Adsorption of benzoic acid on a polycrystalline gold electrode

Ekctrochimica

Pergamon

Acta. Vol. 39, No. 5, pp. 655460.1994

Copyright 0 1994 ElsevierSciace Ltd. Printedin Great B&in. Auri&a memd all3-4686p4

0013-4686(93)E0017-6

s6.al+

o.ml

ADSORPTION OF BENZOIC ACID ON A POLYCRYSTALLINE GOLD ELECTRODE P. ZELENAY, P. WASZCZUK, K. D~BROWOLSKA and J. ~OBKOWSKI* Department of Chemistry, Warsaw University kwirki i Wigury 101,02-089 Warsaw, Poland (Receiued 5 August 1993; in revisedform 11 October 1993) Ah&act-Adsorption of benxoic acid on a polycrystalline Au electrode, obtained by electroplating of gold, has been studied in 0.1 M HClO, using cyclic voltammetry and radiotracer technique. Adsorption has been found to take place in the entire range of studied potentials, from 0.05 to 1.75V (rhe), with the surface concentration of the adsorbate exceeding 5 x lo’* molecules cm-’ at saturation. Desorption into a clean supporting electrolyte is small and extremely slow. On the other hand, surface/ bulk exchange of benxoic acid is much faster, attesting to the dynamic equilibrium between the adsorbed and solution species. Adsorption data and model calculations strongly indicate that two different orientations of the adsorbed molecules are present on the surface. Flat (parallel to the surface) orientation dominates at less positive potentials while the vertical (perpendicular to the surface) orientation dominates at more positive potentials. Regardless of orientation, benxoic acid adsorption on gold falls into the chemisorption category.

General behavior of the system bears close resemblance.to the adsorption of benzeic acid on platinum that was reported earlier in Zelenay and Sobkowski, Electrochim. Acta 29.1715 (1984). Key words: adsorption, benzoic acid, gold, aqueous solution.

INTRODUCTION Benzoic acid adsorption on solid electrodes has drawn considerable attention from several electrochemistry groups investigating possible modes of the adsorbate coordination to the surface. The most distinct ones would involve either a direct binding of the benzene ring to the surface (flat orientation) or a surface bond through free electron pairs on oxygens in the carboxylic group (vertical orientation). Gold is a suitable substrate for the mechanistic studies of this kind for it offers a wide range of nearly ideal polarizability, unavailable on catalytic surfaces of other noble metals, including platinum. For more practical reasons, the original interest in the surface interaction of benzoic acid was also influenced by the reports indicating anti-corrosion properties of benzoic acid and benzoate (eg[l]). Surface behavior of benzoic acid on gold in aqueous solutions has been studied by a number of methods that include differential capacity[2] and conductivity measurements[3], surface-enhanced Raman spectroscopy (SERS)[4] and potentialdifference ir spectroscopy (PDIRS)[S]. These studies unambiguously demonstrate that benzoic acid interacts quite strongly with the gold surface over a broad range of electrode potentials. However, unlike on platinum electrodes[5-81, no chemical transformation of the C,H,COOH molecule adsorbed on Au has been observed. Adsorption tends to rise when the solution

becomes

more acidic[2]

thus indicating

*Author to whom correspondence should be addressed.

that the benzoic acid molecule, rather than benzoate, is a dominant species at the gold surface. Spectroscopic data have provided interesting clues to the surface orientation of adsorbed molecules, although some discrepancies between data obtained by SERS and PDIRS still remain unaccounted for[4,5]. This work reports on the first strictly quantitative investigation of the benzoic acid adsorption on gold. Voltammetric examination of the system is followed by radiotracer determination of the surface concentration of the interphase species as a function of time, electrode potential and solution concentration. Also studied is desorption and surface/bulk exchange of benzoic acid. This involves measuring surface counts as a function of time at the presence of (i) clean supporting electrolyte (benzoic acid completely rinsed out of the cell after adsorption), and (ii) large excess of unlabelled benzoic acid in the solution. The most likely orientation of the adsorbed molecules is discussed, depending on the electrode potential. The study, which is a continuation of the earlier work on benzoic acid adsorption on platinum[8], also offers a comparison of benzoic acid adsorption on two solid electrodes of different electrocatalytic properties but similar surface morphology. EXPERIMENTAL The experimental methods used in this work were cyclic voltammetry (cu) and radiotracing. Cyclic voltammetry was typically used to control the cleanliness and reproducibility of the gold surfaces as well

