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



Bleaching powder was at first regarded as a molecular compound of lime and chlorine, "chloride of lime", CaO,Cl2. Balard in 1835 suggested that it was a mixture of equimolecular amounts of calcium hypochlorite and chloride: Ca(OCl)2 + CaCl2. Stahlschmidt assumed that it contained the compound Ca(OH)(OCl): according to Lunge, free lime is not an essential constituent, but is due to the particles of lime becoming encrusted with bleaching powder and so escaping complete chlorination. He prepared a product of the following composition:

CaOCl2,H2O91.80
CaCO30.95
CaCl20.45
Ca(OH)26.80
total100.00


Balard's formula, Ca(OCl)2 + CaCl2, would require that bleaching powder should contain a considerable proportion of free calcium chloride. If, however, it is treated with successive small amounts of water, the first portions of the extract contain much less chlorine as chloride than would be the case if the latter pre-existed in the powder. Again, alcohol extracts from good bleaching powder only a small amount of calcium chloride, although the latter is readily soluble in alcohol. Moist carbon dioxide at 70° sets free the whole of the available chlorine from bleaching powder, whilst it has no action on calcium chloride: CaOCl2 + CO2 = CaCO3 + Cl2.

These results agree with the formula proposed by Odling, according to which the active constituent of bleaching powder is a compound of the formula calcium chlorohypochlorite, caocl2, i.e., calcium chlorohypochlorite, formed from a molecule each of hydrochloric and hypochlorous acids:

calcium chlorohypochlorite formation

Stahlschmidt's formula, Ca(OH)OCl, is disproved by the fact that although bleaching powder containing as much as 39 per cent, of chlorine which can be liberated by acids, i.e., available chlorine, has been prepared, his formula limits this to 33 per cent.

O'Shea (1883) decided between the three rival formulae of Balard, Stahlschmldt aid Odling as follows. He removed any free calcium chloride by treatment with alcohol, and determined in the residue. (i) the total lime, CaO; (ii) the total chlorine; (iii) the chlorine as hypochlorite.

The residue after treatment with alcohol, and the above ratios, should be in the different cases:

ResidueCaO/total ClCaO/hypochlorite Clhypochlorite Cl/total Cl
BalardCa(OCl)21:21:21:1
StahlschmidtCa(OH)OCl1:11:11:1
OdlingCa(OCl)Cl1:21:11:2
Found1:21:11:2
Thus, only Odling's formula agrees with the experimental results.

Recent experimenters (Ditz; Neumann) consider that the free lime in ordinary bleaching powder is an essential constituent, and that combined water is also present. Normal bleaching powder is regarded as the compound 3Ca(OCl)Cl, Ca(OH)2, 5H2O. In complete absence of moisture a very hygroscopic compound 3Ca(OCl)Cl, Ca(OH)2, 3H, O is formed, whilst at low temperatures Ca(OCl)Cl, Ca(OH)2, H2O can be obtained. The active constituent is Odling's compound.


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