Voltammetry with microelectrodes in wine: determination of the total acidity

Ana&ca Chmuca Acta, 272 (1993) 151-159 Elsevler Science Pubhshers B V , Amsterdam

151

Voltammetry with microelectrodes in wine: determination of the total acidity M Antometta Baldo, Salvatore Damele and Glan A Mazzocchm Department of Physical Chemrrtry, Unrversrtyof Vemce, Calle Lurga S Marta 2137,30123 Vemce (Italy) (Received 20th May 1992, revxsed manuscnpt recerved 21st September 1992)

Abstract A voltammetnc study of wme was performed m order to ascertain Its sultabdlty as a medmm for analysts The electrochenueal processes of the probe molecule Ru(NH,)&l, and of some electroactwe species present m the medmm were mvestlgated unth a platinum nucroelectrode and the relevant responses were compared with those obtamed urlth an electrode of the same matenal of conventIona size The results mdaxted that urlth the latter electrode ohnuc losses affect the voltammetnc processes, whereas no problem anses wth the nucroelectrode It was also found that the acids present m wme are responsible for a large cathodic wave and its measurement, by voltammemc and chronoamerometnc tltratlons, penmtted the determmahon of the total acldlty of wme \Irlth a relatrve standard devlatlon wrthm 2 5% The results obtamed \mth this approach were compared with those obtamed by the classIcal method commonly employed for the determmatlon of the total tltratable acldlty of wme &ywomk Amperometry, Voltammetly, Acidity, Chronoamperometry,

Small electrodes have received mcreasmg attention for both kmetlc studies of the electrode processes and quantitative analyses [l] In geqeral, their use provides a means of slmphfymg the procedure of electroanalysq makmg it more accessible 111more demanding situations [2-51 Some of the advantageous characterlstlcs are the low current mvolved with a consequent neghglble ohrmc drop, the fast response due to a small electrode capacitance and steady-state condltlons achievable m a short tune These properties allow mlcroelectrodes to be employed, for mstance, dlrectly m reslstlve media and without the dehberate addltlon of supportmg electrolytes [6-91 Microelectrodes have been used previously for determmatlons m real samples directly and without pretreatment [3-51 This approach 1s advantaCorrespondence to S Damele, Department of Physlcal Chemstry, University of Vemce, Calle Larga S Marta 2137, 30123 Vemce (Italy)

Mlcroelectrodes,

Wmes

geous because the species naturally present, addltlves and contammants can be revealed and determmed wlthout changing then nature and wthout modlflcatlon of the exlstmg chenucal eqmhbna m the sample lks paper reports an investigation on wme, which 1s a water-ethanol medmm m which the ethanol content ranges from about 7 to 18% (v/v> at 20°C [lO,ll] It also contams acids and salts but It 1s less conductwe and more MSCOUS than an aqueous solution havmg the same lomc strength 1111 Therefore, the use of mlcroelectrodes may be advantageous compared with conventional electrodes, and this aspect was explored EXPERIMENTAL

Reagents Hexamnuneruthenmm(II1) tnchlonde, Ru(NH,),Cl,, was provided by Matthey Brshop and

