Solutions of hypochlorite do not keep indefinitely, they change very slowly in the dark, much more quickly in the light. Daufresne studied the influence of light in the following manner.

Portions of the same solution of known strength being placed in two flasks, one flask was left on the laboratory table exposed to diffused light, while the other was kept in a cupboard. He ascertained that the activity of the solution sheltered from the light had not sensibly varied, whilst the first had lost about 20 per cent. of its hypochlorite.

After

Loss in one month.

0 day.

7 days.

15 days.

21 days.

30 days.

Solution kept in the" light, titration. . .

0.505

0.497

0.452

0.4II

0.380

247 %

Solution kept in the" dark, titration. . .

0.505

0.505

0.502

0.500

0.497

1.4%

When the mass of liquid is considerable, the alteration is extremely slow. Daufresne kept a solution of hypochlorite of 0.502 per cent. of ClONa in a wicker-covered carboy of black glass, 25 litres, without any special precautions as regards light. At the end of 3 1/2 months the titration gave - 0.493 per cent. of ClONa, a loss practically negligible.

What becomes of the hypochlorite? One cannot say with certainty. By analogy with what happens, under the influence of heat, it is thought that the hypochlorite tends towards its two stable forms, chloride and chlorate of sodium:

4C1ONa = 3NaCl + C1O3Na + O

Obviously, this reaction implies a release of oxygen, which is sometimes lacking. Besides, it does not explain all the facts observed. It is sufficient to remember that, in practice, it is better to keep the solutions away from the light, and still more important to renew them as frequently as possible, every ten or fifteen days at least.

Like the chemical hypochlorite, the electrolytic hypochlorite must be kept away from the light. Even under these conditions it rapidly degenerates when it is pure. But the addition of permanganate of potash or an alkaline salt greatly improves its keeping qualities. The following table gives the results of a series of tests carried on over a period of three weeks:-

Titration in ClONa % on the

1st day.

10th day.

20th day.

I.

Electrolytic hypochlorite kept in the light.........

0.502

0.404

0.229

2.

Electrolytic hypoclorite kept in dark.

ness...........................................

0.502

0.422

0.294

3.

Electrolytic hypochlorite with the addition of 1: 20,000 of permanganate of potash, kept in the light...

0.502

0.482

0.398

4.

Electrolytic hypochlorite with the addition of I: 20,000 of permanganate of potash, kept in darkness

0.502

0.485

0.365

5.

Electrolytic hypochlorite with the addition of 0.2 per cent. of silicate of soda, kept in the light ....

0.502

0.492

0.484

6.

Electrolytic hypochlorite with the addition of 0.2 per cent. of silicate of soda, kept in darkness ...

0.502

0.500

0.500

Inspection of this table shows that electrolytic hypochlorite can be so far stabilised as to undergo no alteration of importance for a period of two or three weeks.