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6 3 4 S. L. GUPTA AND L. HOLLECK

leicht polarisierbar sind, so eröffnet sich über diese Komplexe ein Weg zur gezielten Synthese von Fest-körpern, die dem Littleschen Modell voll entspre-chen.

1 H . P . FRITZ U. H . J . KELLER, Z . Naturforsch. 2 0 b , 1 1 4 5 [ 1 9 6 5 ] .

2 H . J. KELLER U. K . SEIBOLD, Z . Naturforsch. 2 5 b , 5 5 1 [1970] ; 25 b, 552 [1970].

3 H . J . KELLER U. K . SEIBOLD, J. A m e r . chem. Soc . 9 3 , 1 3 0 9 [1971].

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6 G . F . KOKOSZKA U. R . W . DUERST , Coord. Chem. Rev . 5 , 209 [1970].

7 P. C. HOHENBERG, Phys. Rev. 158, 383 [1967] 8 J. S. LANGER, J. Polymer Sei. 29 C, 87 [1970]. 9 W. A. LITTLE, Phys. Rev. A 134,1416 [1964].

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11 K. KROGMANN U. D. STEPHAN, Z. anorg. allg. Chem. 362, 290 [1968].

1 2 S. Y A M A D A , J . A m e r . chem. Soc. 7 3 , 1 5 7 9 [ 1 9 5 1 ] .

Der Deutschen Forschungsgemeinschaft, Bad Godes-berg, sowie dem Fonds der Chemischen Industrie dan-ken wir herzlich für die finanzielle Unterstützung die-ser Arbeiten, den Farbenfabriken Bayer, Leverkusen, für die Überlassung verschiedener Isocyanide.

13 E. A. HADOW, Quart. J. chem. Soc. 14, 104 [1861]. 14 K. KROGMANN, Angew. Chem. 81,10 [1969]. 15 L. RAMBERG, Chem. Ber. 40, 2578 [1907]. 1 6 K . A . HOFMANN U. G . BUGGE , Chem. Ber. 4 0 , 1 7 7 2 [ 1 9 0 7 ] . 17 L . TSCHUGAEFF U. P . TEEARU , Chem. Ber. 4 7 , 5 6 8 , 2 6 4 3

[1914], 18 L . MALATESTA U. F . BONATI , Isocyanide C o m p l e x e s of

Metals, John Wiley & Sons Ltd., London 1969. 19 D . S. MARTIN, JR., R . A . JACOBSON, L . D . H U N T E R U. J. E .

BENSON , Inorg. Chem. 9 , 1 2 7 6 [ 1 9 7 0 ] . , 9 A I . (JGI, U . FETZER , U . EHOLZER, H . KNUPFER, K . OFFER-

MANN, in: Neuere Methoden der präp. org. Chemie, Band IV, S. 37, Verlag Chemie, Weinheim 1966.

2 0 M . ATOJI , J. W . RICHARDSON U. R . E . RUNDLE , J. A m e r . chem. Soc. 79 ,3017 [1957].

2 1 G. ROUSCHIAS U. B . L . SHAW, J. Chem. Soc . ( A ) 1 9 7 1 , 2097.

2 2 B . JOVANOVIC, L . MANOJLOVIC-MUIR U. K . W . M U I R , J . Organometal. Chem. 33, C75 [1971].

Adsorption of Triphenyl Phosphine Oxide at the Dropping Mercury Electrode in Methanolic Solutions

S. L . GUPTA and L . HOLLECK Chemisches Institut der Hochschule, Bamberg/BRD

(Z. Naturforsch. 27 b, 631—636 [1972] ; received January 22, 1972)

Triphenyl Phosphine Oxide (TPO) which is known as a strong inhibitor of electrode processes in aqueous solutions is progressively adsorbed at the dme from methanol in the potential range — 0.5 to —1.3 volts as compared with water, —0.15 to —1.6 volts (Ag/AgCl), for the concentra-tions studied. Adsorption activity as well as the sharpness of the desorption peak of TPO decrease in the order: water > 50% methanol > methanol and the adsorption region contracts as the solvent is changed from water to methanol. Adsorption isotherms in methanol and 50% methanol follow L a n g m u i r ' s equation with adsorption coefficients equal to 1.58 x 102 1/mole and 1.62 x 1041/mole respectively.

Triphenyl phosphine oxide (TPO) is known to be a strong inhibitor in aqueous solutions 1. How-ever, the behaviour of this substance in non-aqueous solvents has uptil now remained unstudied. The study of this problem is of definite interest as much as non-aqueous solutions are widely used in electro-chemistry, particularly in polarography, where ad-sorption effects play a large role.

The present investigation, therefore, gives the re-sults obtained in the study of adsorption of TPO

Requests for reprints should be sent to Prof. Dr. L. HOL-LECK, Chem. Institut d. Phil.-Theol. Hochschule, D-8600 Bamberg, Am Kranen 12.

in methanolic solutions and the comparision of its surface activity in aqueous solutions.

Experimental

Tensammetric curves were recorded by Polarecord (type E 261) from Metrohm AG, Herisau/Schweiz, along with A. C. Modulator E 393 using 50 Hz and 10 mv (r.m.s.). Water free pure methanol was used as the solvent and the other substances used were either chemically pure or were recrystallised/redistilled be-fore use. The dissolved atmospheric oxygen was re-moved from the solution with a stream of purified nitrogen which was preliminary saturated with vapours of the test solution. Ag/AgCl electrode dipped in

ADSORPTION OF TPO IN METHANOLIC SOLUTIONS 6 3 5

saturated LiCl solution in methanol was used as a reference electrode. The outer jacket of the reference electrode was filled with 1 M LiCl solution in methanol and this solution was changed from time to time to avoid any contamination of the electrode with sur-factant and the change of the alternating current due to change in the resistance of the system. The capillary used for the dropping mercury electrode (dme) gave m = 2.36 mg/s and £ = 3.50s/drop in 0.1m LiCl in methanol (open circuit) at h = 40 cm (uncorrected for bade pressure). The mercury used was first purified chemically and subsequently distilled under reduced pressure.

