AN ULTRASENSITIVE THERMISTOR JIICROCALORIMETER
Xov., 1961
which are independent of the concentrations. If rnxcr) is linearly dependent on VZJ(,), relation (3) becomes log
=u
+b
m
~
~
(4)
‘Yhe chloride systems were found to vary according to relation (4) a t external electrolyte concentrations above about one molal.3 Below this concentration negative deviations of linearity occur. The devia-
2041
tions are due to the relatively low values of ?i(r) a t low external electrolyte concentrations which has been attributed to consistent experimental errors and the influence of impurities of the resin phase.3 A similar behavior was observed with the nitrate systems as is illustrated in Fig. 1. The results of applying relations 3 and 4 are given in Table 11.
,4W ULTRASENSITIVE THERMISTOR MICROCALORIMETER AND HEATS
OF SOLUTION OF NEPTUNIUM, URANIUM AND URANIUX TETRACHLORIDE1 BY G. R. ARGUE,^ E. E. MERCER^
AND
J. W. COBBLE
Department of Chemistry, Purdue University, Lafayette, Indiana Received M a y Z4. 1961
A sensitive solution microcalorimeter using a thermistor-amplifier bridge and automatic recording has been constructed with magnesium, heats of solution in 1 M HCl have been and evaluated. The sensitivity of the instrument is 1 x measured to =k0.2’3&with samples as small as 19.51 fig. The device has been designed primarily for use in determining the thermodynamic functions of actinide elements and their compounds. The heats of formation of U(1V) in 1 M HCl, U+4(aq.) and UClp(c) have been redetermined and new values are reported for Np( IV), Np( 111)in 1M HCl and N ~ + ~ ( a q .Np+S(aq.). ),
Introduction sensitivity; such instruments probably qualify to be called ultramicrocalorimeters. Microcalorimeters suitable for carrying out heats The calorimeter has been used in redetermining of solution and reaction of actinide and other scarce elements have been reported and described the heats of formation of U(1V) and Np(1V) in by Westrum and E ~ r i n gGunn , ~ and C ~ n n i n g h a m , ~acid solutions, and the heat of formation of UCld(c). and White and Salman.6 In general, these in- Kew measurements on the heats of solution of struments have sensitivities such that precise heat UCI4(c)in perchloric acid solutions, and neptunium data for some reactions can be obtained with in HC1 solutions, allows accurate estimations of samples of a few milligrams. However, still more the heats of formation of U+4(aq) and K ~ + ~ ( a q ) sensitive calorimeters will be required to extend a t infinite dilution. Finally, a new method has heat measurements on the actinides and their eom- been used to directly determine the heat of formapounds beyond the transcurium elements. There tion of n ’ ~ + ~ ( a q ) . are a number of ways in which the sensitivity of Experimental small microcalorimeters can be improved, but Calorimeter.-The microcalorimeter body was a modithe most feasible method appears to be the sub- fication of the design of Westrum and E ~ r i n g . It ~ was constitution of thermistors as the temperature sensing structed from a small tantalum beaker having a volume of 7 cc., being about 4 cm. in length. The element. Such applications are not ne^,^!^ but approximately top was fitted with a tantalum lid which could be fastened the current availability of extremely stable thermis- to make a vacuum tight seal and was suspended by a Kel-F tors with high temperature coefficients makes plastics hanger from the top of a “submarine jacket” their application to microcalorimetry extremely (Fig. 1). A 40 mil diameter Pyrex stirring rod running the center of this hanger and a glass thermistor probe desirable; in addition, such use further lends itself down piercing the lid through a vacuum tight seal provided the t o automatic recording devices. This latter fea- essential parts of the device. An aluminum radiation ture in itself increases the accuracy of micro- shield supported on lucite legs was used. This submarine calorimetry, and this communication reports the type of calorimeter also has been used by otherss.4; the principal advantages of this arrangement have been studied details of an automatic recording thermistor and summarized recently el~ewhere.~ microcalorimeter approximately four times more A synchronous 288 r.p.m. motor was mounted directly sensitive than any yet described. Further, ex- above the stirring rod on a guided spring suspension. A perience with this device has led to recommenda- slight vertical movement of the suspension broke the bulb fastened to the end of the stirring rod.lo The tions for future microcalorimeters of even higher sample whole stirring apparatus could be covered by a plastic bag (1) Supported by the U. S. Atomic Energy Commission. (2) From th