0003-2670/93/$06 00 0 1993 - Elsewer Science Publishers B V All nghts reserved

152

purified as reported [121 All morgamc salts, actds and orgamc compounds were of analytlcal-reagent grade The wme samples analysed were Italian red wines (Chianti, Cabernet and Merlot) and white wines (Tocal and Verduzzo from Frmh and Veneto) Doubly distilled water was used throughout to prepare standard solutions of the reagents When necessary, the samples were deareated with mtrogen (99 99%) from SIAD Electrodes and lnstrumentatwn To prepare a platinum disc mlcroelectrode, a wire of diameter 25 pm was sealed directly m glass as reported previously [13] For a carbon rhar mtmn~l,wtmrl~ 9 unncI’v cmnle 1ca1”“ll rwhnn fihre ”Q p”’ rrm 111 tn UloI ,ILI~I”“I”I~I”““) u ll”l” diameter was connected with a thm copper wire with salver epoxy and sealed with pure resin m a glass capillary This was cut perpendicularly to its length to expose the carbon disc Pnor to each measurement, the electrodes were polished with graded alumma powder (down to 0 05 pm) on a pohshmg mlcrocloth For comparison, a conventional platinum disc electrode of diameter 3 mm was used A saturated calomel electrode WE) was employed as a reference electrode The expenments with mlcroelectrodes were done m a two-electrode cell configuration mamtamed m a Faraday cage made of sheets of alummmm to reduce external noise Chronoamperometnc (Chr), linear-sweep (LSV) and cyclic voltammetrlc (CV) waveforms were generated by a PAR 175 function generator, a Kelthley 485 plcoammeter served as a current-measuring device and data were plotted with Hewlett-Packard 7045 B X-Y recorder For cychc voltammetry at conventional electrodes an Amel 472 multlpolarograph, equipped vvlth a three-electrode assembly, was employed The bulk vlscoslty of the samples was measured wth a Ostwald vlscoslmeter calibrated with pure water pH measurements were made by usmg a Methrohm 605 pH meter Procedure A 20-ml volume of wme, eqmhbrated at room temperature, was withdrawn wrth a syringe from

M.A Bald0 et aL /Anal Chm Acta 272 (1993) 151-159

a lust uncorked bottle, transferred mto the cell previously pre-evacuated and eqmhbrated with nitrogen and kept under a nitrogen atmosphere As the levels of oxygen m bottled wme are very low [10,14,15], measurements were performed without bubblmg nitrogen before the scan This procedure avolds losses from the solution of volatile components of wme, such as acetlc acid and molecular SO,, which would accompany the normal deoxygenatlon step

RESULTS AND DISCUSSION

Probe of the wine matnx with platmum electm#Lx .r “UC” Figure la shows a typical voltammogram recorded at a platinum disc nucroelectrode at a low scan rate on a red wme sample In the cathodic region a well defined reduction wave L characterized by a half-wave potential (E,,,) at -0 710 V 1s observed, whereas the anodlc scan mostly shows emdence of two smaller processes The cathodic and anodlc background current dlscharges, probably due to the reduction and oxldatlon of the water m the wme, occur behyond - 1 2 and + 135 V respectively This behavrour 1s typical also of the white wme samples, partlcularly m the cathodic region For companson, Fig lb shows the voltarnmogram obtamed at a conventional platinum electrode for the same red wme sample as m Fig la The voltammogram shows distorted and badly defined processes before the solvent discharge Here the response 1s probably conditioned by the presence of a considerable ohmic drop In order to obtain an insight mto this aspect, a series of measurements was made Hrlth the two different types of electrodes on a wme sample containing deliberately added hexammmeruthemum(II1) tnchlonde as probe molecule Thus compound 1s known to undergo a reversible oneelectrode process even m media contammg organic substances which can partially cover the electrode surface 116,171 Figure 2 shows cyclic voltammograms recorded vvlth the rmcro- and macro-electrodes on this type of solution The loganthmlc analysis, I e , the slope of the

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Fig 2 Cychc voltammetnc curves obtained for a red wme sample fortified with 1 mM RuNI,),@, at (a) a 125ym radius platinum disc at a scan rate of 10 mV s-* and (b) a 1 5-mm radrus platinum disc at a scan rate of 50 mV s- ’

(b)

