AIARIO CIAMPOLINI AND PIERO P.\OLCTTI
1224
TTol. A3
the equivalent conductance at "infinite dilution" of the alkali metal dissolved in its alkali halide on the nature o f both the anion and the cation of the salt was noted. On this basis, it was expected that .iG of K met2 1 dissolved in RF would reach a value lower than any in the alkali metal-halide systems (except Cs or Rb in their fluorides) and that AG of I< in KI should have a d u e higher than that in the bromide. Also, -\c for ?;a in S a 1 should lie above that of the bromide and represent a maximiim among these systems (except possibly for Lil). This has been verified in the present investigation (Table 11). As proposed earlierl2 the anion effect
is believed to be connected with the polarizability of the anion: electron transfer betm-een t8wooxidation states of a metallic element is known to be greatly faditmatedby greater polarizabi1it.y of the a n i ~ n . ~ The ~ , ' ~polarizability of the fli:oride ion (0.99 A.3) is very low compared with that of the chloride, bromide or iodide ion 13.05, 4.17, 6.28 A.3, respectively), The greater in t'he sodium systems as compared with the potassium systems containing the same anion is not fu!ly understood at present. Possibly a lowering of tile iiegat,ive charge on the anion by a slightly more strongly polarizing cation such as Ka+ might, faci1itat)e elect,ron transfer by lowering the pot,ential barrier TABLE I1 to electron motion, which the anions represent. EQUIVALENT COXDUCTANCE O F METAL IN ALKALI MFTAI,Acknowledgment.-Helpful dimisions with ProHALIDESYSTEMS AT INFINITE I)II,UTIOK (!looo) fessor IVilliam T. Smith, Jr., Univ?rl;ity of TenA@ (ohm-1 cm 2 equiv.-I) - P _ _ c1Halide ~ salt of metalnessee, Chemistry Departnieiit, are grat,efully :tcMetal Br I knowledged. c -
Na
K
800
6000 2800 (820')
13 ,000 6 ,500 (8iO")
,
16 000 8,100
(12) 13. Taube and H. Myers. J . A m . C h m . S o r . , 7 6 , 2103 (19%). (13) C. Zener, Phys. Rev., 82, 403 (1951).
THERRIOCHE~IICALSTUDIES. T7.I HEATS OF STEPWISE NEUTRALIZATION OF ETHYLENEDIAININE ASD DIETHYLENETRIA3IISE BY MARIOCIAMPOLINI AND PIERO PAOLETTI Istituto d i Chimica Generale e Inorganica dell' Vniversitd d i Firenze, Florence, Italy Received J a n u a r y 30, 1961
The rcwlts of a calorimetric investigation of the stepwise heat of nnitralization for ethylenediaminr i n ,If IiCl and for diethylenetrianiine in 0.1 M IiCl are presented.
Introduction I n the course of calorimetric investigations on the heat of form.ation of meha1 complexes with ethylendiamine (en)2 and diethylenetriamine (den),' it was necessary to know the accurat'e values of the stepwise heah of neutralization for these polyamines. Heats of neutralization of ethylenediamine determined both calorimet'rically and potentiometrically a,re reported in the l i t e r a t ~ r e ,whereas ~,~ only potentionietric values are known for dieth~lenetriarnine.~.~ All these values, however, are referred to ionic media different from those used by us a8ndoften disagree one with the other. Hence it seemed worthwhile to det'eimine, by direct, calorirnet'ric measurement, the heat changes for the successive stages of neutralization of these two amines in the same ionic medium used for the metal systems. A ill KC1 solution for ethylenediamine and a 0.1 AI solution for diethylenetriamine were employed. The result's of these investigations are discussed below-. (1) P a r t I V , 11. Cinmpolini, P. Paoletti a n d L. Sacconi, J . Chem. Soc., in press. ( 2 ) M. Cinmpclini. P. Paoletti a n d L. Sacconi, ibid., 4553 (1960). ( 3 ) T. Davies. S.8. Singer a n d L. A. K. Staveley, ibid., 2301 (1954). (4) G. H. LIcIntyre. ,Jr., B. P. Block and W. C. Fernelius, J. A m . Chem. Soa., 81, ,52!9 (1959). ( 5 ) H. B. Jonassen. R . R. LeBlanc, A . Meibohm a n d R. M. Rogan, ibid., 72, 2430 (1950).
W.
Experimental Materials.-Anhydrous ethylenediamine and diethylenetriamine were put twice through a Todd fractionating column. The cuts of these t x o bases used in the experiments were analyzed by potentiometric titration against hydrochloric acid and found to be 99.9 and 99.6% pure, respectively. hpproximiitely M solutions of these amines were made up with carbon dioxide-free potassium chloride solutions of the same ionic strength used iri the measurements. The concentrations of these solutions vere determined by potentiometric titration against ca. 1.5 N hydrochloric acid which had been standardized gravimetrically as silver chloride. This same hydrochloric acid was used in the nieasurements of the heat of neutralization. Calorimetric Measurements.-The calorimeter, described in a previous paper,B was placed in a thermostatic bath a t 25.000 =t 0.005O. Its capabilities have been tested by rneasurcments of the heat of solution of potassium chloride in wat,er, in the molar ratio 1:16? a t 25". The mean of eight determinations was 4194 & 5 cal./mole. The values reported in the literature, corrected for this dilution, are: 4184 f 8,34187.' For each run the bottle was filled with a weighed amount of the amine stock solution. Hydrochloric acid was placed in the dewar flask and,the volume made up by adding carbon dioxide-free potassium chloride solution of the desired ionic strength. The final volumes (Table I ) were evaluated from the known weights of the reactants and the final density of the calorimetric liquid. The heats of dilution of the hydrochloric acid were measured by diluting a weighed sample of it in the same volume of the ionic medium as with the experiments with (6) L. Sacconi, P. Paoletti a n d M. Ciampolini, Ricerca Sci., 29, 2112 (1959); J . An. Chem. Soc., 82, 3828 (1960). (7) K. P. hlischenko a n d Y u . Y a . Kaganovich, J . B p i L Chem., U . S. S. R., 22, 1078 (1949).
