304 QUANTITATIVE AGRICULTURAL ANALYSIS
The calcium thiosulphate thus formed is largely decomposed by
boiling, calcium sulphite and free sulphur being formed:
CaS203 -» CaS03 + S. . (2)
The free sulphur formed in reaction (2) is dissolved by calcium
tetrasulphide to form pentasulphide.
CaS4 + S -» CaS5. (3)
The insoluble sludge remaining consists of a mixture of calcium
sulphite and some calcium sulphate, the latter being formed by
oxidation of sulphite.
Extensive investigations on the fungicidal value of sulphur
of polysulphides were carried on by Syre, Solmon and War-
mall, 1 using the hop-mildew at its most resistant stage as their
standard. They have expressed the opinion that the fungicidal
value depends upon the percentage of polysulphide sulphur in
solution, rather than the total sulphur content.
Lime-sulphur solutions, either upon standing exposed to air
or after being sprayed, slowly react with oxygen, forming calcium
thiosulphate and free sulphur:
2CaS5 + 302 -> 2CaS203 + 3S2
Determination of Total Sulphur.—Weigh a closed weighing bottle then
add about 10 cc of the lime-sulphur solution, close and weigh again. Rinse
into a 250-cc volumetric flask and dilute to the mark with recently boiled
and cooled distilled water and mix thoroughly. Dissolve 2 to 3 gm of
sodium peroxide in 50 cc of cold distilled water in a 250-cc beaker. Pipette
10 cc of the prepared lime-sulphur solution to this solution, keeping the tip
of the pipette just under the surface of the solution until it is to be raised
for drainage at the end of the process. Cover immediately with a watch
glass and warm on a steam bath with frequent shakings until the sulphur is
oxidized to sulphate (the yellow color having disappeared), adding more
sodium peroxide if necessary. Dilute to 25 cc, acidify with hydrochloric
acid, evaporate to dryness, treat with 25 cc of water acidified with 5 cc of
hydrochloric acid, boil and filter to remove silica if present. Dilute the
filtrate to about 200 cc and heat to boiling. Add a drop of methyl red then
neutralize with sulphur-free ammonium hydroxide. Add 1 cc of approxi-
mately normal (1 to 10) hydrochloric acid, then add 10 to 25 cc (as found
to be necessary) of 10-per cent barium chloride solution, slowly from a
pipette, stirring constantly. Digest on a steam bath until the precipitate
1 /. Agr. Sci., 9, 283 (1919).