- Email: [email protected]
Induced adsorption of phosphate (H2PO4−) species by Zn adatoms on platinum
JOUnNALOf
ELSEVIER
Journal of ElectrounnlyticalChemistry 437 (1997) 259-262
Short communication
Induced adsoff tion of phosphate (H2PO4) species by Zn adatoms on platinum 13. Horfinyi a,*, Akiko Aramata h ~' Central Research Institute fi~r Chemistry of the Hungarian Academy t~Seh'nces, Budapest, P.O. Box 17, 1525, Hungmy b Catalysis Research Center, HokkaMo University, Sapporo 6, Japan Received 17 June 1997; received in revised form 11 July 1997
Abstract The specific adsorption of HzPO~- ions accompanying the upd of Zn 2+ ions on platinum electrodes has been studied by a radiotracer technique using 32p labelled phosphate species in the presence of a great excess of CIO4 ions. It has been found that the specific adsorption of species induced by Zn adatoms depends on pH and at a given potential on the concentration of Zn 2+ ions. © 1997 Elsevier Science S.A. Keywords: Zn; Upd; Induced anion adsorption; Phosphate; Radiotracer technique
1. Introduction
2. Experimental
In a previous communication [I] the specific anion adsorption accompanying the upd of Zn 2+ ions on (platinized) platinum electrodes in acidic medium was studied by a radiotracer technique using aSS labelled sulphate species and 360 labelled CI- ions. It was found that sulphate ions adsorb readily on the top of Zn adatoms while the adsorption strength of CI- ions on the Zn adlayer is significantly lower than that of HSO4(SO42-) ions. The potential dependence of the adsorption of labelled species in the presence and absence of Zn 2+ ions and the competitive adsorption phenomena occurring between the species present in the system were considered in detail. Thus competitive adsorption between non-labelled H2PO 4 and labelled HSO 4 species was canied out and the high adsorbability of H2PO 4 ions was demonstrated indirectly. On the basis of the latter preliminary results a systematic study of the sorption of H2PO 4 species labelled by the 32p isotope was suggested as a continuation of the work. The aim of present communication is to report the results obtained from the study mentioned above.
The electrochemical cell and the measuring technique described in previous adsorption studies were used [1-5]. 32p labelled phosphoric acid (specific activity 7.4 GBq mmol- J ) was used in the radiotracer adsorption studies. Platinized electrodes were prepared by the usual electrodeposition process [2,3] on the gold-plated plastic foil forming the bottom of the cell. The roughness factor of the electrodes used (determined by hydrogen adsorption) was about 100. HCIO4 solutions in various concentrations were used as supporting electrolytes. Potential values quoted are given on RHE scale. Zn 2+ ions were added in the form of salts by dissolving ZnO in the supporting electrolyte used.
" Correspondingauthor. E-mail: [email protected].
0922-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII $0022-0728(97)00392-6
3. Results Fig. 1 shows the potential dependence of the adsorption of labelled phosphate species in the absence and presence of Zn 2+ ions (at different concentrations) in the presence of 0.I mol dm -3 HCIO4 supporting electrolyte at a low (5 × 10 -5 mol dm -3) H3PO 4 concentration. It may be seen from this figure (and comparing with the similar results obtained for labelled HSO4 ions [1]) that there is no doubt concerning the occurrence of the induced specific adsorption of phosphate species by Zn adatoms (formed in
G, Hordnyl, A, Aramata/ Journal of Elevtrotmalytlval Chemistry437 (1997) 259-262
260
OmV
700 mV
0,5
¢ i
4
'o0,25 2
0
0
400
200
600
.
.
.
v
.
