1005
NOTES
July, 1957
TABLE I MOLALITIES OF ISOPIESTIC SOLUTIONS OF PICOLINIC ACID(m,)AND SODIUM CHLORIDE (ml)AT 25" mi
mp
CI
mi
mi
0.2552 .3254 .3989 .4568 .5936 1.228
0.1335 .1684 .2046 ,2321 ,2966 .5907
0.973 .960 .948 .939 .921 .888
1.406 1.670 2.306 2.860 3.580 4.072
0.6661 .7830 1.047 1.277 I . 564 1.757
vi 0.876 .870 .851
.845 .839 .837
far from that of sodium chloride at the same total, ionic strength. From the stoichiometric concentra- THE OSMOTICAND tion, 0.3 M , the observed (apparent) osmotic coefficient, 0.965, the concentration of zwitterion, m vi 0.2960 IM, the concent,ration of electrolyte 23 X 0.1 0.988 M , and the osmotic coefficient of sodium .2 .974 chloride a t this concentration, 0.984, the osmotic .3 ,961 coefficient of the zwitterion, p* is calculated by .4 .947 0.963 X 0.3 = 0.2960qi
+ 2 X0.0023 X 0.984 or
'p*
0.961
The correction is therefore small and is even smaller a t higher concentrations. I n this way values of the zwitterion osmotic coefficient pi were calculated a t round concentration of the zwitterion and are recorded in Table 11, the corresponding activity coefficients being calculated by standard procedure. The activity coefficients are remarkably low for a zwitterion. The value at 2 M , 0.700, may be compared with 1.431 for sucrose,2 1.050 for glycerol,2 0.869 for urea,2 1.186 for a-amino-n-butyric acid13 1.028 for alanine4 and 0.899 for glycolamide.6 Only glycines approaches it ( y = 0.786 a t 2 M ) . The activity coefficient of a highly dipolar amino acid in a polar solvent such as water must be dependent on a number of factors of which perhaps the dipole moment of the acid, leading to long-range electrostatic forces, and the partial molal volume, governing the co-volume effect, are most important. For athermal solutions the co-volume effect leads to a n increase in the activity coefficient7s8given by the equation where r is the ratio of the free volumes of solute and solvent. Glueckaufs recently has examined the covolume effect for hydrated electrolytes and has had considerable success in accounting for the activity coefficients by assuming that r is the ratio of the partial molal volumes of solute and solvent. It is known,8 however, that for sucrose, whose molal volume is twelve times that of water, T = 5 gives a good representation of the activity coefficient and an examination of the data for glycerol2 shows that r = 2.6 is to be preferred against 3.9 for the ratio of ( 2 ) G . Scatchard. W. J. Hamer and 8. Sac., 60. 3061 (1938).
E. Wood, J . A m . Chem.
(3) P . K. Smith.and E. R. B. Smith, J . B i d . Chem., 121, 607
(1937).
(4) R. A. Robjnson, ibid., 199, 71 (1952). ( 5 ) R. H. Stokes, Trans. Faraday Sac., 60, 565 (1954). (6) E. R. B. Smith and P. K . Smith, J . Eiol. Chem., 117, 209
(1937).
(7) J. H. Hildebrand and R. L. Scott, "The Solubility of Nonelectrolytes," Reinhold Publ. Corp., New York, N. Y., 1950. (8) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions," Butterworth Scientific Publications, London, England, 1955. (9) E. Glueckauf, Trans. Faradnu Sac., 61, 1235 (19551.
.5 .6 .7
.8 .9 1.0 1.2 1.4 1.6
.932 .919 .911 ,904 .897 .892 .884 .876 .870
m1
mr
91
4.401 4.969 5.416 6.232 7.225 7.977
1.892 2.123 2.307 2.641 3.055 3.382
0.841 .846 .853 .866 .887 .908
TABLE I1 ACTIVITY COEFFICIENTS OF PICOLINIC ACIDAT 25' Yi
m
0.976 .950 .926 ,902 .877 ,854 .836 .819 ,803 ,790 .768 ,748 .731
1.8 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
C*
0.863 .858
,848 .841 .837 .837 .841 .846 .854 ,862 .872 .883 .896 .911
Yi
0.714 .700 .672 .648 ,629 ,615 ,605 .599 .595 .592 .592 .593 .596 ,601
the molal volumes, The quantitative application of the co-volume effect is therefore not clear but we would a t least expect that the increment in the activity coefficient would be greater for solutes of large molal volume and as the molal volumes of picolinic acidlo and glycine'l are 83.8 and 43.2 cc. mole-' respectively, we would expect the co-volume contribution to the activity coefficient of picolinic acid to be considerably greater than for glycine. But the measured activity coefficient of picolinic acid is much less than that of glycine. There must therefore be a large effect depressing the activity coefficient of picolinic acid and measurements of the dielectric constant of its aqueous solutions might contribute to the problem. (10) Unpublished measurements. (11) F. T. Gucker, W. L. Ford and C. E. Moser, THIEJOURNAL, 48, 153 (1939).
