J. Electroanal. Chem., 110 (1980) 259--275 259 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

V O L T A M M E T R I C S T U D Y OF H U M I C A N D F U L V I C S U B S T A N C E S

P A R T III . C O M P A R I S O N OF T H E C A P A B I L I T I E S O F T H E V A R I O U S P O L A R O G R A P H I C T E C H N I Q U E S F O R T H E A N A L Y S I S O F H U M I C A N D F U L V I C S U B S T A N C E S

A. COMINOLI, J. BUFFLE * a'nd W. HAERDI

Department of lnorganic and Analytical Chemistry, University of Geneva, Sciences II, 30 quai Ernest Ansermet, CH-1211 Geneva 4 (Switzerland)

(Received 2nd October 1979; in revised form 3rd January 1980)

ABSTRACTS

A systematic study of the influence of FA and HA on the current measured by the various polarographic techniques is presented. In most of the techniques (ac polarography, pulse polarography, Kalousek technique) one observes mainly a modification of the capacitive current which may be used for the quantitative analysis of FA. For this purpose the best techniques are ac polarography and the decrease in the polarographic maximum of O2. The sensitivity of these techniques is about 1 mg 1-1 . A comparison of the results of the last two techniques is given for FA of various origin. Comparison is also made with model compounds in the case of ac polarography.

INTRODUCTION

Previous ly we r e p o r t e d [1 ,2] t h a t fulvic (FA) and h u m i c (HA) subs tances m a y be adso rbed on the m e r c u r y e lec t rode and c o n s e q u e n t l y m a y cons ide rab ly m o d i f y t he r e d o x p rope r t i e s o f the t r ace m e t a l ions p resen t in the so lu t ion . The a im o f Parts I I I and IV [3] of th is series is t o s t u d y the po l a rog raph ic behav iou r o f F A and H A in o rder to :

(a) d e t e r m i n e the i r in f luence on the e l ec t rochemica l p rope r t i e s o f a n y d e p o l a r izer t h a t is p resen t in the so lu t ion ;

(b) t es t t he poss ibi l i ty o f using the a d s o r p t i o n p rope r t i e s o f these subs tances for the i r quan t i t a t i ve analysis or qual i ta t ive charac te r i za t ion . Indeed , these subs tances have i l l -defined s t ruc tu res and are m a d e u p o f a m i x t u r e o f com- p o u n d s having similar p roper t i es . An e s t i m a t i o n o f the i r c o n c e n t r a t i o n or the i r cha rac t e r i za t ion w o u l d t h e r e f o r e on ly be possible b y using a m a x i m u m n u m b e r o f ana ly t ica l t e chn iques [4 ,5 ] . In this regard , po la rog raph ic t e chn iques m i g h t o f f e r an add i t iona l m e a n s fo r H A and FA cha rac te r i za t ion .

The ac p o l a r o g r a m o f F A wi th a p e a k in the p o t e n t i a l range - -1 .1 to - -1 .5 , has been a t t r i b u t e d to t he e l ec t rochemica l r e d u c t i o n o f FA b y S i l chenko et al. [6] . Moreover , h y m a t o m e l a n i c acids, o b t a i n e d b y e x t r a c t i o n o f h u m i c sub-

* To whom correspondence should be addressed.

260

stances with 95% ethanol have been reported [7] to be electro-reducible in alkaline medium at --1.4 V vs. NCE. However, there seems to be a general agreement amongst most of the authors [7--11], that HA and FA are not reducible at mercury electrode, at least for E > --1.0 V.

While several authors have determined FA and HA concentrations by treat- ing the samples either with HNO3 [12] or NaNO: [13,14] , only Lucena42onde and Gonzales-Crespo [11 ] to our knowledge, have made use of their absorption properties for their determination. These authors characterized HA and FA by measuring the decrease in the polarographic maximum of oxygen. The last technique, as well as other methods using mercury [15--17] or platinum [18] as electrodes, have been shown to be useful for the concentrat ion measurements of anthropogenic, non-humic, surface-active agents of natural waters.

