ANALYTICA
ClitIMICA ACTA =7 'I'hc aluminum cloctrode is a Irighly polarizahlc clcctrode botll nnodically and cnthodically, the passivity of the metal being attributed to a film of aluminum osiclc! covering the clcctrode surface. 'I'hc clcctrodc potentials measured by clcctrochcn~ical techniques do not agree with the values calculatccl from thermodynamic data. I%~sccl on free energy data, ~,ATIhWII' calculated the standard elcctrodc potential for the raaction : Al = Al"" (acl.) + 30 to lx equal to --I.GG V 7wstts the normal hydrogen clcctrode. However, potential mcastmxtxetlts at a "pure" aluminum clcctrocle give values which arc much less negative than those corresponding to the above standard potential. Using an aluminum amalgam elcctrodc, 1-l RYROVSI<Y" calculated a value for the clcctroclc potential of -1.337
V wit11 rcfcrencc to the normal hydrogen clcctrode.
'I'hc use of aluminum sheet (gg.ggOk, pure) as a pll-indicator electrode has been rccornmoncled by SCAIMNO" in potcntiometric titrations of acids with bases. Rcponsc to 1'11, however, has not been reported. It is only stated that a large break in potential occurred at or near the eq~livalencc point, but no potential data arc reported in solution of a given pE1.
After completion of our work, PLUIIB~ using electrodes which were made free of oxide by scraping the surface with alumina, rcportecl values ot potentials after diffcrcnt periods of time in osygcn-satumtcd solutions of various ~II. He cxtrapolatccl the values to zero time (zero osiclc thickness) and considered them to correspond to the potentials of the osidc-free electrode. The potentials tllus obtained diffcrcd by more than 0.6 V from the values cnlculatcd on the basis of LATIBIER'S standard potential, PLUMB offers an intcrprctation for this discrepancy. It should be realized, howcvcr, that the anoclic reaction at an aluminum clcctrodc differs from the cathodic reaction and the potentiirl therefore dots not correspond to that of a rcvcrsiblc system.
Accorcling to our findings, in acid mcclium a pure aluminum clcctrode does not function as a per clectrodc. In agrccmcnt, with PLUMB it was found that the potentials in solutions containing csccss of hydrosyl ions arc fairly rcproduciblc. As a matter of fact, the aluminum clectrodc does not function as a PH electrode but it does as 5 pOH clcctrodc in the lxcsencc of an excess of hydrosyl ions. In acid media containing fluoride the depolarization potential is fairly reproducible and found to vary with plr and fluoride concentration. However, when exact information concerning the change Hcfcrcllccs p. zg IS I. M. I<oI.'rIIoI'I~, c. J. SAM HUCli'rTI VOL.