655

P. ZELENAY

656

to measure the real surface area. The latter quantity was obtained from the surface oxidation charge using the so-called Burshtein minimum current on a positive-going ctr scan as a reference for the charge integration ([9] and references cited therein). Also, cyclic voltammetry allowed preliminary testing of the surface activity of benzoic acid and predetermine the potential range of adsorption. All voltammograms were recorded at the scan rate of 50mVs-‘. The actual measurements of benzoic acid adsorption, in terms of surface concentration (F), were carried out using the radiotracer technique. The equation for the calculation of F, the radioelectrochemical cell, and the method itself were reported and reviewed elsewhere[lO, 111. Benzoic acid was labeled with carbon-14 in the carboxylic group. The specific activity of the radiochemical used in this work was 3.2 Ci mol- i. The linear adsorption coefftcient of the /I- radiation emitted by carbon-14 was taken as 3OOcm-‘[12]. Each working gold electrode was prepared on the surface of a glass scintillator. The electrode preparation involved the vacuum evaporation of a gold film, 300nm thick, that later served as a conductive support for electrolytically formed rough deposits of gold. The deposition was conducted from 6 x 10d3 molar AuCl, (gold chloride) solution, according to the procedure for the preparation of roughened copper and gold electrodes, described by Horanyi et aI.[13, 143. This was carried out for about 60min at a constant potential, chosen in the hydrogen evolution region, and a current density of the order of 1 mAcm_‘. The roughness factor of the electrodes thus prepared was between 20 and 30, which is relatively low for adsorption measurements utilizing radioactive labels. A large surface area Pt gauze was used as the counter electrode. All potentials were measured against the reversible hydrogen electrode (he) in 0.1 M HClO,, except during electrolytic deposition of gold when the mercury/mercury sulfate reference was employed (0.515 V vs. rhe). The supporting electrolyte was 0.1 M HCIO, which is generally considered to be a non-adsorbing medium on both platinum and gold electrodes.

et

al.

as

.2 t

0” f 5 0

/

0.0

0.4

0.6

I

1.2

1

1.6

Fig. 1. Cyclic voltammetry of Au electrode: (----) 0.1 M HClO,, and (-) 0.1 M HClO, + 10s3 M C,H,COOH solution. Scan rate: SOmVs-‘. altered by an injection of benzoic acid to the cell. As indicated in Fig. 1 by the solid line, oxidation of the gold surface in 10e3 M C,H,COOH solution is considerably hampered and shifted towards more positive potentials. Nevertheless, as judged from the oxide reduction charge, the total amount of the oxide is only slightly less than in the solution containing no benzoic acid. Thus, one may conclude that (i) benzoic acid shows substantial surface activity at the potential of the onset of oxide formation, and (ii) whatever amount of adsorbate is formed at lower potentials it tends to desorb from the surface at utmost positive potentials. This is further supported by the voltammetric profiles recorded after holding the potential at 1.20 V for 20min in 10m3M CsH,COOH and subsequent rinsing of the cell with the supporting electrolyte solution (Fig. 2). (The electrode potential remained unaltered during the rinse due to the presence of a thin solution layer that enabled external potential control.) The first scan in the positive direction (solid line) shows that even less oxide is now formed than under continuous scanning conditions. This clearly indicates that blocking of the surface sites by benzoic acid is time dependent. It is also quite apparent from this figure that complete removal of the adsorbate 1

I

I

RESULTS AND DISCUSSION Preliminary examination of the adsorption system was done by cyclic voltammetry in benzoic acid solution and compared with voltammetry in the clean supporting electrolyte solution. The voltammetric profile in perchloric acid alone, shown in Fig. 1 with a dashed line, is characteristic of polycrystalline gold in the absence of surface active compounds. It is basically featureless in the “double layer” region of potentials, from 0.10 to 1.2OV. At more positive potentials, different stages of surface oxidation give rise to several current peaks. Following the reversal of the positive-going scan, a single oxide reduction peak is seen in the potential range substantially more negative than the potential range of oxidation. This attests to the irreversibility of the oxide formation on gold. The voltammogram is

0

PotentialN

L

0.0

0.4

0.6

I

1.2

1

I

1.6

2.0

PotentiatN

Fig. 2. Cyclic voltammetry of Au electrode in 0.1 M HCIO,: (-) 1st scan after holding the electrode at 1.2OV (arrow) in lOm3 M C,H,COOH for 20min and subsequent rinsing of the cell with supporting electrolyte; (. . .) 2nd scan. (-- --) Scan of benzoic acid free Au electrode. Scan rate: SOmVs-‘.