I

‘rod Fig 1 Cychc voltammetnc curves obtained for a red wme sample (Chlantl) at (a) a 12 5-km radius platinum disc at a scan rate of 10 mV s-l and (b) a 15-mm radius disc at a scan rate of 50 mV s-*

plot of log[(r, - 11/i] versus potential (I, = dlffuslon hmltmg current), along wth the E,,, E 3,4 values for the wave obtamed at the rmcro-

electrode [18], and the anodlc to cathodic peakpotential separations (Ep, - Epc) obtamed at the conventional electrode at different scan rates, are reported m Table 1 The values obtained are compared mth those found after add&on of KC1 as supportmg electrolyte From these data It can be mferred that at the mlcroelectrode the responses are almost free from ohmic dlstortlon, whereas at the macroelectrode, 111the absence of the supporting electrolyte, considerable dlstortlon arises, smnlarly to the behavlour observed for wave L m Fig lb The anodlc to cathodic peak separation observed m the reduction process of [Ru(NH&J~+ at the conventional electrode could also be attrlbuted to a slower heterogeneous electron transfer rate, which takes places m wme as a consequence of an mhlbltlon effect caused by

TABLE 1 Effect of solution resistance on voltammetry at micro and conventional electrodes

(mV)

Electrode radius km)

Scan rate (mV s-l)

Slope ’ (mV)

J&/4

Wine

Wme + KCl

Wine

Wme + KC1

12 5 x 10-4

2 5 10 20 50 100

58 59 59

58 59 58

57 56 58

56 56 57

0 15

-

E3,4

b

-

a Slope of the plot of logk, -II/II vs E, expected theoretxal value at 20°C = 58 l/n 20°C = 55 4/n mV ’ Expected theoretical value at 20°C 58 l/n mV

EpI - Epc ’ (mV) Wine

Wme + KCl -

110 140 170

60 70 90

mV b Expected theoretical value at

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Acta 272 (1993) 151-159

slon coefficients obtained for [Ru(NH,),13’ m the three media are also reported The diffusion coefficients were calculated from the expenmental steady-state hmltmg current by employing the relatlonshlp I, = 4nFDcr (where n = number of electrons, D = dlffuslon coefficient, c = bulk concentration, and r = electrode radius) [18] The product 07 (where 17= vlscoslty of the medium) for the different media 1s almost constant, as expected on the basis of the Stock-Emstem equation D = kZ’/67r?7r,, (k = Boltzmann constant, r,, = hydrodynamic radius of the probe molecule) [20] Therefore, the decrease m the hmitmg current values on passing from water to the ethanohc matrices is fully Justified on the basis of an increase m the vlscoslty of the medium The overall results obtained indicate that ethanol does not adversely affect the voltammetrlc behavlour of the probe molecule

adsorption of organic substances at the electrode surface, m particular of ethanol Actually, If this were the case a slmrlar phenomenon should also be observed at the mlcroelectrode However, both the logarlthmlc analysis and the El,4 - E3,4 values reported m Table 1 m the latter instance indicate no departure from the reversible behavlour As at the mlcroelectrode slow rates of the heterogeneous electron transfer would lead to larger devlatlons from the reversible behavlour than that observable at the conventional electrode [18], it can be concluded that the high (Ep, - Ep,) values obtained m the latter instance can conceivably be attributed to ohmic drop effects Because ethanol 1s the mam organic component of the wme matrur, its effects on the electrochemical reduction of [Ru(NH&,13+ were exammed For this purpose measurements were made on a 12% (v/v) aqueous ethanol solution contammg 1 g 1-l KC1 The half-wave potentials and the hrmtmg current values recorded for [Ru(NH3)J3+ m water-ethanol (W/E), water (W) and wme are compared m Table 2 The E1,2 values differ from one another if taken against SCE, whereas they fall wlthm the expenmental error if measured against the Z& of the femcmmm-ferrocene (Fc+/Fc) system, as an mtemal reference, independent on the nature of the solvent [19] This result means that the above differences m El,* are due to liquid Junction potentials The differences observed m the lmutmg current values can be explained by assuming a change m the diffusion coefficient of the probe molecule due to the change m the vlscosltles of the media In Table 2 the measured vlscosltles and the dlffu-

