Vapor pressures of the methanes

Abstract

Experimental meas urements, not previously reported in detail, of the vapor pressure of CH, and the differe nces in vapor press ures of CH. and the deuterom ethanes below the normal boiling point are presented. A critical revi ew of the literature relating to the vapor pressures of m ethane and the deu teromethanes is given. On the basis of the experim ental measure-m ents and t he previous literature t he following best values are assigned to fix ed points on the vapor pressure curves. The triple points are: CH., 87.50 mm , 90.66° ; CH aD , 84.5 mm , 90.41° ; CH 2D 2, 82. 0 mm, 90.17° ; CHDa, 80 .2 mm. 89.96° ; CD., 79.1 mm, 89.79° K. The boiling poin t of CH. is 111.67° K; the critical poin t 45.6 atmosp heres and 190.6° K. Equa-t ions and a table are given for t he vapor press ure of CH. in t he solid region , t he liquid region below t he normal boiling po in t and th e li qu id region above t he no rmal boiling poin t. The ratios of the vapor press ures of the methanes are expressed by equa t ions of th e form T log ,o PD/ PH = A-B/ T. Th e co ns ta nts A and B have t he follow ing va lues, r es pec-t ive ly: in t he solid range CH aD , 1.260, 110.2; CH2D 2, 2.694, 222. 2; CHDa, 4.452, 351.7; CD ., 5.529, 4 1.0.5 ; in t he liquid ra nge, CH aD , 1.328, 129.5 ; CH2D 2, 2.671 , 245.4; CHDa, 3.969, 343.8; C D., 5. 159, 42 1.1. Th e vapor-press ure ratios of the isoto pic m etha nes for m a nearl y geo metri c p rogressio n wi t h in creas ing d eute rium substitution. Dev ia t io ns of t heir mix t lll'eS fr om ideal solu t io ns arc very s m·a ll. 1. A Review of the Vapor Pressure of Methane and the Deuteromethanes 1.1. Methane The vapor pressure of solid and liquid methan e has been determined in various tempera! ure ranges by many investigators, and , in addi tion , t her e have b ee n sever al summari es of t he pu bli shed da ta. S. F. Pickering [1] 2 l'eviewed t he liLCI'atul'e r clatin g to t be cri tical con stant ; Copson and Froli ch [2] summal'ized t he vapor-preSSUl'e data available in 1929; and Stull ummari zed t he data in ] 950 [3]. A series of besL values appeared in t he In tern ation al Critical Tables [4]; Egloff [5] a nd Timmcrmans [6 ] surv eyed Lh e li terature in preparation fOT t heir refer ence books of physical constan Ls of hydrocarbon s; a nd a table of selected values for t he vapor press ure below the normal boiling point was give n by Rossini and co-workers in collaboration with t he American P eLro-leum Institute [7]. Other, Jess critical, surveys have also been made. An important deficiency remainin g after t he above summaries is that t he excellen t API table of vapor pressures in the low-pressure range is based to a consid erable extent on data not generally available. The vapor pressure of olid methan e has been m easured by Hun ter [8], Henning and Stock [9], K arwat [lOi, Freeth and V cl'schoyle [1lJ, Tichn er and Lossing [12] and by Bl'ickwedde and Scott of t he National Bureau of tandards (section 2 of t his pap er). D cterminations of t.h e tripl e poi.nt as Ii tcd in table 1 provide a fixed point t hrough whi ch an equation for the da ta should pass. Of Lh e reported determi ll ations, t he exLensiv e da ta from Clusiu ' labor atory ap pear to be besL l'epresen ted b y Lhe values 90.67 ± 0.03° K and 87.4 ± 0.1 mm , which were determin ed from a sLudy of the melLin g tem-peratul'e as a fun cLion of pressure. Th e value 90.660 ± 0.005° K and 87.60 ± O.I O mm. were foun d by Brickwedde and Scott wi Lh a carefully purified sample. The besL values of t he triple poin L Lempera-ture and press ure appeal' 1,0 be 90.66 ± 0.01 ° K and 87.50 ± 0.10 mm. In an attempt to r emedy this deficien cy t his r e-view has been prepared in which the data from variou sources h ave been compared including those describ ed in section 2, previously unpublished, and available to non e of the previous reviewers except perhaps Rossini. An attemp t has been made to find equations which fit well t he best available data and at the same time pass t hrough selected b est values for the triple point, normal boiling point, and critical poin t. 1 This research was supported in part 'by the-Atom ic Energy Com mission. 2 Figures in brackets indicate t he li terat u re references a t) he end of t his paper.