657

Adsorption of benzoic acid from the surface cannot be accomplished in just one scan. Even after the second voltammetric scan, the gold surface is not completely free of the adsorbate (dotted line), At the scan rate applied, 50mVs-’ between 6 and 8 scans are needed to restore the original voltammetry (dashed line). The potential dependence of benzoic acid adsorption was measured by means of the radiotracer technique. The data recorded in 10m4M C,H&OOH (Fig. 3) show that the surface activity of the acid extends over the entire range of potentials investigated in this work. A measurable amount of adsorbed benzoic acid is already present on the surface at the starting potential of 0.05 V. This result agrees with the earlier SER spectra indicating that considerable adsorption of benzoic acid takes place at potentials as low as -0.2OV vs. sce[4]. The r values increase slightly with potentials up to about 0.9OV, without exceeding the 7.3 x 10” molecules cm-’ level (filled circles). A sharp rise in r is seen when the potential is stepped above 0.9OV. The r vs. E plot culminates in the range of 1.20-1.3OV where surface concentration of the adsorbate reaches more than 3.2 x 1014 molecules cm-‘. A steep drop in r is observed at more positive potentials which coincides with the surface oxidation of the gold surface, beginning at about 1.2OV. Noteworthy is that no total displacement of the adsorbate from the surface is observed, even at the most positive potential studied in this work (1.75V). This leaves no doubt that benzoic acid forms a strong surface bond that allows some of the adsorbate molecules to avoid displacement from the electrode by the highly surfacereactive oxygen-containing species. Following the reversal of the positive-going sequence, virtually no change in the surface concentration is observed until the oxide reduction starts at about 1.3OV (empty circles). Between 1.30 and 1.2OV the adsorption signal rises sharply, up to the level observed before, during the positive-going potential sequence. On one hand, this shows that surface oxide effectively prevents benzoic acid from adsorption. On the other hand, it becomes quite apparent that outside the potential range where gold oxide (hydroxide) is present on the surface the adsorptive

0.6

properties of the metal are the same during either potential sequence. No adsorption is detected only when benzoic acid is injected into the cell on top of a pre-formed layer of gold oxide at 1.75V (squares). The otherwise strongly surface-interacting molecules of the acid appear incapable of either penetrating or displacing the surface oxide. The oxide must first be removed from the surface by reduction before a measurable adsorption can take place. Once the oxide layer is reduced, a “regular” pattern of adsorption at potentials below 1.3OV is observed on the negative-going adsorption plot. When the bulk concentration of benzoic acid is increased by an order of magnitude, up to lo-‘M, the r vs. E plot remains “qualitatively” unchanged. At the same time, however, the surface concentration of the adsorbate increases in both potential regions of adsorption below and above 0.9OV (Fig. 4). The maximum surface concentrations reach 1.8 and 5.0 x 1Ol4 molecules cm-l in these two regions, respectively. The second value in particular should be viewed as very high for adsorption of a large organic acid. The observed character of the potential dependence suggests that benzoic acid undergoes strong adsorption on gold which may involve the formation of a chemical bond between the molecule and the surface. Therefore, it comes as no surprise that the adsorption process is slow. Adsorption during the first 5 min is shown in Fig. 5. Regardless of the initial state of the surface, the adsorption process is complete in only about 70% at the end of the fifth minute. Reaching the saturation value of surface concentration requires much longer adsorption time of 15-30min, depending on adsorption potential. At its early stage in particular, the rate of adsorption also depends on the state of the surface. Of the two plots shown in Fig. 5, the one marked with empty circles pertains to the adsorption process taking place on a partially oxidized surface of gold. In this case, the adsorption potential of 1.2OV was approached from the positive end of the potential scale (1.75V). The plot shown by filled circles represents adsorption at the same potential but approached from the negative side which means that the surface process is taking place on the oxide-free

1.2

PotentialN

Fig. 3. Potential dependence of benzoic acid adsorption on Au in 10e4M C,H,COOH solution using positive- (0) and negative-going (0) potential sequences. Positive-going plot started at 0.05 V, and reversed at 1.75 V. (0) Adsorption plot after C,H,COOH was added at 1.75 V.