5
0
l0
E/mV
15
thnin
Fig. h Potential dependence of the adsorption of labelled H.,PO4- ions in 0.1 tool dm -3 HCIO4 supporting electrolyte in the absence (curve 1) and presence of Zn -~+ ions, (curve 2 ) 2 , 5 X 10 -3, (curve 3) 1,5X 10 -2 tool dm -'~ Zn(CIO4) 2. cH.i,o£ = 5X 10 -'~ tool dm -'~
the potential range 0-300 mV). In comparison with HSO4species (see Ref. [1]), the increase in the H2PO4 adsorption at E < 300 mV is not too high in the strongly acidic solution. However, at lower H + ion and higher Zn2+ concentrations the extent of induced adsorption is significantly higher as will be shown later. The mobility of the adsorbed H2PO4- species can be clearly demonstrated by the exchange of adsorbed labelled species by non-labelled molecules added to the solution phase. This is shown in Fig. 2. On the other hand, the adsorption of H2PO4 species induced by Zn adatoms cannot be influenced by CI- ions at least at low CI- concentrations while, as is well known [6,7], CI- ions displace H2PO~- species adsorbed on platinum surfaces in the absence of adatoms. This is demonstrated by Fig. 3.
Fig, 3. Effect of CI- ions on the adsorption of 1-121'O~ in the presence of Zn 2+ ions Oil Po- = 5X I0 -'~ tool din-3; Oz,..+ = 2 5 × 10 -3 tool d i n - 3 in I X l 0 tool d m • HCIO 4. C I - ions were added to the solution phase at the moment indicated by the arrow (c = 1 X 10-4 tool din-3),
Fig. 3 clearly shows that the addition of CI- ions does not result in significant change in the extent of the adsorbed labelled H2PO~- at 0 mV (formation ofZn adatoms) as only a slight decrease in the count-rate can be observed. (Compare with the phenomenon occurring after the addition of non-labelled H2PO4- species to the solution phase as shown in Fig. 2.) A switch to 700 mV leads to a complete elimination of the labelled species from the surface. The maximum appearing after the potential switch indicates that at the beginning there is an increase in the HzPO4 adsorption on the free Pt sites in accordance with the expectations based on the potential dependence of the adsorption (see Fig. I) and this initial section is followed by the displacement of adsorbed H2PO4 species by CIions. (These phenomena for HSO[ and CI- competition
,L t~
60
15
fJ
3 I~ 40
~10
2
,./ 'T 20
1
i
0
I0
20 Vmin
30
Fig. 2. Exchange of labelled phosphate species adsorbed at 0 mV in the presence of Zn 2 + ions with non-labelled ones added to the solution phase
in a great excess at the moment indicated by the arrow. Supporting electrolyte: 0.I mol dm -3 HCIO4. Initial H3PO 4 concentration: 5X 10 -5 tool dn1-3. Final: 1 × 10 -2 mol dm-3.Czn..,- = 1,5X 10 -z tool dm -'~
0
200
400
600
E/mY Fig. 4. The eftk:ct of HSO4 ions on the potential dependence of ~he adsorption of labelled H2PO4- species in the presence of Zn 2+ ions Cl.I po- = 5 X 1 0 -5 mol din-3; C z n 2 + = 5 X l 0 -3 tool dm -3 in 0.l tool dm:-34HCIO4, cri,po~-:(I)0, (2) 5"x 10-4; (3) 5 X 10-3 mol dm -3.
G, Horfmyi,A, Aramata/ Journal of Electroana/ytlcalChemistry437 (1997)259-262
261
1,2
q~ 0.4
5 o.s
0,4
0.2 ......
