DISSOCIhTlOS O F M O S O M E T H Y L O L V R E l
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SUMMARS
A dew-point method for experimental measurement of the saturated vapor densities of each component of a binary system over the entire range of composition is described. It is particularly applicable to solutes of slight volatility. The equilibrium vapor-phase compositions of aqueous triethylene glycol solutions have been determined by this method. Good definition of the end-point was obtained for partial pressures of triethylene glycol as low as lo-‘ mm. of mercury. h method of analysis of these data is described which permits computation of the composition of the liquid phase in equilibrium with any saturated vapor mixture. The method depends upon evaluation of the derivatives of the activities of the components with respect to each other. Such an analysis of the triethylene glycol-mater system indicates the existence of negative deviations from Raoult’s law, REFEREKCES (1) LEWIS. ~ X D RASDALL: l’herinodynaniics and the Free Energy of Chemical Substances. McGraw-Hill Book Company, I n c . , S e w T o r k (1923). (2) P ~ C . K ~ S D BISE: J . Phys. Chem. 60, 329 (1946). (3) Tovs-c .4sn VOCEI.: J . Am. Chem. Soc. 64, 3025 (1932).
RiITE OF DISSOC1;ZTlOS OF 3IOSO3IETHYLOLUREh I S HYDROCHLORIC ,SCID SOLCTIOW LAURASCE H . DOSKALLY Donnally Research and Comulting Laboratory, 484 Fourth Acenue, S u n Francisco, California
Receited A u g u s t 8 , 1949 IKTRODCCTIOS
Urea and formaldehyde react reversibly in aqueous solutions to form various crystalline substances, such as monomethylolurea and dimethylolurea. Smythe (4)has investigated the kinetics of formation of monomethylolurea in the absence of buffers and in the range of pH from 4 to 6; his initial concentrations were 1.88-2.52 moles per liter. Crowe and Lynch (1) have measured the equilibrium and rates of association and dissociation of monomethylolurea at a single hydroxide-ion concentration of 0.05 M by a polarographic method. I t is the purpose of this investigation to present the rates of dissociation of monomethylolurea at hydrochloric acid concentrations of 0.005 to 0.25 M , with added salt concentrations to 0.35 III in potassium chloride. The dissociation and formation of monomethylolurea may be represented as Follo\\.s: HzSCOSHCH?OH $ HzSCONHZ HCHO (1)
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LACRASCE H . DONNALLY
However, there are evidently side reactions also since, at concentrations of monomethylolurea higher than approximately 0.01 M, a white precipitate is formed. For this reason the measurements \?-ere all made at a concentration of approximately 0.0057 M .
.5
TIME HOURS
*5
i 0
20
-Log HCl
FIG.1. FIG.2. FIG. 1. Dissociation of monomethylolurea a t various hydrogen-ion concentrations, where a is the initial monomethylolurea concentration a t 18.S"C. FIG.2 Plot of - log k, versus - log ("2)
EXPERIMENTAL
Monomethylolurea was synthesized by the method of Einhorn and Hamburger (3). The melting point of the thoroughly dried material v a s l l l ° C . (uncorr.) The rate of the dissociation was followed by measurements of the formaldehyde concentration by a method of Donnally (2) which consists in neutralizing the sample to approximately pH 7 with sodlum bicarbonate, adding excess sodium bisulfite, allowing to stand for 15 min., adding acid to pH 3, titrating the excess bisulfite with iodine solution, finally adding sodium carbonate solution, and titrating the formaldehyde bisulfite compound. The total
COMXZUPiICATIOS TO THE EDITOR
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amount of the monomethylolurea was measured by adding to the sample sufficient of an equimolal mixture of disodium hydrogen phosphate and sodium dihydrogen phosphate to make the total phosphate concentration 0.3 .If. The sample was then heated on a steam bath for 1 hr. and finally assayed for formaldehyde as before. The monomethylolurea concentration should be below 0.006 Jf for 99 per cent dissociation of the compound under these conditions. I n figure 1 is shown a plot of log a / ( a - x), where a is the initial concentration of monomethylolurea and x is the concentration of formaldehyde at time t against the time in hours. Initial monomethylolurea concentrations varied from 0.0054 to 0.0061 Jf and the hydrochloric acid concentrations in different experiments were 0.0058, 0.0295, 0.05, and 0.25 ,If. Owing to the formation reaction and perhaps also to the side reactions the plots deviate toward the end of the reactions from a straight line. However, it is possible to compare the reaction rate constants, kl, by use of the slope of the curve at zero time. In figure 2 is shown a plot of log Izl against log (HC1). In table 1 the data are tabulated; also included are values obtained at concentrations of potassium chloride up to 0.35 31. SUhlhlARY
The rate of dissociation of monomethylolurea at hydrochloric acid concentrations of 0.0058 t o 0.25 -If and at 18.8”C. was measured. The rate was found to be directly proportional to the concentration of hydrochloric acid in that range of hydrogen-ion concentration. REFEREXCES
(1) CROWE,G. h.,JR.,ASD LYSCH,C. C.: J. Am. Chem. SOC.70,3795 (1948). (2) DOSNALLY, L. H . : Ind. Eng. Chern., Anal. Ed. 6, 91 (1933). (3) EISHORN, A , , A K D HAMBURGER, 9.: Ber. 41, 24 (1908). (4) SMYTHE, LLOYD E.: J. Phys. & Colloid Chem. 61, 369 (1947).
CO~IMUXICATIO?iTO THE EDITOR ADDITIOS TO T H E PAPERS “THE ORIGIS OF T H E BI-IONIC POTESTIAL ACROSS POROUS MEMBRASES OF HIGH IONIC SELECTIVITY. I h K D 11.”
J. Phys. Sr Colloid Chem. 63, 1211, 1220 (1949) In reviewing the literature on the bi-ionic potential (B.I.P.) important work by K. H. Meyer and P. Bernfeld (Helv. Chim. Acta 28, 962, 980 (1945)) was missed. These investigators have reported some additional measurements on B.I.P. chains which check with those of previous workers. More important, they have applied the fixed-charge theory t o this problem, anticipating thereby, in a somewhat different terminology, major aspects of our later much more extensive
