INSECTICIDES AXD FUNGICIDES

of the arsenates of calcixun are quite stable:
arsenate, Ca3(As04)2 and dicalcium arsenate, CaHAsO4.

tri calcium arsenate may be made in two ways, as follows:

7*

e

3CaHAs04 + 2NaOH-» Ca3(AsO4)a + Na*HAsO4 +2HaO;
2H3As04 4- 3Ca(OH)a — » Cas(As04)2 + 6H2O.

arsenate dissolves in water to the extent of 0.33
In 100 cc at 25° while tricalcium arsenate is soluble to the

extent of only 0.014 gni at the same temperature. The sola-
bility of the first salt Is so large that there Is danger of damage
when it is applied to tender foliage. Also, unless it has
prepared with c4arCj it may contain quantities of the easily
soluble disodium arsenate, as shown in Eq. (1). This has been
largely overcome by adding an excess of lime water, whirh
reacts with any dicalcium arsenate or disodium arsenate to form
the less soluble tricalcium salt. 1 The powdered calcium arsenates
on the market contain approximately 52 per cent of arsenic,
calculated as pentoxide, while the paste contains less^ according
to the proportion of water retained.

IDetermination of Total Arsenic. — Proceed by the distillation method as
with Paris green, using 2 to 2.5 gm of sample. Calculate as the pentoxide.

LIME-SULPHUR SOLUTION
This spray is important in the control of San Jose and
scales. It is effective also In the extermination of numerous
insects. This is especially true when it is combined with
arsenate and nicotine and it Is used then for the simultaneous
destruction of many sucking and chewing insects and of fungus
diseases. The standard lime-sulphur solution consists of cal-
cium tetrasulphide, pentasulphide and thiosulphate in a water
solution. It is produced by boiling lime water containing
sulphur. The probable reactions are generally understood to be
iFts follows:
3Ca(OH)2 + 10S -» 2CaS4 + CaS203 + 3H2O. (1)
i See also RKEDV and HAA«, J. I nd. Eng. Chem., 13, 1038 (1921).