A. C. MAKRIDES AND N. HACKERMAN
594
formed from CC14-CHCla-CH2C12. Since these binary solutions were reported as very nearly symmetrical, we were interested in the ternary solution. The third column of Table I11 reports our experimental heats for the ternary, and the fourth column is the deviation.of these values from the sum of the binary data of Cheesman and Whitaker, which is given in equation 8.
+
AH?& = 220.4~1~9
+
~1~8[565 - 33.1(~1- XS)] 17.0~~~s (8)
Vol. 63
The deviation is about the order of the experimental error. It is possible, therefore, to predict AH: values for this ternary as accurately as the binaries were measured. Acknowledgment.--The authors gratefully acknowledge the support given this project by the National Science Foundation. We also wish to thank Miss Norene McClellan, who assisted with the freezing point measurements.
HEATS OF IMMERSION. I. THE SYSTEM SILICA-WATEK. BY A. C.
MAKRIDES
AND
N. HACKERMAN
Department of Chemistry, The University of Texas, Austin, Texas Received September 18, 1968
The heat of wetting of SiOz by water depends on temperature of evacuation and on thermal treatment of the Si02 powder. Available data suggest that the heat of wetting may depend on the specific surface area of the powder.
Introduction A number of heats of wetting of silica powders with water have been Values for relatively coarse powders (lo0 m.”/g.) and with gels by a factor of about 2 or more. Data for coarse powders are restricted, however, to a single temperature of evacuation and, in a number of cases, refer to powders of unspecified impurity content. Heats of wetting of silica powders variously pretreated are reported here. Experimental
Each vessel included a heater, a stirrer, a sample holder and breaker and a thermistor. The calorimeter heaters were wound non-inductively from manganin wire. They were coated with Glyptal resin and enclosed in gold containers friction-fitted on Teflon plugs inserted in the brass plate. Transformer oil increased thermal conductivity between heater and container. Both calorimeter stirrers were driven at 150 r.p.m. by a single Bodine precision motor with attached reduction gear. A Gilmer positive-drive belt, with associated pulleys, was used to ensure constancy of stirring. A Bakelite section, threaded a t both ends, reduced heat conduction along the stirrer shaft. Bakelite gave air-tight seals with Neo rene O-rings placed in recesses drilled in the brass plate. ‘&he Bakelite sections were replaced every two or three runs, while O-rings were changed after every run. The breaker rods were in two parts, both ending in BakeCalorimeter.-Heats of wetting were measured with a lite sections. The upper part rested against the Neoprene twin microcalorimeter similar to the instrument of Evans sheet, while the lower end extended from just beneath the and Richards.8 A major modification was the use of ther- sheet to the bulb. The bulb was broken by pressing the upper section against the flexible Neoprene sheet. mistors for measuring temperature changes (see below). Electrical Cricuits .-The calorimeter vessels were caliTwo closely similar Dewar flasks of 500-cc. capacity served as the calorimeter vessels. They were attached to a brated electrically. The potential drop across the heater nickel-plated brass plate with aluminum rings equipped with and across a series standard resistance was measured with Neoprene gaskets. Both vessels were enclosed by a bright a precision (0.05 mv.) Rubicon potentiometer. Current nickel- lated jacket which was also attached to the brass was drawn from a Willard cell which had been stabilized by discharge, for at least 3 hours, through a resistance equal t o plate tffrough an intervening gasket. The calorimeter assembly, suspended with Bakelite rods that of the heating circuit. A double-throw, double-pole from an aluminum plate, was immersed in a constant tem- microswitch closed the heating circuit and simultaneously perature bath. A thyratron circuit temperature controller actuated a precision timer (0.01 sec. sensitivity). The with a thermistor as one arm of an a x . (60 c.p.6.) bridge timer was driven by a constant frequency (60 c.p.5.) source. The electrical circuit used in conjunction with the therregulated the bath temperature within f0.001’ over 24mistors consisted of a Muller bridge, a 70 ohm input imhour periods. Heat exchange between vessels and bath took place across pedance Liston-Becker DC 14 breaker amplifier and a Brown the brass plate. Heat transfer amass the air space between potentiometer. The initial (small) difference in resistance vessels and jacket was comparatively negligible. A Neo- between thermistors was compensated on the bridge, the prene sheet and a Lucite plate, inserted between the brass output of which was fed to the amplifier and the signal replate and the Dewar flasks, reduced heat exchange to a de- corded on the potentiometer. With the amplification ratio generally used, a resistance difference of 0.01 ohm gave 100 sirable level. mv. deflection. The corres onding temperature sensitivity (1) G. E. Boyd and W. D. Harkins, J . A m . Chem. Soc., 64, 1190 was 20 X 10-6 “C./mv. a n 8 the equivalent heat input sensitivity 0.01 cal./mv. (1942); W. D. Harkins and G. E. Boyd, ibid., 64, 1195 (1942). (2) F. L. Howard and J. L. Culbertson, ibid., 72, 1185 (1950). Thermistors.-The high sensitivity of thermistors as compared to other temperature-sensitive devices offers a (3) A. C.Zettlemoyer, G. J. Young, J. J. Chessick and F. H. Healy, THIE JOURNAL, 6’7, 649 (1953). major advantage in calorimetric application. Microcalorimeters employing thermistors have been d e s ~ r i b e d . ~ ~ ~ (4) A. V. Kiselev, N. N. Mikos, M. A. Romanchuk and K. D. Shcherbakova, Zhur. Fia. Khim., 21, 1223 (1947). Thermistors sometimes lack stability, particularly over long periods of use. With some precautions, however, (5) W. A. Patrick, quoted by R. K. Iler, “The Colloid Chemistry of Silica and Silicates,” Cornel1 University Press, Ithaca, N. Y., 1955, stable and reproducible behavior was obtained. pp. 240-242. Disk-shaped thermistors of 100 ohm nominal resistance (6) F. E. Bartell and R. M. Suggitt, THIBJOURNAL, 68, 36 (1954). were used in pairs matched within 0.1%. They were prepared by Victory Engineering Corporation by aging in a (7) M. M. Egorov, K. G. Krasil‘nikov and E. A. Sysoev, Dokladz, Ahad. Nauk S.S.S.R., 108, 103 (1956). constant temperature bath (25’)for about 3 months. They were coated with Glyptal resin, air-dried and then baked a t ( 8 ) D. F. Evans and R. E. Richards, J. Chem. Soc.. 3932 (1952).
HEATSOF IMMERSION OF THE SYSTEM SILICA-WATER
April, 1959
595
TABLE I Powder
A" B* C D" Ed
% AlaOr
% FerOa
% TiOa
% Loss ignition
% min. Si02
Other
0.22 .04 .049 Fez08 AlzOa .014
0.05 ,016 .019
0.034 ,008
0.13 .09
99.56 99.92 99.82 99.0-99.7
Trace MgO, CaO Trace MgO, CaO 0.031% MgO
+
...
...
.Ol
...
.0017
Nil
0.2-1.0
...
...
...
CaO
