A C T A U N I V E R S I T A T I S L O D Z I E N S I S FOLIA CHIMICA 9, 1991
Maria Turowska*, Je rz y Sokołowski* THE pH EFFECT ON THE POTENTIAL OF ZERO CHARGE
OF GOLO ELECTROOES**
P o te n t ia l of zero charge (p z c) of gold in so lu tio n s w ith constant mole co n cen tratio n of ions in the range pH 1,5-11 was measured. Measurements were c a rr ie d out by the in v e rs io n immersion method with a p p lic a tio n of an automatic measuring equipment. E m p irica l formula of lin e a r reciproca- l i t y o£ pzc of gold in the pH fu n ctio n was given.
Values of the zero charge p o te n tia l of p o ly c r y s t a llin e gold e le ctro d e s in non-adsorbing s a lt so lu tio n s found ex p erim en tally by vario u s authors d if f e r co nsid erab ly [l- 1 2 , 23, 24], The d iffe re n c e s r e s u lt from applied measuring methods, of e le c tro d e surface prepa r a tio n as w e ll as e le ctro d e m a te ria l. Examinations of the ele ctro d es made of m onocrystals of gold dependance of pzc on c ry s ta lo g ra p h ic o rie n ta tio n of gold [13-17, 22], The most p o s itiv e pzc values d is p la y c r y s t a ls with o rie n ta tio n (11 1 ), next with (100) and the most n egative with the o rie n ta tio n (11 0 ). In accordance w ith the paper [22, 25] pzc values of p o ly c r y s t a llin e gold e le ctro d e s are clo se to pzc values for m ono -crystalline gold e le ctro d e s of o rie n ta tio n (110).
I t appears purposeful to study the pH e f f e c t on pzc of a gold e le c tro d e by means of measuring that value using id e n t ic a l methods of preparing e le ctro d e s and the a p p lic a tio n of the same measuring method. In the present paper, in flu e n ce of hydrogen ion co ncen tra tio n upon pzc value of p o ly c r y s t a llin e gold in 0.01 mole/dm3 sodium sulphate was measured by means of the in v e rs io n immersion method.
In s t it u t e of Chem istry, U n iv e r s ity of t(5df, Poland. * *
This work was f in a n c ia lly supported by CPBP 01.15 research programm.
S im ila r experiments were c a rrie d out by the scrape method of pzc measurement of gold ele ctro d e [7, 18, 20, 25],
EXPERIMENTAL
Measurements of the p o te n tia l of zero charge were taken by an in v e rs io n immersion method by means of an apparatus constructed in the Department of General and In org an ic Chemistry of the Lodziensis U n iv e r s ity allo w ing an automatic measurements and recording of data by th is method [ l ] .
In th is method surface of the metal is prepared by an induction heating of the ele ctro d e s for s e v e ra l hours in the atmosphere of e l e c t r o l i t i c a l l y obtained hydrogen.
Examined ele ctro d es of a c y lin d r ic a l shape were produced from a gold f o i l of 99,9% p u r ity . As referen ce a s i l v e r e le ctro d e in s o lu tio n 0.01 mole/dm3 was used.
Measurement were c a rrie d out in Na2S04 s o lu tio n as the weak- -adsorbing e le c t r o ly t e . To prepare so lu tio n s of a required pH the so lu tio n s of Na2S0^, HjSO^ and NaOH were used with a p p lic a tio n of a constant ion concen tratio n s (S0^~) e q u a llin g 0,01 mole/dm3.
pH of the so lu tio n s was checked with a Mera-Elwro pH-meter of N-512 type c a lib r a te d fo r seven b u ffe r so lu tio n s with known pH va lu es.
S o lu tio n s of used e le c t r o ly t e s were prepared of weigh po rtion s of high p u rity s a lt s d isso lved in tr ip p le d i s t i l l e d w ater. For de greasing of the ele ctro d e s methanol was used.
Each measuring c y c le of pzc of an e le ctro d e co nsisted o f:
a) flu sh in g of an e le ctro d e with t r ip p le d i s t i l l e d water -5 min,
b) in duction heating of an e le ctro d e at a temp. 1073-1093 K fo r 10 min,
c ) co olin g down of an e le ctro d e - 5 min, d) measurement.
AIX operations were c a rrie d out in the hydrogen stream at the ra te of flow 2,5-3,5 cm3/sec.
apparatus were e sta b lish e d ex perim en tally and allowed o b tainin g the most rep eatable r e s u lts in sh o rte st p o ssib le time of c y c le du ration to prepare the e le c tro d e fo r measurements.
RESULTS AND DISCUSSION
Obtained r e s u lt s c o n s is t of B measuring s e r ie s . One s e r ie s covers 10 measurements of pzc values of the gold e le c tro d e in s o lu tio n of NajSO^ of an e s ta b lis h e d pH valu e.
Obtained values of immersion impulse were scale d on the values re le te d to a normal hydrogen e le c tro d e . The e rro r was determined s t a t i s t i c a l l y , making use of Stu d e n t’ s d is t r ib u t io n of 9 degrees of freedom and of 0,95 confidence le v e l. In c a lc u la tio n s no mistakes connected w ith d e fin in g of the s i l v e r - s i l v e r c h lo rid e e le c tro d e p o te n tia l and liq u id p o te n tia l were taken in to account, but as i t was ex perim en tally proven they were much sm alle r than a s t a t i s t i c a l measurement e rr o r .
