A knowledge of the methods usually employed for the qualitative and quantitative analysis of the various chemical substances met with in paper manufacture is of considerable importance. The scope of this work will not allow us to enter fully into the necessary details of manipulation; for these the reader is referred to standard works on analysis; we will merely indicate the methods.

In testing recovered soda, it is necessary to boil the finely powdered ash for some time with water; unless this be done, the amount of alkali will probably be under-estimated, as part of the soda is present as a difficultly soluble silicate.

It is sometimes desirable to determine the amount both of sodium hydrate and carbonate in a sample of alkali. For this purpose an excess of a solution of barium chloride is added to a solution containing a known quantity of the substance under examination, and the mixture made up to a definite volume. It is then filtered, and an aliquot portion of the filtrate titrated with standard alkali. The filtrate contains only the alkali that was originally present as hydrate, the carbonate of soda having formed with the barium chloride, insoluble barium carbonate, and sodium chloride.

The method of estimating the amount of available chlorine in bleaching powder by determining the quantity of ferrous sulphate it is capable of oxidising is fallacious, as by this means calcium chlorate is included in the result.

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.

• (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.

The same solution serves also for the estimation of sulphurous acid in sodium and other sulphites.

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.

Raw materials intended for the preparation of gelatine may be similarly examined. They yield the whole of their gelatine on heating with water. The effect of prolonged heating on solution of gelatine should be borne in mind. The amount of residue left after treatment with water should be carefully determined.

The chemical examination is both difficult and somewhat misleading. A microscopical examination will often throw much light on their composition.

For comparing the tinctorial power of different samples, weighed quantities should be ground up with starch or some other white powder. In this way some idea of the intensity of colour can be readily arrived at.