In the present paper, a comparative s tudy of the behaviour of HA and FA, by using various polarographic techniques (e.g. dc polarography (DCP), ac polarography (ACP), differential pulse polarography (DPP), the Kalousek tech- nique (KP) and the decrease in polarographic maximum of O2 (02 max techni- que)) has been made in order to find the best method which might be used for the qualitative and quantitative analysis of these substances. Incidentally, this work may also help to obtain a qualitative insight into their behaviour at the mercury--water interface, particularly as the various techniques make use of different time scales, so that they are sensitive to different phenomena. However, it must be emphasized that the aim of this work is indeed to help to choose the best electrochemical conditions and methods when working with FA, but only empirically, i.e. on the basis of the reported experimental results. No assumption is made here about the behaviour of FA, and a semi-quantita- tive interpretation of this behaviour can be made only by taking into account the results reported in Part IV. On the other hand, from the results repor ted in this paper (Sections I and IV.2), as well as from results on other properties of FA [4,5,19], it seems very likely that the behaviour of any fresh water FA will be similar to those reported here.

(I) MATERIALS AND METHODS

The apparatus and the characteristics of the capillaries used with the various techniques are summarized in Table 1.

A scan rate of 2 mV s-' was used in DCP, ACP and O2 max. technique. In ACP the amplitude of the alternating potential was 10 mV and, unless

otherwise stated the frequency used was 400 Hz. The alternating componen t of the current was recorded at a phase angle of 90 °. These conditions were used to eliminate, as much as possible, the possible faradaic component of the current [20--22].

The latter was achieved in DPP [21] by sampling the current, at the very beginning of the pulse (between 2 and 6% of the duration). The ampli tude of the impulses used were 50, 100 or 200 mV. Their duration were 40 ms and they were applied 50 ms before the falling of the drop. A scan rate of 0.5 mV s - ' was used.

The so-called K, [23 ] Kalousek technique (i.e. that corresponding to

TABLE 1

Apparatus and characteristics of the capillaries used with the various

261

polarographic techniques

Technique Apparatus Height of the Drop Flow-rate mercury column time/ of mercury/ cm s mg s- '

DCP Electrochemistry system 32 4.7 1.59 PAR Model 170

ACP Electrochemistry system 29 5.2 1.31 PAR Model 170

02 max. Electrochemistry system 69 2.2 3.30 technique PAR Model 170 DPP Tacussel polarograph -- 3.0 a 0.53

(UAP4 + PRT 30 modulus) KP Metrohm E506 27 3.0 a 1.53

a A hammer was used.

circuit II in ref. 26] was used: the current was recorded at the end of the cycle corresponding to the constant potential. A value of --0.5 V, which is close to the pzc potential in the presence of humic substances, was chosen for this potential (see ref. 3). Under these conditions, the double layer was "charged" during the first half-cycle corresponding to the linearly varying potential ("producing" half-cycle) and the current was measured during the "discharge" of the electrode condenser which occurred during the other half-cycle, at --0.5 V. A scan rate of 1.33 mV s-' and frequencies of 75 and 150 Hz were used.

Unless otherwise stated, Ag/AgC1/sat. KC1//0.1 M KNO3/ /was used as the reference electrode and the potentials are given with respect to this electrode. The pH was measured by means of a Metrohm EA-125 combinat ion glass elec- t rode and a Metrohm E-603 pH-meter. Unless otherwise stated all the experi- ments were carried out at 25 ° + 0.5°C, in 0.1 M KNO3.

All the reagents were pro analysis Merck products. Nitrogen of Bertholet SA (Geneva, Switzerland} was always used to deaerate the solutions. The purity of the gas is quaranteed to be bet ter than 99.995% and was not purified further.

The origins and the pretreatment of the water containing FA and HA were the same as described in Part I of the series [1] and in ref. 4. The numbering of the waters used here corresponds to that given in ref. 4. Unless otherwise stated water No. 50a, Without any fractionation of the organic substances bu t filtered through 0.2 pm, was used for all the measurements. Previously we reported [19,24] that ca. 70% of the organic substances in these water samples have a molecular weight (MW) < 10,000, and a mean value of MW probably ranges from several hundreds to a few thousands [1,25]. Thus, most o f the organic matter is present as fulvic substances. It must be pointed out that some of the experi- ments reported here (e.g. see Section IV .2) were done with various waters, as well as with the fraction of water No. 50a corresponding to MW ranging from 250 to 10,000. The behaviour of these various waters was similar.

