280
and coworkers9 in an early investigation of the amylose-iodine reaction, is in full agreement with this suggested balance between reactions 1 and 4. According to the concept discussed above, the shorter polyiodine chains in the diorter helical regions of the low molecular weight amylose would be more susceptible to this complete depolymeriza tion (reaction 4) than the long complex chains of a high molecular weight amylose. Thus higher equilibrium iodine concentrations would be expected for the former case and lower ones for the latter. The above considerations may also explain why the salt-induced precipiitation of the complex from amylose solutions of heterogeneous molecular weight is selective for high molecular weight a m y l o ~ e . * ~Tn, ~such ~ a solution all amylose fraction8 must be in equilibrium with a, common free iodine concentration; therefore, the high molecular weight fractions will bind much more iodine than the OW molecular weight fractions and will become more susceptible to precipitation by the salt than
NOTES the low molecular weight amylose fractions which are "solubilized" by their uncomplexed regions. Due to the mathematical complexity of the problem in the absence of an initial state assumption it is difficult to compare the entire observed time course of the reaction (e.g., Figure 1 in ref 14) to the concentration vs. time curves predicted from the mechanism. At the present time we are using the analog computer mentioned above to solve this problem and our initial data are very encouraging. We hope that a complete discussion of the results will be a part of our next communication on this subject. Acknowledgment. The authors wish to thank Mr. John W. Chang for the assistance in the analog computer study and Mr. Alex Vivod for the expert help offered in several instrumentation problems. (27) R. E. Rundle, J. F.Foster, and R. R. Baldwin, J . Amer. Chem. Soc., 66, 2116 (1944).
(28) K.Ohashi, J . Agr. Chem. Sac. Jap., 33, 576 (1959).
NOTES
The Partial M O M Volumes of Tetraphenylarsnndum Tetraphenylboron in Water at Infinite Dilution. Ionic Partial Molal Volumes by Frank J. Fdillero Contribution h'o. 119ti from The Unhersity of Miama, Rosenstiel School of Marine and Atmospheric Sciences, Miami, Florida 33149 (Receized August 7, 1570) Publication costs assiated by the Institute of Marine Sciences
The partial. mola,l volumes, V2, of electrolytes have proved to be very useful in elucidating ion-ion and ionsolvent interaction~.l-~The partial molal volumes of electrolytes at infinite dilution, P " 2 , where ion-ion interactions vanish, are particularly appropriate to study ion-solvent interactions since volume properties are easy to visualize (geometrically) and relatively easy to determine (experimentally). The difficulty of using v", data (as well as lother thermodynamic data) to study ion-solvent interactions on an absolute basis stems from the problem of assigning P"'s to individual ions. The division of the Po's of electrolytes into their ionic components, 9" (ion), can normally be made only by nonthermodynamic methods. Lack of adequate theThe Journal of Physical Chemistry, Vol. 76, No. 6,1971
ories or knowledge of such fundamental parameters as ionic radii1,3t4 increase the difficulties of assigning absolute ionic properties. Once the V" of one ion is estimated (usually the proton), the VO's of other ions are fixed due to the additivity principle. Millero1v3and Panckhurst4have recently reviewed the various methods used to estimate ionic " ' 8 . As pointed out else where'^^ to compare the estimates of ionic P"'s obtained by these various methods on a common basis, it is necessary to use the most reliable P" data. For example, Panckhurst4 has criticized most of the methods used by various workers to estimate the P"(H+). He selects P"(H+) = 1.6 cc/mol at 25" in water as the "best" estimate; however, when one uses more reliable P" data one obtains Vo(H+) = -0.1 cc/mol as the "best" estimate (ie., using similar technique^).^ Thus, although some of the criticisms made by Panckhurst4 are valid, most of the methods yield results of P"(H+) between 0 and -5.0 cc/mol at 25" in water. As discussed elsewhere, Panckhurst's criticism4 of Zana and Yeager's6 ionic potential mea(1) F.J. Millero, Chem. Rev,, in press. (2) F. J. Millero in "A Treatise on Skin," Vol. I, H. R. Elden, Ed., Interscience, New York, N. Y., 1971,Chapter 11. (3) F. J. Millero in "Structure and Transport Processes in Water and Aqueous Solutions," R. A. Horne, Ed., Interscience, New York, N. Y., 1971,Chapter 15. (4) M.H.Panckhurst, Rev. Pure A p p l . Chem., 19, 45 (1969).
NOTEP~
281
surements (which yield P"(H+) = -5.4 cc/mol) also appear to be invalid. Due to the internal consistency of the P" (ions) obtained by Zana and Yeager5 and the agreement of hhe ecitimates for V"(H+) obtained by other workers13we feel that P"(H+) should be ca. -5.0 cc/mol a t 25". The purpose of this note is to examine the use of the P" of the largo electrolyte tetraphenylarsonium tetraphenylboron (Ph4AslBPh4)in estimating ionic partial molal volumes. Since Ph4AsBPh4 contains a very large cation and anion of nearly the same size, we hoped to determine ionic partial molal bolumes by the methods used b,y other workei~to calculate ionic heats of transfer6and ionic changefi in partial molal free energies.' The P ' s of aqueous Ph4AsBPh4 solutions a t 0, 25, and 50" were calculated from the P"'s of NaBPh4,* Ph4As(31,9and PJaCI1o#llusing the additivity principle. The Bo's of these electrolytes (as well as HC13) are tabulated in Table I. Table 1: T h e Partial Molal Volumes a t Infinite Dilution, pso,of NaBPhc, PhrAsC1, NaCl, and Ph4AsBPh4 in Water a t 0, 25, and 50' (cc/mol) Temp, OC
0 25 50 0
NaEPhaa
267.16 276.41-1 283.63
P'haAaClb
307.57 318.48 327.94
a Results taken from ref 8. Results taken from ref 9 and 10.
NaCIC
HCld
Ph4AsBPhr
12.90 16.62 17.71
16.45 17.83 18.00
561.83 578.27 593.86
b
Results taken from ref 9. Results taken from ref 3.
d
The simplest method of using the v" of Ph4AsBPh4 to estimate ionic p ' s would be to assume P"(PhdAs+) = Vo(BPh4-). This method yields P"(H+) = -11.5 cc/mol a t 25". This value is more negative by -6.0 oc/mol than t'he estimates made by other methods.lS3 This is not surprising since one would expect the P"(PhdAs+) to be larger than the P"(BPh4-) since the covalent radius of >As< is 1.18 8 compared to 0.88 8 for >BAs A ~ < )and ~ 8"(>B B < ) are ~ the tetrahedral covalent volumesoof the ions >As< and >BAsB
