A treatise on chemistry. Vol. 1, The non-metallic elements / by Sir H.E. Roscoe & C. Schorlemmer ...

  • Henry Enfield Roscoe
Date:
1881
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Licence: Public Domain Mark

Credit: A treatise on chemistry. Vol. 1, The non-metallic elements / by Sir H.E. Roscoe & C. Schorlemmer ... Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

    192/792 (page 176)
    sulphate, a decorniDosition which is rendered visible by the change of colour from the original red to a deep gi-een, thus:— 2Cr03 + 3H2SO, = Cr^ (SOJg + SH^O + O3. 85 (8) Oxygen can be obtained by the decomposition of bleach- ing powder (Mitscherlich, 1843 ; Fleitriiann, 1865). For this pre- paration a clear concentrated solution of bleaching powder—a substance containing calcium hypochlorite, CaCljOg—is placed in a flask, and a few drops of cobalt chloride added. Cobalt oxide is precipitated on heating the mixture to about 80°, and a rapid effervescence of oxygen occurs. The cobalt oxide, which is formed, is left unchanged after the operation, and may be employed again; it probably acts, like the manganese dioxide, by the formation of a higher oxide, which is again quickly reduced, the oxygen being liberated as a gas, thus:— - CaCl.,0, + 4CoO = CaCl^ + 2Co,03, Co.Og - 2CoO + 0. Instead of a clear solution, a thick paste of bleaching powder may be used, with the addition of a little cobalt salt and a small quantity of paraffin oil, to prevent the frothing which usually occurs. Tiie best temperature for the evolution of gas is from 70°-80° C. CaCl, + CaClg 0.^ = 2CaC]2 + 0.,. The same decomposition and the replacement of chlorine for oxygen may be shown in a striking manner by passing chlorine gas, generated in a flask from manganese dioxide and hydro- chloric acid into a second flask, which contains boiling milk of lime, to which a little copper nitrate solution has been added. Oxygen gas is then liberated in the second flask, and may be collected as usual. The following equation explains the replacement, and we see that two volumes of chlorine yield their equivalent, or one volume of oxygen. CI2 -I- Ca (IIO), = CaClg -h H,0 + 0. 86 (9) In order to prepare oxygen cheaply on (he large scale several other processes luive been suggested. Amongst them tlia following appears to be the most useful:—