584
HUGHM. HUFFMAN, SAMUEL S. TODD AND GEORGE D. OLIVER
[CONTRIBUTION No. 12 FROM
THE
Vol. 71
THERMODYNAMICS LABORATORY OF THE PETROLEUM EXPERIMENT STATION, BUREAU OF MINES,BARTLESVILLE, OKLA.]
Low Temperature Thermal Data on Eight CsH,, Alkylcyclohexanes B Y HUGH
M.
HUFFMAN, SAMUEL
In continuation of the program of the Bureau of Mines, to investigate the thermodynamic properties of petroleum hydrocarbons and related substances, low-temperature thermal studies have been made on eight C ~ H M alkylcyclohexanes over the temperature range 12 to 300'K. This investigation of a group of isomers represents one of the most comprehensive studies of isomers yet made in the field of low temperature calorimetry. A number of interesting and unusual phenomena were observed which will be discussed. Insofar as the authors are aware this is the first published work on the low temperature heat capacity of these substances, Experimental The Materials.-The materials used in this investigation were API-NBS "best" samples prepared by the A.P.I. Research Project 6 a t the !Sational Bureau of Standards3and certified by them in regard to their purity. The Apparatus.-The measurements were made in the apparatus described by Ruehrwein and Huffman.* Very briefly, the method is as follows About 0.4 mole of the material under investigation was contained in a sealed copper calorimeter, which was mounted in the adiabatic calorimetric system. A measured amount of electrical energy was supplied to the calorinieter, and at all times the temperature of the environment was maintained a t that of the calorimeter to prevent heat interchange. The initial arid final temperatures of the calorimeter were measured by means of a platinum resistance thermometer. The electrical measurements required for determining the resist nnce of the thermometer and for the electrical energy were made 011 a '34-hite" double potentioineter in conjunction with a high-sensitivity gahd
s. TODDAND GEORGED. OLIVER1 nometer and accurately calibrated resistances. The potential was in terms of a bank of six saturated cadmium cells, which had been certified by the National Bureau of Standards. Time measurements were made with an electric stopclock driven by alternating current, the frequency of which was controlled with an accuracy of 0.001%. The precision of the measurements was, in general, better than 0.17;; and above 30°K., it is believed that the accuracy uncertainty, unless otherwise stated, should not be greater than 0.2%. The energy rneasurenients were made in terms of the international joule and were converted to calories by dividing by 4.1833. Melting-point studies were made on all of these compounds as a part of the routine measurements and the results are summarized in Table I. I t should be borne in mind that the equilibrium temperatures given correspond to triple points since the material in all cases was sealed in the calorimettr under its awn vapor pressure. Three of these T A B L1 ~ CsHie AI KkLCTC1,OIIEXANE MELTING POINT Ethy~cycloht.xdrie
96 II'i
'
Jb6.
lr, 30 1 G I h315 41 37 8351" ;I srz %.3'75 'io J i .;i'382" 100 0 Pure = 0.03b2Ar ].mpurity = 0 008 * 0.001 mole
SCJX\.1.lf4 R Y
T'enled
161.8279 .8354
70 82 b3K) 161 8400
1,l-I)i~netliylcyclobexarle 24 :: 239 393 239.154 40 (! 49 1 414 63 7 540 515 76.4 .603" 603 '79.2 1604 Gll '92 0 R3R" ,638 100.0 662 Pure 230 811 \ -- u 00423A1 I i i i p i ~ ~ i t= y 0 07 * 0.07 iiiole yo
(1) Prevent address C?I bide and Carbon Chemical Cnriiwa tirr+ Oak Ridge, Tennessee. (21 Knowlton and Huffmaa, THISJOUKNAL, 66, 1492 (1944) (3) These samples of API-NBS hydrocarbons have been made available by the American Petroleum Institute and the National Bureau of Standards through the A. P. I Research ProJeet 44 on the "Collection, analysis, and calculation of data on the properties of bydrocarbons " The samp1es were purified at the National Ruredii i of Standards hg the A I' I Research Project 8 on the ''Amily\~\ purification and properties of hydrocarbons," under the sui)er\iTin*i of Frederick D Ros.;int f r m n ,?aterial suppired b y t h e follnr-rng ~ z b - 1,~--Dimethylcycloliexane liborator~e, 3 '3.7 2257 22,; 1420 Ethylcyclohexane, crs-f ,J-dimtlhylcyclohcrnnc, trans-1.J-drmcfkyl cy
