284 PHILIP: THE SOLUBILITY O F

XXXV.--l’he Solubility of SuZphaniZic Acid and its Hydrates.

By JAMES CHARLES PHILIP. IN the cours3 of another research i t became desirable to ascertain exactly the conditions of stability of the different hydrates of sulphanilic acid. According to earlier investigations (see Laar, J . pr. Chem., 1879, 20, 242; Ber., 1881, 14, 933), both dihydrate

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SULPHANILIC ACID AND ITS HYDRATES. 285

and monohydrate exist, but the conditions of temperature and concentration under which they separate from solution have not been determined. The solubility of sulphanilic acid has accord- ingly been measured a t a number of temperatures between Oo and 5 5 O , whilst, in addition, the composition of the solid phase in equili- brium with the saturated solution was determined in each case.

The investigation has shown that the stable solid in contact with the saturated solutions is dihydrate from Oo to 21°, mono- hydrate from 21° to 40°, and anhydrous acid from 40° upwards. In previous determinations of the solubility of sulphanilic acid (see Dolinski, Ber., 1905, 38, 1835), this change in the character of the solid phase as the temperature rises has been overlooked, and the attempt has been made to represent the variation of solubility with temperature as a continuous curve. As will be seen from the determinations of solubility recorded below, this cannot be done. In some cases, indeed, two distinct values were obtained for the solubility a t a given temperature, and investigation revealed the fact that the difference in solubility was associated with a difference in the solid phase present.

The equilibrium between the solid hydrates and the water vapour in the surrounding atmosphere also presents points of interest. Chief among these is the difference in the behaviour of the dihydrate and the monohydrate in regard to dehydration. Under conditions in which the monohydrate is unaltered, the dihydrate loses all its water, and leaves the anhydrous acid. This is probably due to suspended transformation in the case of the monohydrate, but it has been frequently observed that the rate of dehydration of the dihydrate exhibits no change a t the point corresponding with the monohydrate. This fact suggests that the molecules of the dihydrate, when deprived of water, may yield those of the anhydrous acid directly, without passing through the monohydrate stage, and further evidence of this kind, if obtainable, would throw light on the manner in which the water molecules are attached in compounds containing water of crystallisation.

Whilst this appears to be the normal course of dehydration of the dihydrate, it was observed in a few cases that the process stopped a t the monohydrate stage, although the conditions were those in which other samples of the dihydrate had lost all their water.

From experiments in which the hydrates were kept in closed vessels over sulphuric acid of different strengths, and weighed from time to time, the dissociation pressures were deduced. At 20° the dihydrate remains of constant weight in an atmosphere with an aqueous vapour pressure higher than about 16.5 mm. of mercury,

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286 PHILIP: THE SOLUBILITY OF

whilst the corresponding limit for the monohydrate is about 12 mm. of mercury. The latter figure was confirmed by direct measurement of the dissociation pressure in a tensimeter.

EXPERIMENTAL.

The sulphanilic acid employed in the investigation was obtained by recrystallisation of the best commercial acid. The purity of the recrystallised material was established by dissolving a known quantity in water, and titrating with a carefully standardised sodium hydroxide solution.

In the determination of solubility the point of equilibrium was reached from both sides. I n one tube, water was stirred up with excess of sulphanilic acid; in another tube, a solution which had been saturated at a somewhat higher temperature was treated similarly. The two tubes were immersed in a constant temperature bath, and the stirring was continued for six to seven hours a t least. The equality of the values obtained for the solubility in the parallel experiments was a guarantee that equilibrium between solid and solution had really been reached. In some cases, where intermittent shaking was employed instead of continuous stirring, the time of contact of solid with solution was correspondingly longer.

When sufficient time had been allowed for the attainment of equilibrium, two samples of the saturated solution were drawn into separate pipettes through mtton-wool plugs, and immediately dis- charged into weighed flwks. In this way, known weights of satur- ated solution were obtained, and the sulphanilic acid present was then estimated by titration with standard sodium hydroxide solution.

When the extracta had been made, the remaining solution, along with the excess of solid, was thrown on a filter, and the liquid was removed as completely as possible with the aid of a pump. After the solid had been further dried by pressing between filter paper for about ten minutes, a portion was weighed out and left overnight in a vacuum desiccator containing concentrated sulphuric acid. In these circumstances the water of crystallisation of sulphanilic acid is rapidly and completely removed. The dihydrate contains 17.2 per cent. of water, and the monohydrate 9-42 per cent., so that this method of exwination showed clearly whether the solid in equili- brium with the saturated solution had been dihydrate, mono- hydrate, or anhydrous acid.

The results obtained for the solubility are recorded in the following table, and are represented graphically in Fig. 1. The temperatures shown in the table are corrected values, based on

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SULPHANILIC ACID AND ITS HYDRATES. 287

a comparison of the thermometers actually used with a standard instrument.