~, 300
0 0
200 E/mY
100
were discussed in the previous paper; for the details, see Ref. [!]) The study of the competitive adsorption of HSO 4 and H2PO 4 species using labelled H2PO 4- and non-labelled HSO 4 confirms the results presented in the previous communication [1] (where the opposite labelling was used), i.e., the adsorption strength of H2PO ~- species is highe r than that of HSO4- ions. This is demonstrated by Fig. 4. It may be seen from this figure that a significant displacement of H2PO 4 species can be attained only by the presence of a great excess o f HSO 4 ions in the solution phase in comparison to the concentration of phosphate species. The result presented in Fig. 3. allows us to determine the potential dependence of the adsorption of H2PO 4 ions induced by Zn adatoms without the interference o f the
0.2
:
"" ..i,:.. ,. q~ : 200 300 E/mV Fig. 7. Potential dependence of the induced adsorption of H2PO~" species in the presence of CI- ions at two different H+ concentrations cH21'O;"= I X 10-4 tool dm-'~; Czr,2÷= 2 × If,'- I mol dm -'a; Coo-= 1× 10-4 tool dm -3. In tide presence of: 0.1 mol dm-3 (dotted line); and 5× I0 -:* mol dm -3 HCIO4 (full line). 0
JJ
0.1
,
,
",
adsorption phenomena occurring on free Pt sites (see Ref. [1]), Fig. 5 shows the F vs. E and A F / A E vs. E curves obtained by potentiostatic point by ~oint method (in 25 mV steps) in the simultaneou);'pr'esence of C I - , Zn 2÷ and labelled H2PO 4 ions in 0.I tool dm -3 HCIO4 supporting electrolyte. The shape of both curves practically coincides with those obtained for H S O 4 ions reported in Ref. [1], i.e., the potential dependence of the anion adsorption is determined by that of the formation of the upd layer. The extent of induced adsorption at a fixed potential depends on the concentration o f Zn 2+ ions. This is shown ill Fig. 6. On the other hand, a decrease in the hydrogen ion concentration also leads to a significant increase in the extent of the anion adsorption as shown in Fig. 7.
30]
o
i 0
-'1
0.1
.
100
Fig, 5, Potential dependence of the adsorption (F, full line) H2PO~" of ions (induced by Zn adatoms) in the presence of CI- ions• c~l po- "= 1 X " ~4 10-'1 tool dm-'~; CZnv.=l.5Xl0-2 mol din-'; ccl-=lX10 tool dm -3 in 0.1 tool dm -3 HCIO4. TIDecorresponding AF/AE vs. E curve (obtained in 25 mV steps) is given by the dotted line.
o,
13•. o ~ . °.l-
,
0.2 c, zn(clq) 2
Fig. 6. The influence of the concentrationof Zn"+ ions on the extent of induced adsorption at 50 mV in the presence of IX10 -4 mol dm-3 H3PO4 in 0.1 tool dm-3 HCIO4 supporting electrolyte.
10
20
30
t/min Fig. 8. The count-rate vs. time curve in the pr.sence of labelled HaPO4 (c = 1X 10-4 tool dm -3) in 5:< 10-2 tool dm-3 Zn(CIO4)2 at pH = 3.5 following sprinkling ZnO (full line) and formationofa Zn(OH)z layer on the surface of the electrode. CDIvo- = I × 10-4 tool dm-3; Czn2*= 1.5 --3 2 4_4 ×10 -2 tool dm "; Cct-=l×10 mol dm -3 in 0.1 tool dm -3 HCI04).
G, tlor(myi, A, Ammam ~Journal tf Electmtmalytlcal Chemisto' 437 (1997) 259-262
262
I
2
400
200
0 0
100
200
A similar increase ot' the count-rate can be observed in the course of electrodeposition of Zn at pH = 5 from a 2 × 10-j tool dm -3 Zn(CIO~)2 solutioll at very low potentials ( - 7 0 0 mV). This is shown in Fig. 9. Interrupting the electrodeposition process, the sorption can be studied under open circuit condition. The results presemed in Fig. 9 demonstrate that phosphate species adsorb on the surface of electrodeposited Zn. The elimination of tile adsorbed labelled molecules can be achieved only by the dissolution of the Zn layer.
t/rain Fig. 9. Count-rate vs. time curve in the course of the electrodeposltion of Zn at -720 mV (I); at open-circult conditions ,'tt - 6 8 0 mV (2) in 0.2 tool dm -3 Zn(CIO4)2 solution at pH = 5; and following the addition of 3X 10-" tool dm -3 HCIO4 (3).
As the pH increases the formation of Zn... OH surface species seems to be a probable process and in the first approximation a decrease in the adsorption of other anions would be expected. It can be shown, however, that on ZnO or Zn(OH)2 surfaces, an accumulation of phosphate species OCCURS.
