146 ROSCOE ON THE COMPOSITION OF AQUEOUS XVIII.-On the Composition of the Aqueous Acids of constant Boiling Poini. BY HENRYENFIELD ROSCOE. IT is still a very generally received opinion that a liquid which boils unchanged at a fixed temperature must be regarded as a chemically homogeneous body. This supposition is not how- ever borne out either by theoretical considerations or by practical experience The proportions in which the constituents of a mixed liquid pass on boiling into the state of vapour depend upon the propor- tions in which the constituents are contained in the liquid and upon the separate tensions of the vapours of these constituents at the temperature of ebullition. Although the laws which regulate the vaporization of mixed liquids are as yet far from being under- stood it is easy to see that the relations between the several tensions and the proportions by weight of the two or more con- stituents may be such that at a given temperature of ebullition the composition of the vapour is identical with that of the liquid.As soon as this point is reached the mixed liquid boils at a constant temperature without undergoing any change of com-position arid in this respect does not diRer from ft uniform chemical combination Such mixed liquids possess however other proper- ties by means of which they can be easily distinguished from definite chemica lcombinations. The characteristics by which a chemical compound is recognized are generally considered to be (l),that the components of such a combination are united in quantities represented by some simple atomic relation ; and (it) that this relation rernaihs unaltered under a certain change of physical conditions Prom the experiments of Dalton Mitscherlich Millon Bineftu and others it has been hitherto supposed that most of the aqneous aci&+and especially hydrocl~loric hydrobromic hydriodic hydro-fltioric nitric and sulphuric acids of very various degrees of strength not only attain a fixed composition when boiled under the ordinary atmospheric pressure but that the liquids thus pre- pared are definite chemical compounds of acid and water.In the present communication my aim xt7ill be to show that this latter supposition is incorrect ; that a1though liquids possessing constant ACIDS OF COISBTANT BOILIKG POINTI composition are obtained by boiling the above-mentioned acids under the ordinary atmospheric pressure these bodies cannot be regarded as definite hydrates but that the phenomenon of constant composition and fixed boiling-point is to be ascribed to the establishment of that particular relation between the weights and tensions of the constituents by virtue of which acid and ivater are contained in the vapour in the same proportions in which they are present in the liquid.I shall show in the first place that the compositioh of the acids obtained by ebullition under the ordinary atmospheric pressure does not in the case of any of the acids examined with one exception agree with that of a simple hydpate; and secondly that in every case when these constant acids are brought under other physical conditions such as when boiled under different pressures or when a current of dry air is passed through them at different temperatures they are decom-posed and attain a different but constant composition so that a mixture of acid and water may be made which on vaporization at a given temperature does not undergo any alteration in composition I.Nitric Acid. Dalton was the first clearly to point out that mixtures of nitric! acid and water when boiled undergo such a change that the weak mixtures lose \later and the strong mixtures lose acid until the residual liquid attains a specific gravity of 1-42,and boils under the ordinary atmospheik pressure steadily at 120" C.This observatiun has since been corifirmed by the experiments of Mitscherlich B ineau Millon and Smith. The residual acid obtained by boiling either a weaker or a stronger acid was found by these chemists to possess an almost constant composition the analyses giving an amount of nitric acid (HNO,) varying from 66 to 70 per cent. on the liquid. From his own experimeuts Mitscherlich