ACIDIM′ETRY

Syn. Acidom′etry ; Acidime′tri a , ., L.; Acidimétrie , Fr. The estimation of the strength or quantity of acid, in a free state, contained in any liquid. It is the reverse of ‘alkalimetry.’ Acidimetrical a ays are understood to refer to the relative strengths of the same acids ( i. e. , the quantity 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 proce es. 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 ve el, 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), di olved in a like quantity. This liquid is then exactly neutralised with an alkali. This point is usually determined, by the addition of a small quantity 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 quantity of the alkaline solution consumed for this purpose represents an equivalent quantity of acid, and thus gives us the acid content of the sample under examination. The common practice is to di olve 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 quantity, 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 nece ary to point off the first right-hand figure of the result as a decimal. Thus; repeating the above example— 50: 49:: 250: 24·5 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 36·5: therefore— 100: 36·5:: 45: 16·425% and, since only 50 gr. (instead of 100 gr.) were examined— 16·425 × 2 = 32·85% 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 ⁄ 10 th, 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 analysis it is more convenient if the number of acidimeter divisions of the ‘test-liquid’ consumed expre the per-centage strength of the acid, without further calculation. For this purpose the number of grains of the acid taken for the a ay 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 ⁄ 10 th; the per-centage answer, in the last case, being doubled, quadrupled, ., 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 di olving in water their constituents except ammonia, of which 1000 grains, or one litre, of solution of specific gravity 0·992 contains exactly one equivalent. 53 grains (or grammes) of pure anhydrous carbonate of soda, prepared by gradually heating to redne the crystallised salt, constitute 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 you sand parts, 50 of pure carbonate of lime or chalk, or 28 of pure caustic lime, are used. Besides these, a proce 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 redi olved, carefully avoiding exce . Or better, by adding a rather strong solution of sulphate of copper, to a quantity 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 redi olved 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 nece ary. 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 proce 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 di olved 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. 1·1812. Of course it only answers with acids whose calcium salts are readily soluble in water. b. Graver metrically: — 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 ⁄ 10 th gr. of alkali; and every 10 gr., 1 gr. of alkali; or respectively, 1 ⁄ 10 th per cent. and 1 per cent. The ve el 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 nece ary) so as to restore the balance to an equilibrium. After the proce 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 lo of weight), gives the exact weight of the test-liquor consumed. In all other respects the proce 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 lo of weight in the acidimeter, or apparatus, after the gas, rendered perfectly dry by pa ing through sulphuric acid, has escaped into the air. Table I. — Weights of the respective acids equivalent to the given weight of the principal bases, hydrogen being taken as unity. | 17 | gr. | of pure ammonia. [8] | are exactly neutralised by | 51 Acetic acid (anhydrous). | | 31 | ” | anhydrous soda. [9] | 60 Acetic acid (crystallised or glacial). | | 40 | ” | hydrate of soda. [9] | 99 Arsenious acid (dry). | | 53 | ” | dry carbonate of soda. [10] | 35 Boracic acid (anhydrous). | | 143 | ” | crystallised carbonate of soda. [11] | 62 Boracic acid (crystallised). | | 84 | ” | crystallised bicarbonate of soda. | 22 Carbonic acid (dry). | | 47 | ” | anhydrous pota a. [9] | 67 Citric acid (crystallised). | | 56 | ” | hydrate of pota a. [9] | 85 Gallic acid (dried at 212°). | | 69 | ” | dry carbonate of pota a. [10] | 94 Gallic acid (crystallised). | | 100 | ” | crystallised bicarbonate of