Cooley's Cyclopaedia of Practical Receipts Volume I Part 10

This method answers well with all the stronger acids (excepting oxalic acid), even when dilute; and it has the advantage of not being affected by the presence of a neutral metallic salt with an acid reaction, as sulphate of copper, or of zinc.

Besides this process a solution of lime in sugar may be used, as proposed by M. Peligot, and made as follows:--

Pure caustic lime is carefully slaked by sprinkling with water, and 50 grains (or grammes), made up by water to a milky solution, and 100 grains of pure sugar candy dissolved in 1000 grains of water, are added, and the whole well shaken. It is allowed to settle in a closed bottle, and the clear solution poured off and diluted, until 1000 grains neutralise exactly 100 grains of pure hydrochloric acid of sp. gr. 11812. Of course it only answers with acids whose calcium salts are readily soluble in water.

_b._ GRAVERMETRICALLY:--

The test-liquors or standard solutions of the above methods are made up so as to _weigh_ exactly 1000 grains, instead of to 'measure' 100 acidimeter divisions. Every grain of the test-liquor thus represents 1/10th gr. of alkali; and every 10 gr., 1 gr. of alkali; or respectively, 1/10th per cent. and 1 per cent. The vessel used for containing the solutions is carefully weighed whilst empty, and 1000 gr. being placed in the opposite scale, the test-solution, containing exactly one equivalent of base, is poured in, and the whole made up with distilled water (if necessary) so as to restore the balance to an equilibrium. After the process of neutralisation, the acidimeter, with its contents, is again placed in the scales; its previous weight still remaining there. The number of grains required to restore the equilibrium of the balance (_i.e._, the loss of weight), gives the exact weight of the test-liquor consumed. In all other respects the process is the same as in the 'volumetrical method' already described.

Another method for estimating the strength of the sample of acid is by weighing the amount of carbonic acid expelled during saturation. (Method of Fresenius and Will.) This depends on the weight of gaseous carbonic acid which a given weight of the acid-sample under examination is capable of expelling from pure bicarbonate of soda (or of potash), which is estimated by the loss of weight in the acidimeter, or apparatus, after the gas, rendered perfectly dry by pa.s.sing through sulphuric acid, has escaped into the air.

TABLE I.--_Weights of the respective acids equivalent to the given weight of the princ.i.p.al bases, hydrogen being taken as unity._

{51 Acetic acid (anhydrous).

{60 " " (crystallised or glacial).

{99 a.r.s.enious acid (dry).

{35 Boracic acid (anhydrous).

17 gr. of pure ammonia.[8] } {62 " " (crystallised).

31 " anhydrous soda.[9] } {22 Carbonic acid (dry).

40 " hydrate of soda.[9] } {67 Citric acid (crystallised).

53 " dry carbonate of soda.[10] } {85 Gallic acid (dried at 212).

143 " crystallised carbonate of } {94 " " (crystallised).

soda.[11] } {127-1/2 Hydriodic acid (dry or gaseous).

84 " crystallised bicarbonate } {27 Hydrocyanic acid (anhydrous).

of soda. } {36-1/2 Hydrochloric acid (dry or gaseous).

47 " anhydrous pota.s.sa.[9] } {109 " " (liquid, sp. gr. 1162).

56 " hydrate of pota.s.sa.[9] } are {166-1/2 Iodic acid.

69 " dry carbonate of pota.s.sa.[10]} exactly {54 Nitric acid (anhydrous).

100 " crystallised bicarbonate } neutralised {63 " " (liquid, _monohydrated_, sp. gr.

of pota.s.sa. } by { 1517 to 1521).

50 " {pure chalk. } {67-1/2 " " (liquid, _sesquihydrated_, sp. gr.

{pure marble. } { 15033 to 1504).

28 " pure caustic lime. } {72 " " (liquid, _binhydrated_, sp. gr.

37 " hydrate of lime (fresh). } { 1486).

44 " dry carbonic acid (when } {90 " " (liquid, sp. gr 142).

the bicarbonate of } {36 Oxalic acid (anhydrous).

pota.s.sa or soda is } {63 " " (crystallised).

used for testing in } {72 Phosphoric acid (anhydrous).

the process of Fresenius } {81 " " (glacial).

and Will). } {50 Succinic acid (dry or anhydrous crystals).

22 " dry carbonic acid (when } {59 " " (ordinary crystals).

a dry carbonate is } {40 Sulphuric acid (anhydrous).

used). } {49 " " (liquid, _monohydrated_, sp.

{ gr. 18485).

{75 Tartaric acid (crystallised).

{212 Tannic acid (carefully dried).

[Footnote 8: 1000 water-grains measure of pure liquor of ammonia, sp. gr.

0992, contains exactly 17 gr., or 1 equiv. of pure gaseous ammonia. A standard liquor of this strength may be most conveniently prepared by cautious dilution of a stronger solution, until a hydrostatic bead, corresponding to the sp. gr., floats indifferently in the middle of the new solution, at 60 Fahr. By keeping two hydrostatic beads in the solution--the one made barely to float, and the other barely to sink--we shall always be able to detect any change of strength or temperature which it may suffer; since the "loss of a single hundredth part of a grain of ammonia per cent., or the difference of a single degree of heat, will cause the beads to" vary their positions. To preserve its integrity it must be kept in a well-stoppered bottle. (See below.)]

[Footnote 9: These substances, as well as 'test-solutions' containing them, must be perfectly free from carbonic acid, and must be carefully preserved to prevent the absorption of carbonic acid from the atmosphere.