concluded that this acid of constant boiling point contains 4 atoms of water to 1 atom of NKO, mhilst Bineau and others supposed that this acid consists of 3 atoms of water and 1 of nitric acid (HNO,) corresponding to the well-known series of magnesian nitrates of the formula RNO,+ 3H0. Owing to the discrepancy in these statements it appeared of interest to determine as accurately as possible in the first place whether the acid obtained by distillation under the ordinary L2 148 ROSCOE ON THE COMPOSTTTON OF AQUEOUS atmospheric pressure has a constant composition ; and secondly whether the relation between acid and water is one capable of expression in simple numbers.For this purpose a quantity of strorig acid was prepared by the usual processes,* and freed completely from lower oxides of nitrogen by passing a current of dry carbonic acid through the warm liquid. ‘l’he acid thus prepared was perfectly colourless and free from every trace of chlorine and sulphuric acid. The method of determination adopted consisted either in volumetric analysis with a standard solution of caustic soda of exactly known strength or in neutralizing the acid with a weighed quantity of pure fused carbonate of sodium boiling the solution and adding a small quantity of test acid or alkali to reach the exact point of neutrality which was supposed to be attained when the litmus becsme blue.The quantity of acid employed for each deter- mination was such that the maximum analytical error the amount of which was determined by control experiments never attained 0.2 per cent. on the liquid. In order to check the two methods employed an acid which with test alkali was found to contain 68-00 per cent. of HNO, and with pure carbonate of sodium gave 68.02 per cent. of HNO, was neutralized with freshly precipitated carbonate of barium the barium in solution being estimated as siilphate; 1.3368 grms.of the nitric acid yielded 1.6815 grms. of sulphate of barium corresponding to 67.95 per cent of HNO,. The composition which aqueous nitric acid of various degrees of strength attains when boiled under the ordinary atmospheric pressure was determined by diluting portions of the pure can- centrated acid to a given extent with water and after analysis distilling them in a small retort the strength of the residual liquid being then accurately estimated. The experimental results are contained in the following Table in which Column I gives the volume of acid employed ; Column 11 the percentage quantity of real acid (HNO,) contained in the liquid before distillation ; Column 111 the volume of liquid remaining in the retort; and Column IV the percentage quantity of real acid (HNO,) con-tained in the residual liquid after distillation.* Mil Ion’s statement respecting the difficulty of scparating from the distillate the sulphuric acid used for concentrating the nitric acid aas nut confirmed. Strong nitric acid was easily obtaiued free from every trace of either sulphuric or hydro- chloric acid. ACIDS OF CONSTANT BOILING POINT. 149 TABLE1. NO. 1 i 99.8 5 cbc. 95.8 95.2 5 J? 84.8 84.7 5 ?Y 71.8 77.8 20 y> 69.1 74.7 Jf 68.1 70-5 20 y 68.6 '10.5 15 y 68.6 65.1 30 , 68.0 68.3 ! I NO. I. IT. 111. IV. --_I_-- (1) 20 cbc. '70.2 5 cbc. 68 1 (2) 20 7f 68*3 J> 68 -0 (3) 20 >I 68-3 5 ?> 67 -9 (4) 20 Y 66.9 5 J? 68 *O (5) 20 1) 66.2 5 YI 68 *O (6) 20 3) 66 -2 5 Y) 68 *O Mean 68-00 150 ROSCOE ON THE COi!vZPOSITKON OF AQUEOUS Hence we may conclude :-(1.) That the residual liquid obtained by boiling aqueous nitric mid of various degrees of concentration under the ordinary atmospheric pressure possesses a constant composition.(2.) That the liquid thus obtained contairis 68.0 per cent. of real acid arid that therefore the proportion between acid and water cannot he represeiited by any simple atomic relation the formula HNO +3H0 reqiiiring 70.0 per cent. of FINO,.