SOTEY
494
1-01. ti4
NOTES THE STASDARD HE-ITS OF FOKMATIOS OF TIIF, 10s PAIRS CdI+ ASD ZnI+
siderubly in error because of lack of information concerning the values of the activity coefficients. But if the concentrat,ioIi of the cadmium ions in the BY D. K. -%AIIEF.SOA, G . S . L I ~ L C O L \.\AD I 11. ?; PARTUAsolution is made several times greater than that of it Oliigo Dunedzii .\ Ledand the iodide ions the conceiitrations of Cd13- and XeceiLed October 1 , 1959 Cd142- will be ext,reniely small since the values of the association const,aiits of these species which -4ustin, Matheson aiid Parton' have reported Uiider values of the thermodynamic properties of some of have so far beell published are all these circumstances the last, two ternis in equation the complex ioiiq formed in aqueous solutions of lead and cadmium halides. Calorimetric enthalpies 3 may be neglected. The values of AH1 and AH1 of formation Jvere reported for the lead halide com- t'hen can be calculated from at least tivo equations of plex ions of type I'bX+, but not for the correspond- this simplified form. ing cadmium halide complexes. Approximate Experimental enthalpies of formatioii for the latter species were Apparatus.-The dilution experiments wert~ performed calculat(>dfrom the heat of dilution measurements in a calorimeter of about one-liter capacity zimilar to t h a t of Robiiiwii and lTallace.2 described by Young and 1'ogel.j The potassium iodide We present here the result of a calorimetric solution was contained in a t h i n - I d l e d glass bulh of about and was mixed with Y90 ml. of siirrouridiny determiiiatioii of the enthalpy of formation of the 10 ml. capacity by breaking the bulb n i t h the I h d e of the staincomplex ion CdI+ nhich is in satisfactory agree- solution less steel stirrer. T h e temperature change caused b>- the ment n i t h the calculated value. The result of lieat of reaction ITUS measured by means of tn-o thermistors a sirnilal. mea.urement for the ion ZnI+ is also re- connected in opposite arms of an EL.?. bridgr, and TVW compared both before and after the breaking of thc. bulb v i t h the ported. change produced by a measllred :tmount of The rc-sultb were ohtained by measuring the heat temperature heat generated electrically in a h-ichrome n-ire liwter. effects produced 1)y the hundred-fold dil&oii of a The heating cxrrent of about 0.3 ampere v a s measured l>y potassium iodide solution firstly n i t h 0.054 m means of a potent,ionieterand a one ohm stunthird r c s k t m c c . Cd(CIOI)n mid iecondly with a perchloric acid Thp heating period of aboiit four minutw wvizs mcamred a on(,-tenth second stop-n:itch. A minimum ten?solution of w c h colicelitration that the filial mix- with peratiire change of 0.001" could be detectcd Iiy the change i n tures had the same ioiiic strength ( I ) . These tIvo hicli corrcspondrti to a hetit tcm. Thi' cdorimt~tcrn-as dilution procew's may he represented as 01
KI(n2')
0 0.54 --__
i)/
CIL
rd(c10
ostat :tt 25 f 0 . 0 1 O .
1)
--____f
hj- nirasurinp t,he heat of tiilution of :i sodium chloridc solution. Tlic results of this test itn' given in Txble I. :iJst(d
K I ( 0 01 111') Ionic strength I I Ql cal.
(I)
Ionic strr,ngtli I,.& cal.
(2)
If it is assumed t1i:it the difference htn-eeii and Qz is due eiitirply t o the forination of complex ions, n+iicli i i i wcin.ii-,iii iodide solutions may he of the fclrni C'diT, Cdl2?CdI,--. CdI4'-, then A& = 81- 0 2 = [CdIi-lAHl
+ [CIII,]AII, r [CdI:j-]AH:i + [CdI:'-]AH,
(3)
I n this aquation [CdI+], etc., are the numbers of gram moles of t'he various complex ions in the final solution. and AHl, etc.. are the molar enthalpies of formation of the complex ioiis at the particular ionic strength of the solution. The c.oiicentrations of the rornples ioiis call be calculated from the assocGitioii coiist:ints of the complexes mid the cwicentratioiis of cadniium aiid iodide ioiis. It has tieel pointed out 13)- Touiig and Jo11es3that' the reported values of t h e association coiistaiits for coniplcso:: surh as CdI,,- and C & - niay )IC coli11) .J. 31 A I I S ~ I I1:. I . -1.3Intlirsuii nnd I[. S . P a r t o n . "The Stiiictrirc ited by I\-. J. Ilaiiies, Jnlin X i l e y and
( 2 ) .i. 1.. I
llol~iiisoii nncl IV. E. IVallavr, Ciiem. Ileus., 3 0 , 1!)3
!!442),
( 3 ) 1' t'
y
.I
( ' , ~ u l l e s.11in. ,
/
