Cooley's Cyclopaedia of Practical Receipts Volume I Part 9

=ACID=, _Syn._ ACIDUM, L.; ACIDE, Fr.; ACIDO, Ital.; SaURE, G. In familiar language, any substance possessing a sour taste. In _chemistry_, substances are said to be acid, or to have an acid reaction, when they are capable of turning blue litmus red. In _chemistry_, also, the term acid is applied to a very large cla.s.s of compounds containing hydrogen (hydrogen salts), and in which one or more atoms of that element may be replaced by an equivalent quant.i.ty of a metal or other basic radical; _e.g._--

1. The one atom of hydrogen in hydrochloric acid (HCl) may be replaced by sodium, producing the salt sodium chloride (NaCl).

2. The one atom of hydrogen in nitric acid (HNO_{3}) may be replaced by silver, producing the salt silver nitrate (AgNO_{3}).

3. One atom of hydrogen in acetic acid (HC_{2}H_{3}O_{2})[6] may be replaced by the basic radical ammonium (NH_{4}), producing the salt ammonium acetate (NH_{4}C_{2}H_{3}O_{2}).

[Footnote 6: Symbols indicating the number of atoms of replaceable hydrogen occupy the foremost position in the formulae of acids, as shown in the text.]

Acids which, like those mentioned in the foregoing examples, contain one atom of replaceable hydrogen are called mon.o.basic; those which contain two such atoms (_e.g._ sulphuric acid, H_{2}SO_{4}; tartaric acid, H_{2}C_{4}H_{4}O_{6}),[7] dibasic; those which contain three such atoms (_e.g._ phosphoric acid, H_{3}PO_{4}; citric acid, H_{3}C_{6}H_{5}O_{7}),[7] tribasic; and so on with acids of higher basicity. Acids of greater basicity than unity are frequently termed polybasic.

[Footnote 7: See footnote, p. 26.]

Besides containing replaceable or basic hydrogen, acids are further characterised by the property of combining with alkaloids to form salts; _e.g._--

Sulphuric Acid. Quinia.

H_{2}SO_{4} + 2C_{24}H_{24}N_{2}O_{2} =

Quinia Sulphate.

(C_{20}H_{24}N_{2}O_{2})_{2} . H_{2}SO_{4}

Acetic Acid. Morphia.

HC_{2}H_{3}O_{2} + C_{17}H_{19}NO_{3} =

Morphia Acetate.

C_{17}H_{19}NO_{3} . HC_{2}H_{3}O_{2}

=Dibasic Acids.= See ACID.

=Fatty Acids.= Acids separable from fats or oils; _e.g._ stearic acid, oleic acid, butyric acid, &c.

=Inorganic Acids.= Same as MINERAL ACIDS (which _see_).

=Mineral Acids.= Acids chiefly or wholly derived from the mineral kingdom.

In _medicine_, sulphuric, hydrochloric, and nitric acids, are commonly so called.

=Mon.o.basic Acids.= See ACID.

=Organic Acids.= Acids formed by, or derived from organic substances; _e.g._ acetic acid, tartaric acid, uric acid, &c.

=Polybasic Acids.= See ACID.

=Pyro-acids.= Acids resulting from the decomposition by heat of other acids, _e.g._ gallic acid, when heated, yields pyro-gallic acid.

=Tribasic Acids.= See ACID.

=ACIDIFICA'TION.= [Eng., Fr.] _Syn._ ACIDIFICA'TIO, L. In _chemistry_, the act, process, or state of acidifying, or of making, becoming, or impregnated with acid.

=ACIDIM'ETER.= _Syn._ ACIDOM'ETER; ACIDIME'TRUM, &c., L.; ACIDIMeTRE, Fr.

An instrument or apparatus employed in acidimetry.

The ordinary acidimeters of the chemist are small tubes, constructed to hold exactly 1000 grains of distilled water, at 60 Fahr., within the limits of their scale, which is accurately graduated into 100 divisions.

They are used to contain the alkaline solutions (TEST-LIQUORS, NORMAL or STANDARD SOLUTIONS) employed in the following processes.

Beaume's Acidimeter, and others of the same cla.s.s, are HYDROMETERS, and are described under that 'head.'

=ACIDIM'ETRY.= _Syn._ ACIDOM'ETRY; ACIDIME'TRIA, &c., L.; ACIDIMeTRIE, Fr.

The estimation of the strength or quant.i.ty of acid, in a free state, contained in any liquid. It is the reverse of 'alkalimetry.' Acidimetrical a.s.says are understood to refer to the relative strengths of the same acids (_i. e._, the quant.i.ty of real acid of the same kind contained in the solutions examined), and not to the comparative strengths of acids of different composition or names.

_Acidimetrical processes._ These are founded chiefly on the capacity of the acids to saturate the bases; and, in some of the liquid acids, on the specific gravity.