0.0

0.4

0.6

1.2

1.6

2.0

PotentialN Fig. 4. Potential dependence of benzoic acid adsorption on

Au in 10m3M C,H,COOH solution. See Fig. 3 for description.

P. ZEL!ZNAY

658

et a!.

4

"E

3

%

g6

f 82

2

2

“0

4

! b -1

J

0 0

1

2

3

4

0

5

10

20

30

40

Timelmin

Fig. 5. Rate of C,H,COOH adsorption on the oxide free (0) and oxidized (0) surface of Au. E_,, = 1.2OV; c= 10-3M.

surface. As seen from this figure, the surface oxide not only lowers the amount of the organic adsorbate but also slows down the process of its accumulation. Once formed, the adsorbate is not easily removable from the surface. Desorption into clean supporting electrolyte, after the rinse of the bulk benzoic acid, is very slow, resulting in a mere 25% of the adsorbate gone from the surface after about 1 h (Fig. 6). A considerable and fast drop in the amount of surface benzoic acid is observed after applying first and second voltammetric scans which are marked in Fig. 6 with arrows numbered 2 and 3, respectively. A few more cu scans are needed before desorption of the benzoic acid from the surface is complete (arrow number 4). This provides one more indication that benzoic acid undergoes chemisorption rather than physical adsorption on gold. Despite the apparent strength of the surface bond, there is a clear dynamic equilibrium established between the surface and bulk species during adsorption. This is best illustrated by the surface/bulk exchange experiment in which a large excess of unlabelled benzoic acid is added to the cell after adsorption of the radioactive acid is complete at 1.3OV (Fig. 7). As much as 75% of the radioactive

0

10

20

30

40

50

50

60

70

en

90

Timelmin

60

70

80

Fig. 7. Surface/bulk exchange of benzoic acid at 20-fold excess of unlabelled benzoic acid injected to the cell at arrow 1. Adsorption and exchange potential 1.3OV; adsorption from 10e3M C,H,COOH solution. The start of the cu scans is marked with arrow 2.

adsorbate undergoes exchange over 30min, with the remaining 25% being exchanged over an even longer time (not shown). This kind of experiment attests to the reversible behavior of the system and allows for adsorption isotherms to be used in this case. The concentration dependence of benzoic acid adsorption was measured at 0.30 and 1.3OV, that is, in two markedly different potential regions of adsorption (Fig. 8). The range of solution concentration extended over about three orders of magnitude, from 4 x 10m6 up to 3 x 10m3M. The r vs. c plots at both potentials tend to flatten out above lo-’ M, thus indicating that near-saturation conditions are reached. The Gibbs energies of adsorption were calculated using the Frumkin isotherm equation: cexp(-AG’/RT)=&exp

m

The fitting procedure of the experimental data to the Frumkin model employed a non-linear regression method that used a Simplex algorithm to calculate the set of isotherm parameters for best fit conditions[lS]. In the case of the isotherm used, the parameters are as follows: the Gibbs energy of adsorption (AC’), the maximum surface concentration of the adsorbed benzoic acid at each potential

90

Timelmin

Fig. 6. Desorption of adsorbed benzoic acid into clean supporting electrolyte at 1.3OV. The adsorbate was formed in the 1Om3M C,H,COOH solution at the same potential. (1) bulk benzoic acid removed after adsorption; (2) 1st cu scan applied in the full potential range (0.05-1.75V), starting from the adsorption potential into positive direction; (3) 2nd cv scan; (4) continuous scanning.

0

1

2

3

1 O%/mol.dmJ

Fig. 8. Adsorption isotherms for benzoic acid on gold at (0)0.30Vand(@) 1.3OV.

