ELECTROCHEMICAL PROPERTIES OF MINERAL MEMBRANES
325
(3) LOEB,J.: Proteins and the Theory of Colloidal Behavior, Chap. VI, XI. McGraw-Hill Book Company, Inc., New York (1922). (4) MILLIGAN, W. O., AND WEISER,H. B.: J. Phys. Chem. 40, 1095 (1936). (5)OWENS,H.S.,AND MORRIS,R.: J. Phys. Chem. 42, 563 (1938). (6) PFEIFFER,P.: Ber. 40, 4036 (1907). (7) STIASNY,E.:Gerbereichemie. Steinkopf, Dresden (1931). (8) THOMAS, A. W.: Colloid Chemistry. McGraw-Hill Book Company, Inc., New York (1934). (9)THOMAS, A. W., A N D KREMER, C. B.: J. Am. Chem. Soc. 67, 1821,2538 (1935). (10)THOMAS, A. W., A N D MILLER,H. S.: J . Am. Chem. SOC.58, 2526 (1936). A. W., AND OWENS,H. S.: J. Am. Chem. SOC.67, 1825,2131 (1935). (11)THOMAS, A. W., A N D STEWART, W . G.: Kolloid-Z. 88, ‘279 (1939). (12)THOMAS, (13) THOMAS, A. W., AND TAI,A. P.: J. Am. Chem. SOC.64,841 (1932). A. R‘.,AND VOX WICICLEN, F . C.: J. Am. Chem. SOC.S6, 794 (1934). (14)THOMAS, A. W., A N D WHITEHEAD, T. H.: J. Phys. Chem. 36, 27 (1931). (15)THOMAS, (16) WEISER,H.B.: Inorganic Colloid Chemistry. Volume I I . The Hydrous Oxides and Hydrozides. John Wiley and Sons, Inc., New York (1935). (17)WEISER,H.B., A N D MILLIGAN, W.0.: Chem. Rev. 26, 1 (1939). (18) WERNER,A.-PFEIFFER,P. : Neuere Anschauungen auf dem Gebiete der anorganischen Chemie. Vieweg und Sohn, Braunschweig (1923). (19)WHITEHEAD, T . H.:Chem. Rev. 21, 113 (1937).
T H E ELECTROCHEMICAL PROPERTIES OF MINERAL MEMBRANES. I11 AND IV’.2 3 3
111. THEESTIMATIOK O F AMMONIUM-IOX ACTIVITIES4 C. E. MARSHALL
AND
W. E. BERGMASS
Department of Soils, Missouri AgricultuTal Ezperiment Station, University of Missouri, Columbia, Missouri Received August 11, 1941 I. INTRODUCTIOS
The experiments described in papers I and I1 of this series (1, 2) have demonstrated how potassium-ion activities, below 0.1 molal, may be determined both in true solutions and in suitable colloidal systems. In view of the close similarity 1 Paper No. 768,Journal Series, Department of Soils, Missouri Agricultural Experiment Station. 2 The authors are indebted to the Research Council of the University of Missouri for a grant-in-aid which has made this work possible. 3 In the papers of this series such terms as “potassium-ion activity” and “ammonium-ion activity” are frequently employed. This does not imply that we are dealing with an independent method of determining the activities of cations alone. This cannot be done without making non-thermodynamic assumptions. The methods used have the same validity as pH measurements and the cationic activities referred to have the same meaning as hydrogen-ion activities calculated from such measurements. Paper IV begins on page 327. Research Assistant.
326
C. E . MARSHALL A S D W. E . BERGMAN
in ionic radius (K+ = 1.33 A.; SH: = 1.48 Ai.),hydration (K+ = 1.1H10 and KH: = 4.4HzO when Cl- = 3.OH20), and velocity in the electrical field (K+ = 6.6 X and SH: = 6.6 X cni. per second per volt per centimeter) between potassium and ammonium ions, it seemed likely that the same technique might be applied to determine ammonium-ion activities as was successfully used for potassium. The following experiments demonstrate that this is the case. B.Y.P.’S
TABLE 1 obtained using electrodialyzed bentonite membranes (dried at 490°C.) with various ammonium salt solutions (al = 0.0712 molal S H L ) SALT
‘ONCENTRATION
COMPUTED ACTIVITY
NH:
N
NH,Cl. . . . . . . . . . . . . . . . . . . . . . . . . .
(SH4)&,04.. . . . . . . . . . . . . . . . . . . . .
+ +
XHICl HC1 pH = 3 05 SH4C1 HC1 pH = 4 11 SH,Cl CSCl, SHCl CaCI,
OBSERVED
COKPUTED E.M.P.
millinolri
millivallr
0.1 0.05 0.01 0.1 0.01 0.001 0.0001 0.01 0.001
0.0712 0.00859 0.000957 0,0000985 0.0683 0.0366 0.00832 0.0683 0.00832 0,000946 O.ooo0981 0.00832 0.000946
0.0 53.3 112.4 167.7 4.1 16.8 54.0 2.8 54.1 111.6 160.6 56.3 110.9
0.0 54.3 110.9 169.1 1.1 16.7 55.2 1.1 55.2 111.0 169.2 55.2 111.0
0.001
0.000938
89.8
94.1
0,001
0.000944
109.6
109.0
0,000930
109.3
111.4
0.00801
55.2
56.1
0.1 0.01 0.001
o.ooo1 ( K H ~ ) ~ S O. .I . . . . . . . . . . . . . . . . .
MEAN E.M.F.
I;:: 0.01
11. EXPERIMESTAL
In view of the esperience gained with potassium ion and its general similarity to ammonium ion, it was decided to investigate the applicability of the Sernst equation over a range of concentrations from 0.1 N downwards. As previously described (I), the membrane separating the solutions to be compared consisted of a film of bentonite clay prepared by evaporating a colloidal, electrodialysed suspension (particles all < 0.2 p in equivalent diameter) to dryness and subsequently heating the film to 490°C. Such membranes are sensitive to hydrogen ions as well as to other monovalent cations; hence whenever the hydrogen-ion activity formed an appreciable proportion of the total cationic activity, a modified form of the Sernst equation was used. As previously pointed out, high
ELECTROCHEMICAL PROPERTIES OF MIXER.4L YEMBRASES
327
acidities are not tolerated by these membranes, which give erratic results a t pH values below 4. The cell used may be represented as follow:
Hg I HgtC12 1 satd. KCI 1 Salt solution I 1 Membrane, 1 Salt solution I satd. KCI I Hg?CI?,Hg a?
a1
in which a1 is the activity ( = 0.0712) of-the 0.1 molal ammonium chloride ~ o l u tion used as the reference solution, and a2 is the total activity of hydrogen plus ammonium in the solution with which it is compared. The Sernst equation for 25°C. then takes the form: 0.0712 E.M.F. (in millivolts) = 59.15 log10 [SH:I [H+l
+
the square brackets indicating individual activities in as. When [H+]is small compared lyith [NH;], as was generally the case, we then have: E.M.F.
0.0712 (in millivolts) = 59.15 log,o -[SH:]
The hydrogen-ion activities were determined by the use of the glass-membrane electrode. The clay membranes employed were soaked first in 1.0 molal ammonium chloride solution until zero asymmetry potential was attained and then with the 0.1 molal solution on both sides of the membrane. Only membranes which also showed zero asymmetry potential in this solution were used. The experimentally determined potentials given in table 1 are mean values for two or more membrane.
