I 168
[CONTRTBUTION FROM
ALBERT BRAmT. THE
DEPARTMENT OF CHEMISTRY, UNIVERSITY OF WISCONSIN. ]
THE EFFECT OF DISSOLVED SUBSTANCES ON THE VELOCITY OF CRYSTALLIZATION OF WATER. III. FURTHER EVIDENCE THAT THE EXISTENCE OF HXDRATES IN SOLUTION EXPLAINS THE RETARDING EFFECT OF THE SOLUTE ON THE VELOCITY OF CRYSTALLIZATION OF WATER. BY ALBERT BRA".'
Received May 7 , 1918.
In two pxevious papers2 it has been shown that hydration, in all probability, is the biggest factor in the retarding effect of dissolved substances on the V. C. (velocity of crystallization) of water. It was pointed out, however, that some of the data from which conclusions were drawn were obtained in dilute solutions, with the result that the values of the V. C., in many cases, were so close numerically, that the differences might be accounted for by experimental error. Furthermore, the study had been almost entirely limited to the use of inorganic compounds as the retarding agents. Consequently, the present work was undertaken to broaden the scope of the experiments to several distinct and varied classes of compounds, to abolish doubt as to the validity of the results by using more concentrated solutions, and to substantiate the view that hydration in solution is the chief factor in the retardation of the V. C. of water solutions. The problem has been supplemented from a three-fold standpoint: the effects produced by certain classes of ( I ) inorganic compounds, ( 2 ) organic compounds, and (3) colloids.
Experimental. Apparatus.-A new apparatus was constructed, differing in several particulars from the old Two of the most important changes are worth mention. The secondary bath was lined inside with sheet tin instead of galvanized iron, because the latter was attacked chemically by the solution in the bath to such an extent that a heavy precipitate formed which interfered with observations in the tube. This bath was also made 3 inches wider so that observation windows could be built into both sides of it. With this arrangement, two tubes can be placed in the bath simultaneously, thus doubling the number of readings in a given time. Calibration aqd Choice of Tubes.-Certain dBculties had arisen in connection with the tubes that had been used in the work up to this point. It was therefore decided to make a critical study of different types of tubes in order to determine the best kind of tube for the work. In selecting tubes, 3 important questions arise: (I) What are the effects of 1 Part of a thesis submitted to the graduate school of the University of Wisconsin as partial fulfilment of the requirements for the degree of Doctor of Philosophy * Walton and Brann, %IS JOURNAL, 38, 317 (1916); 38, 1161(1916) Walton and Judd, J. Phys. Chem., 18, 7 2 2 (1914)
I 169
VELOCITY OF CRYSTALLIZATION OF WATER.
the size of the bore and the thickness of wall on the V. C.? ( 2 ) In which type is there the least spontaneous crystallization? (3) Which type will give the most reliable and constant readings? To determine the effects of these variables, Stone' carried out a series of experiments with a large number of soft glass tubes, selected with particular care as to uniformity of bore and thickness of wall. These were standardized against pure water and 0 . 5 M potassium chloride, but only those which checked closely were used. Table I shows that of the 3 types selected, the tubes of Type I and those of Type 3 had approximately the same thickness of wall, but the diameter of the bore of the former was twice that of the latter. The tubes of Type 2 and those of Type 3 had about the same sized bore, but the walls of the latter were approximately twice as thick as those of the former.
TABLEI. Showing the Bore and Thickness of Walls of the Three Types of Tubes Used.
---Type 1 .
Diameter of bore. Mm. 5.6
5.6 5.5 5.7 5.3 Av., 5.5
Type 2 .
Type 3 .