Study of the cathodtc regwn of wane

A series of measurements performed on Italian red and white wines showed that the cathodic wave L m Fig la was present m all the samples analysed, this wave was characterized by a halfwave potential of -0 705 V (50 010 V>, and its height was correlated with the pH of the samples, that is, the higher the wave height, the lower was the pH (see Table 3) Moreover, the voltammograms recorded with a carbon disc electrode on wme samples showed a negative shift of the posltlon of the wave L, and also of the cathodic limit, typical of the overpotentlal effect for hydrogen evolution on this electrode material with respect to platinum [21] These results suggested that the cathodic process L could be ascribed to the reduction of

TABLE 2 Half-wave potentials (* 2 mV), dlffuslon hmltmg currents and dlffuslon coefficients for [Ru(NHJ613+ and vlscositles of different media Medmm

W+KCI W/E (12%) + KCI Wine (115%)

4,~

E,,,

6’)

vs Fc+/Fc

Ru3+/Ru2+

Fc +/Fc

Ru3+/Ru2+

-0 183 -0 170 -0 176

0 150 0164 0 156

-0333 -0334 -0332

I, (nA)

106D km2 s-l)

10-2 7 -1 (km

176 131 122

7 21 5 41 5 04

098 142 153

s-1)

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Chm Acta 272 (1993) 151-159

TABLE 3 Comparison between he&t of wave L and pH for different wme samples Sample a Tocal Frmlano 1 2 Tocar Veneto 1 2 3 4 Pmot Gnpo 1 2 Verduzzo Chianti 1 2 3 Cabernet Merlot

rfl(nA)

pHfOO1

134 130

3 35 3 37

147 142 145 164

3 30 3 32 3 31 3 16

138 150 176

3 34 3 27 3 08

14.5 156 1.50 144 125

331 3 21 328 331 3 45

a Numbers represent samples from different Italian companies

protons released by acidic species present m the samples In order to ver& this hypothesis, some typlcal organic acids of different strength and morgamc bases were added to wme The results obtamed indicated that the wave L increased owmg to addition of acrds characterized by dlssoclatlon constants higher than about 10m5 M, e g , tartanc, acetlc, c&x and lactic aads, whereas it decreased after addltlon of bases, e g , sodium hydroxide A parallel pH check of the same sample showed that a decrease m the pH corresponded to an mcrease m the wave height and vice versa Hence the process under mvestlgatlon should include the reduction of the organic acids charactenstlc of the matrq particularly tartaxx acid and hydrogentartrate Ions present m relatively high concentrations [lo] The contnbutlon of each acid should m prmclple be dlscnmmated, as the half-wave potential for the reduction of a Bransted acid to hydrogen becomes mcreasmgly negative as the acldlty of the Brgnsted acid decreases [22,231 In contrast, the shape of the cathodlc wave L mdxates the presence of only one wave, probably due to overlappmg reduction pro-

Rg 3 Cychc voltammetnc curves obtained for solutions m water-ethanol containing 1 g 1-l KC1 of (a) (full hne) 1 mM lactic acid and (dashed lme) 1 mM acetic acid and (b) (full line) 1 mM tartanc acid and (dashed line) 1 mM atnc acid 12 5-pm radius platinum disc at a scan rate of 10 mV s-l

cesses of the acids involved These m fact are characterued by drssoclatlon constants that are too close and they probably cannot be dlfferentlated by linear-sweep voltammetry To demonstrate this hypothesis, measurements were made TABLE 4 Half-wave potentials obtained from organic acids at a platmum disc mlcroelectrode (12 5 pm radms) m both water (W) and water-12% (v/v) ethanol (W/E), contammg 1 g I-’ of KC1 Acid

Tartanc c1tnc Lactic

Acetlc

Eqmhbnum constants a ph

PKa,

PKa,

3 04 3 13 386 476

437 476 -

640 -

a From [24]

