The rate of solution of solid substances in their own

930

ARTHUR A . NOYES A N D WILLIS R . WHITNEY

Chlorine

. . .. . . . . . . . .... . . . . .. . . . .. . . . .... 2 1 . 7 7

Per cent.

Monochlor naphthol requires 19.66 per cent. ; dichlor naphthol requires 33.02 per cent. There was evidently no simple substitution of C1 for NO,, and it was plain that more complicated changes than those involved by substitution had taken place. T h einability to obtain a product offering the assurance of purity, caused a discontinuance of the work. T h e expulsion of the nitro group by chlorine under these conditions, is, however, worthy of note.

THE RATE OF SOLUTION OF SOLID SUBSTANCES IN THEIR OWN SOLUTIONS.
BY ARTHCR A . NOYES A N D WILLIS R W H I T J E Y
Reiei5ed October
ir, r 8 9 j

at which a solid substance dissolves in its ownsolution, has not heretofore been investigated. This is probably due to the experimental difficulties in the way of keeping the surface of the dissolving substance constant during the solution. T h e question is an important one, especially because of the light its solution would throw upon the attainment of the state of saturation, which is a matter of importance in all solubilitydeterminations. Therefore we have investigated the phenomenon and have succeeded in experimentally es!.ablishing the law according to which the process takes place. The experiments were carried out in the following manner : I n order to keep the alteration of the surface-area of the dissolving substances as small as possible, the slightly soluble substances, benzoic acid and lead chloride, were chosen.Cylinders of these two substances, about eight cm. long and two in diameter, were prepared upon cores of glass. I n the case of benzoic acid, this was done by simply pouring the melted acid into a testtube containing a glass rod flattened a little at the lower end and protruding at the other end above the test-tube. When the whole had cooled, the cylinder of acid adhering well to the glass rod couldusually be easily removed from the tube. It was found necessary in the case of lead chloride to proceed differ-

A

S far as we know, the effect of the concentration on the rate

SOLUTION OF SOLID SUBSTANCES.

931

ently, because the cylinders of it, made as above, broke upon cooling. They were finally made by dipping the glass rodsinto the molten salt, removing then a second or two to allowthem to cool, and repeating the operation until a 3ufficient amount had solidified upon the rods. Both ends of the cylinders of the two substances were coated with parasin, and the free ends of the glass rods were forced through the centers of numbered paraffined corks, each of which fitted into a wide-mouthed bottle of 130 cc. capacity. I n conducting the experiments, exactly roo cc. ofdistilled water were placed in each of the six bottles, and these were suspended in a thermostat kept a t 2 ' C. within a few thousandths 5 of a degree. After the temperature of the bath had been attained, the corks bearing the sticks of substance were inserted in the bottles so that the cylinders were in the middle of the bottle, and covered with water, as shown in the cut. T h e

-- r r \-f-- -- - --- ---- - - - - -. - - i

-

- -, -

time was then noted and the bottles were rotated' in a horizontal position in the bath exactly ten (or ten and a half) minutes, a t the rate of sixty rotations per minute. T h e sticks were then removed from the bottles : and the solutions titrated directly, in the case of the benzoic acid, with sodium hydroxide solution, and in that of the lead chloride,with silver nitrate and potassium thiocyanate. T h e same sticks were then returned to the bottles, which were charged with new portions of roo cc. of water, and were rotated as before for thirty, and finally for sixty minutes, when the resulting solutions were analyzed. I n order to have as nearly as possible the same conditions in the different experiments, each series of three determinations...