of M A are about 0.5 e.v. with an unusual situation of 1.5 volt deviation in the Cr+ ion. Consequently the errors in the determination of the absolute value of appearance potentials are magnified by the difficulty in calibration. An alternative to the use of rare gas calibration is the assumption that, in C1?HloNi,for example, the sum of the ionization potential of Ni and the thermochemically determined bond dissociation energy is equal to the appearance potential of Xi'. This gives a value 0.7 e.v. lower than obtained using a K r calibration. This still gives poor internal consistency in the iron case with a n estimated bond energy higher than obtained from the thermochemical data. Results obtained for metal-ring bond energies by subtracting ionization potentials of the free metal atom from the appearance potential of the corresponding ion from C,oHloM indicates weaker bonding in CloH&g, CloHloMn and CloHloNi. The values for the compounds of Fe, Co, Cr and lrare higher but in view of the uncertainties these can be considered only as showing a trend indicating stronger bonding, Subtraction of the estimated bond energy from appearance potentials of C5H511+ ions gives an upper limit for the ionization potential
of the CsH5Xradical. If these potentials are compared with ionization potentials of the respective metal atoms a similar correlation is made separating ionization mechanisms for C5Hs3Ig) C5H5Mn and C5H5V radicals into one category with potentials 0 7 and 1.5 volts above the atomic ionization potentials and remaining ions with potentials ranging from 2.4 t o 3 2 . e.v. higher. Higher ioniation potentials indicate ionization mechanisms involving removal of electrons either from the C5H5ring or from ring-metal bonds rather than loss of electrons from non-bonding metal orbitals. Consideration of ionization mechanisms leads to the row in Table 111 containing molecular ionization potentials. In the Mg, Cr, M n and V coriipounds these values are all close to free metal ionization potentials. Differences of approximatelv 0.6 and 0.9 e.v. below atomic ionization potential are observed in Ni and Fe, respectively, with Co lower by the largest amount, 1.7 e:,-. The experimental data on molecular ionization potentials are probably least subject to error of all the appearance potentials Some qualitative arguments may be suggested for the low CloH1iCo+value on the basis of a closed electronic configuration for the positive ion
[CONTRIBUTIOS FROM THE POLYTECHNIC I S S T I T C T E O F B R O O K L Y S ]
Chelation of Copper (TI) with Polyacrylic and Polymethacrylic Acid BY -1. 11. KOTLIAR~ AND H. NOR.IWETZ R E C E I V E D J.4SUARP
6,1%?12
The binding of copper( 11) ion by poly(acry1ic acid) and poly(methacr>-licacid) was itivestigated by compzriiig titratioii data, absorption spectra and dialysis equilbria. The extent of copper(I1) binding cannot be calculated from thc titratioii shift without assuming the nature of the chelate formed. Spectroscopic evidence indicates the persistence of a Tingle chelate, containing probably four carboxylates bound to a copper(I1) ion, over a wide range of conditions. Dialysis rquilibrium measurements can be made with dilute polymer solutions and high pol>-rricr/copper(11) ratios and they yield a detailed picture of the dependence of chelation on electrostatic factors, polymer concentration arid the density of ligaiitl groups in t h e individual macromolecule.
The interaction of polyions with monovalent counter-ions has been thoroughly investigated in recent years and an excellent review of the work has been published by Doty and Ehrlich.? Much less is known about the behavior of polymeric acids in the presence of multivalent cations, where both electrostatic interactions and specific complex formation have t o be taken into account. A previous communication from this Laboratory3 dealt with the binding of alkaline earth ions by hydrolyzed maleic anhydride copolymers, a particularly simple case, since there is no uncertainty about the nature of the chelate complex. This paper deals with the chelation of copper(I1) by poly(acry1ic acid) (PALA)and poly(inethacry1ic acid) (PIZIX) . The procedure outlined I)reviously3 for the interpretation of the pH shift produced by a cation in terms of complex formatioil with 3 polymeric acid does nut yield unambiguous results if the number of ligands bound is unknown atid the titration &ita were therefore supplemented with :I ( 1 ) A b s t r k c t e d from t h e 1R.i.5 Ph.11. theais of A. 11. K u L l i u , AI l o r a n e t z , A .\I. Kotlizir a n d H XIark, J I'hy.s ( ' h c n x 5 8 , 19 (1951).
direct determination of copper(I1) binding by dialysis equilibrium and spectroscopic studies of the nature of the chelate complex..' Results and Discussion LVhen (PMA) is titrated in 1 Npotassiuni nitrate, a pronounced shift t o lower p H values is observed on addition of small concentrations of copper(I1) ion (Table I). This was ascribed to coinplcx formation, since similar concentrations of all; a 1'l l l C earth ions produce no shift in the titration curve ( r i ' PAM-1in strong salt solution. T o interpret titration shifts brought about by :i chelating ion, the procedure introduced by Bjerruin' has t o be modified in accordance with polyelecirolyte theory. SiTe shall assume that the electrical free energy o f dissociation LIE',,^ o f t h e carbos!.l groups oil the polynieric chain is independent oE tlic iiuiiiber o f bound metal ions as long as 3 , the avcrage uegative charge per carboxyl group is held con( 4 )