659

Adsorption of benzoic acid (F,), and the interaction coefficient within the adsorbate (g). The calculated Gibbs energies of adsorption are -16.4 and -23.1 kJmol_’ at 0.30 and 1.30 V, respectively, whereas the corresponding values of the interaction coefficient at these potentials are 1.5 and -0.3 (Table 1). this indicates that relatively weak attractive forces are operational within the adsorbate at low potentials which change to a very weak repulsion at more positive potentials. The calculated values of FoD are high: 2.4 and 5.1 x 1Ol4 molecules cm-‘, at 0.30 and 1.3OV, respectively. We compared these experimentally determined values of F, with the expected values of maximum surface concentration of differently oriented benzoic acid (Table 1). Calculation involved the determination of the cross-section areas of benzoic acid molecule differently oriented on the gold surface, using known crystallographic data on interatomic distances and angles between bonds in the molecule[16]. The calculation gives 0.419nm2 for the flat orientation and 0.198nm2 for the vertical orientation (with no free rotation allowed in both cases). As shown in Table 1, the respective values of Too obtained from such a model agree very well with the experimental data. This may indicate that, in the potential range below OMV, the gold surface is predominantly populated by benzoic acid molecules oriented parallel to the surface (Fig. 9), with the surface bond involving II electrons of the benzene ring. On the basis of the observed downshifts in the v1 SERS frequency (symmetric ring breathing mode) upon transition from the solution to the adsorbed state, Gao and Weaver[4] postulated formation of such a bond between the gold surface and a number of aromatic adsorbates, including benzene, several of its alkyl substitutes and, notably, benzoate and benzoic acid. The authors explained the observed downshifts in vi by back donation of electron density from gold to the benzene n* antibonding orbital which occurs when the benzene ring is parallel to the surface. Detected weak attractive forces (ca. 4 kJmol_‘) suggest that hydrogen bonds may be formed between carboxylic groups in two adjacent acid molecules. The presence of dimers of different carboxylic acids in aqueous solutions (formic acid, acetic acid, benzoic acid), linked by hydrogen bonds, was postulated and spectroscopically verified a long time ago.

Table 1. Experimental and calculated parameters of benzoic acid adsorption on gold obtained by using Frumkin isotherm (AC”, g and experimental F,) and crystallographic data (model F,) lo-l4 l(molecules RK - 2, Potential V 0.30 1.30

AC” kJmol-’ - 16.4 -23.1

g

Experimental

Model

1.5 -0.3

2.4 5.1

2.4= 5.5t

* Flat orientation, no free rotation, calculated cross-section area 0.419 nm’. TVertical orientation, no free rotation, calculated crosssection area 0.198 nm’.

6

I

0.0

0.4

0.6

1.2

1.6

2.0

Potential/V Fig. 9. The model of benzoic acid adsorption on gold in two different regions of potential. For the sake of clarity, the suggested orientations of the adsorbate are shown on top of the F-E plot, as measured in the lo-‘M C,H,COOH solution (Fig. 6).

Very high values of F, measured at more positive potentials render the flat orientation quite unlikely near the potential of maximum adsorption and indicate that the acid molecules are “standing-up” on the electrode surface near the potential of maximum adsorption, E,, = 1.20-1.3OV (Fig. 9). The most likely vertical orientation would be with carboxylic groups towards the surface and free electron pairs on oxygen coordinated by the positively charged surface. This, in turn, corresponds with the conclusion of the PDIR study of benzoic adsorption on gold by Corrigan and Weaver[S]. The appearance of the symmetric carboxylate stretching mode v&CO;) with the absence of the observed in solution asymmetric stretching mode V.&CO;) implied a carboxylate (vertical) binding geometry for benzoic acid on gold, with oxygens oriented towards the surface. In view of the present work, the results presented in [4 and 51 can easily be compromised. The SERS study in [4], which postulates flat orientation of benzoic acid on gold, is limited to relatively low potentials, not higher than 0.35 V (rhe). In this potential range, virtually no reorientation from the flat to vertical orientation is expected to take place (cf relatively flat part of the F-E plots in Figs 5 and 6). A much broader range of potentials (up to about 1.65V vs. rhe) is dealt with in [S] in which the adsorption of benzoic acid has been studied using the PDIR spectroscopy. In agreement with the present work, the latter study shows a gradual increase in the intensity of the ir active vibrations (ie the vibrations parallel or somewhat tilted with respect to the surface), up to about 1.25V (rhe). At the same time, the expected decrease in the modes that could unambiguously be related to the flat orientation cannot be seen, since, according to the surface selection rules, they are inactive in ir. Furthermore, the v1 downshifts observed by SERS tend to decrease as potential becomes more positive for all aromatic compounds studied in [4]. This is a clear indication that the contribution of the flat orientation to the overall adsorption picture of benzoic acid, and many other benzene substitutes, may indeed decrease as the electrode surface becomes positively charged.

