III.-On the Relative Bulk of Weak Aqueous Solutions of certain Sulphates and their Constituent Water.

By CHARLES M. PASEA, B.SC., Dalhousie College, Halifax, N.S.

(Communicated by Prof. J. G. MacGregor, and read May 29, 1900.)

In a paper communicated to the Royal Society of Canada,' Dr. MacGregor has shown that in the case of weak aqueous solutions of certain sulphates, the solutions have a smaller volume than the water which they contain would have in the free state. To determine how many of the sulphates exhibit this property, he collected the previously published observations of the specific gravity of solutions of these salts and made some additional observations himself. In the case of some of the sulphates previously examined the solutions were not sufficiently dilute to settle the question under consideration. Accordingly at his suggestion I have examined the sulphates of sodium, manganese, cadmium, and iron (the ous salt), as in these cases the data did not extend to very dilute solutions, and from the data available for the most dilute solutions it appeared possible that at a greater dilution the volume of the solution might become less than that of its constituent water. With this object in view it was only necessary to examine solutions more dilute than those referred to in the above-mentioned paper. I have also used Dijken's observations on ammonium and lithium sulphates for the same purpose.

2

The salts examined had been purchased as chemically pure, the cadmium and manganese from Messrs. Eimer and Amend of New York, and the sodium and ferrous sulphate from E. Merck, Darmstadt.

In the determinations of the specific gravity of the solutions, the pyknometer employed was of the Sprengel form as modified by Ostwald. It weighed about 23 grams, and had a capacity of about 25 cubic centimetres. The pyknometer after being washed out with the solution whose specific gravity was required, was filled and placed in a waterbath, the temperature of which was kept at 18° C. The water in this bath was kept constantly stirred by means of two vanes set obliquely in and rotating round, a vertical axis. The stirrer was driven by means of a small hydraulic motor. The thermometer employed to indicate the temperature of the bath was graduated to fiftieths of a degree centigrade, and its errors had been determined at the Physikalisch-technische

1 Trans. Roy. Soc. Canada (1), 8, Sec. III., 19, 1890.

2 Ztschr. f. phys. Chem., 24, 80, 1897.

Reichsanstalt, Berlin. With a little care the temperature of the bath could be kept at 18° C., as the temperature of the room was generally not far from that. A change of 04 degree would cause a perceptible displacement of the meniscus of the solution relative to the mark on the stem of the pyknometer. After the pyknometer had been for some time in the bath, the liquid was gradually withdrawn until the meniscus coincided with the mark on the stem. A few minutes later, if the coincidence still existed, the pyknometer was taken out, the outside washed by means of a jet of water from a wash bottle, dried with a soft cloth and weighed.

The balance used was made by A. Collot of Paris. As the weight of the pyknometer and solution was approximately 48 grams, a body of about 50 grams was weighed several times, the weighings being performed on several days, but in the same manner as was employed in weighing the pyknometer, which will be described further on. The maximum deviation from the arithmetic mean was 0002 grams. Regarding this as the possible error of a weighing, it can be determined to what degree of accuracy the density of the air should be known, in order to correct for the buoyancy of the air. In these corrections the air in the balance case was regarded as kept quite dry by the calcium chloride placed there.

On making the calculation it is found that the density of the air should be known to 5 in the sixth place of decimals. From this can be calculated how accurately the barometer and thermometer should be read at the time of a weighing. The barometer by means of a vernier could be read to 002 inch; but as the instrument had been in use for some time and its error was not known, I thought it advisable to read the thermometer in the balance case as accurately as possible, and deduce the necessary accuracy in the reading of the height of the barometer. The thermometer in the balance case was compared with the thermometer already mentioned, and its errors noted. It could be read to of a degree centigrade. Hence the height of the barometer should be known to 2 millimetres, or for convenience in reading 05 inches. In the correction for buoyancy the density of the air was taken from Table 6, Kohlrausch's Physical Measurements (1883).

