Grains.
Acetic acid (anhydrous) 11·59
" " (hydrated or glacial) 13·64
Citric acid (crystallised) 15·23
Hydrochloric acid (dry or gaseous) 8·29
" " (sp. gr. 1·16) 24·78
Nitric acid (anhydrous) 12·27
" " (sp. gr. 1·5) 15·23
" " (sp. gr. 1·42) 20·45
Oxalic acid (crystallised) 14·32
Sulphuric acid (anhydrous) 9·09
" " (sp. gr. 1·845) 11·14
Tartaric acid (anhydrous) 15·00
" " (crystallised) 17·05
2. A convenient modification of the preceding method of acidimetry
consists in using the common apparatus figured in the margin and employing
fused chloride of calcium to dry the evolved carbonic acid gas, instead of
concentrated sulphuric acid. The mode of conducting the process and
obtaining the results is precisely the same as in that last explained, and
need not, therefore, be repeated. In this case, however, suction must be
applied to the small tube (_g_), instead of (_d_) in the accompanying
engraving.
_Obs._ These methods, though apparently complicated, are not difficult to
perform, when once well understood. The application of heat after the
completion of the operation is indispensable, as, if it were neglected,
from 0·3 to 0·4 of a gr. of carbonic acid would be retained in the liquid.
The bicarbonate of soda must be pure, and perfectly free from any neutral
carbonate or sesquicarbonate of soda. To ensure this, the bicarbonate of
commerce is reduced to a uniform powder, put into a glass jar, and covered
with its own weight of cold distilled or rain water, and allowed to stand
for twenty-four hours, with frequent stirring. It is then placed upon a
funnel, the tube of which is stopped with loose cotton, so as to allow the
lye to drain off. It is next washed several times with small quantities of
cold distilled or rain water, and after being dried by pressure between
some sheets of blotting-paper, without the aid of heat, is kept for use in
a well-closed glass bottle. Before use, it may be tested to ascertain its
purity. If pure, it neither reddens turmeric paper, nor gives a brick-red
precipitate with a solution of bichloride of mercury. Pure bicarbonate of
potassa may be used instead of bicarbonate of soda; but in either case it
is always proper to use an excess, so as to leave some undecomposed
carbonate after the operation has ended. The presence of a little sodium
chloride or sulphate in the bicarbonate will not interfere in the least,
but the absence of every trace of neutral carbonate is a _sine quâ non_.
[Illustration:
(_a_) Wide-mouthed flask, containing the sample for examination,
hermetically stopped by the cork (_e_) and supporting the
tubes (_b_) and (_c_).
(_b_) Bulbous tube, containing fragments of fused chloride of
calcium, terminating in a capillary tube (_g_).
(_c_) Bent tube, reaching nearly to the bottom of the flask (_a_).
(_d_) Small tube containing bicarbonate of soda.
(_e_) Cork fitting bottle (_a_), and the tubes (_b_) and (_c_),
hermetically.
(_f_) Silken thread, suspending the small tube (_d_).]
The two above methods of estimating the amount of acid are only superior
to the generally used methods first described, when the presence of
colouring matter interferes with the reaction of the litmus used to show
the point of neutralisation.
_Observations._ When great accuracy is required in conducting the
neutralisation of the solution in estimating volumetrically with litmus as
an indicator, it is proper to prepare and keep standard solutions of
sulphuric acid and oxalic acid, with which occasionally to try the
alkaline test-liquor. The only difficulty in the process is to avoid
over-saturation of the acid-sample. Great care must be taken not to exceed
the precise point of neutralisation of the acid. After adding each portion
of the test-liquor, the solution should be well stirred up, and as soon as
the effervescence becomes languid the greatest caution must be observed in
adding more. The proper point is arrived at when the liquor ceases to
redden litmus, and does not alter the colour of turmeric paper; if it
turns the latter brown, too much of the test-liquid has been added, and
the operation becomes useless. Towards the end of the experiment, when
great precision is required, a gentle heat may be applied, in order to
expel the free carbonic acid in the liquor; but otherwise this is
unnecessary. The peculiar soapy odour gradually acquired by the liquor as
it nears saturation will materially assist the operator when testing
vinegars, and some of the other vegetable acids. A good method is to tint
either the acid-sample or the test-liquid with a few drops of litmus, as
noticed under ACETIMETRY; when the reddish shade will gradually deepen
into 'purple,' or the purple into 'red,' as the point of saturation is
approached; and the blue colour will be perfectly restored as soon as this
point is reached. Dr Ure recommends keeping the ammonia-test ready tinged
with litmus, and the same applies to other test-liquors.
In commerce, the strength of acids is frequently reckoned with reference
to a standard, termed 100 acidimetric degrees. This is taken from the
circumstance that 91 gr. of commercial oil of vitriol, of a sp. gr. of
1·845, exactly saturate 100 gr. of dried carbonate of soda. An acid
requiring only 35, 50, or any other number of grains of the carbonate to
saturate it, is in like manner termed of so many degrees strong; the
number of grains representing in each case an equal number of degrees.
