Chem. Bull, 107.