The operation of weighing the pyknometer when filled was conducted as follows: A counterpoise was placed in the left pan, and in the right the pyknometer together with weights necessary to cause the pointer to oscillate about a point near the centre of the scale. The pyknometer was then removed, and weights added until the pointer. oscillated about some point near the former. The sensitiveness of the balance for the weights employed being known, the apparent weight of the solution and pyknometer could be calculated. The point of oscillation of the beam when loaded with counterpoise and weights was

regarded as constant throughout a morning's or afternoon's work, provided the balance had not been used for weighing a much heavier object. In all cases the weighings were performed as above, the counterpoise method being employed.

The weights used were a small set obtained from a trustworthy firm, and were made up of brass weights amounting to 20 grams, the smaller weights being of platinum. They were calibrated according to the method advised by Kohlrausch,' the sum of the weights being put as correct. Thus in the calculation of specific gravity all errors due to inaccurate weights were eliminated. Also as the corrections were small and only required in a few of the weights, I have great confidence that the error due to this cause in the determinations of the concentrations would lie within the limits of my experimental error.

The pyknometer having been carefully dried and treated as described was weighed, the mean of 10 weighings being used. To get the weight of water contained in the pyknometer, the distilled water supplied to the laboratory was carefully redistilled. The weights of water when corrected for buoyancy showed a maximum deviation of 003 per cent for their mean value as calculated from 12 observations. Hence in the determination of the specific gravity the possible error would be ⚫006 per cent. To reduce this error to 005 per cent on an examination of the 12 values it was found to be necessary to take the weight of a solution as the mean of three determinations. This was therefore done. Hence the specific gravity of a solution, and therefore its density has an error less than 5 in the fifth decimal place. The density was found by multiplying the value of the specific gravity by the density of the water at 18°, as given by Landolt and Börnstein.2

The water used in making up the solutions was the distilled water supplied to the laboratory, which would contain some air dissolved in it. But it was found that 16 samples of this water, taken from supplies received at different times, gave as their average specific gravity referred to the redistilled water 99998, the values ranging from 1.00003 to .99996, which lie within the limits mentioned above.

In all cases at least two solutions of any one salt were prepared directly, the concentrations of which were determined by analysis, the sulphate being precipitated in the form of barium sulphate which was collected and weighed. The amount of the original salt was deduced by means of the chemica equation, the atomic weights of the elements. being taken as given by Clarke, quoted in Landolt and Börnstein. The solution to be analysed was placed in the bath at 18°, and after a certain time known volumes were taken out in a pipette, and analysed. The amount of barium sulphate precipitated in one case from a solution Kohlrausch, Physical Measurements, p. 35.

2 Physikalisch-chemische Tabellen, 2 ed., 1894.

delivered by a 50 c.c. pipette had the following values: 0:4493, 0-4490, 0.4487, 0-4487, 0-4493, 0-4495, giving as mean value 0-4491 grams and indicating a possible error of about 0.1 per cent.

The pipettes used for analysis and in dilution were of the approximate volumes 25, 50 and 100 c.c. They were treated as nearly as possible in the same way as when calibrated, Ostwald's suggestions' being carried out, as to time of delivery and mode of handling. They were calibrated by weighing the amount of water they delivered at 18°, the mean of at least three determinations being taken.

Other solutions were then made up from those prepared directly, by taking out a certain quantity in a pipette and adding water from the same or another pipette, the temperature of the solution and water being 18°.

a

The percentage concentration of the solutions i.e., the number of grams of salt in 100 grams of solution, was calculated as follows :-Let M be the mass of salt found in volume P. of pipette used for analysis, P, the volume of the pipette used for measuring the solution which was to be mixed with water, P, the volume of the pipette used for measur ing the water which was to be added to the solution, D the density of the solution from which the mixture was made up, D, the density of the water used for dilution, and p the percentage concentration. Then

The second term in the denominator did not need to be calculated, as it had been found experimentally when the pipettes were calibrated, this being the mass of the water delivered, since the calibrations of the pipettes and the making up of the solutions were performed at the same temperature. Also in the solutions analysed directly, the above formula gives their concentration by putting P, equal to P. and the last term of the denominator equal to zero.

The weights of water delivered by a pipette in no case differed from their mean value by more than 015%. Hence the possible error in the numerator is 12%, and in the denominator is 035%, giving a possible error in the result of about 15%.

Knowing the concentration and the density of a solution, (1) the volume of 1 gram of the solution at that temperature, and (2) the

1 Physico-Chemical Measurements, p. 85.