Aqueous Sulfuric Acid. Heat Capacity. Partial Specific Heat Content of Water at 25 and -20''
F.GL~UQVS
BY J , E. KUWLLER AND M'.
RECEIVFD FERRVARU 25, 1952 'I'he heat capacity of aqueous sulfuric acid arid the partial specific heat3content of water has been determined over the range
1 molal to pure fi?SO, at 25 '. Some concentrations, still liquid a t - "1 , :yere iiivestigated a t that temperature. The tneasurements were made to enable the calculation of the change in the partial molal free energies of the components from freezing ~'oiiitdata of t h c v,iriouq hyiirutc.-. .Ininiprovcd wlorimeter :itid procedure for making such measurements is described.
'The investigation reported here had its origin in accuracy needed in the present research, the unsuccessful atteinpts to use existing data to evalu- partial molal properties would have to be redeterLite the entropy ~f hydration of sulfuric acid. mined. The method adopted was making small 'The entropy of hyclration was desired for c o i n increments of water to the sulfuric acid solution. parison with results obtained from low teiiipera- observation of the temperature rise, subsequent ture heat capacities and the third law of theniio- cooling, followed by the introduction of electrical dynamics. The most promising method of ob- energy to evaluate the heat of solution and the heat taining the entropy of hydration, especially in the capacity. It was clear that diluting sulfuric acid inore concentrated solutions, depends on the use of by small increments, over the whole range of confreezing point data on the several hydrates. The centration, wduld require some improvement over ireezing point curves have been investigated by previous procedure i i i order to bring the amount of Gable, Betz and Maron. However, these authors experimental work within reasonable limits. Such did not plan their irieasurenieuts for the purposc ;L method as bringing a saiiiple of water to temperaof obtaining partial iiiolal free eiiergitts anti their ture equilibrium within the calorimeter, with subobservations are being supplemented by additional sequent mixing, is impracticable, not only because nieasurenients, especially in the itriportail t regions of the labor involved, but also because the repeated near the melting points ol' tht- inoiiohytlrate a11d disassembling and assembling of the necessarily somewhat fragile apparatus would cause its detethe anhydrous acid. The free energy aiid eiitropy calculations will be rioration. presented in a later paper. The calculatioiis require The Dilution Calorimeter .--The calorimeter was de;in accurate knowledge of the partial molal heat &yecl along the lines of one used by Randall and Bissong con tents and heat capacities in sulfuric acid solu- for dilution experiiiierits. The details of the present cal:ire iridic:tted iii Fig. l . The dilution is carried tioris. h t a are avaibdhle, but on trying tu llse orinicter o u t in .-L oiie liter silvered dewar, 7, which was specially COIIhe basic observations n-e found that it was i i i i - Ytructc(1 \\-ithoui yxiceri. to reduce heat leak. The dewar pussible to reproduce the partial nlolal quailtities i:, ciiciobeil ill a nickel plated brass container, 1. The shaft giver1 in the tables of Lewis and Karidall,' and by of ii Pyres glass stirrer, 9, was sealed t o a brass shaft, 3, by ( I C Khotinsky cement at a point under a clamp which supCraig and Vinali in an utiambiguous inaniier. pori iii ruhher cup, 6 . 'This cup is used to catch any greasr -1 principal difficult!- is that total heat \-due5 or dirt t h ~ niiglit t Tvork it.; n a y doivii the stirrer shaft. Rcgiyeii by Hronsted,:' and others, a~icl t c ~ t d Ileiit !ial,lc itirrer (ilivratioii m t l uiiiforni speed are essential iri capacities given hy Uiron," werc nut iiieasurrti ' t t wi.11 :tu apptr:ttuc. :itit1 thehe were not obtained until :in sufiiciently close intervals. \Laiiy additiolial val~tes "0ilii.c" l w , i i i i i ; jii*t I)CI