A CRYOLITE MELTING POINT THROUGH QUENCHING METHODS B Y PERRY
A. FOSTER, JR.
Alcoa Research Laboratories, Aluminum Company of Alnerica, New Kensington, Pennsglvania Received January 91, 1967
Several values for the melting point of cryolite have been reported. These values were all obtained by thermal arrest methods and range generally from 1000 to 1009°.1-6 During the course of our studies on the constitution of the cryolitealumina electrolyte, which is employed in the com(1) P. P . Fedotieff and W. Iljinski, 2.anorg. Chem., 80, 113 (1413). (2) N. W. F. Phillips and others, J . Electrochem. Sac., 102, 648 (1955). (3) M. Rolin, Ann. Phua., 6, 970 (1951). (4) N . Ssabo and Gy. Sigmond, Mat. termesz. d e s , 60, 364 (1941). (5) A . Vojna, Alluminio, 22,035 (1953).
1000
NOTES
mercial production of aluminum, a cryolite melting point of 1004 f 1" was obtained using quenching techniques, essentially as described by Roedder.6 Briefly, the method consists of suspending a small amount of test material in the hot zone of a constant temperature furnace for a period of time sufficient to obtain equilibrium. The charge is then dropped from the hot zone into cold mercury. Microscopic examination of the quenched charge will determine the state of the system a t temperature. Phase changes and their relation to temperature are readily observed by quenching a series of samples over a range of temperatures. Melting points of pure substances are fairly simple to obtain. Their small melting ranges permit one to make full use of precise furnace temperature controllers in establishing solid-liquid transformation temperatures. The accuracy of any given experimental result is limited only by the accuracy of calibration, if the precision of the thermostat is within these limits. Extraneous thermal effects that could influence the experimental results are minimized or eliminated by performing the thermocouple calibration in exactly the same manner as the sample runs. The material to be quenched is enclosed in a platinum foil envelope. The envelope is threaded a t the top with platinum fuse wire, suspended by means of the fuse wire from two platinum wire hooked conductors, and located about 1 millimeter from the thermocouple. Calibration of the Pt, Pt-lO% Rh Thermocouple.The e.m.f. of the thermocouple was measured by a Type K-2, Leeds and Northrup potentiometer. The reference junction was at 0'. The melting points of 99.99% NaCl (SOO.S'), recrystallized NaF (992'), and 99.999% old (1063") were used as calibration oints. The NaC!, wfich was obtained from the Delta 8hemical Corporation, exhibited no significant melting range. The gold was obtained from the Sigmund Cohn Corporation in the form of 0.01 inch diameter wire. To determine the gold point, the wire was contained in a small ceramic capsule to prevent access to the surrounding platinum envelope. The gold had no observable melting range. Sodium fluoride was J. T. Baker C.P. grade and required recrystallization from water solution in order to reduce the melting range to approximately 1". In this case the temperature where the last crystals melted was considered the melting point. The deviation of the observed thermocouple readings from standard reference tables was plotted as a function of the observed readings. This curve was linear over the 263 degree calibration range. ?he accuracy of the calibration was considered to be f l . The thermoregulator maintained the temperature i o within f0.2' of the mean both during calibration and during the cryolite melting point determinations. The precision of the regulator was well within the accuracy of calibration. Cryolite Melting Point Determination.-The cryolite employed in these determinations was of the highest purity known to the aluminum industry. A semiquantitative spectroscopic analysis of representative Sam les of the material and a quantitative determination o f the more prevalent impurities are given in Table I. X-Ray powder diffraction could not detect other phases, such as chiolite. Three or four fragments taken from particularly clean clumps of Greenland cryolite were sealed in each of several platinum envelopes. Each specimen was quenched from a successively higher temperature until one had been uenched from the liquid state. This procedure establiLed the "bracketting temperature." The magnitude of the bracket was then reduced until approximately 0.6" separated the completely solid from the totally liquid state. The cryolite quenched from 1003.9 f. 0.2' (max. deviation), and main( G ) E. Roedder, A m . J . Sci., 249, 81 (1951).
Vol. 61 TABLE I Elements detected
Fluorine Aluminum Sodium Silicon ( SiOz) Potassium (KF) Calcium (CaF2) Lithium (LiF) Magnesium Strontium Iron Copper
Qualitative estimate of concn., %
10-100 10-100 10-100 0.01-0.1 0.01-0.1 0.01-0.1 0.01-0.1