This method originated with the French chemists, and though only
conventional, and principally confined to commercial purposes, is
especially adapted to practical men but little conversant with chemistry,
yet very ready in retaining or calculating anything on the centesimal
scale, from its similarity to monetary language and reckoning.
=ACID'ITY.= _Syn._ ACID'ITAS, L.; ACIDITÉ, Fr.; SÄURE, Ger. In
_chemistry_, the state of being acid. In _physiology_, &c., the impression
given to the organs of taste by tart or acid substances. Sourness. See
FERMENTATION, MALT-LIQUORS, WINES, &c.
=Gas'tric Acidity.= Acidity of the stomach; a common and well-known
symptom of weak or disordered digestion.
_Treat., &c._ Small doses of absorbents or antacids, three or four times
daily, to which some tonic bitter, as calumba, cascarilla, chamomile,
gentian, or orange-peel, may be added. Stomachic stimulants, as capsicum,
ginger, mustard, or wine, &c., taken with, or after, meals, are also
useful. The diet should be light and nutritious; and acescent vegetables,
over-ripe fruit, and weak new beer or other liquors avoided as much as
possible. The bowels should be kept regular, but not open, by the
occasional use of mild aperients, as rhubarb, aloes, castor oil, senna, or
mercurial pill, or compounds containing them. Excessive looseness or
diarrh[oe]a may be checked by a few doses of carbonate of soda,
chalk-mixture, or astringents.
In INFANCY this affection is usually accompanied by restlessness,
continual crying, drawing up of the legs forcibly towards the body,
hiccups, vomiting, diarrh[oe]a, sour eructations, griping pains, green
stools, and debility; often followed, when the irritation is considerable,
by convulsions. The treatment consists in relieving the bowels of all
offending matter by a few doses of rhubarb-and-magnesia. The looseness or
diarrh[oe]a may be checked by a few small doses of carbonate of soda or
chalk mixture; or better, in an infant which is fed by lime-water (1 or 2
fl. oz.) mixed with as much milk. Two or three drops of caraway, cinnamon,
dill, or peppermint water, on sugar (not with the food) will tend to
promote the expulsion, and prevent the undue generation of gases. The
flatulence usually disappears with the acidity. The occasional
administration of 1 to 3 gr. of quicksilver-with-chalk ('grey powder'),
will frequently remove the complaint, and prevent its recurrence, when all
other means fail. The diet of both nurse and infant should be carefully
regulated.
See ANTACIDS, DYSPEPSIA, &c.
_Treatment for Horses._ Alkalies, their carbonates and bicarbonates;
alterative doses of aloes with alkalies; chalk, carbonate of magnesia;
mineral acids; bismuth, arsenic, nux vomica, or strychnia.
=ACIDS, EFFECTS OF, ON VEGETATION.= This subject has been ably
investigated of recent years by Dr Angus Smith and Mr Rothwell, and the
practical importance of their labours is shown by the circumstance that an
Act of Parliament passed in 1875 renders it penal for the proprietors of
alkali works to condense not less than 95 per cent. of the hydrochloric
acid evolved in the process of manufacturing 'soda,' also to allow air,
smoke, or chimney gases to escape into the atmosphere containing more than
one fifth of a grain of hydrochloric acid per cubic foot. Every owner of
an alkali work is likewise required to 'use the best practical means of
preventing the discharge into the atmosphere of all other noxious gases
arising from such work, or of rendering such gases harmless when
discharged.'
The injurious effects of acids on vegetation are indicated chiefly by the
shrivelled-up appearance which the leaves of herbage, trees, &c., exhibit
in the vicinity of chemical works in which the condensation of noxious
gases (hydrochloric acid, sulphurous acid, sulphuric acid, sulphuretted
hydrogen, nitric acid, and oxides of nitrogen and chlorine) is not
effectually carried out. According to Mr Rothwell, 'in fields exposed to
acid vapours handfuls of dead grass may be pulled up in the spring,
smelling strongly of the vapour, and that trees, under similar influences,
become bark-bound.'
The following is a list of trees arranged in the order of their
susceptibility. (Rothwell.)
_Forest Trees._ Larch, spruce fir, Scotch fir, black Italian poplar,
Lombardy poplar, ash, oak, elm, birch, alder, sycamore.
_Fruit Trees._ Damson, greengage, Halewood plum, Jacob plum, pears,
apples, cherries.
_Shrubs, Evergreens, and Wild Plants._ British laurels, Portugal laurels,
_Aucuba_ _Japonica_, Barberry evergreen, hazel, guelder rose, sloe thorn,
hawthorn, raspberries, gooseberries, blackberries, gorse, hollies.
_Farm Crops._ Potatoes, mangel, white clover and rhubarb, red clover,
trefoil, rye-grass, wheat, oats, barley, common turnips, swedes.
_Second list of Plants affected by Noxious Vapours, mixing the classes
according to the effects produced on each._