* The boilitig-point of the acid containing 68.0 per cent. mas fourid to be 120°*5C. under a barometric pressure of 0"'*735;its specific gravity at l5O-5 C. was showir to be 1.414 as a mean of two deterin inations.Distilled at other temperatures the relation of real acid to water will if the phsnomenon of constant boiliug-point depends alto- gether on physical causes he found to be a different one. In the following experiments aqueous nitric acid of various degree8 of strength was distilled under diniinivhed atmospheric pressure effected by placing the acid in 8 retort the neck of which drawn out and bent at an acizte angle passed through a solid caoutchouc stopper into a large bolt-head of 20 litres capacity furnished with a divided manometer-tube and cammunicating with the cylinder of a large air-pump. By this means the acid could be distilled under any wished-for pressure less than that of the atmosphere; by cooling the bolt-head agid by having a large absorbent surface of caustic soda in the interior of the flask it was easy to keep the mercury in the manometer tube to within 5 millimetres of the required height during the whole coiirse of the distillation.Table 3 contains the results of a series of such experiments; Column I gives the prcsswe in millimetres of mercury under which the distillation took place (i. e. the barometric pressure,-the height of the mercurial column in the manometer-tube) ; Column 11 the volumes of liquid employed; Column 111 the percentage of real acid contained in this liquid; Column IV the volume remainiug after distillation ; Column V the percentage of real acid contained in the residual liquid. * Calculated percentage composition of the hypothetical hydrates of nitric acid :-(1st) (2nd) (3rd) H.NO .. 87 5 77 8 70 -0 HO . . 12.5 22 .2 30 .O 100-0 100'0 100.0 ACIDS OF CONSTANT BOILING POINT. TABLE3. Y. - 69 $9 66-7 68 *3 67 *2 66 *9 66 *7 66 *5 66 *S 66.2 66 -5 65 -1 66 *7 66 -9 67 '6 1-1'i obtained whilst under a pressure of 150 millimetres the equilibrium occurs when the percentage of acid reaches 67.6. The distillation of nitric acid under pressures greater than that of the atmo-sphere is accompanied by considerable experimental difliculties owing to the impossibility of bringing the acid into contact with mercury. These difficulties were overcome by employ-[====Ifl A ing the following arrangement :-The bulb-retort (a) Fig.1 152 ROSCOE ON THE COMPOSITION OF AQUEOUS containing the acid was connected by means of a solid caoutchouc stopper with a strong bottle (b) of one litre capacity containing Eome dilute nitric acid ; through the stopper passes a second tube communicating with another similar bottle (c)containing mercury (m)and a strong solution of caustic soda (s) ; a divided manometer tube (d) dipped into the mercury and a third tube (e) passing through the stopper of the battle (c) could be placed to any required depth into a reservoir of mercury. The bottle (6)being immersed in cold water the increase of pressure was attained by pushing the glass bucket (91 filled with pieces of marble into the dilute acid by means of the stiff iron wire (f) working perfectly air-tight through the caoutchouc stopper.As soon as the mercury in the manometer tube had attained the wished-for height and the gas issued from the extremity of the tube (e) under the mercury a portion of the acid in the retort was distilled over and the residue afterwards analyzed. By withdrawing the marble from the acid and again immersing it vhen required the pressure was kept tolerably constant during the whole course of the experiment. The numbers in the following Table show that distilled under a mean pressure of 1,220 millimetres of mercury beyond which it was found inconvenient to operate the composition of the residual liquid reached 68-6per cent. of real acid being a deviation from the acid distilled under ordinary pressures of 0.6 per cent.TABLE 4. NO. I. 11. v. (1) 1260 mn. 20 cbc. '10.5 5 68.8 (2) 1210 ) 20 , 68.3 5 68.7 (3) 1190 Yf 20 JY 68 0 5 68-4 Mean 68.6 The numbers in the columns have the same signification as those in Table 3 except in the first in which the sum of the heights of the barometric column and the column of mercury in the manometer tube is given. As it has thus heen shown that aqueous nitric acid is a liquid wtiich does not give rise on distillation to bodies possessing a simple atomic constitution but that for each temperature a liquid ACIDS OF CONSTANT BOILING POIST. 