HEATSOF STEPWISE NEUTRALIZATION OF ETHYLENEDIAMIKE
July, 1961
amines. I n every case the hent of dilution of the amine solution by the writer contained in the hydrochloric acid solution was found to be negligible.
Results In order to determine the over-all and stepwise heats of neutral zation it is necessary to measure the heats evolved for different hydrochloric acidamine ratios and to calculate the exact amount of the protonnted forms of the polyamines before and after the reaction. The basicity constants employed were those of Edwards8 in $1 KC1 a t 25" for ethy1enedi:imine and those of Prue and Schwarzenbachg in 0 1 X IiCl a t 20" for the diethylenetriamine. The latter constants were corrected to 23" by using approximate A H values. The experimental results are shown in Table I. To check the reproducibility of these measurements, three independent sets of Fix runs were carried out for ethylenediamine but not all of these are reported here. The heat evolved, corrected for the heat of dilution of hydrochloric a:id. is reported in the third column. Allowance must also he made for the heat effect due to the neutralization of hydroxyl ions which arise from the hasic dissociation of the amine. This correction is recorded in the last column. The heats {of stepwise neutralization mere ohtained by comlhiing the reqults of runs carried out for different hydrochloric acid-amine ratios and solving the set of simultaneous equations
1225
neutralization is of the same order of magnitude as the reproducibility of the calorimetric measurements, viz., better than AO.27,. The heat changes for the successive stages of neutralization are, on the contrary, less accurate because of their critical dependence on the values of the basicity constants. TABLE I1 THERXODYNAMIC FUNCTIONS FOR
THE
S u c c c s s ~ vNsr~
TRALIZATION ST.4GES O F POLY.4MINEb A T 25'
-AF
-AH
(kcd /
(ked/
+ + + + +
en H + +enH+ enH+ H + - + e n H 2 * + den H++tfenH+ denH+ H + --f dcnH22+ denHzt+ H++denIl33+
iiiole)
mole)
13 90 10 15 1 3 3.5 13 25 5 80
12 20 10 65 11 20 11 95 i 20
AT
( e . u.)
5 7 5 7 2 1 0 -4 7
--I
Discussion The ionic strength markedly affects the heats of neutralization of ethylenediamine, as is found by comparing our values in M KCI with those of Davies, Singer and Staveley in 0.1 M IJp,the former renresenting a region of low dielectric consta~it.’~This effect will increase in the di- and tri-ammonium ions on account of the repulsion between the positive charges. Acknowledgments.-The authors ale indebted to Prof. L. Sacconi for helpful suggestions and criticism. Acknowledgment is made to the Italian “Consiglio Nazionale delle Ricerche” for the support of this research. (13) E. Q Adams abrd, 38, 1507 (191F), .I Bjerrum ‘Metal 4nimine Formation in Aqueous Solution Haasp h son Copenhagen 19.57, p 21. ‘I
CRITICAL PHENOMENB I N THIN FILMS USISG THE BRAGG-WILLIAMS APPROXIMATIOS BY E(. NISHIKAWA, D. PATTERSON AXD G. DELMAS Unzverszty of Montreal, Montreal, Canada Rerewed February 1 , 1961
The critical properties of thin films of binary regular solutions are treated using the Bragg-Williams approximation. The lowwing of the critical solution temperature is discussed as a function of film thickness, surface tension of the pure cor-ponents and adsorption properties of a supporting solid snbstratr. The possibility of the existence of more than one criticd point is considered.
Introduction Some theoretical and experimental importance is attached to the effect of size and of the surface on the bulk properties of a system. These may be the melting temperature, critical solution point, heat capacit.y, etc. Saraga and Prigogine,‘ for instance, conclude that a surface layer of molecular thickness on a binary regular solution will have a critical tempwature different from that exhibited by the bulk of the solution, and that, under certain conditions, an independent phase separation should be observable in the surface layer. We have considered the critical properties of films of a binary regular solution and the effect on the film thickness (1) L. Saraga and I. Prigogine, C. R. $e Rdundon Parrs, 458 (1932).
doc.
chim. Phys.,
and surface properties, including the role of adsorption on a supporting substrate. The results obtained are somewhat different from those of Saraga and Prigogine except for a particular case. The lattice model with the Bragg-TVilliams approximation has been employed, as originally introduced and used in the study of surface properties of solutions.2--4 Although fluctuations in the composition will be important near the critical point, the treatment should be adequate to give its position. After presenting the model, we treat the special and (2) E. A . Guggenheim, “Mixtures,” Oxford University Press, 1 9 2 , Chapter IX. (3) I. Prigogine a n d R. Delay, Trans. Faraday Soc., 46, 199 (1956). (4) F. Murakami, S. Ono, M. Tamura a n d M. Kurata. J . P t w . So, Japan, 6, 309 (1951).