EI,m f0002 (VI

ID&,~,~) f0002 (VI

-0450 -0455 -0460 -0495

-0445 -0450 -0450 -0490

156

MA Bald0 et al /Anal Chzm Acta 272 (1993) 151-159

on solutions of tartanc, cltnc, lactic and acetic aads m water-ethanol rmxtures contammg KC1 to give the typical lomc strength of wme Fig 3 shows the voltammograms obtained m these solutions and Table 4 gwes the relevant parameter values compared with those observed m aqueous solutions containing KC1 as supportmg electrolyte From Fig 3 it 1s evident that the processes are well defined for all the acids mvestlgated, and that they are not inhibited by bubbles of hydrogen formed during the reduction The half-wave potentials obtained m water-ethanol are more posltlve (wlthm 10 mV) than those m water These slight differences can be attributed to the June++r\W. ..,,0..+...1 GILGU, &?&.+ aa n” u~II‘“IIDcIab~u ~,%.n,M.,,S.n+zWl h., s.af,atr.nn ll”‘, y”LGl‘ual “J Icllbs,ll”l5

water-ethanol contammg acids and chlonde ions, other substances could play a role m the performance of the electrode, particularly surface-active compounds other than ethanol which may adsorb and consequently change the structure of the double layer For instance, when to a waterethanol mixture, contammg tartarlc aad and KCl, a solution of two other typical components such as sodium metablsulphlte and acetaldehyde was added, and after adJustlng it to pH 3 3, the cathodic process shifted negatively from - 0 445 to about - 0 700 V, very close to the E1,2 of wave L of wme, whereas the overall hmltmg current did not change The shape of the lmear-sweep voltammogram B.P,.,W.#I&arl n, ,hP m..-.rr\~l‘X,-t*firlrr n+ 0 lfi... ct.em m+Ll lrWlUCl” ac Lilti Ilub~“~‘~~cI”us# 4c 4 ‘“vv rYcIaI1 IQCW

of the acids against that of ferrocene m the El,2 the two media The trend of the half-wave potentials correlates with the relative strength of the different acids employed, the higher the first dlssoclatlon equlllbrmm constant, the less negative IS the El,2 value (see Table 41, however, these values from the highest to the lowest fall m the range of about 50 mV, thus precluding the posslblllty of their separation A mixture of these acids gave m fact only one wave charactemed by an El,2 value that was wlthm the potential range reported m Table 4, depending on the mutual proportions of the different acids The half-wave potentials reported in Table 4, even for the weaker acid, are almost 200 mV less negattve than the EI,z of wave L observed m wme Actually, wme 1s more complex than a

suffrclently slgrnoldal for most of samples examined, as expected under non-planar dlffislon control [18] The reduction process responsible for wave L can conceivably be represented by the known CE mechanism

Fig 4 Cychc voltammetnc curve obtamed for a white wme sample (Verduzzo Veneto) at a 12 S-pm radus platinum disc at a scan rate of 10 mV s -I

IS

H,A”-= H(,_,,A(“+‘)-+

H++ H(,_,)A (m+l)-

(1)

H++ H, ” _2)A@“+2)-

(2)

H++ e - it 1/2H,

(3)

where H,A 1s a generic polyprotlc acid present m wine For some white wmes the wave exhibited an unexpected current drop, as shown m Fig 4, probably due to substantial electrode passlvatlon The most likely explanation may be the formation of a preclpltate on the electrode surface, e g , of the sparingly soluble potassium hydrogen tartrate and calaum tartrate, the preapltatlon of which can be enhanced by the higher concentration of hydrogen tartrate and tartrate ions achievable at the electrode surface than m the bulk solution, due to the electrode process accordmg to the above general mechanism Determmatwn of the total aed@ Voltammetrtc measurements The total amount of the species grvlng rise to the cathodic process L under mvestlgatlon was quantified by voitammetric titrations by using both the strong base NaOH and tartanc acid as tltrants

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Chtm Acta 272 (1993) 151-159

