Bleaching powder should contain about 35 per cent. of available chlorine, but the percentage frequently falls below this amount, especially in warm weather.
Great care should be exercised in the sampling of bleaching powder, as indeed of all chemicals, in order to insure an average result. Small portions should be taken from different parts of the bulk; the whole should then be carefully mixed, and, if necessary, reduced to powder. Portions of the mixture should be taken, pounded, and again thoroughly mixed.
If this process be repeated once or twice, a perfectly uniform sample will be obtained.
Alum, Sulphate of Alumina, Alum Cake, &c.
- (1) The alumina is estimated as such by precipitation with ammonia. Ferric oxide is also thrown down with alumina, and must therefore be deducted from the amount of the mixed oxides obtained.
- (2) The percentage of iron oxide present is determined in the following way. A considerable quantity of the sample under examination is dissolved in water, and to the boiling solution a large excess of caustic potash solution is added. The caustic potash precipitates both the alumina and ferric hydrates, the former, however, redissolves. The solution is filtered, and the precipitate of ferric hydrate washed, dissolved in dilute hydrochloric acid, and reprecipitated with excess of caustic potash. It is again thrown on to a filter, washed, redissolved in hydrochloric acid, and again reprecipitated with ammonia. In this way it is obtained free from alumina.[17]
- [17] For a method of estimating minute quantities of iron in alums see ‘Journ. Soc. Chem. Ind.,’ April 1887, p. 276.
- (3) Free Acid.—This can be estimated indirectly, i.e. by estimating the alumina and ferric oxide, calculating the amount of sulphuric acid necessary to combine with them, and deducting this from the total amount of sulphuric acid obtained. In the case of alum it is of course necessary to make due allowance for the sulphate of potash or ammonia present. The sulphuric acid is estimated as barium sulphate in the usual way.
- It should be borne in mind that sulphates of alumina are sometimes met with containing uncombined alumina. A convenient method of estimating free acid consists in digesting a weighed quantity of the finely powdered sample in strong alcohol. The alcohol dissolves away the free acid, which can be estimated in the solution by means of a standard solution of alkali.
- Free acid can be detected by means of a solution of {208} Congo-red, which is turned blue with free acid, but not with pure sulphate of alumina.
Antichlor, Sodium Thiosulphate, Sodium Sulphite, &c.
The same solution serves also for the estimation of sulphurous acid in sodium and other sulphites.
Starches.
When perfectly cold, the “stiffness” of the pastes should be compared. This may be done by noting the relative resistance to weights placed upon their surface. Information on this subject is contained in a paper by W. Thomson in the ‘Journ. Soc. Chem. Ind.,’ March 1886.
The identification of starches by means of their microscopical appearances is tolerably simple, as each particular kind possesses characteristic differences.