pota a. | 1271⁄2 Hydriodic acid (dry or gaseous). | | 50 | ” | pure chalk or pure marble. | 27 Hydrocyanic acid (anhydrous). | | 28 | ” | pure caustic lime. | 361⁄2 Hydrochloric acid (dry or gaseous). | | 37 | ” | hydrate of lime (fresh). | 1091⁄2 Hydrochloric acid (liquid, sp. gr. 1·162). | | 44 | ” | dry carbonic acid (when the bicarbonate of pota a or soda is used for testing in the proce of Fresenius and Will). | 1661⁄2 Iodic acid. | | 22 | ” | dry carbonic acid (when a dry carbonate is used). | 54 Nitric acid (anhydrous). | | 671⁄2 Nitric acid (liquid, sesquihydrated, sp. gr. 1·5033 to 1·504). | | 72 Nitric acid (liquid, bin hydrated, sp. gr. 1·486). | | 90 Nitric acid (liquid, sp. gr 1·42). | | 36 Oxalic acid (anhydrous). | | 63 Oxalic acid (crystallised). | | 72 Phosphoric acid (anhydrous). | | 81 Phosphoric acid (glacial). | | 50 Succinic acid (dry or anhydrous crystals). | | 59 Succinic acid (ordinary crystals). | | 40 Sulphuric acid (anhydrous). | | 49 Sulphuric acid (liquid, monohydrated, sp. gr. 1·8485). | | 75 Tartaric acid (crystallised). | | 12 Tannic acid (carefully dried). | [8] 1000 water-grains measure of pure liquor of ammonia, sp. gr. 0·992, 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 “lo 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.) [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. [10] Prepared by gradually heating the pure crystallised carbonate to redne . From being uniform in composition, and easily procured or prepared, they are much employed; preference being usually given to the soda-salt. [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 di olved in 6 or 8 times its weight of water. The little gla 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. IMG:596405606043057898_i043.png: ( 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 lo of weight represents the exact quantity 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 quantity 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: 3·349 Consequently the sample operated on contained 3·5 (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, ., as the case may be. Table II. | Multipliers. | | Acetic acid (anhydrous) | 1·159 | | Acetic acid (hydrated or glacial) | 1·364 | | Citric acid (crystallised) | 1·523 | | Hydrochloric acid (dry or gaseous) | ·829 | | Hydrochloric acid (sp. gr. 1·16) | 2·478 | | Nitric acid (anhydrous) | 1·227 | | Nitric acid (sp. gr. 1·5) | 1·523 | | Nitric acid (sp. gr. 1·42) | 2·045 | | Oxalic acid (crystallised) | 1·432 | | Sulphuric acid (anhydrous) | ·909 | | Sulphuric acid (sp. gr. 1·8485) | 1·114 | | Tartaric acid (anhydrous) | 1·500 | | Tartaric acid (crystallised) | 1·705 | Even this easy calculation may be avoided, in technical analysis, by simply taking for the a ay 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 lo 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: 11·1318 or 11·13 nearly. On this principle are obtained the weights to be taken, as given in— Table III. | Grains. | | Acetic acid (anhydrous) | 11·59 | | Acetic acid (hydrated or glacial) | 13·64 | | Citric acid (crystallised) | 15·23 | | Hydrochloric acid (dry or gaseous) | 8·29 | | Hydrochloric acid (sp. gr. 1·16) | 24·78 | | Nitric acid (anhydrous) | 12·27 | | Nitric acid (sp. gr. 1·5) | 15·23 | | Nitric acid (sp. gr. 1·42) | 20·45 | | Oxalic acid (crystallised) | 14·32 | | Sulphuric acid (anhydrous) | 9·09 | | Sulphuric acid (sp. gr. 1·845) | 11·14 | | Tartaric acid (anhydrous) | 15·00 | | Tartaric acid (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 proce 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 gla 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 pre ure between some sheets of blotting-paper, without the aid of heat, is kept for use in a well-closed gla 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 pota a may be used instead of bicarbonate of soda; but in either case it is always proper to use an exce , 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 . IMG:596405606043057898_i044.png: ( 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 proce 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 usele . 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 unnece ary. The peculiar soapy odour gradually acquired by the liquor as it nears saturation will materially a ist 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 Ace time try ; 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.