Using a cell with a gold-plated bottom (without platinization) and sprinkling on the bottom, a small amount of ZnO powder or depositing a thin Zn(OH)2 layer by addition of small droplets of diluted NaOH solution to the solution phase, the sorption of H2PO4 species can be observed. This is shown in Fig. 8 where the transient increase in the apparent phosphate sorption is demonstrated, following tile sprinkling of a small amount of ZnO on the bottom of the cell (full line) or formation of Z n ( O H 2) at the surface in the presence of a slightly acidic medium. Since the medium is slightly acidic, following the formation of the ZnO or Zn(OH)2 layer, the sorption of the labelled species and the slow dissolution of the newly formed layer proceed simultaneously. Thus, the count rate vs. time curve goes through a maximum. It follows from these observations that at high pH values the formation of surface oxide, hydroxide species could play an important role.
4. Conclusions The experimental results presented above confirm the assumptions made previously that the specific adsorption of phosphate species cannot be neglected in the evaluation of the data obtained from the study of the formation of Zn adlayers. Thus data obtained for the upd of Zn in systems containing phosphate species should be reconsidered in the light of these results.
Acknowledgements Financial support from Hungarian Science Fund (OTKA T 023056 and T 014446) is acknowledged.
References [1] [2] [3] [4]
G. Horfinyi, A. Aramata, J. Electroanal. Chem. 434 (1997) 205. G. Hor,Snyi. G. Vrrtes, J. Electroanal. Chem. 45 (1973) 295. G. Horfinyi, J. Electroanal. Chem. 55 (1974) 287. G. Horfinyi, E.M. Rizmayer, P. Jo6, J. Electroanal. Chem. 152 (1983) 211. [5] G. Horfinyi, M. Wasberg, J. Electroanal. Chem. 431 (1996) 161. [6] G. Horfi~lyi,G. Inzelt, J. Electroanal. Chem. 86 (1978) 215. [7] G. Horfinyi, E.M. Rizmayer, G. lnzelt, J. Electroanal. Chem. 93 (1978) 183.
ELSEVIER
Journal of ElectrounnlyticalChemistry 437 (1997) 259-262
Short communication
Induced adsoff tion of phosphate (H2PO4) species by Zn adatoms on platinum 13. Horfinyi a,*, Akiko Aramata h ~' Central Research Institute fi~r Chemistry of the Hungarian Academy t~Seh'nces, Budapest, P.O. Box 17, 1525, Hungmy b Catalysis Research Center, HokkaMo University, Sapporo 6, Japan Received 17 June 1997; received in revised form 11 July 1997
Abstract The specific adsorption of HzPO~- ions accompanying the upd of Zn 2+ ions on platinum electrodes has been studied by a radiotracer technique using 32p labelled phosphate species in the presence of a great excess of CIO4 ions. It has been found that the specific adsorption of species induced by Zn adatoms depends on pH and at a given potential on the concentration of Zn 2+ ions. © 1997 Elsevier Science S.A. Keywords: Zn; Upd; Induced anion adsorption; Phosphate; Radiotracer technique
1. Introduction
2. Experimental
In a previous communication [I] the specific anion adsorption accompanying the upd of Zn 2+ ions on (platinized) platinum electrodes in acidic medium was studied by a radiotracer technique using aSS labelled sulphate species and 360 labelled CI- ions. It was found that sulphate ions adsorb readily on the top of Zn adatoms while the adsorption strength of CI- ions on the Zn adlayer is significantly lower than that of HSO4(SO42-) ions. The potential dependence of the adsorption of labelled species in the presence and absence of Zn 2+ ions and the competitive adsorption phenomena occurring between the species present in the system were considered in detail. Thus competitive adsorption between non-labelled H2PO 4 and labelled HSO 4 species was canied out and the high adsorbability of H2PO 4 ions was demonstrated indirectly. On the basis of the latter preliminary results a systematic study of the sorption of H2PO 4 species labelled by the 32p isotope was suggested as a continuation of the work. The aim of present communication is to report the results obtained from the study mentioned above.
