Page:Outlines of Physical Chemistry - 1899.djvu/265

 �CHEMICAL DYNAMICS

the modus operandi. This can best be done by tnV|<3l!on£ ing two examples.

1. Inversion of Cane Sugar. — According to Ostwald, the following procedure may be adopted : 10 cubic centi- metres of sugar solution (200 grams per litre) are mixed with 10 cubic centimetres of normal hydrochloric acid solution in a small 20 c.c. flask which is kept in a bath (furnished with a thermostat) at 25°C. Before mixing, the liquids must be at the temperature 25°C, and this temperature must be carefully maintained during the whole course of the experiment. From time to time determinations of the rotation of the solution are made, and the number of minutes from the commencement of the experiment until each determination is made is noted. The polarimetric tube must, of course, be kept at 25°C, and after each determination of the rotation the solution should be put back into the small flask. Ten or more observations at convenient intervals of time l should be made, then, after an interval ten times longer than that required for the inversion of half of the sugar, the angle of rotation is determined for the last time. We may then calculate for each value of t the corresponding value of x (decomposed sugar), and make use of equation 2 to study the constancy of the coefficient k. Or we may calculate directly from the rotations observed. In the equation

_ It a Q — A K = -. log -Z -,


 * >n <*n — A

ci is the initial angle of rotation, a n the angle observed at the time t ni and a the final angle. This formula is only a modification of equation 1 and is based on this considera- tion — namely, that the initial quantity of sugar (a in equation 1) is proportional to the difference between the initial and the final rotations, exactly as the quantity

1 The first observation is made immediately after starting the reaction ; the following observations after intervals of 30 minutes, 60 minutes, 120 minutes, and so on.

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