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Influence of quaternary ammonium ions the electrodeposition of cadmium
Influen©e of Quaternary A m m o n i u m Ions an the Ele©tredepesitien ef Cadmium by T.S.N. Sankara Narayanan* Department of Chemistry, Baylor University, Waco, Texas *Current address: National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Channai, India
tudies on the catalysis and inhibition of reactions occurring at the electrode/electrolyte interface during electrochemical oxidation and reduction reactions are of great significance since such a f u n d a m e n t a l u n d e r s t a n d i n g will ultimately lead to the impr ove m ent of the existing processes and/or development of newer processes. From the studies made in our laboratory it was realized t h a t certain organic additives, because of their adsorption at the metal/solution interface, were capable of altering the reaction kinetics of the competing reactions t h a t occur at the interface and accordingly can either catalyze or inhibit the desired electrochemical reaction. The detailed mechanisms by which these additives influence the reactions at the metal/solution interface were discussed elsewhere. 1-5 Among the v ar iety of additives studied, q u a t e r n a r y ammonium salts seem to be an interesting class of compounds as th ey exhibit both the catalyzing and inhibiting influence on the desired electrochemical reaction, such as the electrodeposition of metals, depending upon the experimental conditions used. The q u a t e r n a r y a m m o n i u m salts are very well known for their phase-transfer catalyzing ability. ~'7 They are also proven corrosion inhibitors, s - l l Besides these their utility in the development of sensors and other applications are discussed elsewhere. 12-17 Earlier studies have shown the ability of t e t r a b u t y l a m m o n i u m perchlorate (TBAP) in catalyzing the electrodeposition of metals and alloys;lS'19 however, some exceptions to the catalyzing influence are also observed. The pr es e nt communication is intended to discuss the conditional influence of the ion pairing q u a t e r n a r y a m m o n i u m halides on the type of e f f e c t - - c a t a l y z i n g or i n h i b i t i n g - - d u r i n g the electrodeposition of metals with special reference to cadmium.
S
EXPERIMENTAL DETAILS
Materials U s e d Cadmium(II) was chosen as the metal ion for the present study as it forms a n u m b e r of chloride corn94
plexes, each having a different charge. Sodium chloride solution was used as the supporting electrolyte. The q u a t e r n a r y am m oni um salts used in this study, t h a t is, t e t r a m e t h y l a m m o n i u m chloride (TMAC), t e t r a b u t y l a m m o n i u m chloride (TBAC), tetramethyla m m o n i u m bromide (TPAB), and tetrahexylammonium bromide (THAB), were purchased from Aldrich Chemical Co. and used as received without fu rth e r purification.
Methods of Study Voltammetric m e a s u r e m e n t s were used as the exploratory technique to determine the effect of the q u a t e r n a r y am m oni um compounds on the rate of deposition of cadmium. A platinum wire (0.16 cm 2) was used as the cathode whereas a platinum foil wire (3 cm 2) and a s a t u r a t e d calomel electrode served as the counter and reference electrodes, respectively. The current-voltage curves were obtained using a BAS voltammograph (CV-27) using a 5-mM CdC12, 0.01 M NaC1 solution at a voltage sweep rate of 20 mV/sec. The current efficiency measurements were made based on the amount of charge utilized and weight of the deposit after depositing t hem on copper foil cathodes having an exposed surface area of one cm 2 using a 5-raM CdC12, 0.01 M NaC1 solution at a constant potential of - 1 . 2 V for 15 minutes. RESULTS A N D D I S C U S S I O N
Figure 1 shows the effect of different tetralkylammonium ions at a concentration of I x 10 3 M on the reduction of Cd(II) ions from a chloride solution. It is evident t h a t with the exception of TMAC the other t hree q u a t e r n a r y am m oni um halides decrease the limiting current density of the Cd(II) ion reduction. There is also a considerable am ount of shift in the hydrogen evolution potential with the increase in the chain length of the alkyl group of the tetralkyla m m o n i u m ions. It was thought t h a t the size of the alkyl chain of the q u a t e r n a r y a m m o n i u m halides is a deciding factor in determining the limiting current density of the Cd(II) ion reduction. Accordingly, because of the Metal Finishing
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Figure 1. Effect of various ion pairing quarternary ammonium compounds on the current voltage curves for the reduction of Cd(ll) ions in a 0.01-M sodium chloride solution; voltage sweep rate of 20 mV/sec. (a) 5omM CdCI 2, 0.01 M NaCI; (b) (a) + 1 × 10 -3 M TMAC; (c) (a) + 1 x 10 -3 M TBAC; (d) (a) + 1 x 10 -3 M TPAB; and (e) (a) + 1 × 10 -3 M THAB.