153 having a constant composition is obtained it appeared of interest to determine for lower temperatures the composition of the acid unalterable by vaporization.For this purpose air completely dried over sulphuric acid and phosphoric anhydride was passed through the nitric Fig. 2. acid contained in the burette-shaped vessel Fig. 2 until no further change in composition was observed. The requisite temperature was obtained by immers-ing the burette in a large water-bath the heat of which was kept constant a weighed U-tube con- i taining phosphoric anhydride being placed bet ween the burette and the drying apparatus to ensure all absence of moisture. Experiments thus conducted for every temperature show that a point is reached at which the composition of the aqueous acid undergoes no further change by vaporization. In Table 5 Column I gives the percentage of real acid in the liquid before the experiment; Column 11 the per- centage after the experiment; Column III?the fraction of the original volume of acid remaining after the experiment; Column IVY the duration of the experiment in hours.TABLE5. NO. 3;. ---111. --IT. 7-1 I (11 68.0 66.2 2 hrs. (2) 64.9 66.1 -4 z 1 I:, 1 (3) 64.9 66.3 1 + 3 >? Passage of dry air through acid at 60"C. 68.0 66.9 66-9 65.7 65.7 65.2 65-22 64.9 64.9 68.8 64.9 64.8 6443 64.5 64.5 64.4 64.0 64-5 64.5 64.5 154 ROSCOE ON THE COMPOSITION OF AQUEOUS It is thus seen-(1,) That weak or strong aqueous nitric acid through which dry air is passed at looo attains a constant com-position of 66.2 per cent.of real acid. (2.) That when air is passed through the same acid at 60*,the composition becomes constant at 64%per cent.; and (3.) That at the ordinary atmospheric tempera- tures-ip mean 13"-the equilibrium is reached when the liquid wntains 64.0 per cent. of real acid. The ease with which strong nitric acid is decomposed even at temperatures below its boiling point is well known. It seems to he impossible to prepare the real acid HNO by employing the usual method of distillation rectification over sulphuric acid and volatilization of the oxides of nitrogen by means of a current of dry air or carbonic acid. None of the chemists who have worked upon this subject appear to have had the real acid and I have also failed to obtain it although every care to ensure absence of moisture was taken; one colourless acid contained 99%per cent.of real acid another 99-47 per cent The former nearly anhy- drous acid not only suffered decomposition on boiling as is seen by refererice to Table I but underwent alteration when a current of air perfectly dried over a large quantity of phosphoric anhy- dride was passed through the liquid at 15". In seven hours after which time three-fourths of the acid had been volatilized the per- centage of real acid had sunk to 98.77. This shows that nitric acid ("0,) undergoes decomposition not only when vaporized at the boiling point but also at the ordinary atmospheric temperatures. n.-&h@huric Acid. Ve owe to Marignac the interesting and important observa- tion that real sulphuric acid (HSO,) cannot be obtained by the 155 ACIDS OF CONSFANT BOILING POINT.distillation either of a weaker aqueous acid or of a stronger fuming acid. The real acid can only be prepared by crystdlisa- tion and on boiling is decomposed into anhydride which is seen in the receiver and a weaker acid remaining in the retort. Marignac showed that this weaker residual acid boils at 338"C without undergoing ohange; and he further proved that aqueous sulphuric acid of every degree of concentration whether containing more or less water than the real acid (HSO,) attains this constant composition on boiling. From four separate deter- minations Xarign ac found that this residual liquid oontained 98.70 per cent. of real acid.The followipig determinations have confirmed in every par-ticular M ar i gn ac' s conclusions. The analyses were made by weighing out the requisite quantities of pure fused carbonate of sodium the exact point of neutrality being estimated by addition of small volumes of standard alkali and acid of well-established strengths. (1.) Pure strong sulphiiric