In the tltratlons with NaOH, the decrease m the current height of wave L was recorded as a function of the amount of NaOH added For concentrations of NaOH ranging from 1 to 20 mM, the cahbratlon graphs were linear, with an average correlation coefficient of 0 998 The end-points were then determined, by extrapolation to zero current, from the straight lmes obtamed m the titrations Table 5 gives the acidity values so determmed, expressed m meq per htre of NaOH added, for a red wme sample (Chianti) and a white wme sample (Tocal from Frmh) The relative standard deviation of about 15% obtamed for five determmatlons indicates the good reproduclblhty of the method In order to verify whether the values obtained could reliably express the tltratable acidity of wme, they were compared with those determined m the same samples by usmg the classical and standardized titration methods with bromothymol blue as indicator and with a potentlometrlc pH measurement that adopts as titration end-point a pre-established pH value [25] In this instance the end-point adopted was pH 7 0, as recommended m the standard procedure The agreement between the voltammetrlc and potentlometrlc results 1s fairly good, the drfferences among the acid@ values found bemg wthm 2%, and the pH value at the end-point of the voltammetrlc titration bemg very close to that

TABLE 5 Aclcllty mdex of wme Method

Sample Chianti

ToGU

NaOH pH b consumptlon (meq 1-l) a

NaOH pH b consumptlon (meq I-‘) B

Bromothymol blue 75 (3 8) pH-metnc 69 (2 1) Voltammetnc 70 c (15)

7 1.5 81(35) 703 77(23) 7 07 d 78 ’ (14)

720 701 705d

a Values m parentheses are R S D (o/o) from five rephcate determmatlons b Expenmental pH at the end of the tltratlons ’ Obtamed by extrapolation d Obtamed after the extrapolated amount of NaOH was added to the wme samples

157 TABLE 6 Parameters of calibration graph and titrable acidity values obtamed m voltammetnc htratlons of two wme samples spiked Hrlthtartaruzacid Sample

Slope (nA meq-‘1

Intercept (nAI

ra

Tltrable aclchty (meq 1-l)

Chlantl Tocal

4 650 4 43s

137 151

0998 0996

69 76

’ n=8

pre-establhshed m the potentlometrlc titration (see Table 5) The bromothymol blue method yielded higher acidity values mostly m red wines (+6%), because of the evident dlfflculty of generating a vlslble colour change of the dye m this coloured matrix In the tltratlon with tartarlc acid (which can also be regarded as a standard addition procedure), an increase m the current height of wave L was recorded as a function of the tltrant volume Tartarlc acid was added at concentrations ranging between 4 and 16 meq l- ’ to obtain linear trends Table 6 gives the tltrable acid@ evaluated for the Chlantl and the Tocal samples, expressed m meq 1-l The R S D was 2 1% and the mean value thus obtained, compared with that found by voltammetnc titration with NaOH, gave dlfferences not greater than 3% The influence of oxygen m the determmatlon of the total acidity was also exammed Bottled wine contains very low concentrations of oxygen [10,14,151, m any case, even consldermg a wme sample saturated with oxygen (the solublhty of 0, m wme 1s m the range 5 6-6 ml 1-l at 20°C [14]), the hmltmg current from its reduction, calculated by the equation I,_ = 4nFDcr and by assuming D, = 2 12 X lo-’ cm2 s-l [21], ranges from 4 8 to 5 1 nA, which cause a maximum error of 4% for the lowest current reported m Table 3 Chronoamerometnc measurements To obtain steady-state currents, the measurements can be made by chronamperometry at a time sufficiently long after the unposltlon of a potential step to a value where the reduction is diffusion controlled However, to reach true steady-state condltlons,

M.A Bald0 et al /Anal Chtm Acta 272 (1993) 151-159

158

wtthm l%, the theoretical analysis predicts 202 s [26] for a &SCelectrode of 25 km m diameter and assummg 10m5 cm* s-l as an average d&&on coefficient for the species involved Conversely, d a closeness of about 5% IS taken as a good approxlmatlon to the steady-state current, the time required decreases to 10 s, which 1s a good arrangement for faster measurements Figure 5a shows a typical transient recorded under the latter conditions The comparison between the expenmental currents obtained by chronoamperometry and lmear-sweep voltammetry, also reported m Fig 5a, indicate that the former technique gave current responses 3 4% higher than the latter, hence the contribution of the tnne-dependent term falls mthm the theoretl-

0

10

I-

J --C_

-.

