Oxides And Oxy-acids Of Chlorine : Bleaching Powder, Available Chlorine

Bleaching powder is mainly employed as an oxidising agent, and the active agent is really nascent oxygen. Usually the chlorine equivalent of this active oxygen is returned as the available chlorine: O (16) = Cl₂ (71). If bleaching powder consisted entirely of the compound Ca(OCl)Cl, the chlorine equivalent of the active oxygen atom of the hypochlorite radical would be O = Cl₂, i.e., the total chlorine. This would, in fact, be wholly expelled by acids: CaOCl₂ + H₂SO₄ = CaSO₄ + H₂O + Cl₂, in accordance with the former definition of available chlorine.

Commercial bleaching powder always contains some free lime, as well as calcium chloride, CaCl₂, and possibly calcium chlorate, Ca(ClO₃)₂, and since the chlorine of these compounds is not liberated as such by acids, and the oxygen of the chlorate is not available for the usual oxidising purposes of bleaching powder, a distinction is made between the total and available chlorine.

The estimation of the available chlorine of bleaching powder is carried out by one of the following methods:

1. Penot's method: A suspension of bleaching powder is titrated with decinormal sodium arsenite solution, until a drop of the liquid, placed by means of a glass rod on a piece of filter-paper which has been soaked in potassium iodide and starch solution and dried, no longer gives a blue colour owing to liberation of iodine. The reaction is: As₂O₃ + 2CaOCl₂ = As₂O₅ + 2CaCl₂.

Thus As₂O₃ requires 2O or 4Cl (4Cl + 2H₂O = 4HCl + 2O), so that 1 c.c. of N/10 As₂O₃ = 0.00355 gm. of active Cl.

2. Bunsen and Wagner's method: A suspension of bleaching powder is treated with excess of potassium iodide solution, and acidified with acetic acid. Iodine is liberated: 2KI + HOCl + CH₃-COOH = CH₃-COOK (potassium acetate) + I₂ + H₂O + KCl. This is titrated with decinormal sodium thiosulphate solution until the yellow colour has practically vanished: 2Na₂S₂O₃ + I₂ = Na₂S4O₆ (sodium tetrathionate) + 2NaI. A little starch-paste is then added, and the titration continued until the "blue colour, due to the iodine, vanishes, 1 c.c. of N/10 Na₂S₂O₃ = 0.00355 gm. of active Cl.

Hypochlorous acid, or hypochlorites, are estimated in presence of free chlorine by means of the following reactions:

2KI + HOCl + HCl = 2KCl + I₂ + H₂O

2KI + Cl₂ = 2KCl + I₂.

Each molecule of HOCl neutralises one equivalent of acid, whilst chlorine does not affect the acidity of the solution. By titrating the iodine and the remaining acid, the amounts of HOCl and Cl₂ may be calculated.

Protein crystallography
Chemistry guide
Oxides And Oxy-acids Of Chlorine
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