The electrochemical cell and the measuring technique described in previous adsorption studies were used [1-5]. 32p labelled phosphoric acid (specific activity 7.4 GBq mmol- J ) was used in the radiotracer adsorption studies. Platinized electrodes were prepared by the usual electrodeposition process [2,3] on the gold-plated plastic foil forming the bottom of the cell. The roughness factor of the electrodes used (determined by hydrogen adsorption) was about 100. HCIO4 solutions in various concentrations were used as supporting electrolytes. Potential values quoted are given on RHE scale. Zn 2+ ions were added in the form of salts by dissolving ZnO in the supporting electrolyte used.
" Correspondingauthor. E-mail: [email protected].
0922-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII $0022-0728(97)00392-6
3. Results Fig. 1 shows the potential dependence of the adsorption of labelled phosphate species in the absence and presence of Zn 2+ ions (at different concentrations) in the presence of 0.I mol dm -3 HCIO4 supporting electrolyte at a low (5 × 10 -5 mol dm -3) H3PO 4 concentration. It may be seen from this figure (and comparing with the similar results obtained for labelled HSO4 ions [1]) that there is no doubt concerning the occurrence of the induced specific adsorption of phosphate species by Zn adatoms (formed in
G, Hordnyl, A, Aramata/ Journal of Elevtrotmalytlval Chemistry437 (1997) 259-262
260
OmV
700 mV
0,5
¢ i
4
'o0,25 2
0
0
400
200
600
.
.
.
v
.
5
0
l0
E/mV
15
thnin
Fig. h Potential dependence of the adsorption of labelled H.,PO4- ions in 0.1 tool dm -3 HCIO4 supporting electrolyte in the absence (curve 1) and presence of Zn -~+ ions, (curve 2 ) 2 , 5 X 10 -3, (curve 3) 1,5X 10 -2 tool dm -'~ Zn(CIO4) 2. cH.i,o£ = 5X 10 -'~ tool dm -'~
the potential range 0-300 mV). In comparison with HSO4species (see Ref. [1]), the increase in the H2PO4 adsorption at E < 300 mV is not too high in the strongly acidic solution. However, at lower H + ion and higher Zn2+ concentrations the extent of induced adsorption is significantly higher as will be shown later. The mobility of the adsorbed H2PO4- species can be clearly demonstrated by the exchange of adsorbed labelled species by non-labelled molecules added to the solution phase. This is shown in Fig. 2. On the other hand, the adsorption of H2PO4 species induced by Zn adatoms cannot be influenced by CI- ions at least at low CI- concentrations while, as is well known [6,7], CI- ions displace H2PO~- species adsorbed on platinum surfaces in the absence of adatoms. This is demonstrated by Fig. 3.
Fig, 3. Effect of CI- ions on the adsorption of 1-121'O~ in the presence of Zn 2+ ions Oil Po- = 5X I0 -'~ tool din-3; Oz,..+ = 2 5 × 10 -3 tool d i n - 3 in I X l 0 tool d m • HCIO 4. C I - ions were added to the solution phase at the moment indicated by the arrow (c = 1 X 10-4 tool din-3),
Fig. 3 clearly shows that the addition of CI- ions does not result in significant change in the extent of the adsorbed labelled H2PO~- at 0 mV (formation ofZn adatoms) as only a slight decrease in the count-rate can be observed. (Compare with the phenomenon occurring after the addition of non-labelled H2PO4- species to the solution phase as shown in Fig. 2.) A switch to 700 mV leads to a complete elimination of the labelled species from the surface. The maximum appearing after the potential switch indicates that at the beginning there is an increase in the HzPO4 adsorption on the free Pt sites in accordance with the expectations based on the potential dependence of the adsorption (see Fig. I) and this initial section is followed by the displacement of adsorbed H2PO4 species by CIions. (These phenomena for HSO[ and CI- competition
,L t~
60
15
fJ
3 I~ 40
~10
2
,./ 'T 20
1
i
0
I0
20 Vmin
30
Fig. 2. Exchange of labelled phosphate species adsorbed at 0 mV in the presence of Zn 2 + ions with non-labelled ones added to the solution phase
in a great excess at the moment indicated by the arrow. Supporting electrolyte: 0.I mol dm -3 HCIO4. Initial H3PO 4 concentration: 5X 10 -5 tool dn1-3. Final: 1 × 10 -2 mol dm-3.Czn..,- = 1,5X 10 -z tool dm -'~
0
200
400
600
E/mY Fig. 4. The eftk:ct of HSO4 ions on the potential dependence of ~he adsorption of labelled H2PO4- species in the presence of Zn 2+ ions Cl.I po- = 5 X 1 0 -5 mol din-3; C z n 2 + = 5 X l 0 -3 tool dm -3 in 0.l tool dm:-34HCIO4, cri,po~-:(I)0, (2) 5"x 10-4; (3) 5 X 10-3 mol dm -3.