Grams of anhydrous acid per 100 grams

Temp. of solution. Solid phase. 0.0" 0'444 dih ydra te 7-2 0.622 9 9

13.3 0.841 > Y

18*9 1.093 > I

18.9 1.137 monohydrate 25.1 1.384 9 9

Grams of an hydrous acid per 100 grams

Temp. of solution. Solid phase. 31.1 1.662 monohydrate 37.2 2.004 Y9

44'0 2'44 Y Y

44.0 2-36 anhydrous acid

54.5 2.85 7 2

4'p-5 3'52 3 )

Particular interest attaches to the figures obtained a t 18.9O and In two experiments at the former temperature, different 44.0°.

FIG. 1.

10" 20" 30" 40" 50" 60"

Temperature.

valnes of the solubility were recorded, and examination of the solid phase in each case showed that the higher value was obtained in presence of the monohydrate, which is metastable at 18*9O, whilst the lower solubility was given by the stable dihydrate. Similarly, a t 44'0°, the higher value of the solubility was given by the mono-

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288 PHILIP: THE SOLUBILITY OF

hydrate, which is the metastable phase a t this temperature also, and the lower value by the anhydrous acid.

The dihydrate of sulphanilic acid, obtained by crystallisation from its solutions below 20°, is a highly efllorescent substance, and, if exposed to the air of the room, loses almost all its water in twelve hours, ultimately becoming anhydrous. The monohydrate, on the other hand, has on several occmions been similarly exposed, side by side with the watch-glass coytaining the dihydrate, withou: losing weight at all. I n one experiment, indeed, the monohydrate retained its weight unchanged for over a fortnight, and even then showed no sign of losing water. This may be an instance of suspended transformation, but the comparative rates of dehydra- tion of the two hydrates, obtained in other experiments, indicate that the process is different in the two cases.

A specimen of the monohydrate, kept in a cloBed vessel (a desic- cator) over 49 per cent. sulphuric acid, lost weight slowly, and after two days still retained half its water of crystallisation. A specimen of the dihydrate, on the other hand, exposed in the same vessel, lost weight rapidly, as shown by the following record:

Time of exposure. Per cent.

loss of weight. 3.5 hours 6.7 7 . 0 9 9 13 '3

22-5 ,, 17.0 17'4 Atter 3 hours further

in a vacuum desiccator 1 It will be seen that the dihydrate loses weight a t a nearly uniform

rate, whether its content of water is above or below that corre- sponding with the monohydrate. It is true that the rate of loss of weight becomes lower when dehydration is nearly complete, but the uniform rate persists until the point corresponding with the monohydrate is well passed.

The difference in the behaviour of monohydrate and dihydrate is clearly brought out by Fig. 2, in which two dehydration curves, similar to many others which have been recorded, are reproduced. Slightly moist crystals of the monohydrate were weighed out on a watch-glass, and this was kept all the time in the balance case, the air in which was partly dried by a few dishes containing concen- trated sulphuric acid. Weighings made from time to time showed how the dehydration was proceeding. The curve A , obtained by plotting the weight of water (in centigrams) held by 1 gram of anhydrous acid at various timee, shows that in the case of the mono- hydrate the mechanically held moisture come's off rapidly, and that when the composition has reached the point corresponding with a water content of 1 molecule per molecule of anhydrous acid, there

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SULPHANILIC ACID AND ITS HYDRATES. 289

is practically no further loss of weight under the conditions of the experiment. Curve B is a graphical reproduction of the corre- sponding record obtained with slightly moist crystals of dihydrate exposed t o the same conditions. The chief features of curve B are the change of slope about the point corresponding with the dihydrate, and the absence of a break, or even a change of direc- tion, a t the point corresponding with the monohydrate. It is perhaps worth noting that the crystals, whether of monohydrate o r dihydrate, employed in these experiments, had been filtered from their mother liquors and then transferred directly to the watch- glasses without grinding or further drying.

It has been suggested above that the absence of change in the monohydrate under certain conditions is probably due t o suspended

FIG. 2.

37-5

transformation. I n connexion with this, a comparative experiment was made with two specimens of monohydrate derived from the same crystallisation, one of which, however, wi18 finely ground, whilst the other was not so treated. On exposure to the air of the room, the untreated acid remained of constant weight, whilst the finely ground specimen lost weight dowly, much more slowly than unground dihydrate under similar conditions. Although, therefore, the state of division has undoubtedly some influence on the progress of dehydration, this factor cannot adequately account for the very different rates of dehydration observed for the two hydrates.

With the object of ascertaining the dissociation pressures of the two hydrates, specimens of each were exposed in closed vessels (desiccators) containing sulphuric acid of various strengths. It wi18 found that the monohydrate lost weight slowly over 35.4 per cent-

VOL. CHI. U

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‘290 JOWETT AND PYMAN: TH& ALKALOIDS OF

acid (aqueous vapour pressure about 11-6 mm. a t 20°), but retained its weight unaltered over 30.9 per cent. acid (aqueous vapour pres- sure about 12.9 mm. at 20O). The dihydrate lost weight slowly over 14.9 per cent. acid (aqueous vapour pressure about 16.5 mm. at 20°), but showed no loss over 9.4 per cent. acid (aqueous vapour pressure about 16.9 mm. at 20O).

The author desires t o express his thanks to Mr. H. R. Courtman and Mr. E. Jobling, who assisted in the preliminary stages of this investigation.

IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY, LONDON, S.W.

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