P. ZELENAYet al.

660

CONCLUSIONS Adsorption of benzoic acid on gold is not only strong but also occurs in the entire range of potentials available to electrochemical studies in aqueous solutions. This kind of behavior is likely to result from the presence of two surface active centers in the benzoic acid molecule, one involving 71electrons of the benzene ring, and the other, the free electron pairs on carboxylic oxygens. These centers are responsible for adsorption in two distinctively different regions of potentials in which benzoic acid molecules are either lying flat or standing up on the surface. A region of mixed orientation cannot be ruled out in the intermediate region near 0.90 V. Despite well known differences in the electrocatalytic activity of platinum and gold, adsorption of benzoic acid on both metals is quite similar. Earlier published data on electroplated platinum[S, 171 clearly indicate that, also in this case, one deals with a typical chemisorption system. However, the adsorption maximum is reached on platinum at much less positive potentials than on gold and the postulated flat orientation of the adsorbed molecules applies to the entire range of potentials, starting with the hydrogen adsorption region and ending with potentials where gold oxide already covers the platinum surface[8]. Consequently, the maximum surface concentration of the adsorbate is considerably lower on platinum than on gold which allows for the molecules to assume a flat orientation. The presence of only one orientation on platinum may be explained by the higher surface bond energy of the flat-oriented molecules on platinum than on gold that should prevent reorientation to the vertical mode, even at very positive potentials. The surface bond energies for benzoic acid on both metals are not yet available but a much more negative value of the Gibbs energy platinum than on gold adsorption on (-55.1kJmol-‘[8) and -15.8kJmol-‘, for flat orientation on Pt and Au, respectively) may point to the stronger bond on the former metal. Benzoic acid is one of a few known compounds undergoing strong adsorption on a variety of substrates and, as such, should be further examined as a

protective substance for the metallic surfaces, first and foremost, as a corrosion inhibitor for iron and its alloys. Acknowledgement-This

work was financially supported by the Committee for Scientific Research under the grant number 2 0520 9101.

REFERENCES 1. H. S. Venkataraman and B. Dandapani, 1. Proc. Inst. Chemist. (India) 37, 71 (1967); 37, 82 (1967). 2. K. Katoh and G. M. Schmid, Bull. Chem. Sot. Jpn. 44, 2007 (1971). 3. K. Niki and T. Shirato, J. electroanal. Chem. 45 App. 7 (1973). 4. P. Gao and M. J. Weaver, J. phys. Chem. 89, 5040 (1985). 5. D. S. Corrigan and M. J. Weaver, Langmuir 4, 599 (1988). 6. G. Horanyi and F. Nagy, _~ J. electroanal. Chem. 32, 275 (1971). 7. T. Shim& A. Kunugi and S. Nagaura, Nippon Kagaku Kaishi 2, 250 (1975). 8. P. Zelenay and J. Sobkowski, Electrochim. Acta 29, 1715 (1984). 9. H. Angerstein-Kozlowska, in Comprehensive Treatise of EIectr&hemistry Vol. 9, (Edited by E. Yeager, J. O’M. Bockris and S. Sarangauani) Plenum Press, New York __ (1984). 10. A. Wiqckowski, J. electrochem. Sot. 122,252 (1975). 11. A. Wieckowski, in Modern Aspects of Electrochemistry (Edited by J. O’M. Bock+ B. E. Conway and R. E. White) Vol. 21. Plenum Press, New York (1990). 12. P. Zelenay, L. M. Rice-Jackson and A. Wieckowski, Langmuir b, 974 (1990).

13. G. Ho&vi.

E. M. Rizmaver and P. Joo. J. electroanal. Chem. 146,221 (1983). * 14. G. Horinyi, E. M. Rizmayer and P. Joo, J. electroanal. Chem. 152,211(1983). 15. J. Gawtowski, M. Szklarczyk, K. Franaszczuk and P. Zelenay, ELectrochim. Acta submitted. 16. Tables of Interatomic Distances and Confiauration in Molecule> and Ions, p. M212, The Chemical Society, London (1958). 17. G. HorLnyi, J. Solt and F. Nagy, Acta Chim. Acad. Sci. Hung. 67,425 (1971).