acid eontaining 93.4 per cent. a€ real acid was distilled in a retort until two-thirds of tlie liquid had been volatilized The liquid remainizlg in the retort con-tained 98.7 per cent. of real acid. (2.) An acid containing 98.37 per cent. of HSO was distilled in a small retort until one-fifth of the original volume remained. The residue contained 98-32per cent. of HSO,. (3.)An acid containing 98-37 per cent.of ffS0 was boiled down in a porcelain capsule to one-third of its original volume. The residual liquid contained 9845 per cent. of real acid. (4,) An acid containing 100.33 per cent. of HSO, and fuming in contact; with the air at ordinary temperatures was boiled down to one-eighth of its orignal volume. The residual liquid con-tained 98%per cent of real acid. (5.) Another portion of the same fuming acid which a second analysis showed to contain 100.34 per cent. IISO, was boiled to one-fourth of its original volume. The residual liquid contained 98-40per cent. of' real acid. It thus appears not only that aqueous sulphuric like aqueous nitric acid attains an unalterable cornposition not correspoiiding to any definite hydrate on distillation under the ordinary atmo-spheric pressure ; but we also notice what is very remarkable that both the real acids HNO and HSO, bodies which passess in other respects the characteristics of well-defined chemical corn- 156 ROSCOE ON THE COMPOSITION OF AQUEOUS pounds are decomposed on boiling yielding that aqueous acid which remains unchanged at the temperature of ebullition.III.-Hydrochloric Acid. Bineau found that aqueous hydrochloric acid boiled under the ordinary atmospheric pressure attained a composition of 20.2 p.c. of HC1 corresponding exactly to the formula HC1+16RO and this liquid he conceiwd to he a definite hydrate. In a research upon the absorption of hydrochloric acid and ammonia in water published in vol. XI page 1.28 of the Journal of the Chemical Society which I made in conjunction with Mr.Dittmar this result of Bineau's was to a certain extent confirmed inas- much as the acid of constant composition obtained by ebullition under the ordiuary atmospheric pressure was found to contain f20*24 per cer,t. of HCl; but it was at the same time shown that distilled under other pressures or vaporized at other temperatures this acid was decomposed other liquids of constant composition being then produced and the conclusion was there-fore drawn that this hydrate has no red existence. The following Table 6 extracted from the above-mentioned memoir gives the relation between the pressure under which the acid is distilled and the cornposition of the constant liquid.The Column F shows the pressure in metres of mercury under which aqueous hydro-chloric acid must be distilled to attain the constant composition given in the next column. TABLE6. -PercentagePm* i ofHC1. Pm. Percentageof HC1. pm. Percentageof HC1. Pm. Percentageof HCl. --._. -I_ 0.05 23*2 0-6 20.7 2-0 18.5 0.1 22-9 0.7 20.4 2.1 15.4 0.2 22'3 0 76 20.24 23 18-3 0-3 21.8 0.8 20.2 2.4 18.1 0'4 21.4 0.9 19.9 25 18 0 0.5 21'1 1-0 19.7 1-1 19.5 1-2 19.4 ACIDS OF CONSTANT BOILING POINT. Hence it is evident :-(1.) That there exists for each pressure a corresponding aqueous hydrochloric acid which undergoes no change in composition when distilled under this pressure and therefore has a constant boiling point. (2.) That the com-position of these aqueous acids is different for each pressure a gradual change in pressure being accompanied by a gradual zlteration in the percentage of hydrochloric acid.When aqueous hydrochloric acid is vaporized by passing a current of dry air through the liquid at a given tempemture a similar point is reached beyond which no decomposition occurs. Table 7 contains the interpolated values obtained from the expe- rimental results given in the original paper. The first column gives the temperatures ; the second column gives the percentage of HCl contained in the acid unalterable at the corresponding temperature. TABLE-7. -TO. Percentage TO. Percentage TO. Percentage TO. 