Obtained re s u lts are shown in F ig . 1. On the diagram ( F ig . 1) c a lc u la te d s t a t i s t i c a l e rro rs of pzc were p lo tte d and e rro rs of pH measurement values estim ated. Obtained pzc values of the gold e le c tro d e are p o s itiv e and increase accord in g ly with the growth of pH of s o lu tio n . At pH < 7 changes reaching se ve ra l times m iliv o lt s are observed, above th at values of pH are more d is t in c t .
i
F ig . 2. L in e a r iz a tio n of pH-dependence of the in verse p o te n tia l of zero charge
In F ig . 2 r e p r ic o c it ie s of pzc in pH fu n ctio n were presented. The lin e a r dépendance of the type
Eqio (pH) = a PH * b (1 )
was obtained. Constants a and b of the equation (1 ) were designated by le a s t square method and
are.-a = (-0,77 - 0,10) V "1 b x ( i l , 5 - 0 ,7 ) V '1
The e rro rs a and b were determ inated fo r confidence le v e l c* = 0 ,9 5 .
In F ig . 2 confidence le v e l curve d esignating confidence region were marked, in which the p r o b a b ility of fin d in g measurement points P i (pH^; Eq=q ) amounts 0,95.
Basing on c a rrie d out c a lc u la tio n s the dependence of p o te n tia l of zero charge of p o ly c r y s t a llin e gold e le c tro d e in described con d itio n s may be presented by the fo llo w in g e m p iric a l eqution:
AuEQ^-0 (pH) = ( - ° ' 77 * 11»5>V~1
Obtained r e s u lts c o rr e la te rath e r w e ll with lit e r a t u r e data. Obtained in th is work pzc values of gold fo r values of pH = 7 amount AuEQ=0 1 0,17 V whereas recommended by T r a s a t t i [ 11 ] , a f te r a n a ly s is of experim ental data obtained by a number of authors amounts AuCQ=0 = 0,18 V* and b y K h r u s z c z e v and K a z a- r i n o v [12] auEq-q = 0,19 V. P o te n t ia l of zero charge of gold obtained from c a p a city minimum in so lu tio n s of sulphates amount: in work [5] AuEQ=0 * 0,23 V ( c N SQ = 0,01 mole/dm3) , in work [7] AuEQ=0 = 0,13 V ( c Na2S04 = 0,05 "ole/dm3), in work [8] AuEq=0 = 0,1 V ( CK^S0^ = ° . ° 167 mole/dm3) and in work [ IB ] auEq.q * 0,13 V (cK SQ = - 0,02 mole/dm3, pH = 6 ,4 ).
pH in flu e n ce upon value of p o te n tia l of zero charge of poly- c r y s t a lin e gold were examined in the works [ l 8, 20, 25, 27]. In papers [18] and [25] the b a sis of data were the curves of d if f e r e n
t i a l c a p a c ity of examined e le ctro d e s in aquous s o lu tio n of NaF [18] and NaUH [25 ]. I t appears d isp u tab le whether the minima observed on the curves can be in te rp re te d as pzc values of these e le c tro d e s . The course of curves is being deformed by the appearance of pseudo ca p a c ity brought about by adsorption processes occuring on the e le ctro d e w ith in th is range of p o te n tia l at which minimum. of
d iffu s e la y e r cuuld be expected. Thus s h if t in g of the observed minima can be fo rm ally in te rp re te d as superim position of two s im u l
taneous e f f e c ts in the compact (a d s o rp tio n ) and d iffu s e (minimum)
la y e rs . Form ally s im ila r proces was analyzed in the paper [2 6 ]. View of the above i t is d i f f i c u l t to compare ob ta in ed by us r e s u lt s
with the data of above authors. The values of pzc of gold measured by the scrape method are more n egative than obtained by another methods.
As can be seen from c a rrie d stu d ie s gold d is p la y s increased s e n s i t i v i t y to changes of the s o lu tio n a c id it y compared to copper
[19] and s i l v e r [ 2 l ] . I t is comendable, th e re fo re , th at measurement of the p o te n tia l of zero charge of gold ele ctro d e be c a rrie d out at a c o n tro lle d pH of the s o lu tio n . For measurement in sulphate s o lu tio n s i t Is purposeful to use a b u ffe r c o n s is tin g of sodium sulpha te and sodium hydrosulphate [27 ].
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Maria Turowska, Je rz y Sokołowski
WPŁYW pH NA WARTOŚĆ POTENCJAŁU ŁAOUNKU ZEROWEGO ELEKTROD ZŁOTYCH
Zmierzono p o ten cja ł ładunku zerowego ( p łz ) z ło ta w roztworach o stałym stężeniu molowym jonów siarczanowych ( c Qn2-= 0,01 mol/dm’ ) wwUl granicach pH 1,5-11. Pomiary prowadzono inwersyjną metodą zanurze niową sto su jąc automatyczną aparaturę pomiarową. Stwierdzono, te wartość płz elektrod y z ło te j wzrasta wraz ze wzrostem pH roztworu. Podano empiryczny wzór lin io w e j odwrotności w arto ści p o ten cjału płz z ło ta w fu n k c ji pH roztworu.