Hereafter no distinction will be made between FA and HA. Their concen-

262

tration will be expressed in g 1-1 ({FA}). The chemical composi t ion of the waters under s tudy is described in ref. 4. In particular, the analyses of these waters by AAS, for (FA} = 100 mg 1-1 showed that the concentrat ion of Cu(II), Pb(II) and Zn(II) were most ly < 10 -7 M and in any case always < 10 -6 M, and that, except for water No. 50d, their molar ratio of total iron to FA concen- trations is less than 5%.

(II)RESULTS OF THE VARIOUS METHODS

(H. 1)DC polarography

The dc polarograms of solutions containing various concentrations of FA are shown in Fig. 1. In agreement with most of the authors, no well-defined reduction wave is observed. However, the current increases negatively at any potential with an increase in {FA} (Fig. 2). Moreover, it was observed that this increase is more pronounced as the pH decreases, the change in i being fairly small at pH 7.0.

The influence of pH on the dc current is even more apparent from Fig. 3 which shows the difference Aide between the currents in the presence and in the absence of FA, as a function of pH, at various potentials.

(H.2) The Kalousek technique

Figure 4 shows the variation of the current recorded at constant potential (--0.5 V) during the "recording" half-cycle, as a function of the potential of the "producing" half-cycle. It is seen that on the positive side of the pzc (see ref. 3), the positive current increases with (FA}, while on the negative side

-0.8

0.6

-0.4

- 0.2

0

i / ,uA f

o.2 o.4 06 o.B -1.o E / V 1.2

Fig. 1. DC polarograms of FA, pH = 2.85. (FA} = (a) 0; (b) 20 mg 1-1; (c) 40 mg 1-1 ; (d) 60 mg 1-1;(e) 100 mg 1-1 ;(f) 140 mg 1-1 .

I i / n A

-60 I

pH 50

• / 0 C

/ 2 o

2'o 4b 6'o 8b Ido 1~o I~o - { F A } / m g V'

Fig. 2. Change in de current with {FA}, at various potentials. E = (a) --0.4 V; (b) --0.6 V; (c) --0.8 V.

263

of the pzc, the current remains relatively unchanged. Figure 5 shows that the difference A i K in the currents measured in the presence and in the absence of FA increases with {FA}, irrespective of the potential. The results obtained with f = 75 Hz and f = 150 Hz also showed that Aik increases with f, the shape of the curves Aik = f ( ( F A } ) , being similar at both frequencies. A more detailed study in the low concentration domain ( (FA} < 100 mg 1-1) showed that, in this domain the relationship between Ai K and ( F A ) is not fully linear and that the linearity depends on the value of the potential.

200

100

0

- 100

- 200

- 300

-400

& i d c / n A

. /

2,0 3.0 4.0 S.0 6.0 7.0 8.0 9,0 10.0 II.0 12.0 pH

Fig. 3. Inf luence o f pH on the difference, Aidc , be tween the de currents in the presence o f FA ( ( F A } = 55 mg 1-1 ) and in its absence. E = (a) 0 V. (b) - -0 .3 V. (c) - -0 .6 V; (d) - -0 .9 V; (e) --1.0 V.

, i / / J A

1.50

0,75

0

0.75

LS0

- 2,25

Q

264

J J

0 -0,2 -0.4 -0.6 0.8 -I.0 E / V

Fig. 4. Polarograms obtained by the Kalousek technique in the presence of various concentra. tions of FA. Frequency: 150 Hz, pH = 2.9. {FA~ = (a) 0; (b) 100 mg 1-1 ; (c) 200 mg 1-1 ; (d) 300 mg 1-1 ; (e) 400 mg 1-1 ; (f) 780 mg 1-1.

(II. 3) Pulse polarography

.The influence of {FA} on DPP curves is shown in Fig. 6. In this technique, the capacitive component of the current was amplified as compared with the faradaic, by sampling the current between 2 and 6% after the beginning of the pulse. The influence of the height of the pulse is shown in Fig. 7. In this figure the current; corresponding to the hump of the i = f(E) curve was recorded as a function of {FA}, for three values of the height of the pulse.