s m a l l e r size of t h e a l k y l chain, only T M A C is catalyzing t h e r e d u c t i o n of Cd(II) ions, a n d t h e r e m a i n ing t h r e e q u a t e r n a r y a m m o n i u m ions fail to exhibit s u c h a n effect. B u t a n e a r l i e r s t u d y h a s s h o w n t h a t t h e a d d i t i o n of i × 10 3 M of t e t r a b u t y l a m m o n i u m p e r c h l o r a t e (TBAP) is c a p a b l e of c a t a l y z i n g t h e elect r o d e p o s i t i o n of tin; is hence, in o r d e r to h a v e a b e t t e r u n d e r s t a n d i n g of this effect, c u r r e n t - v o l t a g e c u r v e s w e r e r e c o r d e d for t h e r e d u c t i o n of Cd(II) ions f r o m a chloride solution in t h e p r e s e n c e of T M A C (Fig. 2) a n d T B A C (Fig. 3) in t h e c o n c e n t r a t i o n r a n g e o f l × 10 -4 M to 1 × 10 2 M. I t is e v i d e n t f r o m F i g u r e s 2 a n d 3 t h a t t h e a d d i t i o n of T B A C did not c a t a l y z e t h e r e d u c t i o n of t h e CD(II) ions in t h e
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Potential, V vs SCE Figure 2. Effect of varying concentration of TMAC on the current voltage curves for the reduction of Cd(ll) ions in a 0.01-M sodium chloride solution; voltage sweep rate of 20 mV/sec. (a) 5-mM CdCI 2, 0.01 M NaCI; (b) (a) + 1 × 10 -4 M TMAC; (c) (a) + 1 x 10 -3 M TMAC; (d) (a) + 1 × 10 -2 M TMAC.
96
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Potential, V vs SCE Figure 3. Effect of varying concentrations of TBAC on the current voltage curves for the reduction of Cd(ll) ions in a 0.01 M sodium chloride solution; voltage sweep rate of 20 mV/sec. (a) 5-mM CdCI 2, 0.01 M NaCI; (b) (a) + 1 x 10 -4 M TBAC; (c) (a) + 1 x 10 -3 M TBAC; (d) (a) + 1 x 10 -2 M TBAC.
c o n c e n t r a t i o n r a n g e s studied, a n d e v e n t h e addition of T M A C w a s found to d e c r e a s e t h e l i m i t i n g c u r r e n t d e n s i t y of the Cd(II) ion r e d u c t i o n a t a c o n c e n t r a t i o n of 1 × 10 2 M. T h e r e s u l t s of t h e c u r r e n t efficiency m e a s u r e m e n t s (Table I) f u r t h e r s u p p o r t t h e observ a t i o n s m a d e f r o m t h e c u r r e n t - v o l t a g e curves; T B A C did not c a t a l y z e t h e electrodeposition of cadm i u m in t h e c o n c e n t r a t i o n r a n g e s studied, a n d t h e a d d i t i o n of TMAC exhibits a c a t a l y z i n g effect only at c o n c e n t r a t i o n s below 1 × 10 3 M; hence, it is a p p a r e n t t h a t it is not only t h e size b u t also the concent r a t i o n of t h e a d d e d q u a t e r n a r y a m m o n i u m ions t h a t decides t h e i r c a t a l y z i n g effect. A l t h o u g h t h e c o n c e n t r a t i o n d e p e n d e n c e on t h e c a t a l y z i n g effect w a s realized earlier, in the case of t h e addition of Table I. Effect of Quaternary Ammonium Ions on the Current Efficiency of Cadmium Deposition from a 5-mM CdCIJ0.01 M NaCI Solution at a Constant Applied Potential of -1.2 V for 15 min. (Coppper cathodes: 1 cm 2)
2 -0.4.
