The presence of iron is indicated by a yellow colour, and may be confirmed by the usual tests as in sulphuric acid.
Oxalic acid should be pure white and soluble in distilled or rain-water. 6·3 grm. may be weighed out, and made up to 200 c.c. If 20 c.c. of the solution for a test be used, each c.c. of normal soda solution equals 10 per cent. of pure crystallised acid, C2O4H2 + 2 Aq. The end-reaction with methyl orange is rendered sharper by the addition of a few drops of neutral calcic chloride towards the end of the titration.
Acetic acid may be similarly determined, each c.c. of normal alkali being equivalent to 0·06 grm. of C2H4O2. Caustic soda, or lime-water and litmus, give sharper results than sodic carbonate and methyl orange. Brown pyroligneous acid is difficult to test from the dark compounds formed with soda, but may be indirectly determined by the quantity of marble, baric carbonate, or magnesia which it will dissolve (compare [p. 100]), or very possibly by lime-water like tan-liquors with a little tannin as indicator.
EXAMINATION OF LIME AND LIME-LIQUORS.
The quantity of caustic lime in either quicklime or lime-bottoms may be determined by weighing a quantity of the finely powdered material containing not more than 1 grm. of caustic lime, and shaking it thoroughly with 1 litre of distilled water and filtering. 100 c.c. should be taken, and decinormal acid, sulphuric or hydrochloric (or if oxalic, with addition of neutral calcic chloride, or with litmus instead of methyl orange as indicator). Each c.c. of decinormal acid corresponds to 0·0028 grm. of CaO. If the filter and residue be treated with sufficient normal acid to dissolve the whole of the carbonates, and then titrated back with normal sodic carbonate and methyl orange, the loss (less soda solution required than acid was originally employed) is equal to the carbonate of lime and carbonate and hydrate of magnesia present. 1 c.c. of normal acid = 0·05 grm. of CaCO3.
Fig. 17.