'ercentage of HCL. of HCl. of HCI. of HCI. -- -_I 0" 25.0 30" 24.3.60" 23-0 goo 21.4 5" 24.9 35" 23'9 65" 22.8 95" 21'1 loo 24.7 40" 23.8 '70" 22.6 100" 20-7 15O 24.6 45" 23.6 7 5" 22.3 20° 24.4 50" 23 4 80" 22.0 25" 24.3 55"- 23.2 85" - 21-7 - IV. Hydrobromic Acid. Lowig first observed that water saturated at the ordinary atmospheric temperature with hydrobromic acid gas loses acid when boiled and that water containing but little gas in solution loses water under similar circumstances. Bineau showed that the composition of the acid obtained by boiling mas constant and his experiments proving that such acid contained from 46.1 to 47.4 per cent. of HBr he concluded that 017 distillation the hpdrate containing 10 atoms of water is formed. According to theory this hydrate should contain 47*38per cent.of HBr when the equivalent of bromine is taken as 80. On exposing aqueous acid of the above strength in a closed vessel over dried potash or on passing a currerit of dry air through the acid Bineau found that the residual liquid contained from 48.7 to 51.7 per cent. of 158 ROSCOE ON THE CONPOSITION OF AQUEOUS hydrobromic acid; this he assumes to be a hydrate containing 9 atoms of water composed theoretically of equal weights of water and real acid (HBr.) For the purpose of determining the exact relation of the composition of aqueous hydrobrornic acid to the temperature of ebullition the acid was prepared by shaking together pure bromine water and phosphorus added in small pieces from time to time until the liquid became colourless the strong fuming solution being freed by distillation from phosphorus and phospharic acid.The pure and colourless acid thus prepared was diluted to it given extent with water; and as soon as the percentage of real acid (HBr) which the diluted liquid contained had been estimated by accurate volumetric ahalysis with silver it was boiled in a bulb- tube retort under the ordinary atmospheric pressure until a certain pc;rtion of the acid had distilled over when the composition of the residue was determined by exact analysis with silver. In Table 8 are seen the results of eight such distillations with aqueous hydrobromic acids of various degrees of concen-tration Column I gives the barometric pressure under which the acid boiled; Column If the volume in cubic centimetres of acid employed Column 111 the perccntage of HBr contained in the original liquid ; Column IV the volume in cubic centimetres of acid remaining in the rctort after the distillation; and Column V the perceBtage of HBr contained in the residual liquid being the mean of two aiialy~es which generally differed orily in the second decimal place.TABLE8. BO. I. 11. 111. IV. v. rn 0.752 25 cbc. 45-54 8cbc. 47.28 0 752 j 45.68 47.39 0.753 47.30 47 78 0 -762 I 47 *65 47 86 0 T53 47 .'18 47 61 0 T62 47-87 47 -73 0 752 49 -00 47 -71 0 962 49.51 47 *84 ACIDS OF CONSTANT BOILIXG POINT. If the distillations are conducted under the same conditions the compositions of the residual acids are identical (see Nos.4 and 8); if the physical conditions (barometric pressure and volume of liquid employed) are different the acid may vary in composition about 0.1 or 0.2 per cent. as is seen in one or two of the numbers. From the foregoing experiments we see that aqueous hydrobromic acid when boiled under the ordinary atmospheric pressuw of 0.76 of mercury attains a fixed composition of 4'7% per cent. of real acid or contains 0.5 per cent. more acid than Bineau's hypo-thetical hydrate. Under these circumstances it was found to boil constantly at 126OC. That the point of constant composition which aqueous hydrobromic acid attains on vaporization is not solely dependent upon chemical attractions but is mainly influ- enced by physical circumstances is still more distinctly seen on examining the change which the aqueous acid undergoes when a current of dry air is passed through the liquid at a constant temperature.Through a liquid containing 4'7.65 per cent. of real acid dry air was passed at the temperature of 16' for 50 hours; after the lapse of this time the strength of the acid reached 51.8 per cent. and after the air had passed for 30 hours longer the liquid contained 51-65per cent. of real acid as a mean of two analyses and had therefore attained the point of constant compo- sition. Hence it is clear that neither of the supposed hydrates of hpdrobromic acid has a real existence. In order to determine the point of equilibrium for other temperatures perfectly dry air was passed through the acid at 100' contained in the burette Fig.2 and the alteration which the liquid underwent was deter- mined. The following results were obtained :-Dry air passed through aqueous acid at 100OC. P.C.reai acid. (1) 1 vol. acid containing 48 0 p. c. when reduced to Q vol cohtained 49 *59 (2) JI 7) 48-05 17 2? 