GLYCINE I N WATER SOLUTION* BY JESSIE Y. CANN
A stJudyof the structure of glycine from the absorption spectra of the compound dissolved in water, in hydrochloric acid solution and in sodium hydroxide solution has just been made by Anslow, Foster and K1ingler.l They also made a study of the freezing points of glycine in water solution. Using their experimental data for freezing points, we, in this paper, have calculated various thermodynamic functions, and have considered glycine as a micelle. The term micelle, meaning an aggregation of ions or molecules in solution, acting as a unit, has been uaed by several authors.* Hoskins, Randall and Schmidt have studied the activity coefficients of glutamic and aspartic acids. Although these are di-carboxylic acids, the authors assumed that the ionization of the second carboxyl was negligible. Glycine, or amino-acetic acid, has been compared with these acids, considered as micelles, and striking resemblances occur. Table I contains all the calculated thermodynamic data. The first two columns give the molalities, m, and the freezing point depressions, 8, obtained by Anslow, Foster and Klingler.’ In column eleven are given the activity coefficients, calculated according to the equations for concentrated solutions given by Lewis and RandalL3 In the fifth colum are given the values of ofj =
I
e --vXm’ where e = the depression of the freezing point, v the number
of ions assumed at infinite dilution, X the molecular depression of water i.e. 1.858’, and m the molality. The value for v was taken as 2 , on the assumption that glycine is a uniunivalent electrolyte. Log j was next plotted against log m, and the values of a and /3 determinedS3 The value for a was taken as 0.3 and that for /3 as 0.534. Then j was plotted rn
against log m, and the values of the function f j d log m obtained from the 0
areas under the curve, to each of which was added the area from infinite dilution up to molality = 0.01,obtained from the equation,
* Contribution from the Chemical Lahoratory of
Smith College. Anslow, Foster and Klingler: Unpublished work. * a. McBain and Salmon: J. Am. Chem. Soc., 42 26 (1920).b. Randall, MoBain and White: 48, 2517 (1926);c. Randall and Cann: 50, ?1928).d.L n d a l l and Cann: Chem. Reviews, 7, 369 (1930);e. Hoskins, Randall and &midt: Biol. Chem., 88, 215 (1930). 8 Lewis and Randall: “Thermodynamics,” pp. 342, 346, 347 (1923). 1
JESSIE 1'. CANN
2814
TABLE
P
a = 0.3 m
0
0.010
0.029
0.IO0 0.210
0.195 0.389
0.385
0,710
0.572
0.765 0.986
1,035 I ,382 I . 769
1.200
2.121
I .500
2.679
m
0
log m
2 . 0000 I .oooo I
,3222
~.5855 1.7574 i.8837 1.9939 0.0792 0.1761 Tj d log m 0
1 0.534
=
m? 0.1000
0.3162 0.4583 0.6205 0.7563 0.8746 0.9930 1,0955
0,5171
0 .j 2 0 7
7.7136
0.5244
0.4787
I ,2248
0.5194
0.4241
7.7196 1.7155
1 __
2.303
-1%
Y
Y
0.2896
0.5129
0.7695 0.9381
0.1;oo
1.382
0.9402 1.0039
0.0954 0.2064 0.2178 0.2187 0.2228 0.2231
1.769
1.oj51
0.2245
1.2796
0.0525
1.200
2.121
1.1002
0.2277
0.0470
1.500
2.679
I . I ~ I ~0.2255
1.3279 1.3772
0.010
0.029
0.1942
0.100
0.j631 0.7203
0.385
0,195 0.389 0.710
0.572
1.035
0.765 0.986
0.210
0.8515
0.1151
1.0702
0.085 I
1.1630
0.0687
1.2270
0.0593
0.0420
K 5 . 4 0 x IO-^ 3.48 X IO-^ 3.14x10-3 3 . 0 5 x IO-^ 2.90X10-~ 2.86X10-~ 2.87X IO-^ 2.78X1o-3 2.76X IO-^
The values for the total areas are given in column ten. Then the activity coefficients, y, were calculated from the equation,
The last term was neglect,ed. The values of the activity coefficients are listed in the thirteenth column. It will be noticed that the values of y are small, as was to be expected. This is in agreement with the values obtained by Hoskins, Randall and Schmidt2"for glutamic and aspartic acids. We therefore assume that glycine associates or forms micelles in concentrated solut,ions. This is also in agreement with the results of Anslow, Foster and Klingler' in postulating polymerization in concentrakd solutions. In Curve I we have plotted the values of y against m. In Curve I1 we have plotted the values of j/mi against mi. This curve shows t,hat the results
GLYCINE IN WATER SOLUTION
2815
CURVEI
are in agreement with those obtained for other substances by the authors2 who have assumed micellation. In Curve I11 we have plotted -log y against mf and find that it can be compared favorably with the composite curve of Randall.4 For curiosity, we have calculated values of K on the basis that glycine dissociates in water solution into two ions. We have therefore taken K to be equal to (m y)2/m(I -7). These values are listed in column fourteen of
CURVEI1 Randall: J. Chem. Educ., 8, 1066 (Fig. 2 ) (1931).
2816
JESSIE Y. CANN
] IS!
CURVEI11
Table I. The values of K, and Kb, as determined by Branch and Mijarnoto,j are really hydrolysis constants. They are also considered as such by Adams6 and by B j e r r ~ r n . ~ The conclusion of this study is that glycine, considered as a micelle in concentrated solutions, shows good agreement with other substances considered thermodynamically on the same basis. 6 Branch and Mijamoto: J. Am. Chem. Sac., 52, 863 (1930). 6Adams: J. Am. Chem. Sac., 38, 1503 (1916). Bjerrum: Z. physik. Chem., 104, 147 (1923).
