SULPHURIC ACID. 
§ 7°*] 
99 
extracted by boiling water, and precipitated by ammonia, the end result 
being quinine hydrate (which may be filtered off and used again for similar 
purposes) and a sulphate, nitrate, or chloride of ammonia in solution. 
It therefore remains to determine the nature and quantity of the acids 
now combined with ammonia. The solution is made up to a known 
bulk, and portions tested for chlorides by nitrate of silver, for nitrates 
by the copper or the ferrous sulphate test, and for sulphates by BaCl 2 
solution. If sulphuric acid is present there will be a precipitate of 
barium sulphate, which, from its density and insolubility in nitric or 
hydrochloric acids, is very characteristic. For estimating the sulphuric 
acid thus found, a known bulk of the same liquid is heated to boiling 
after acidifying by hydrochloric acid, and a sufficient quantity of baric 
chloride solution added. Unless this exact process is followed, the 
analyst is likely to get a liquid which refuses to filter clear ; but if the 
sulphate be precipitated from a hot liquid, it usually settles rapidly to 
the bottom of the vessel, and the supernatant fluid can be decanted clear ; 
the precipitate is washed by decantation, and idtimately collected on a 
filter, dried, removed from the filter and burnt up in the usual way. 
The sulphate of baryta found, multiplied by *3434, equals the 
sulphuric anhydride. 
The older process was to dissolve the free sulphuric acid out by alcohol. 
As is well known, mineral sulphates are insoluble in, and are precipitated 
by, alcohol, whereas sulphuric acid enters into solution. The most valid 
objection, as a quantitative process, to the use of alcohol, is the tendency 
which all mineral acids have to unite with alcohol in organic combination, 
and thus, as it were, to disappear ; and, indeed, results are found, by 
experiment, to be below the truth when alcohol is used. This objection 
does not hold good if either merely qualitative evidence, or a fairly 
approximate quantation, is required. In such a case, the vomited matters, 
the contents of the stomach, or a watery extract of the tissues, are evapo¬ 
rated to a syrup, and then extracted with strong alcohol and filtered ; a 
little phenolphthalein solution is added, and the acid alcohol exactly 
neutralised by an alcoholic solution of clear decinormal or normal soda. 
According to the acidity of the liquid, the amount used of the decinormal 
or normal soda is noted, and then the whole evaporated to dryness, and 
finally heated to gentle redness. The alkaline sulphate is next dissolved 
in very dilute hydrochloric acid, and the solution precipitated by chloride 
of barium in the usual way. The quantitative results, although low, 
would, in the great majority of cases, answer the purpose sufficiently. 
A test usually enumerated, Hilger’s test for mineral acid, may be 
mentioned. A liquid, which contains a very minute quantity of mineral 
acid, becomes of a blue colour (or, if 1 per cent, or above, of a green) on 
the addition of a solution of methyl aniline violet ; but this test, although 
useful in examining vinegars (see Foods), is not of much value in 
