function of the volume concentration, c, over the whole range at 30' and up to 3 molar a t 25'. Urea therefore behaves like sucrose and not like typical electrolytes, the apparent molal volumes of which increase linearly with clb. The experimental results and the partial molal volumes of
[CONTRIBUTION FROM THE
urea and water are tabulated. Equations giving the apparent molal volume and density as functions of concentration are also included. The density of solid urea is found to be 1.329 g./ml. a t 25' and its molal volume 45.19 ml. RECEIVED AUGUST1, 1938
EVANSTON, TI,I.INOIS
DEPARTMENT OF CHEMISTRY, COLUMBIA
UNIVERSEY]
Kinetics of Neutralization of Pseudo Acids in H,O and D,O BY SAMUEL H. MARON'AND VICTOR K. LA MER In this paper are reported kinetic measurements aci acid -+ aci ion. However, Pedersen' has by a conductance method on the neutralization a t shown that this view is inconsistent with present 0 and 5' of nitromethane, nitroethane, and nitro- ideas of acid-base catalysis as outlined by Bronsted.& The isomerization of a nitropar&n must isopropane by hydroxyl and deuteroxyl ions. The familiar protolytic processes of ionization be considered a prototropic process, and as such and neutralization of acids are in most cases not it cannot take place spontaneously, but must inamenable to kinetic study for they proceed, if not volve the intermediary of an acid or basic catainstantaneously, at least with velocities too high lyst. In conformity with this view he actually to measure. In the class of substances designated found the isomerization of nitromethane to the as pseudo acids, however, ionization and neutrali- aci form to be a reaction subject to general base zation proceed at a much slower rate, and in cer- c a t a l y ~ i s . ~He consequently postulated as the mechanism of neutralization tain cases can be followed kinetically. This dif€erence in rates of protolytic transfer RCHzNO:! f B RCH-aNOz BH' k k has been employed by Hantzsch* to define a RCH=N02BH+ (2) pseudo acid as a substance whose salt formation is a function of time, as against the acid whose z . e., the nitro form donates a proton to the base, neutralization is essentially instantaneous. He and the residue is converted rapidly by an elecstates, further, that "if the salt formation is a tron shift to the ion. Thus k:
