1656 J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y August1986

r each ing , at equ i l i b r ium, a less nega t i ve (more act ive) va lue than the one observed for the same e lemen t s if used a lone as cathodes. Metal ions of iron group (Fe, Co, and Ni) b e l o n g to th is ca tegory . These sys tems , d u r i n g opera t ion , have the t endency to e lec t rodepos i t as meta l in the form of t he dendr i t i c deposi t : f rom one side, t h i ck and c o m p a c t enough to cover comple te ly the under ly ing act ive coat ing and, f rom the o the r side, h a v i n g an open a r ch i t e c tu r e main ly in the outer layer so as to create a n e w surface, less act ive than the L H E O C but more act ive than the same ele- men t s i f used alone as the cathode.

Nondeactivating agents.--This group, w h i c h is an ex- cep t ion , cons is t s on ly of copper , w h i c h does no t s eem to p lay any role in the ca thode po ten t i a l for all t e s t ed LHEOC.

Copper , unde r these condi t ions, has a t endency to elec- t rodepos i t in a ve ry porous, open, and thin form, creating, on one side, a large area (a condi t ion necessary but not suf- f ic ient for p r e v e n t i n g or k e e p i n g u n d e r con t ro l the cata- lyt ic aging) and, on the o the r side, l eav ing f ree the ma jo r par t of t he ac t ive coat ing. These , toge ther , m i g h t be the m o s t p r o b a b l e reasons for th is u n e x p e c t e d and u n u s u a l e lec t rochemica l per formance .

Conclusions Mercury , o rgan ics con t a in ing " N " and " S " func t iona l

g roups , c h r o m i u m , nickel , and i ron deac t iva te L H E O C . Coppe r seems to have no effect. By looking in detai l at the m o r p h o l o g y of the var ious cases of two typical impuri t ies , i ron and copper , the above resul t s can be i n t e r p r e t e d in t e rms of surface phenomena .

Manuscr ip t submi t t ed Oct. 23, 1985; revised manusc r ip t r ece ived Apri l 7, 1986. This was Pape r 429 presen ted at the Toron to , Ontar io , Canada, Mee t i ng of the Socie ty , May 12-17, 1985.

Oronzio de Nora Impianti Elettrochimici S.p.A. assisted in meeting the publication costs of this article.

R E F E R E N C E S 1. A. Nido la and R. Schira , P a p e r 236 p r e s e n t e d at The

E l e c t r o c h e m i c a l Soc i e ty Meet ing , Cincinnat i , OH, May 6-11, 1984.

2. A. Nido la and R. Schira , p r e s e n t e d at J o u r n ~ e s d ' E l e c t r o c h i m i e 1985, F lo rence , Italy, May 28-31, 1985.

3. A. Nidola and R. Schira, p resen ted at the 3rd Interna- t iona l S y m p o s i u m of S c i e n c e and Techno logy , Ma- dras, India, Dec. 1984.

4. A. J. Arvia and D. Posadas, in "Encyc loped ia of Elec- t rochemis t ry , A. J. Bard, Edi tor , Vol. III, p. 262 (1975).

5. K. L o h r b e r g and P. Kohl , Electrochimi. Acta, 29, 1557 (1984).

6. T. J. Gray, U.S. Pat. 4,430,186 (1984). 7. K. Kinoshi ta and P. Stonehar t , in "Modern Aspec ts of

E l e c t r o c h e m i s t r y , " J. O'M. Bockr i s and B. E. Conway, Editors, Vol. 12, p. 205, P l e n u m Press, N e w York (1977).

8. D. S. C a m e r o n and S. J. Copper , U.S. Pat. 4,414,071 (1983).

9. F. A. Lowenhe im , "Modern Electroplat ing, 3rd ed., pp. 342-357, J o h n Wiley, New York (1974).

10. P. Pascal, "ComplOments au nouveau trait6 de chemie min6ra le , " T o m e 9, p. 113, B. Masson & C., Par is (1977).

11. R. B u c u r and P. Marg inean , Electrochimi. Acta, 29, 1297 (1984).

The Solubility Product Constant of ZnO Thedford P. Dirkse*

Department of Chemistry, Calvin College, Grand Rapids, Michigan 49506

Many a t t empts have been m a d e to de te rmine the solubil- i ty p r o d u c t va lue for zinc hyd rox ide , and a va r i e ty of ex- p e r i m e n t a l m e t h o d s have. b e e n en l i s t ed in these efforts. The resul ts , however , are far f rom cons is ten t . At 25~ or about room tempera ture , they range f rom 10 -14 (1) to 10 -21 (2).

The re are severa l r easons for th is d i sc repancy . I t has b e e n k n o w n for s o m e t i m e tha t f resh ly p rec ip i t a t ed zinc h y d r o x i d e undergoes an aging process (3). Dur ing this pro- cess, t he p r ec ip i t a t e changes phys ica l ly and its so lub i l i ty decreases . The process of aging is cons idered to be due to a loss of wa te r f rom the precipitate. Many of the a t tempts to d e t e r m i n e the so lub i l i ty p r o d u c t of zinc h y d r o x i d e in- vo lved m a k i n g concen t ra t ion m e a s u r e m e n t s in a solut ion f rom w h i c h zinc h y d r o x i d e had ju s t b e e n p rec ip i t a t ed , and, thus, t hese va lues w e r e m e t a s t a b l e ra ther t han equ i l i b r ium values.

F u r t h e r m o r e , t he p r e p a r a t i o n of pu re Zn(OH)2 is diffi- cult. A prec ip i ta te fo rmed by adding OH- ions to aqueous solut ions of zinc salts gives a precipi ta te that conta ins the an ion of the zinc salt. Most of the a t t empts to measu re the solubi l i ty p roduc t of Zn(OH)2 used a m e t h o d that invo lved the addi t ion of alkali to aqueous solut ions of a zinc salt. In t h e s e cases, t he re is l eg i t ima te d o u b t abou t the compos i - t ion of the sol id phase in e q u i l i b r i u m wi th the sa tu ra ted solution. It is more l ikely to be a basic salt than Zn(OH)2. A more comple t e descr ip t ion of these difficult ies is g iven in Ref. (4).

The aging of zinc hyd rox ide even tua l ly leads to the for- ma t ion of ZnO, wh ich is a stable solid phase. Thus, to avoid metas tab le s i tuat ions in seeking to eva lua te the solu- bi l i ty p roduc t of Zn(OH)2, it is necessary to use ZnO as the sol id phase . Yet, a sea rch of the l i t e ra tu re has r evea l ed that , whi le so lub i l i ty p r o d u c t va lues as a c o n c e n t r a t i o n

* Electrochemical Society Life Member.

p r o d u c t have been r epo r t ed for ZnO in e q u i l i b r i u m wi th aqueous sa tura ted solutions, no work has been repor ted in w h i c h a t h e r m o d y n a m i c va lue of the so lub i l i ty p r o d u c t has been obta ined for such solutions.

The work repor ted here was carried out in an a t t empt to rec t i fy th is lack. The m e t h o d used i n v o l v e d the measure - m e n t of the vol tage of the fol lowing cell at 25.0~

Zn ] 1.0M MOH saturated with ZnO I 1 . 0 M M O H ] H g O I H g [1]

The e lec t rode react ions and E ~ values (5) were cons idered to be

Zn 2+ + 2e = Zn E ~ = 0.763V [2]

HgO + H20 + 2e = Hg + 2 OH- E ~ = 0.098V [3]

g iv ing a cell react ion

Zn(s) + HgO(s) + H20 = Hg(1) + Zn 2+ + 2 OH- [4]

w i th E~ = 0.861V. The ac t iv i t ies of the sol id and l iqu id phases are c o n s i d e r e d to be uni ty. There fo re , the equi l ib- r i u m cons t an t for the cell r eac t ion c o r r e s p o n d s to the quo t i en t

(az,2+)" (aon- )2/(aH2o) = K%o/(aK2o) [5]

for a solut ion of alkali sa turated wi th ZnO. Us ing the Nerns t equat ion, the vol tage of the cell in Eq.

[1] t hen becomes

Ecen = 0.861 - (RT/2F) in (K%o/aH2o) [6]

Experimental The cell case was a Py rex glass H-type cell wi th a fr i t ted

porous glass d i sk in the c rossp iece . The e l ec t rode com- pa r tmen t s were 2.5 cm in diameter . One c o m p a r t m e n t con-

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.97.58.73Downloaded on 2014-05-30 to IP

Vol. 133, No. 8 S O L U B I L I T Y P R O D U C T C O N S T A N T 1657

Ld

1.3600

1.3580

1.3560

1.354C

I I I I I

~ 0 o 0

0

0 0

% %

ooo 0 o

I I I 2 0 40 60

days Fig. 1. Variation of the EMF of the cell: Zn I 1.0M KOH saturated

with ZnO[1.0M KOHiHgOJHg.

O~oo~ I 0 0 0 0 ~ G0o

80 I00

tained AR grade red HgO and triple-distil led mercury. A p la t inum wire was used to make contact with the HgO/ Hg/OH- electrode. The other compar tment contained a length of 99.9% zinc rod (1.5 mm diam) in the alkali electro- lyte, which had previously been saturated with AR grade ZnO. Twice-dist i l led deionized water was used to make the solutions. Two cells were prepared: in one the electro- lyte was a 1.0M KOH solution; in the other, a 1.0M NaOH solution was the electrolyte. The cells were placed in a con- stant tempera ture bath at 25.0~ The electrolyte in each compar tmen t was then deaerated by bubbl ing ni t rogen through the electrolyte for 20-30 min. After this, each cell compar tmen t was sealed to maintain a ni t rogen atmo- sphere above the electrolyte and to exclude the access of air.

Cell voltages were measured several times a week with a cal ibrated Kei thley Model 179 TRMS digital mult imeter . This ins t rument has an input resistance of 10 Mf~. Mea- surements were made over a period of about 31/2 months.

Results and Discussion The cell voltages showed a steady decrease over the first

months (Fig. 1). The cell voltages selected as the equilib- r ium values were those for which the cell voltages re- mained approximate ly constant for about 3-4 weeks: 1.3521 -+ 0.00005V for the cell containing the 1.0M KOH, and 1.3544 -+ 0.0001V for the cell containing the 1.0M NaOH electrolyte. No corrections were made for the junction po- tential because the alkali concentra t ion was the same on ei ther side of the junct ion and the am'ount of ZnO dis- solved in these solutions was considered to be too small to change the activity coefficients of the ions and the H20 appreciably.

To calculate K~ o for Zn(OH)2 from Eq. [6], it is necessary to know the activity of the water at 25.0~ in these alkali so- lutions. The values selected are: log aH2o = --0.016 for the 1.0M KOH solutions (6) and -0.0148 for the 1.0M NaOH so- lut ions (7). Both these values were de termined by EMF measurements over a wide range of temperature and alkali

concentrations. More recently, these values have been cal- culated from vapor pressure measurements (8). The values so obtained are: log aH20 -- -0.0173 and -0.0159 for the KOH and NaOH solutions, respectively.

Additional values used in these calculations are: R = 8.314 J K -1 mol-1; T = 298.2 K; and F = 96,485 C mo1-1. Substi tution of these values in Eq. [6] gives the following values for the K~ of Zn(OH)2:2.4 • 10 -17 in the 1.0M KOI-I solutions, and 2.0 • 10 -~7 in the 1.0M NaOH solutions. Within the limits of significant figures, the different values for the activity of water make no difference in the final cal- culated results.

An analysis of the values used in making these calcula- tions shows that the least precisely known value is that for the activity of the water. This is the limiting numerical•ac- tor in arriving at the final calculated result, and the values given above represent this uncertainty. For this reason, no a t tempt was made to measure the cell voltages more pre- cisely than to 0.1 mV.

The voltage for the cell containing the KOH electrolyte was a little lower than that for the cell containing the NaOH electrolyte. Al lmand (9) also noticed differences in cell voltages between cells containing NaOH and KOH so- lutions as the electrolyte. These observations were made in the course of a study in which he sought to determine the solubility product constant for Cu(OH)2 and for CuOH. He consistently observed that cell voltages were 1-2 mV more negat ive for cells containing aqueous KOH than for cells containing aqueous NaOH.

Conclusions The work reported here gives a value of 2.2 • 10 -1T for the

K~ of ZnO. This is in good agreement with the value of 3 • 10 -17, which was determined by EMF measurements , of a cell similar to that used in this work except that the solid phase was a rather pure form of Zn(OH)2 and the al- kali was 1M NaOH (10).

Manuscript submitted June 24, 1985; revised manuscript received March 17, 1986.

Calvin College assisted in meeting the publication costs of this article.

REFERENCES 1. O. Fruhwirth, G.W. Herzog, I. Hollerer, and G.

Reitsamer, Surf. Technol., 15, 43 (1982). 2. G. C. Bauer, Iowa State College J. Sci., 13, 37 (1938). 3. F. Fricke and T. Ahrndts, Z. Anorg. Allg. Chem., 134,

344 (1924). 4. W. Fe i tknecht and P. Schindler, Pure Appl. Chem., 6,

130 (1963) 5. W.M. Latimer, "Oxidation Potentials," 2nd ed. pp. 168,

179, Prentice-Hall, Englewood Cliffs, NJ (1950). 6. G. Aker lof and P. Bender, J. Am. Chem. Soc., 70, 2366

(1948). 7. G. Akerlof and G. Kegeles, ibid., 62, 620 (1940). 8. B. Pound, B. Sunderaraj , R. P. Singh, and D. D.

Macdonald, "Thermodynamic Framework for Est imat ing Efficiencies in Alkaline Batteries," Re- port LBL-16806, Lawrence Berkeley Laboratories, Berkeley, California (1983).

9. A. J. Allmand, J. Chem. Soc., 95, 2151 (1905). 10. H. G. Dietrich and J. Johnston, J. Am. Chem. Soc., 49,

1419 (1927).

Involvement of Incipient Metal Oxidation Products in Organic Oxidation Reactions at Noble Metal Anodes

L. D. Burke and V. J. Cunnane

Department of Chemistry, University College, Cork, Ireland

The object of the present work is to demonstrate that in many organic electro-oxidat ion reactions at noble metal anodes in aqueous media, interfacial cyclic redox behavior involving incipient metal oxidation products (as interme-

diates) may be more impor tant than the widely accepted adsorption-based view (1-3) of this type of reaction. The ac- t ivated chemisorpt ion model is still accepted as being valid for many reactions involving hydrogen gas, e.g., hy-

) unless CC License in place (see abstract).  ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.97.58.73Downloaded on 2014-05-30 to IP