(II.4) AC polarography

The influence of { FA} on the ACP curves is similar to that obtained for DPP (Fig. 8). It must be noted that the curves of Fig. 8 were recorded at a

AiK/IMA

1.2 . ~ . . ~ ¢1

°. - ' I ~ ° . - b

05 / ' J J ' J ' J ~ "

0.3

~oo 2oo 3°° ~no 50o 6°° 7°o 8°0 { ~ - A } / m g l-'

Fig. 5. Influence of {FA} on the difference, AiK, in the currents measured by the Kalousek method in the presence and absence of FA. Frequency = 75 Hz, pH = 2.9. E = (a) 0 V; (b) --0.1 V; (c) --0.2 V; (d) --0.3 V.

265

8.0

6.0

4.0

d

b

o

0.1 0 - 0.1 - 0 2 - 0 . 3 - 0 . 4 - 0 . 5 - 0 .6 - 0 .7 - 0 .8 E/V

Fig. 6. Di f ferent ia l pulse polarograrns of F A : p H = 7.0; scan rate = 0.5 m V s - ] ; the pulse was applied during 40 ms and started 50 ms before the falling of the drop. The sampling of current was made between 2 and 6% of the pulse duration. (FA} = (a) 0; (b) 20 mg 1-1 ; (c) 40 mg 1-1 ;(d) 80 mg 1-1.

p h a s e ang le 0 o f 9 0 ° in o r d e r t o a m p l i f y t h e c a p a c i t i v e c o m p o n e n t o f t h e a l t e r n a t ing c u r r e n t as c o m p a r e d t o t h e faradaic . H o w e v e r , a s m o o t h c u r v e w i t h n o p e a k , v e r y c l o s e t o t h e z e r o c u r r e n t l ine , w a s a l so o b t a i n e d at 0 = 0° , w h i c h i n d i c a t e s t h a t n o faradaic p r o c e s s , at l eas t s u f f i c i e n t l y fast t o b e n o t i c e d in

11.0

10.0

7 . 0

6.0

5.0

3.0

2.0

\

~"-~"~ ~ " ~ - " - ~ c

\ \ .

\

+ ~

~ b

~ 20 3b 4b s~ 60 70 Bb " { F A } / m g t - '

Fig. 7. Influence of the height of the potential pulse, ZkE, on the current measured in the presence of FA by DPP. Experimental conditions, see Fig. 7. In each case the current was measured at the potential of the hump, E h. (a) AE -- 50 mV, E h = --350 mV; (b) ZkE = 100 m V , E h -- --321 mV; (c) ZkE = 200 mV, E h -- --257 mV.

266 200 ~0:2 0:3 -0:~ -0:s -& -0:7 -0:8 -0:9 1:0

E/v -o~

10.0

Fig. 8. AC po la rograms of FA: pH = 3.8; scan ra t e = 2 m V s -1 ; a m p l i t u d e = 10 m Y ; fre- q u e n c y = 400 Hz, phase angle = 90. °. ( F A ) = (a) 0; (b) 20 mg 1-1 ; (c) 60 mg 1-1 ; (d) 120 m g 1-1.

ac polarography, takes place in the potential range studied. In ac polarography the capacitive current is theoretically proport ional to

the frequency of the applied alternating voltage [21 ]. Figure 9 shows that this relationship is obeyed irrespective of the value of (FA) up to abou t 300 Hz, and that no great departure of the curves from linearity occurs below 450 Hz.

The influence of pH on the alternating current measured on the hump of the i = f(E) curves (E = --0.365 V) is shown in Figs. 10 and 11. It is seen that the alternating current decreases linearly with (FA} for {FA} ~< 30 mg 1 -~, and that this decrease, Ai, is more pronounced as pH decreases.

The ionic strength (IS) of natural waters varies widely. Now ACP is sensitive

20,0 i~/pA .,~°a

° * ID

12.0 o~i ~ 8.0 ;~£/*

4.0

I00 200 300 400 ~/Hz

Fig. 9. I n f l uence of t he f r e q u e n c y co o n the a l t e rna t ing c u r r e n t m e a s u r e d at the p o t e n t i a l o f t h e h u m p ( - -0 .365 V). (FA} = (a) 0; (b) 30 m g 1-1 ; (c) 60 mg 1-1 ; (d) 90 mg 1-1 . F o r o t h e r c o n d i t i o n s see Fig. 8.