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System Studied Current 5-mM CdC12/0.01 M NaC1 5-mM CdC12/0.01 M NaC1 1 × 10 2 M T M A C 5-mM CdC12/0.01 M NaC1 1 × 10-3MTMAC 5-mM CdC12/0.01 M NaC1 1 × 10 2 M T M A C 5-mM CdCle/0.01 M NaC1 1 × 10 4 M T M A C 5-mM CdC1J0.01 M NaC1 1 × 10 ~ M TMAC 5-raM CdC12/0.01 M NaC1 1 × 10 - 2 M T M A C "Average of five determinations TMAC-Tetramethylammonium chloride TBAC-Tetrabutylammonium chloride
Efficiency, %" 26.0 42.4 33.0 20.8 23.2 18.6 10.4
Metal Finishing
TBAP during the electrodeposition of tin the reason for such a behavior was attributed due to the grouping of the q u a t e r n a r y a m m o n i u m ions with the tin ions. is It is interesting to recall the results on the effect of varying concentrations of t e t r a b u t y l a m m o n i u m ion on the limiting current density of the reduction of tin ions in the concentration range of i × 10 -9 M to 1 × 10 2 M; the limiting current density increases with increase in concentration of TBAP up to 1 × 10 -3 M and decreases thereafter; however, a similar addition of the t e t r a b u t y l a m m o n i u m ion does not seem to cause any catalyzing influence on the electrodeposition of cadmium even at a concentration of 1 × 10 -4 M. The added q u a t e r n a r y ammonium ions can exert an influence on the reduction of metal ions and other electroactive species in two possible ways: (1) by forming an ion pair with the negatively charged electroactive species, catalyzing their reduction rate; and (2) by blocking the surface of the electrode, inhibiting the reduction rate of metal ions. The increased rate of deposition of zinc, tin, and cadmium when they are present in the electrolyte as zincate, stannate, stannite, and cadmium cyanide complexes are proven examples of the effect of the first type whereas the corrosion-inhibiting effect of the q u a t e r n a r y ammonium ions reported elsewhere s-l~ and the noncatalyzing or inhibiting effect observed in the case of the electrodeposition of metals from positively charged complexes, such as the electrodeposition of copper from [Cu(ED)2] 2+ complex, etc., are examples of the latter type; hence, it is evident t h a t besides the concentration of quaternary a m m o n i u m ions and their size of the alkyl chain, the type of electroactive species present in the electrolyte and the concentration of the negatively charged electroactive species should also be considered when studying the influence of the q u a t e r n a r y ammonium ions on the electrodeposition of metals. In the electrolyte used for the present study the added cadmium chloride is expected to form at least three different chloro complexes, 2°'21 namely CdC1 +, CdC12, and CdC13-, of which only the latter one is capable of forming an ion pair with the added quat e r n a r y a m m o n i u m ions. As the concentration of CdC12 in the electrolyte used is 5 mM, the concentration of CdC13 will be very low. Since it is believed t h a t the added q u a t e r n a r y ammonium ions form a 1:1 ion pair complex with the negatively charged CdC13 , the excess concentration of the quat e r n a r y a m m o n i u m ions will tend to compete with the other positive and neutrally charged predomin a n t electroactive species, namely CdC1 ÷ and CdC12 for adsorption of the cathode. If this is the case then 98
the adsorbed quaternary ammonium ions will inhibit the reactions occurring at the surface of the electrode, the reduction of the Cd(II) ions, and water or hydronium ions leading to the respective reaction of the deposition of cadmium and the hydrogen evolution. Moreover, such an adsorbed quaternary ammonium ion, depending upon the bulkiness of its alkyl group, is also capable of affecting the transport of the electroactive species approaching towards the electrode surface. CONCLUSIONS
The effect of four different quaternary ammonium ions, which differ mainly with respect to their alkyl chain length, on the electrodeposition of cadmium was studied. When added at a concentration of i × 10 3 M, with the exception of TMAC, the other three quaternary ammonium ions exhibit an inhibiting effect. When studied for varying concentrations in the range of I × 10 -4 M to 1 × 10 2 M, TBAC fails to show any catalyzing influence for the entire concentration range studied and even TMAC showed an inhibiting effect at concentrations above I × 10 3 M. The concentration of the negatively charged electroactive species present in the system is a deciding factor in determining whether the added quaternary ammonium ion will exert a catalyzing or inhibiting effect on the electrodeposition of t h a t metal. The bulkiness of the alkyl chain of the quaternary ammonium ions can affect the transport of the electroactive species approaching the electrode surface. From the study it is valid to conclude t h a t the addition of quaternary ammonium ions can catalyze the electrodeposition of metals provided (1) the size of the alkyl chain is relatively small; and (2) their concentration is in excess of what is required for forming a 1:1 ion pair complex with the negatively charged electroactive species present in the system. ACKNOWLEDGEMENT
