NOTES
Ueo., 1954
detergent molecules, oriented radially, and entrapping some 6000 molecules of water. The
1167
radius of the sphere is that of a nearly conipletely extended detergent molecule.
NOTES POTENTIOMETRIC STUDIES ON SODIUM ACETATE-SODIUM PERCHLORATE SYSTEMS I N ACETIC ACID1
the dissociation behavior of a base such as sodium acetate in acetic acid. By means of a glass-calomel electrode assembly, it was possible to obtain results which are in accordBYTAKERU HIGUCHI, MARIA L. DANGUILAN AND ance with those obtained by Hall and Werner,4the AAROND. COOPER latter workers having employed a chloranil-calomel School o/ Pharmacy, University of Wisconsin, Madison, Wisconsin electrode system as a basis for their potentiometric Received June 1 4 ~ 1964 measurements. In our laboratory it has been found The present communication is concerned with the that the glass electrode responds to the hydrogen influence of sodium perchlorate on the relative ion activity of acetic acid systems (or equivalently, basicity of sodium acetate in acetic acid. Although to the acetate ion activity). Figure 1 shows, for the acetate salt is presumed to be nearly completely example, the effect of variation in the relative basicionized (in the sense that electron transfer from the ity of acetic acid containing varying amounts of cation to the anion is essentially complete), such acetate as a function of the concentration compounds have been found to exist mainly in the sodium of base present as obtained by the present potentioform of neutral “ion-pairs” in solvents of low dielec- metric method. In accordance with the work of tric constants.2p3 No previous attempts seem to the previous author^,^ the slope of the dilution curve have been made, however, t o study the possible is close to 0.5, Le., 0.059/2 volts, per tenfold change influence of other salts having a common cation on in concentration, the solid line representing the theoretical slope and the points the experimental values. I n the presence of a constant sodium perchlorate concentration, as shown in Fig. 2, this slope becomes 0.0500
0.02000
0.0250
d
0.01000
El 0
d
42 P)
g 0.0100
8 8
a
3 0.00500
6
5
0
2
3
a a
$ 0.0050
m 0.00250
0.0025
0.00125
0.0010 20 30 40 50 60 70 Millivolts. Fig. 1.-Potentiometric study of the behavior of sodium acetate in glacial acetic acid. 0
10
__
( 1 1 Supported in part by a grant from the Research Committee from funds supplied by the Wisoonsin Alumni Researoh Foundation. (2) I. M. Kolthoff and A. Willman, J . A m . Chem. SOC.,156, 1014 ( 1 931). (3) E . Griswold, M. M. Jones and R. K. Birdwhietell, ibid., 71, 5701
(1953).
80 100 120 Millivolts. Fig. 2.-Potentiometric determination on sodium Derchlorate-sodium acetate system; concn. of sodium perchlorate in M : A, 0.00125; B, 0.00250; C, 0.00500; D, 0.01000; E, 0.02000. 20
40
60
unity, Le., 0.059 volt per tenfold change in the sodium acetate concentration, while in the alternative situation, where the sodium acetate concentration (4) N. F. Hall and T. Werner, ibid., SO, 2367 (1928).
NOTES
1168
is fixed and that of the perchlorate varied (Fig. 3), a slope of 0.5 for a tenfold change in concentration is observed. In both cases, a linear relationship is present as the Concentration of one of the moieties is varied while the other is maintained constant. These results may be rationalized by assuming that sodium perchlorate undergoes greater dissociation than sodium acetate (a greater degree of "ion-pair" separation), hence the sodium ion concentration of the solution may be attributed mainly to the sodium perchlorate. By expressing the dissociation constants for the two salts as K N ~ C I=O(Naf)(C104-)/(NaCl04) ~
(1)
K ' N ~ =A (Na+)(Ac-)/(NaAc) ~
(2)
and employing the simplifying assumption6 above that the concentration of sodium ion is determined essentially through the dissociation of sodium perchlorate, we obtain the relationship
+ 3)
(2
(Na +) E { K( NaClO4)] '/n (Ac-) S K'(NaAc)/K'/z(NaC104)'/2
(3)
(4)
Equation 4 shows a first-order dependency on the sodium acetate concentration and an inverse square root dependency on the sodium perchlorate concentration. Since experimental data are in close agreement with those predicted by equation 4, it may be concluded that sodium perchlorate is a ('stronger electrolyte" than sodium acetate in acetic acid, hence repressing the dissociation of sodium acetate, resulting in a decrease in the relative basicity of the solution.
VOl. 58
It is evident from these behaviors that the common practice of estimating pK values of acids and bases in water by determining the pH of half neutralized solutions cannot be used in certain nonaqueous systems. The apparent basicity of a base! as in the present instance, is dependent in part on the strength of the acid employed in its neutralization. Stronger the acid, weaker the base would appear since the resulting salt formed would be more highly dissociated, leading to a greater suppression of the dissociation of the parent base. Experimental Analytical grade reagents were used throughout. Sodium acetate solution, 0.05 molar, was standardized against perchloric acid with quinaldine red as indicator. Sodium perchlorate, 0.025 molar, was prepared in situ by mixing equal volumes of 0.05 molar acetic acid solutions of perchloric acid (previously rendered anhydrous with acetic anhydride) and sodium acetate. All other solutions were prepared from these stock solutions. The potentiometric measurements were carried out using a Beckman Model G pH meter, e uipped with a glass electrode and an external calomel eqectrode joined with the solution by means of an agar-gel bridge (4% agar and 5% potassium chloride). All potentiometric readlngs were determined relative to that of a 0.100 molar solution of sodium salicylate in acetic acid taken just prior to the determination of the unknown solution. Duplicate determinations on independently prepared solutions agreed, in most part, within one or two millivolts.
DENSITIES OF MOLTEN SODIUM AND RUBIDIUM HYDROXIDES BY DONALD BOQART N A C A Lewis Flight Propuleion Lab., Cleveland, Ohio Received J u l y 86, 1964
The densities of molten sodium and rubidium hydroxides have been determined up t o temperatures of 920" using a method based on Archimedes' principle of buoyancy. The present data for sodium hydroxide in the temperature range 690 to 920" are shown in Fig. 1.
0.02000
2 0.01000 8 0 J
a8$ 0.00500 a
s
3
I
m 0.00250 ' -
I.. .____
60 80 100 120 Millivolts. Fig. 3.-Potentiometric determination on sodium acetatesodium perchlorate system; concn. of sodium acetate in M : ( l ) ,0.00125; (2), 0.00250; (3) 0.00500; (4),0.01000.
40
(5) This assumption may be shown valid b y employing the alternative case, that the sodium ion concentration from the acetate is greater than that from the perchlorate, whereby experimentally determined data do not conform to the derived equation.
The data for rubidium hydroxide (measurements taken on two successive days) are shown in Fig. 2. (1) K. Arndt and G. Ploetz, Oak Ridge Nat. Lab. Index No. Y-F35-5, Mar. 21, 1952 (translated from Z . physik. Chem., 121, 439 (1926)). (2) M. Nishibayashi. Wright Air Development Center Teohnical Report 53-308, Nov. 1953.