2~.o { i~/,uA

17.0 x . ~

A~. ~ × b

15.0 a ~

13.0

l0 20 30 40 50 60 70 80 90 100 {FA} /m 9 ~-'

Fig. 10. Influence of {FA} on the alternating current measured at the potential of the hump. For experimental conditions see Fig. 8. pH = (a) 3.8; (b) 7.3; (c) 11.0.

267

to change in the cell res istance. Moreover , a l t h o u g h the results o f refs. 1 - -4 and 25 seems t o indicate that the p o l y e l e c t r o l y t i c nature o f F A d o e s n o t s e e m t o play the m o s t i m p o r t a n t role in their g lobal behaviour , the IS might i n f l u e n c e their a d s o r p t i o n propert ies . H e n c e the e f f ec t o f ion ic s trength changes was te s ted by adding various a m o u n t s o f KNO3 to water sample N o . 50a w h o s e IS was l o w (IS = 10 -3 M, for { F A ) = 60 m g 1-1). It can be seen f r o m the results s h o w n in Fig. 12 that the res i s tance o f the s o l u t i o n s having IS < 0 . 0 6 M have a s trong e f f ec t o n capaci t ive currents , but for s o l u t i o n s w i th IS ~>0.1 M, small vat ia t ion in IS have very l i tt le e f f ec t o n the m e a s u r e d current , irrespect ive o f the value o f ( F A ) .

8.0

6.0

4.0

2.0

~ ' ~ t , b

0 Q

3.O 42 5.0 6.O pH

Fig. 11. Influence of pH on the difference in the alternating current, Ai~, measured in the absence and in the presence of FA, at the potential of the hump. For experimental condi- tions see Fig. 8. (FA~ = (a) 30 mg 1-1 ; (b) 60 mg 1-1 ; (c) 90 mg 1-1.

268

24.0

20.0

16.0

120

8.0

4.0 /

oC1 __o j l

~.J°- b + / , ~

- - a ~ C

d

0.04 0.08 0.12 0.16 0.20 [KNO3]/Ivl

Fig. 12. Influence of the concentration of electrolyte on the alternating current measured at the potential of the hump. pH = 2.9. For other experimental conditions see Fig. 8. (FA} = (a) 0; (b) 25 mg 1-1; (c) 50 mg l - l ; (d) 75 mg 1-1.

(II. 5) Suppression of the polarographic maximum of 02

The current of the polarographic maximum of O:,/max, decreases with increasing concentrations of FA. (see Fig. 13), and simultaneously, one observed a positive shift in the potential of the maximum (from --0.420 V for {FA} = 0 to --0.250 V for (FA} = 10 mg 1-1). It has been shown that the decrease in this maximum may be expressed empirically as

log(i~ax/i max) = K s + S(FA}

where i~ ax and/max are the currents at the maximum in the absence and in the

120

100 ~ ~ x

8o

40

20

(~

10 20 30 { F A } / m g t -1 40

Fig. 13. Influence of (FA} on the current of the polarographic maximum of 02 . [KNO3[ = 1.4 X 10 -3 M; pH = 2.35 (a) Solution saturated with air; (b) solution saturated with O2.

m o x o0 ] %0

O.B ~ O

0,6 o~°

OA o ~ / ~ , b

~.o 2.0 ~..o ~.o s.o ~'.o 7:0 8'.o ~'.o ~do ~,:0 Zo " {FA~,/rn g r '

Fig. 14. Change of log[ iomaX /i max] with {FA}. For experimental condit ions see Fig. 13.

269

presence of the adsorbing agent, FA, and K s and S are specific constants of the adsorbing agent; S, in particular, is related to its adsorption capacity and increases with the suppressing capacity of the test compound.

Figure 14 shows that the above linear relationship is obeyed in 02 saturated solutions for {FA} ~<40 mg 1-1 and in air-saturated solutions for (FA} <10 mg 1-1. This figure also shows that the value of S depends on the concentration of 02 in the solution.

Figure 15 shows the dependency of i max on the IS, the maximum decreas- ing with increasing IS which is in agreement with the observations reported in the literature [26]. For IS >10 -2 M, the maximum becomes too drawn out or is not sufficiently well defined for it to be used for analytical purposes.