The tensammetric curves in presence of TPO in various solvents represent the equilibrium curves as determined by the variation of the drop-time.

Results and Discussion

Figs. 1, 2, and 3 give the adsorption of TPO at various concentrations in pure methanol, 50% methanol and pure water respectively. As can be seen, TPO gets adsorbed at the dme from methanol in the potential range —0.5 to —1.3 volts (Ag/ AgCl) although its adsorption activity decreases markedly and the adsorption region contracts con-siderably as compared with that from water and 50% methanol. Only cathodic desorption peak is observed from methanolic and 50% methanolic solu-tions as compared to cathodic and anodic desorption

t JlA

1.0

0.5

0 0 -0.5 -1.0 -1.5 -2.0 V [Ag/AgCl]

Fig. 1. Adsorption of TPO in methanol, 0.1M LiCl; curve 1 — without TPO, curves 2 to 8 - 3.6, 7.2, 10.8, 14.4, 21.6,

36.0, and 57.6 mM respectively.

1.0

1 HA

05

0 0 -0.5 -1.0 -1.5 -2.0

V[Ag/AgCl] Fig. 2. Adsorption of TPO in 50% methanol, 0.1m LiCl; curve 1 — without TPO; curves 2 to 8 — 0.144, 0.216, 0.288,

0.36, 0.72, 1.44, and 2.88 mM respectively.

2.0

flA

1.0

0 0 -0.5 -1.0 -1.5

V[Ag/AgCl]

Fig. 3. Adsorption of TPO in water, 0.1 M LiCl; curve 1 — without TPO; curves 2 to 6 — 0.144, 0.18, 0.36, 0.72, and

1.44 mM respectively.

peaks of TPO in water. The adsorption activity and the sharpness of the cathodic peak of TPO de-crease in the order water > 5 0 % methanol > me-thanol. Further, the potential corresponding to the maximum adsorption of TPO varies with concen-tration of TPO in methanolic and 50% methanolic solutions at lower concentrations.

The fact that on passing from water to methanol the adsorption activity of TPO decreases markedly and its adsorption region contracts considerably

6 3 6 S. L. GUPTA AND L. HOLLECK

can be explained firstly by the effect of the adsorp-tion of solvent molecules and secondly by the ad-sorption of cations of the supporting electrolyte and the increase in the solvation of TPO in methanol which leads to a decrease in the surface activity of TPO in methanol.

Fig. 4. Dependence of £des. on log conc. of TPO; curves 1, 2, 3 — in methanol, 50% methanol and water respectively.

Fig. 4 gives the dependence of the desorption po-tential (£des.) o n concentration of the surfactant in methanol, 50% methanol and water respectively. The relations are linear as expected from F r u m -k i n - D a m a s k i n theory.

HANSEN et al.2 showed that at the potential cor-responding to maximum adsorption, the fraction of the surface covered, 6 is related to the differential double layer capacity per unit area C, by the equa-tion:

0=(C0-C)/(C0-C')

where C0 is the value of C at the same potential in the absence of surface active agent and C' is the value corresponding to a saturated monolayer. The value of C is given by extrapolating the dependence of the reciprocal of the capacitance at the minimum of the C vs. V curve from the reciprocal of the con-centration to l / c = 0.

If the tensammetric currents are taken propor-tional to the differential capacity of the double

1 (a) L . HOLLECK, B . KASTENING , and R . D . WILLIAMS, Z . Elektrochem. 66, 396 [1962] and subsequent papers, (b) L. HOLLECK and D . JANNAKOUDAKIS, Z . Naturforsch. 1 8 8 b , 4 3 9 [ 1 9 6 3 ] . (c) L . HOLLECK and G. HOLLECK , M h . C h e m . 95, 990 [1964]. (d) B. KASTENING, Ber. Bunsenges. phy-

layer, Fig. 5 gives the adsorption isotherms of TPO in methanol and 50% methanol at potentials cor-responding to their maximum adsorption. It can be seen that the adsorption isotherms follow L a n g -m u i r's equation in the concentration ranges 0.011 M to 0.072 M and 0.028 x 10~2 to 0.288 x 1 0 _ 2 m of TPO with adsorption coefficients equal to 1.58 X 102 1/mole and 1.62 x 104 1/mole in methanol and 50% methanol respectively (vide Fig. 6 ) .

0 0.1 0.2 03*10'2

1.0

t Q

0.5

0 , 0 0.2 OA 0.6 0.6x10''

CM

Fig. 5. Adsorption isotherms of TPO; curves 1 and 2 — in methanol and 50% methanol respectively.

0 20 40 60 60X102

% 1.5

0.5 0 10 , 20 30x10

;/c M — Fig. 6. Plot of 1 / 0 vs. 1/c; curves 1 and 2 — in methanol

and 50% methanol respectively.

One of the authors (S. L. G.) is indebted to the Alexander von Humboldt Foundation, BRD, for the award of a senior fellowship and to the B.I.T.S., Pilani (India) for sanctioning leave of absence.

sik. Chem. 688, 979 [1964], (e) B. KASTENING and G. KAZEMIFARD, Ber. Bunsenges. physik. Chem. 74, 551 [1970].

2 R . S . HANSEN. R . E . M I N T U R N , and D . A . HICKSON, J . Phy-sic. Chem. 60, 1185 [1956].