THE DT980CIATTON PRESSURES OF SOME NTTRIDES. 215 X XV . -The Dissociation Pressures of some Nit rides. By ROLAND EDGAR SLADE and GEOFFREY ISHERWOOD HIGSON. Equilibria of the type 2M+N 2MN, where ill is three equivalents of a metal have been investigated. In this system there are two components namely metal and nitrogen and three phases namely metal nitride and nitrogen. The number of degrees of freedom is thus 2 + 2 - 3 = 1 and there-fore a t one temperature there is one pressure of the nitrogen which determines the equilibrium of the dissociation. If the nitride dis-solves in the metal as a solid solution there are only two phases, and the number of the degrees of freedom is therefore two. The equilibrium will then be determined not only by the pressure of the nitrogen but also by the composition of the solid phase.Applying the Nernst heat. theorem to the equilibrium M + &N,=MN + &, where Q is tbe heat evolved when half a gram-molecule of nitrogen combines with the metal we have and where Qo and Qt respectively are the heats of reaction a t absolute zero and some other temperature T. As the heats of formatioii of the nitrides investigated were unknown i t is impossible to apply these equations to determine the value of pNp but it will be seen that the pressure of nitrogen a t 1127O would be 1 mm. if the heat of formation of the nitride were 70,600 cals. and 60 mm. if the heat of formation were 59,000 cals. Qt=&-,-3*5T, EXPERIMENTAL. The furnace used for the determination of the equilibria is described in the preceding paper.Nitrogen was prepared by heating a solution containing ammonium chloride potassium nit'rate and potassium dichromate. The dichromate served to oxidise any oxides of nitrogen to nitric acid which was absorbed by passing the gas through a long tube of soda-lime. The nitrogen was then dried by passage through a long tube of phosphoric oxide. The equilibria were determined by heating small quantities o 216 THE DISSOCIATION PREBSURE8 OF SOME NITRIDES. the metal to a known temperature introducing nitrogen into the furnace and determining the value to which the pressure fell. Nitrogen was then pumped out and the equilibrium was determined from the low-pressure side. TTanadiunt.-This metal is known to form two nitrides VN and VN,. A t 1203O the equilibrium pressure was found to' be slightly less than 0.2 mm.and a t 1 2 7 1 O slightly less than 1.5 mm. The equil-ibrium pressure is somewhere near these values but equilibrium was attained very slowly and it was impossible t o heat the platinum tube in use a t that time to a higher temperature. Boron ,-An attempt was ninde to determine the dissociation pressure of boron nit.ride at 1 1 0 0 O and 1240O. A t l l O O o the velocity was too low for the equilibrium t o be determined. A t 1240° the pressure of nitrogen fell from 26-4 mm. to 9.4 mm. in six hours and appeared to be approaching the con-stant value of about 9 mm. but the boron attacked the platinum tube and caused i t to leak so that further experiments could not be made. Tantalum.-Tantalum is known to form two stable nitrides TaN and Ta3N,.I n our experiments it is probable that the lower nitride TaN was formed. When the metal was heated a t 1170O in nitrogen under 15 rnm. pressure the gas was slowly absorbed until the pressure fell to 0.5 mm. The furnace was then exhausted to 0.05 mm. The pressure rose in two hours to 0.4 mm. In another experiment' a t 1308O the pressure fell from 9 m, to 1.2 rnm. in one and a-half hours and remained constant for half an hour. Next day it was completely exhausted and again heated t o 1308O. The pressure rose to 0.8 mm. and remained constant a t this value for two hours. The furnace was then allowed t o cool. Summary of Results. Heat of formation of nitride, that is Q0 calculated from Tempera-ture. Pressure of nitrogen. Nernst's formula. 1271 9 9 1.5 7 77,200 ,, Vanadium 1203" Not greater than 0.2 mm. 79,200 cals. Boron ... 1222 9 ? ? 9.4 7 About 69,000 Tantalum 11 7 0 0.4-0.6 mm. 74,700-75,5;0 cals. This investigation was carried out in the Muspratt Laboratory 1308 0.8-1.2 , 79,900-82,800 ,, of Physical Chemistry University of Liverpool. BRITISH PHOTOGRAPHIC RESEARCH ASSOCIATION LABORATORY. [Received February 14th 191 9.