G, Horfmyi,A, Aramata/ Journal of Electroana/ytlcalChemistry437 (1997)259-262
261
1,2
q~ 0.4
5 o.s
0,4
0.2 ......
~, 300
0 0
200 E/mY
100
were discussed in the previous paper; for the details, see Ref. [!]) The study of the competitive adsorption of HSO 4 and H2PO 4 species using labelled H2PO 4- and non-labelled HSO 4 confirms the results presented in the previous communication [1] (where the opposite labelling was used), i.e., the adsorption strength of H2PO ~- species is highe r than that of HSO4- ions. This is demonstrated by Fig. 4. It may be seen from this figure that a significant displacement of H2PO 4 species can be attained only by the presence of a great excess o f HSO 4 ions in the solution phase in comparison to the concentration of phosphate species. The result presented in Fig. 3. allows us to determine the potential dependence of the adsorption of H2PO 4 ions induced by Zn adatoms without the interference o f the
0.2
:
"" ..i,:.. ,. q~ : 200 300 E/mV Fig. 7. Potential dependence of the induced adsorption of H2PO~" species in the presence of CI- ions at two different H+ concentrations cH21'O;"= I X 10-4 tool dm-'~; Czr,2÷= 2 × If,'- I mol dm -'a; Coo-= 1× 10-4 tool dm -3. In tide presence of: 0.1 mol dm-3 (dotted line); and 5× I0 -:* mol dm -3 HCIO4 (full line). 0
JJ
0.1
,
,
",
adsorption phenomena occurring on free Pt sites (see Ref. [1]), Fig. 5 shows the F vs. E and A F / A E vs. E curves obtained by potentiostatic point by ~oint method (in 25 mV steps) in the simultaneou);'pr'esence of C I - , Zn 2÷ and labelled H2PO 4 ions in 0.I tool dm -3 HCIO4 supporting electrolyte. The shape of both curves practically coincides with those obtained for H S O 4 ions reported in Ref. [1], i.e., the potential dependence of the anion adsorption is determined by that of the formation of the upd layer. The extent of induced adsorption at a fixed potential depends on the concentration o f Zn 2+ ions. This is shown ill Fig. 6. On the other hand, a decrease in the hydrogen ion concentration also leads to a significant increase in the extent of the anion adsorption as shown in Fig. 7.
30]
o
i 0
-'1
0.1
.
100
Fig, 5, Potential dependence of the adsorption (F, full line) H2PO~" of ions (induced by Zn adatoms) in the presence of CI- ions• c~l po- "= 1 X " ~4 10-'1 tool dm-'~; CZnv.=l.5Xl0-2 mol din-'; ccl-=lX10 tool dm -3 in 0.1 tool dm -3 HCIO4. TIDecorresponding AF/AE vs. E curve (obtained in 25 mV steps) is given by the dotted line.
o,
13•. o ~ . °.l-
,
0.2 c, zn(clq) 2
Fig. 6. The influence of the concentrationof Zn"+ ions on the extent of induced adsorption at 50 mV in the presence of IX10 -4 mol dm-3 H3PO4 in 0.1 tool dm-3 HCIO4 supporting electrolyte.