The influence of pH on i max is shown on Fig. 16. A decrease in/max observed, at low pH, for any concentration of FA may be attributed, at least partially, to increase in IS caused by the addition of strong acid used to obtain the desired pH. This figure also shows that for a given concentration of FA, the decrease in i m~x is maximum for pH close to 3. The results obtained in this section are summarized in Table 2.

An 0.1 M NH4OH medium recommended by Lucena-Conde and Gonzales- Crespo [11 ] for such a study was also used for comparison purposes. It must be noted that , in this medium, as well as at pH = 2.9 in KNO3 8 mM, the region of linearity of the log(i~ax/im~x) vs. {FA} plot was found to be narrower than in other media ({FA} ~<7 mg 1 -~ instead of (FA} <11 mg 1-1).

It is also interesting to note that, by using an average MW of 1000 for FA [19,25], ava lueof S = 5.6 X 1041 mo1-1 is obtained at pH = 2.90 and 1.4 mM KNO3 which is comparable with the values obtained for strong suppressors of oxygen maximum.

(II.6) Discussion on the nature of the measured currents for the various polaro- graphic techniques

Fulvic and humic substances may react at the electrode by two basic processes giving rise to two kinds of currents:

(1) A modification of the capacitive current due to a change in the double- layer structure by adsorption of FA.

270

50.0

40.0

30.0

20.0

10,0

,~o

~ b

c

0.002 0.004 0 006 O,OOB 0.Q1 [KNO~]/M

Fig. 15. Inf luence o f the concentrat ion of the e lectrolyte on the current o f the polarographic m a x i m u m of 02. So lu t ion saturated wi th air; pH = 2.85. {FA} = (a) 0; (b) 3.0 mg 1-1 ; (c) 6.9 mg 1-1 .

(2) A faradaic current due to the oxido-reduction of some functional group of FA or of a species (metal ion or organic compound) which is attached to FA by some kind of chemical bond (complexation, hydrogen bonding [27] , etc.).

The adsorption phenomenon was already observed during the reduction of Pd--FA complexes [1,2], and seems to be confirmed by the fact that FA suppresses the maximum of oxygen. Also, the decrease of the alternating current at 0 = 900 with the increase of {FA}, as well as the linear increase of

50 1 ir~°×/l~A

I / ' \ 01i \

30 ÷ / ~ ' ~ - - + - ~ + _ _ . ~ b

/ s// 20 ÷/ ./

/ . / /

m /

J.0 L0 si0 6.0 pH

Fig. 16. Influence of pH on the current of the polarographic maximum of 0 2. Solution saturated v¢ith air. IKNO31 : 1.4 X 10 -3 M. {FA} = (a) 0; (b) 3.0 mg l -I ; (c) 6.9 mg 1-1.

271

TABLE 2

Values of S under different conditions. 02 saturated solutions; T = 20 + 0.5°C. Water No. 50b

Added electrolyte c/mM pH S/1 mg -1

KNO3 8.0 2.90 0.103 KNO 3 4.0 2.90 0.065 KNO3 1.4 2.90 0.056 KNO3 1.4 3.80 0.047 KNO3 1.4 5.00 0.040 NH4OH 100.0 11.35 0.063

this current with frequency, confirms the fact that a modification of the capa- citive component of the current, due to adsorption of FA, is observed with this technique. This adsorption seems to occur mainly in the vicinity of the pzc (see Fig. 8 and ref. 3), or at more positive potentials, which is likely for negatively charged compounds. DPP and Kalousek techniques seem to confirm the above observation.

On the other hand, the existence of a faradaic current might play an impor- tant role in the results obtained with DCP. Indeed, although we did not observe any well-defined dc polarographic wave, in agreement with most of the authors we observed that an increase in (FA} led to a decrease in positive current or an increase in the negative current, together with a positive shift in the potential corresponding to zero current. Moreover, the negative current increased when the pH was lowered, which would be likely for the reduction of some organic component of FA. The existence of a faradaic component in DCP is confirmed by the more detailed results discussed in ref. 3 which also indicate that the faradaic current is due to a slow electrode process. This may explain why the results observed here with DCP are very different to those obtained with ACP, DPP and Kalousek techniques which are fast relaxation techniques insensitive to slow processes. A slow faradaic process is also in agreement with the obser- vation that no peak was observed in ACP, even at ~ = 0 °.