_a._ VOLUMETRICALLY:--

1. The sample of the acid to be examined (100 gr., or any convenient aliquot part thereof) is placed in a suitable gla.s.s vessel, and if it be one of the stronger acids, diluted with six or eight times its weight of water, or if solid (as oxalic, or citric acid), dissolved in a like quant.i.ty. This liquid is then exactly neutralised with an alkali.

This point is usually determined, by the addition of a small quant.i.ty of litmus solution, which turns just blue when the solution is neutralised, but when a carbonate is used for the alkaline solution, the acid must be boiled a short time after each addition to expel the carbonic acid. The quant.i.ty of the alkaline solution consumed for this purpose represents an equivalent quant.i.ty of acid, and thus gives us the acid content of the sample under examination. The common practice is to dissolve one equivalent of the alkaline test in grains or grammes in water, and to make up the solution to exactly 1000 parts by measure (_i. e._, 1000 'water-grains' or grammes), so as to accurately fill the 100 divisions of an acidimeter; when the quant.i.ty, in grains or grammes, of the sample tested, bears the same proportion to the equivalent number of the acid under examination, that the number of acidimeter divisions of the test-liquor consumed bear to the per-centage of acid sought.

Thus:--suppose 50 gr. of a sample of sulphuric acid take 25 acidimeter divisions (300 parts or water-grains measure) of the test-liquid to neutralise it, what is its content of real acid?

The equivalent of sulphuric acid is 49 (half its atomic weight); so, by the rule of proportion,

50 : 49 :: 25 : 24-1/2

It therefore contains 24-1/2 parts of real sulphuric acid, in 50.

If the 1000 parts or grain-measures, instead of the number of the acidimeter divisions, be taken for the calculation, it will, of course, be necessary to point off the first right-hand figure of the result as a decimal. Thus; repeating the above example--

50 : 49 :: 250 : 245

Or, since the equivalent of the test-liquid is 100, it will bear the same proportion to the equiv. of the acid examined as the number of the acidimeter divisions of the test-liquid consumed in neutralising 100 gr., do to the per-centage sought. Thus:--50 gr. of hydrochloric acid take 45 acidimeter divisions to effect neutralisation, what is its real strength?--The equiv. of hydrochloric acid is 365: therefore--

100 : 365 :: 45 : 16425%

and, since only 50 gr. (instead of 100 gr.) were examined--

16425 2 = 3285%

Some operators prefer employing 100 gr. instead of the equivalent weights of the given tests in making their test-solutions, in which case each gr.

or 1000th part represents 1/10th, and each acidimeter degree 1 gr. of the alkali or carbonate employed; when a similar proportion will obtain to that first above given.

In technical a.n.a.lysis it is more convenient if the number of acidimeter divisions of the 'test-liquid' consumed express the per-centage strength of the acid, without further calculation. For this purpose the number of grains of the acid taken for the a.s.say should correspond to the equivalent number of such acid (see _Table_ I, below); or to some convenient aliquot part of it, as the 1/2, 1/4, 1/5, or 1/10th; the per-centage answer, in the last case, being doubled, quadrupled, &c., according to the aliquot part taken. The reason of this is obvious.

For the test-solutions, ammonia, and the dry and crystallised carbonates and bicarbonates of potash and soda, are used, and are made by dissolving in water their const.i.tuents except ammonia, of which 1000 grains, or one litre, of solution of specific gravity 0992 contains exactly one equivalent.

53 grains (or grammes) of pure anhydrous carbonate of soda, prepared by gradually heating to redness the crystallised salt, const.i.tute one equivalent (half the atomic weight), and 69 grains (or grammes) of pure dry carbonate of potash. Of the crystallised salt 143 grains of carbonate of soda will be required, and 84 grains (grammes) of the crystallised bicarbonate of soda, and 100 of the crystallised bicarbonate of potash.

Occasionally solutions containing in one thousand parts, 50 of pure carbonate of lime or chalk, or 28 of pure caustic lime, are used.

Besides these, a process known as Kiefer's is practised, and an ammoniacal solution of oxide of copper is employed as the 'test-liquor,' and the 'point of neutralisation' is known by the turbidity observed as soon as the free acid present is completely saturated.

The normal solution or test-liquor is prepared by adding to an aqueous solution of sulphate of copper, pure ammonia water, until the precipitate, which at first forms, is just redissolved, carefully avoiding excess. Or better, by adding a rather strong solution of sulphate of copper, to a quant.i.ty of a rather strong solution of ammonia containing exactly 17 gr., or one equiv. of pure ammonia, as long as the precipitate which forms is redissolved on agitation; the resulting liquid being afterwards diluted with pure distilled water, until it accurately measures 1000 water-grains, or fills 100 divisions of an acidimeter, at 60 Fahr. In either case, the strength of the resulting 'test-solution' must be carefully determined by means of standard sulphuric acid, and adjusted, if necessary.