10
20
30
t/min Fig. 8. The count-rate vs. time curve in the pr.sence of labelled HaPO4 (c = 1X 10-4 tool dm -3) in 5:< 10-2 tool dm-3 Zn(CIO4)2 at pH = 3.5 following sprinkling ZnO (full line) and formationofa Zn(OH)z layer on the surface of the electrode. CDIvo- = I × 10-4 tool dm-3; Czn2*= 1.5 --3 2 4_4 ×10 -2 tool dm "; Cct-=l×10 mol dm -3 in 0.1 tool dm -3 HCI04).
G, tlor(myi, A, Ammam ~Journal tf Electmtmalytlcal Chemisto' 437 (1997) 259-262
262
I
2
400
200
0 0
100
200
A similar increase ot' the count-rate can be observed in the course of electrodeposition of Zn at pH = 5 from a 2 × 10-j tool dm -3 Zn(CIO~)2 solutioll at very low potentials ( - 7 0 0 mV). This is shown in Fig. 9. Interrupting the electrodeposition process, the sorption can be studied under open circuit condition. The results presemed in Fig. 9 demonstrate that phosphate species adsorb on the surface of electrodeposited Zn. The elimination of tile adsorbed labelled molecules can be achieved only by the dissolution of the Zn layer.
t/rain Fig. 9. Count-rate vs. time curve in the course of the electrodeposltion of Zn at -720 mV (I); at open-circult conditions ,'tt - 6 8 0 mV (2) in 0.2 tool dm -3 Zn(CIO4)2 solution at pH = 5; and following the addition of 3X 10-" tool dm -3 HCIO4 (3).
As the pH increases the formation of Zn... OH surface species seems to be a probable process and in the first approximation a decrease in the adsorption of other anions would be expected. It can be shown, however, that on ZnO or Zn(OH)2 surfaces, an accumulation of phosphate species OCCURS.
Using a cell with a gold-plated bottom (without platinization) and sprinkling on the bottom, a small amount of ZnO powder or depositing a thin Zn(OH)2 layer by addition of small droplets of diluted NaOH solution to the solution phase, the sorption of H2PO4 species can be observed. This is shown in Fig. 8 where the transient increase in the apparent phosphate sorption is demonstrated, following tile sprinkling of a small amount of ZnO on the bottom of the cell (full line) or formation of Z n ( O H 2) at the surface in the presence of a slightly acidic medium. Since the medium is slightly acidic, following the formation of the ZnO or Zn(OH)2 layer, the sorption of the labelled species and the slow dissolution of the newly formed layer proceed simultaneously. Thus, the count rate vs. time curve goes through a maximum. It follows from these observations that at high pH values the formation of surface oxide, hydroxide species could play an important role.
4. Conclusions The experimental results presented above confirm the assumptions made previously that the specific adsorption of phosphate species cannot be neglected in the evaluation of the data obtained from the study of the formation of Zn adlayers. Thus data obtained for the upd of Zn in systems containing phosphate species should be reconsidered in the light of these results.
Acknowledgements Financial support from Hungarian Science Fund (OTKA T 023056 and T 014446) is acknowledged.
References [1] [2] [3] [4]
G. Horfinyi, A. Aramata, J. Electroanal. Chem. 434 (1997) 205. G. Hor,Snyi. G. Vrrtes, J. Electroanal. Chem. 45 (1973) 295. G. Horfinyi, J. Electroanal. Chem. 55 (1974) 287. G. Horfinyi, E.M. Rizmayer, P. Jo6, J. Electroanal. Chem. 152 (1983) 211. [5] G. Horfinyi, M. Wasberg, J. Electroanal. Chem. 431 (1996) 161. [6] G. Horfi~lyi,G. Inzelt, J. Electroanal. Chem. 86 (1978) 215. [7] G. Horfinyi, E.M. Rizmayer, G. lnzelt, J. Electroanal. Chem. 93 (1978) 183.