Mohr states that a dilute solution of either of them is best preserved in a flask or bottle well closed with a cork fitted with a small bulb tube (resembling a chloride of calcium tube), filled with a finely triturated mixture of sulphate of soda and caustic lime, and bearing a very thin open tube in the exit aperture. Fresenius, and most other foreign chemists, prefer 'test-solutions' of pure soda. With test-solutions containing caustic alkalies, exact neutralisation of an acid is not only more easily effected, but more readily perceived, particularly when either solution is tinted with litmus.]

[Footnote 10: Prepared by gradually heating the pure crystallised carbonate to redness. From being uniform in composition, and easily procured or prepared, they are much employed; preference being usually given to the soda-salt.]

[Footnote 11: The crystals must be free from attached water, but not the least effloresced.]

_Oper._ A determined amount of the acid under examination is accurately weighed into the flask _A_ (see _engr._); and if it be a concentrated acid, or a solid, it is mixed with or dissolved in 6 or 8 times its weight of water. The little gla.s.s tube (_e_) is then nearly filled to the brim with pure bicarbonate of soda, in powder, and a fine silken thread is tied round the neck of the tube, by means of which it can be lowered down into the flask (_A_), so as to remain perpendicularly suspended when the cork is placed in the latter; the cord being held between the cork and the mouth of the flask. The flask (_B_) is next about half filled with oil of vitriol, and the tubes being arranged in their places, as represented in the _engr._; and time having been allowed for the mixture of acid and water to cool completely, after the increase of heat caused by mixing, the whole apparatus is very accurately weighed. The cork in the flask (_A_) is then slightly loosened, so as to allow the little tube containing the bicarbonate of soda to fall into the acid, and is again instantly fixed AIR-TIGHT in its place. The evolution of carbonic acid now commences, and continues until the acid in the flask (_A_) is neutralised. When this takes place, which is easily seen by no bubbles being emitted on shaking the apparatus, the flask (_A_) is put into hot water (120 to 130 Fahr.), and kept there, with occasional agitation, until the renewed evolution of gas has completely ceased. The little wax stopper is then taken off the tube (_a_), the apparatus taken out of the hot water, wiped dry, and suction applied, by means of a perforated cork, or a small india-rubber tube, and the mouth, to the end of the tube (_d_), until the sucked air no longer tastes of carbonic acid. The whole is then allowed to become quite cold, when it is replaced in the balance (the other scale still containing the original weights), and weights added to restore the equilibrium.

[Ill.u.s.tration:

(_A_) A wide-mouthed flask, capable of holding 2-1/2 to 8 oz., containing sample for trial (_f_).

(_B_) Ditto, capable of holding 1-1/2 to 2 oz., partly filled with oil of vitriol (_g_).

(_a_, _c_, _d_) Tubes fitting air-tight in the flasks by means of the corks (_i_) and (_j_).

(_b_) Piece of wax fitting air-tight on the end of _a_.

(_e_) Small tube capable of holding about 1 drachm of powdered bicarbonate of soda or potash.

(_h_) Open end of the tube (_d_).

(_k_) Silk cord fastened to the tube (_e_).]

The loss of weight represents the exact quant.i.ty of dry carbonic anhydride, or anhydrous carbonic acid gas, that has been expelled from the bicarbonate of soda, by the action of the acid in the sample examined.

The quant.i.ty of real acid it contained is then deduced by the following calculation:--One equivalent of gaseous carbonic anhydride, or anhydrous carbonic acid (= 44) bears the same proportion to one equivalent of the acid in question, as the amount of the carbonic anhydride expelled does to the amount of the acid sought. Thus, suppose a dilute sulphuric acid expels 3 gr. of carbonic anhydride, the arrangement is--

44 : 49 :: 3 : 3349

Consequently the sample operated on contained 35 (nearly) grains of true sulphuric acid.

Instead of the above calculation, we may multiply the weights of the respective acids required to expel 1 gr. of carbonic acid (as exhibited in the following table) by the number of gr. of dry carbonic acid evolved during the above operation. The product represents the per-centage strength, when 100 gr. of the acid have been examined. When only 50, 25, 20, or 10 gr. have been tested, this product must, of course, be doubled, quadrupled, &c., as the case may be.

TABLE II.

Multipliers.

Acetic acid (anhydrous) 1159 " " (hydrated or glacial) 1364 Citric acid (crystallised) 1523 Hydrochloric acid (dry or gaseous) 829 " " (sp. gr. 116) 2478 Nitric acid (anhydrous) 1227 " " (sp. gr. 15) 1523 " " (sp. gr. 142) 2045 Oxalic acid (crystallised) 1432 Sulphuric acid (anhydrous) 909 " " (sp. gr. 18485) 1114 Tartaric acid (anhydrous) 1500 " " (crystallised) 1705

Even this easy calculation may be avoided, in technical a.n.a.lysis, by simply taking for the a.s.say such a weight of the respective acids as is capable of disengaging exactly 10 gr. of dry carbonic acid from the bicarbonate. In this case, the loss of weight in grains, from the operation, multiplied by 10, at once indicates the exact per-centage strength sought. The proper weight of any acid to be taken to give per-centage results is found by simply dividing ten times the equiv. of that acid by 44. For, taking sulphuric acid as an example,

as-- 44: 49 :: 10 : 111318

or 1113 nearly.

On this principle are obtained the weights to be taken, as given in--

TABLE III.

Grains.

Acetic acid (anhydrous) 1159 " " (hydrated or glacial) 1364 Citric acid (crystallised) 1523 Hydrochloric acid (dry or gaseous) 829 " " (sp. gr. 116) 2478 Nitric acid (anhydrous) 1227 " " (sp. gr. 15) 1523 " " (sp. gr. 142) 2045 Oxalic acid (crystallised) 1432 Sulphuric acid (anhydrous) 909 " " (sp. gr. 1845) 1114 Tartaric acid (anhydrous) 1500 " " (crystallised) 1705

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.