Hence, from the results obtained here, together with those discussed in ref. 3, it is likely that, in the potential range studied, the presence of FA may both modify the capacitive current due to their adsorption, and give rise to a faradaic current corresponding to their reduction in accordance with a slow electrode process. ACP, DPP and Kalousek techniques are probably mainly sensitive to the first phenomenon, while the second one plays an important role in DCP. The relative importance of the capacitive and faradaic currents in DCP as well as the possible nature of the reducible compound, are discussed in ref. 3.

(III) APPLICATION OF THE POLAROGRAPHIC TECHNIQUES FOR THE ANALYSIS

AND CHARACTERIZATION OF FA

(III.1) Choice of the best technique

Although the domain of linearity of i vs. {FA} plot in DCP falls in thecon- centration range of FA usually encountered ((FA} <80 mg l -~), the precision

272

of the values obtained from the plot at low concentrat ion ({FA} <10 mg 1-1) and at pH >5.0 is not very good. In addition, sufficiently large changes in current are obtained only at negative potentials (E <--0.6 V) and this may pose serious problems when electroactive species are present in the water. The DPP is not recommended because of the fact that a linear port ion is never observed in the current vs. (FA} curves.

Finally, the Kalousek technique has the advantage over the two preceding methods in that the changes in current may be measured at a relatively "posi- t i re" potential (between 0 and --0.3 V). Unfortunately, the drawbacks of this technique are that the i vs. (FA} plot is non-linear and also that the precision of the method is not good at levels of (FA} found in natural waters.

Thus, ACP and the O2 max, technique are the best techniques for the measure- ment and characterization of FA for the following reasons:

(1) The methods are sensitive (limit of detection: for ACP 2 mg 1-1 ; for the 02 max technique 0.5 mg 1-1 in air-saturated solutions and 1 mg 1-1 in oxygen- saturated solutions).

(2) "Posit ive" potentials with respect to pzc, may be applied (--0.2 V to --0.5 V).

(3) In both cases an adsorption parameter [S for the 02 max. technique, and A for ACP, where A = slope of the linear por t ion of the current vs. the (FA} curve (Fig. 10)] may be defined which is characteristic of the adsorbing com- pound.

It should be noted that the applicability of the 02 max. technique is more restricted than ACP because experiments have to be carried out under very carefully controlled conditions as it is very sensitive to pH and ionic strength changes. Also, for measuring currents the entire polarogram has to be recorded, whereas in the case of ACP i can be measured at a constant value of potential. However, the 02 max technique is useful when a high sensitivity is needed.

The fact that the 02 max. technique is sensitive to experimental condit ions is further illustrated by the results of the s tudy of adsorption of FA on kaolinite: we found that the products of dissolution of kaolinite, in the absence FA slightly modify the oxygen maximum, and thus had to be taken into account in interpreting the results obtained in the presence of FA. This complication does not arise in ACP.

(III.2) Comparison of the results obtained for FA from various origins

Figure 17 shows that there is a good correlation between S and A measured for a series of FA of various origins (as described in ref. 4). These results indi- cate that either S or A may be used to characterize the adsorption properties of FA at the mercury electrode. It is interesting to note that, when A and S are measured in replicates, standard deviations of + 5% and +3% respectively, are obtained. However, from the dispersion of the points in Fig. 17 it may be seen that the variability is not bet ter than 10--15%. This higher variability probably reflects the influence of the variation in the chemical composit ions of the various waters tested on the measured parameters themselves.

The values of A for a series of FA and HA of various origins, along with those of a series of compounds which might be taken as models of FA, are given in Table 3. Fulvic substances are believed [27] to contain mainly ben-

273

220

200

180

160

140

120

100

- A / , u A l g -'~

, , /

/ , /

/

/ /

32~ / / / e31b ~i /

/ / / , /

2 2b / ~b • / 41b ~0

/; / 5Oa

60 80 lO0 120 140 160 S / l g -1

Fig. 17. Correlation of the parameters A and S measured from ACP and the polarographic maximum of 02 respectively, for a number of different FA samples.

