X Axr> FTXGICIDES

settles readily, then filter through quantitative filter paper. Wash until
free from chlorides and burn tht* paper in an inclined weighed crucible at a W
temperature (not above dull redness). When the precipitate is white, cool
and. \veigh. Calculate the sulphur from the weight of barium sulphate.
Corrections should lie made for any sulphur present in the reagents, deter-
mined by a blank experiment. Sodium peroxide, especially, is liable to
contain sulphates.
Determination of Total Sulphide Sulphur. — Dissolve 50 gm of zinc chlo-
ride in about 500 cc of water and add 125 cc of concentrated ammonium
hydroxide, which will redissolve the precipitate first formed. Add 50 gm
of ammonium chloride and dilute to about 1 liter.
Pipette 10 cc of the lime-sulphur solution (freshly made as for the
total sulphur determination) Into a 250-cc beaker, dilute to 100 cc and add
ammoniacai zinc solution until the sulphide sulphur is all precipitated, as
indicated by the failure of a drop of the clear solution to darken a few drops
of dilute nickel sulphate solution. Filter immediately, wash the precipitate
thoroughly with cold water and return it and the filter to the beaker. Cover
with water, disintegrate with a glass rod and slowly add about 3 gin of
sodium peroxide, keeping the beaker well covered with the watch glass.
Warm on the steam bath, with frequent shaking, until all of the sulphur is
oxidized to sulphate and the precipitate is all dissolved, adding more sodium
peroxide if necessary. Make slightly acid with hydrochloric acid, filter to
remove shreds of filter paper, wash thoroughly with hot water, heat the
filtrate and washings to boiling and determine the sulphur as described for
total sulphur, neutralizing and acidifying in the same manner. Calculate
the per cent of sulphide sulphur in the sample.
Total Calcium. — The per cent of calcium in a lime-sulphur
solution will depend upon the character and purity of the lime
used in its preparation, as well as upon dilution and degree of
hydrolysis. It will vary over wide limits but as this element
is of relatively small importance in connection with insecticidai
properties, its determination is not often required. The fol-
lowing method is official:
Determination of Total Calcium. — To 25 cc of the lime-sulphur solution,
prepared as for the preceding determination, add 10 ce of concentrated
hydrochloric acid and evaporate to dryness on the steam bath. Add 25 cc
of water and. 5 cc of concentrated hydrochloric acid, warm until all of the
calcium chloride is dissolved and filter from sulphur and any silica that may
be present. Make slightly ammoniacai, boil and filter from iron and
aluminium hydroxides if these are produced. Heat to boiling and precipi-
tate the calcium with am&ionium oxalate solution and finish the determina-
tion as described on page 64 or 69, Part I. Calculate the per cent of calcium
oxide in the sample.
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