THE DETERMINATION OF CHLORINE IN BLEACHING POWDER BY IODINE AND STANDARD HYPOSULPHITE.

Apparatus, Reagents, etc.—As before. The student may take for analysis a sample of ordinary commercial bleaching powder.

Method Reactions.—If, to the milky liquid obtained by treating the powder with water, excess of KI be added, and then HC2H3O2 the chlorine is liberated from the bleaching powder, which consists largely of Ca(OCl)Cl, and sets free an equivalent quantity of iodine.

Ca(OCl)Cl + 2HC2H3O2 = Ca(C2H3O2)2 + H2O + Cl2
2KI + Cl2 = 2KCl + I2

This liberated iodine is then titrated with the hyposulphite, using starch as an indicator.

If the bleaching powder consisted wholly of Ca(OCl)Cl it contains 55% of available chlorine—that is, chlorine which can be liberated by treatment with dilute acid; but the commercial variety generally contains other substances, and does not yield more than 36% available chlorine.

The Analysis.—Weigh out 5 gms. of the bleaching powder and transfer to a porcelain mortar. Rub up with about 50 c.cs. water till of the consistency of a thin cream. Allow to settle for a few minutes, and pour off the liquid into a litre flask. Repeat with another 50 c.cs. of water, and again with another lot, till the whole of the sample is transferred to the flask. Dilute to the mark with water.

Shake well, and remove with the pipette 50 c.cs. Add excess of KI; about 16c.cs. of E/5 KI will do. Render acid with 5E. HC2H3O2. Titrate with the N/10. Na2S2O3, using the starch indicator as before.
Repeat the titration on another 50 c.cs. from the litre flask.

Calculation.—The N/10 Na2S2O3 solution has been standardised on pure copper. Assume it was found that 1 c.c.  N/10 Na2S2O3 = .00631 gm copper.

Theoretically 1 c.c. N/10 Na2S2O3 = .0127 gm. iodine = .0063 gm. copper = .00355 gm. chlorine.
Therefore .1 c.c. of the N/10 Na2S2O3 = .00355 x 631/630 = .003555 grams chlorine.
The percentage of chlorine is then calculated as usual.

Concluding Note.—The analyses given in this chapter have been selected both on account of their utility to the technical chemist and on account of the variety of practice afforded the student in such work. It must be remembered that only a few of the most important methods have been described. The student who desires to follow the matter further should consult the authorities previously mentioned (Sutton and Schimpf), and for their special application to the work of the metallurgical chemist he may consult Furman’s Manual of Assaying and Beringer’s Text Book.