‘\

r

t Z5nA

I

I I f

(=I

I

I

‘\I ‘I

I

-.-0.05

J-l(b)

I

: 0

::” 01

t/r lb a!b Fig 5 (a) Comparisonbetween(full hne) LSV curve at a scan rate of 10 mV s-l and (dashed lme) Chr response to a potential step from 0 00 to -0 95 V vs SCE obtamed for a wme sample at a 12 5qrn radms platinumdisc (b) Waveform apphed m the chronoamperometncexpenments

TABLE 7 Chronoampexometrlccurrents recorded for two wme samples Sam- 1 b ple a A

B

No of cycles’ 123

4

56

7

8

9

10

lChr 181 176 174 173 172 171 171 171 171 170 h-5 175 2 (2 04) ~1-10 173 (193) lc,, 162 156 154 153 153 153 153 153 152 152 h-5 155 6 (2 43) 11-10 154 1 (195)

a A= Verduzzo, B = t&anti b l1_5 and zl_lo refer to the mean values for 5 and 10 cycles, respectively ’ Valves m parenthesesare R S D (%I

cal approxlmatlon estnnated above It was also necessary to select the base potential, to avoid mterferences, and a delay between steps to allow the electrode actlvatlon The waveform applied that gave the best performances IS reported m Fig 5b To obtain a datum pomt, the cycle was generally repeated five times without pohshmg the electrode, lower current values were observed for the cycles after the first, caused by a slight fouling of the electrode, and the R S D found was within 2 5% Ten replicates, however, gave a better R S D ,I e , less than 2%, the current values after 4-5 cycles being almost constant (R S D < 0 5%) This result suggests the choice of a larger number of cycles than five, however, the time for the analysis would become too long Hence five cycles can represent a good compromise between reproduclbdlty and time consumption for the analysis Table 7 gives typical results for a datum pomt The chronoamperometrlc determmatlon of the total acidity was made by titration vvlth NaOH, using the same concentration range as reported for hnear-sweep voltammetric measurements Each titration point was obtained by running, after the NaOH addition, a series of five chronoamperometrlc measurements with the waveform shown m Fig 5b The mean value after each addition plotted agamst the volume of tltrant gave straight lines with an average correlation coefficient of 0 997 Figure 6 shows typical tran-

MA Bald0 et al /Anal

I

0

Chrm Acta 272 (1993) 151-159

I

1'0

ih

Rg 6 Chronoamperometrlc responses to a potenhal step as m Fig Sb recorded at a 12 5 Frn-radius platmum chsc m the tltratlon of a wme sample with NaOH (0) blank and (I), (2) and (3) with 5, 10 and 20 mM NaOH,respectwely

scents for three addltlons of NaOH to a sample of wme The acldlty values found by this method were compared with those determmed on the same samples by voltammetrlc titration with NaOH, and the agreement was satisfactory For instance, for a white wme sample Werduzzo from Veneto) the NaOH consumption for linear-sweep voltammetry was 62 04 meq I-’ and for chronoamperometry 62 56 meq l-‘, the dlfference being < 1%

The authors thank Professor I Moret for helpful dlscusslons and Mr D Rude110 for expenmental assistance Financial aid from the Italian Nazlonal Research Council (CNR) and Ministry of University (MURST), Rome, 1s gratefully acknowledged REFERENCES 1 R M Wghtman and D 0 Wlpf, m A J Bard (Ed ), Electroanalytlcal Chenustry, Vol 15, Dekker, New York, 1989, p 267

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