TABLE 3

Values of the parameter A obtained from ac polarographic measurements, for various compounds

Compounds or samples nos. --A/pA 1 g-1 --A/pA l mo1-1

1 +150 +1.5 X l0 s 2 +110 +1.1 X l 0 s 3 ~:180 +1.8 X 10 s

10 +170 +1.7 X 10 s 11 +180 +1.8 X 10 s 22 +180 +1.8 X 10 s 24 +140 +1.4 X l 0 s 50a +150 +1.5 X 105 41a +100 +1.0 X 107 F3 +230 +2.3 X 107 F4 +660 +6.6 X 107

Phenol CeHsOH Pyrogallol C6H3(OH)3 Pyrocatechol Celia(OH)2 Hydroquinone CeH4(OH)2 Mellitic acid C6(COOH)6 Phthalic acid CeHa(COOH)2 Benzoic acid CeHsCOOH Hemimellitic acid CeH3(COOH)3 4-Hydroxy-benzoic acid C6H4(OH)(COOH) Gallic acid CeH2(OH)3(COOH) L-dopamine L-phenylalanine ~-alanine Citric acid Tiron C6H2(OH)2(SO3Na)2 Purpurogalline Alizarin complexon Alizarin S Tannic acid

2.25 X 102 5.6 X 103 5.8 X 103 1.6 X 104 1.5 X 103 1.4 X 104 1.7 X 104 2.0 X 1 0 4

2.9 X 104 4.1 X 10 a 3.7 X 104 4.3 X 104

--6.8 X 102 --5.9 X 103 --2.64 X 103

4.6 X I0 a 1.3 X 10 s 2.0 X l 0 s 3.7 X l 0 s

274

zene (or aromatic) rings with a certain number of phenolic groups and of carboxylic groups, fixed on the ring. The values of A are expressed in pA 1 mol -~ for the compounds whose structure is known and in pA 1 g-~ for FA and HA. For these compounds the value of A in pA 1 mol -~ was calculated by using either a mean value of 1000 for MW of FA for water samples Nos. 1, 2, 3, 10, 11, 22, 24, 50a [19,24,25] or a value of 100,000 for HA (waters Nos. 41a, F4, F3) [19] as estimated from ultrafiltration measurements. The latter value corresponds to the minimum value of MW.

The results in Table 3 show that FA of the same nature (Nos. 1--24; 50a: FA from lakes and ponds) have very similar absorption properties, while HA with much higher MW (from peat or decomposi t ion of leaves) have stronger adsorption properties when their concentrat ion is expressed in m o l l -1 . It must be pointed out that a more precise investigation of A and the nature of the organic compound should take into account the fact that the solution contains a mixture of organic compounds and that A is dependent on kinetic and equi- librium properties of each of the components of the mixture and on their com- petitive interaction at the interface.

A comparison of A values of model compounds with those of FA and HA, show that only the A values of polycyclic compounds resemble that of natural compounds. All the other aromatic compounds yield low A values, suggesting that they cannot be used as a model for FA, at least in so far as the absorption properties are concerned. It is also interesting to note that A values for carbox- ylic compounds are higher than the corresponding phenolic compounds.

(IV) CONCLUSION

The preceding results give an order of magnitude of the modification of the polarographic current, produced by the presence of FA in a test sample; they also show how this modification is influenced by the experimental conditions.

Moreover, the above results show that the polarographic techniques, in particular ACP, may be useful for the analysis of FA and HA in natural waters, provided that well-controlled conditions are chosen. Indeed, it is obvious that these methods, in which the adsorption of FA and HA play ab important role, are hardly selective and that, in particular, any other adsorbable compound present in the water under s tudy may interfere with the measurement of the concentrat ion of FA. Hence a correct analysis of an unknown water sample should involve a separation of other adsorbable compounds of the water, and moreover the type of FA or HA of the test sample must be known, e.g. from A or S measurements (Section IV.2) or by some other measurements (ref. 4).

Despite their limitations, these methods are useful at least for two kinds of purpose:

(1) They offer an additional means for characterization of FA and HA from A or S measurements.

(2) They enable FA or HA to be monitored during their chemical reaction with other compounds. For example, adsorption of FA on kaolinite at pH 2.9 was followed by using the 02 max technique in our laboratory.

2 7 5

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