8 79 49 07 (3) ?> f> 49.10 79 1s 4 7f Y? 49 35 ?7 (4) 73 3t 50 *lo 4 7) 7$ 49 35 ?7 17 The aqueous acid was also distilled under a greatly increased pressure by help of the arrangement represented in Fig. 3. (a) Fig. 3 is a small bulb blorPn before the lamp to which is fused the glass tube (b) communicating ,by a solid caoutchouc stopper with 100 ROSCOE ON THE COJIPOSITION OF AQEEOVS Fig. 3. the divided manometer tube (c) containing mercury.After the tube (c) had been filled with mercury to the requisite height the acid was placcd in the bulb u and then the drawn-out end closed before the blow- pipe. The distillation was now commenced and con- tinued until the acid had diminished to a given volume when the pressure was read off' from the ditTereiice of height of the columns of mercury in the two tubes. For the purpose of obtaining the boiling point of the acid under these circumstances a thermometer was inserted into the bulb-tube and the apparatus made air-tight by a joint of caoutchouc and lead-foil carefully wrapped with copper-wire. Uiider a total pressure of 5 1.952 metres of rnercury the acid boiled at 153"C; analysis of the residual acid gave the following results :-(1) Acid containing 48.0 per cent.left after distil- lation a residual liquid containing 46.30 per cent. of real acid (HBr). (2) Acid containing 46.07 per cent. left after distil- lation a liquid containing 46-36 per cent of real acid. v. Hydriodic Acid. Bineau concluded that the liquid of constant boiling point obtained by distilling a diluted or a saturated aqueous solution of hydriodic acid was composed of I1 atoms of water to one of real acid (HI). The residual pcid contained according to Elis experi-ments from 56.3 to 57.2 per cent. of hydriodic acid whilst according to calculation the 11-atom hydrate requires 56.39 per cent. of real acid. Aqueous hydriodic acid was prepared by leading into distilled water the gas evolved by heating a mixture of 20 parts of pure iodine 14 parts of iodide of potassium and 14 parts of phosphorus mixed with a little water.The acid thus obtained of any reqtiisite degree of coiiceii tratiori was perfectly oolourless when preserved out of contact with air and although the acid was always distilled in a current of hydrogen it still became slightly ACIDS OF CONSTANT BOILING Pomr. lei coloured but was not decomposed to such an extent as to affect the results of the analysis. The composition of the acid before and after the distillation mas determined by volumetric analysis with silver; the results of these determinations as is seen by reference to the following numbers are closely concordant ; care however must always be taken to have excess of silver present at the commencement of the analysis otherwise some iodine is liberated by the free nitric acid.TABLE9. Distillation of aqueous hydriodic acid under the ordinary atmospheric pressure. Volume of liquid employed in each experiment 25 cbc. > , remaining after each distillation 13 cbc. Percentage of real acid taken. Percentage of real acid in the residue. (1) 56.50 .. .. ". 5 7.03 56-93 (3) 56.70 .. .. .. 56-94 56.90 (3) 57524 f. .. .. 57.03 (57.16 (4) 57.17 .. 