[ ( ~ O N T R I I l I J T I ( O NF R O M TEIE STEEL CHEMISTRY
LABORATORY AT I ~ r l R T M O U T I IC O L L E G E ]
A Polarographic Study of the Perborate Complex BY DAVIDhl. KERN RECEIVi$rJ h r A R C I l
28, 1953
Potential nie~lsurernetitsof the 02-Ha02 couple in 0-0.3 .lr borate arid inilliniolar H 2 0 2solutioiis support the formula H ~ O Z . B ( O H )for ~ - the perborate species, with a dissociation constant K = 0.03 mole/l., in agreement with the results of previous workers. At low borate ion concentrations, however, the apparent value of K increases t o ca. 0.05. The half wave potentials and diffusion currents of the HgOa reduction wave have been determined as a function of the borate ion concentration. The potential data indicate that perborate reduction occurs I>>- way of the HpOr molecule.
I n a recent study1 of the 02-HzO2 couple a t the dropping mercury electrode, the diffusion current of H202 was observed to be much depressed in the presence of borate ion. This effect was attributed to the formation of a perborate complex.? The present paper deals in detail with the influence of borate on the 02-H20zpotential, the diffusion coefficient of H 2 0 2and the half-wave potential of i t 4 reduction wave. The complexation of hydrogen peroxide and borate ion was studied initially by Menzelj3 using a solvent distribution technique, and recently by Edwards,4who observed the effect of hydrogen peioxide on the pH of boric acid and borax solutions. I t appears from these investigations t h a t Hz02and borate ion form a whole series of complexes of the type B,(H202)n, where B represents the borate ion B ( 0 H ) - 4 ; however, both authors found t h a t their results could be explained fairly well assuming that the only important species was B.H2O2, a t least when the initial concentrations of the two substances were in the range 0.05-0.5 M . There is no evidence for a complex with boric acid. The 02-H202Couple in Borate Buffers.-The potential of the 02-H202couple is conveniently measured a t the dropping mercury electrode bj' observing the potential a t which the composite wave crosses the residual current. At that point the mercury drop is functioning as a null point electrode, there being no net conversion of one conipound to the other. The crossing potential is related to the standard potential a t 25" by the equation E,
=
E"
+ 0 0296 log P O ~ / C E-~ O0 O592pH ~
(1 I
This equation represents a linear relation between E , and p H in solutions of constant O2 and H2O2 concentrations. On addition of borate to the solution, complexation will reduce the H202concentrntion and cause the observed crossing potential E: to diverge from the straight line in the positive direction. If it be assumed t h a t the borate complex has the simplest formula, then where CR?O>is the original H202 concentration.5 (1) 1).K e r n , 'L'IIIS JUTJR~YAI,, 7 6 , 4208 (l!lLX). ( 2 ) '4 similar phenomenon i n t h e polarography of glyoyal in b o r a t e bi,Eers was ul,ser\ed b y P IIlvinL a n 8 C Bennett Lhit!., 76, 1112
€1. LIenzei. Z p h y s i b . Chem., 1 0 5 , 427 ( l Y 2 : 3 ) . J . Edrr-ards, T H I SJ i i L? T h e eu,,erirnents d e ionic strenqtli. so it was considered admi,sihle t o use concentration7 r a t h e r t h a n activities in these equations. T h e 01activity was autonintically maintained const:int throuchmit b r c n u i r t h e s d u t i i i n s were iii qtiiiihriiimw i t h thra a i r .
Substituting the reduced H202 concentration into equation I , we find for the new crossing potenfinl E,: = E" f 0.0290 log P , , J C H ~ O ~ 0.0296 log (1
+ + (B),'K)
- 0.0592PH
(;3)
The e.m.f. shift occasioned by the presence of borate in a solution of fixed H + , 0 2 and total peroxide concentrations is then given by E: - E, = A& = 0.0296 log (1
+ (U)/S)
(-4)
A complication arises for solutions of p H greater than 10.6 because of the acid dissociation of IlSO?. At these acidities the crossing potential even in borate-free solution diverges from the value calculated from equation 1 by an amount AEi, = 0.0296 log ( 1 - I
