Focus; and in the path of the light is placed a glass beaker containing a dilute solution of sodium thiosulphate (hyposulphite of soda).
The mixture is then cooled, acidified by means of sulphuric acid, and titrated with decinormal sodium thiosulphate solution.
The mixed solution of poiysulphides and thiosulphate of calcium thus produced is clarified, diluted largely, and then mixed with enough of pure dilute hydrochloric acid to produce a feebly alkaline mixture when sulphur is precipitated.
The addition of more acid would produce an additional supply of sulphur (by the action of the H2S203 on the dissolved H 2 S); but this thiosulphate sulphur is yellow and compact, while the polysulphide part has the desired qualities, forming an extremely fine, almost white, powder.
It may also be obtained by heating carbon, sulphur and many metals with concentrated sulphuric acid: C + 2H 2 SO 4 = 2SO 2 }- CO 2 + 2H 2 O; S + 2H 2 SO 4 = 3S0 2 + 2H 2 0; Cu + 2H 2 SO 4 = SO 2 -fCuSO 4 + 2H 2 0; and by decomposing a sulphite, a thiosulphate or a thionic acid with a dilute mineral acid.