1. .. Hence it is seen that the acid of constant composition which is obtained when aqueous hydriodic acid is boiled under the ordi- nary atmospheric pressure contains 57.00 per cent. of real acid and cannot therefore be considered to be a definite hydrate of hydriodic acid.The boiling point of this acid was found to be 127O C. under a barometric pressure of 0.774 metre. In order to ascertain the point of equilibrium for other temperatures aqueous hydriodic acid was vaporized in a current of dry hydrogen gas at the temperatures of 16' and 100". Table 10 gives the results of such experiments. VOL. XIII. M 162 ROSCOE ON THE COMPOSTTFON OF AQUEOUS TABLE10. Vaporization of aqueous hydriodic acid in a current of dry hydrogen at ordinary atmospheric temperature. Temp. Duration of the experiment. Percentage of real acid taken. Percentage of real acid in residue. (1) 15O .. 32 hours .. 57.0 .. 59.44 (2) 15" .. 41 , .+ 59.44 . (60.36160.33 (3) 17" ..15 , . . 60.03 . . 60.27 (4) 17" .. 15 , . . 60.27' . {60,3960.42 (5) 19O *. 10 , .. 60.21 .. 60.68 (6) 19' . 6 7 . 60*68 .. Vaporization of aqueous hydriodic acid in a ciirrent of dry hydrogen at looo until half the liquid was volatilized. Percentage of real acid taken. Percentage of real acid in residue. (1) (2) 56.98 59-78 .. .. .. .. *. .* 58-49 58-26 (3) 58-21 .. .. ' f58-24 158.20 VI. HydroJluoric Acid. Aqueous hydrofluoric acid when boiled under the ordinary atmospheric pressure attains according to Bineau a constant composition corresponding to the formula €IF1 + 4 HO and containing 35.9 per cent. of anhydroiis acid. In order to verify this assertion pure aqueous hydrofluoric acid was prepared by leading the gas evolved from a strong acid heated in a platinum retort into water also contained in a platinum vhssel; the acid thus obtained being then boiled under the atmospheric pressure and the quantity of acid contained in the residual liquid determined by throwing the acid weighed in ACIDS OF CONSTANT BOILING POINT.a platinum capsule into a vessel containing an excess of standard soda solution. The quantity of free soda was estimated by adding a slight excess of test acid and the point of neutrality was considered to be reached when the litmus again became coloured blue on adding soda solution drop by drop in the cold. The results of a series of experiments thus conducted are seen in Table 11 in which Column I gives the volume of' acid before boiling; Column I1 the percentage of €IF1contained in the liquid before boiling; Column 111the volume of HF1 after boiling ; and Column IV the percentage of HF1 contained in the liquid after boiling.TABLE11 11. IY. I. 11. 111. 1 -2 '*a 12 cbc 36 *9 6 6 -2 11 -8 38 *1 12 16 -5 38 -1 6 32 .O 38 9 12 35 *4 39 .o 20 r 36 -4 136 8 41 -6' 40 26 -5 From these experiments it is evident that wlien aqueous hydro-fluoric acid is boiled in platinum vessels under the ordinary atmo- spheric pressure it attains a composition varying from 36 to 38 per cent. of HFI. The great differences observed arising from the impossibility of keeping the physical conditions constant under which the ebullition takes place.We may take the mean 37.0 per cent. as representing pretty closely the composition of the acid unalterable by bailing in the air. If a true chemical compound between acid and water were thus produced such large 3% 2 164 ROSCOE ON COMPOSITION OF AQUEOUS ACIDS ETC. variations in the composition of the residual acid could not have been found. Vaporized at other temperatures this constant hydro- ffuoric acid containing 37.0 per cent. of PEFl undergow change. A portion of acid of the above strength obtained by ebullition was placed in a platinum crucible over quick lime inside a leaden exsiccator closed 'iFith a lid having a sulphuric acid joint. After standing at a temperature of 15°C. for 4 days the acid was found to contain in two analyses 36.4 an6 36.6 per cent.of HF1; after remaining 4 days longer at the sRme temperature the liquid contained 33.5 per cent. of acid; and when the acid had been standing for 2 more days at the same temperature over quick lime it was found to contain 32.5 and 32.7 per cent. of acid. A weaker acid was next vaporized under like conditions ;an acid containing 31.6 per cent. of HFI placed over quick lime at 15" attained after 2 days a composition of 32.1 per cent. and after a further exposure in the exsiccator for 4 days two analyses showed that it contained 32*4and 32.3 per cent. of WF1. I beg to thank my assistant Mr. Schorlemmer for the able help which he haa given me in carrying out the foregoing experi- ments