Financial assistance from the Robert A. Welch Foundation of Houston is gratefully acknowledged. REFERENCES
1. Franklin, T.C., Surface and Coatings Technology, 30(4):415; 1987 2. Franklin, T.C., Plating and Surface Finsihing, 81(4): 62; 1994 3. Franklin, T.C. and T.S.N. Sankara Narayanan, Journal of the Electrochemical Society, 143(10):2759; 1996 4. Franklin, T.C. et al., Journal of the Electrochemical Society, 144(9):3064; 1997 5. Franklin, T.C. et al., Journal of the Electrochemical Society, 145(3):801; 1998 Metal Finishing
6. Starks, C.M. and C. Liotta, "Phase Transfer Catalysis: Principles and Techniques," Academic Press, N e w York; 1978 7. Dehmlow, E.V. and S.S. Dehmlow, "Phase Transfer Catalysis," 2nd Ed., Verlage Chemic, Weinheim, Germany; 1983 8. Schweinberg, D.P. and V. Ashworth, Corrosion Science, 28:539:1988 9. Frignani, A. et al., Corrosion Science, 28:539; 1988 10. Zvauya, R. and J.L. Dawson, Journal of Applied Electrochemistry, 24:943; 1994 11. Vasudevan, T. et al., Corrosion Science, 37:1235; 1995 12. Wotring, V.J. et al., Analytical Chemistry, 62:1506; 1990 13. Katsu, Takashi, Analytical Chemistry, 65:176; 1993 14. Ma, S-C. et al., Analytical Chemistry, 65:2078; 1993 15. Afifi, S. et al., Journal of the Electrochemical Society, 134:2169; 1987 16. Franklin, T.C. and T. Jimbo, Journal of the Electrochemical Society, 134:2169; 1987 17. Tanaka, K. et al., Journal of Chemical Society, Perkin Transactions, 2:1571; 1995 18. Franklin, T.C. et al., Journal of the Electrochemical Society, 133:893; 1986 19. Franklin, T.C. et al., Journal of the Electrochemical Society, 135:1638; 1988 20. Sillen, L.G. and A.E. Martell, "Stability Constants for Metal-ion Complexes," 2nd Ed., The Chemical Society, London; 1964 21. Sillen, L.G. and A.E. Martell, "Stability Constants for Metal-ion Complexes," Supplement No. 1, Part II, The Chemical Society, London; 1971 MF
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June 1999
99
tudies on the catalysis and inhibition of reactions occurring at the electrode/electrolyte interface during electrochemical oxidation and reduction reactions are of great significance since such a f u n d a m e n t a l u n d e r s t a n d i n g will ultimately lead to the impr ove m ent of the existing processes and/or development of newer processes. From the studies made in our laboratory it was realized t h a t certain organic additives, because of their adsorption at the metal/solution interface, were capable of altering the reaction kinetics of the competing reactions t h a t occur at the interface and accordingly can either catalyze or inhibit the desired electrochemical reaction. The detailed mechanisms by which these additives influence the reactions at the metal/solution interface were discussed elsewhere. 1-5 Among the v ar iety of additives studied, q u a t e r n a r y ammonium salts seem to be an interesting class of compounds as th ey exhibit both the catalyzing and inhibiting influence on the desired electrochemical reaction, such as the electrodeposition of metals, depending upon the experimental conditions used. The q u a t e r n a r y a m m o n i u m salts are very well known for their phase-transfer catalyzing ability. ~'7 They are also proven corrosion inhibitors, s - l l Besides these their utility in the development of sensors and other applications are discussed elsewhere. 12-17 Earlier studies have shown the ability of t e t r a b u t y l a m m o n i u m perchlorate (TBAP) in catalyzing the electrodeposition of metals and alloys;lS'19 however, some exceptions to the catalyzing influence are also observed. The pr es e nt communication is intended to discuss the conditional influence of the ion pairing q u a t e r n a r y a m m o n i u m halides on the type of e f f e c t - - c a t a l y z i n g or i n h i b i t i n g - - d u r i n g the electrodeposition of metals with special reference to cadmium.
S
EXPERIMENTAL DETAILS
Materials U s e d Cadmium(II) was chosen as the metal ion for the present study as it forms a n u m b e r of chloride corn94
plexes, each having a different charge. Sodium chloride solution was used as the supporting electrolyte. The q u a t e r n a r y am m oni um salts used in this study, t h a t is, t e t r a m e t h y l a m m o n i u m chloride (TMAC), t e t r a b u t y l a m m o n i u m chloride (TBAC), tetramethyla m m o n i u m bromide (TPAB), and tetrahexylammonium bromide (THAB), were purchased from Aldrich Chemical Co. and used as received without fu rth e r purification.
Methods of Study Voltammetric m e a s u r e m e n t s were used as the exploratory technique to determine the effect of the q u a t e r n a r y am m oni um compounds on the rate of deposition of cadmium. A platinum wire (0.16 cm 2) was used as the cathode whereas a platinum foil wire (3 cm 2) and a s a t u r a t e d calomel electrode served as the counter and reference electrodes, respectively. The current-voltage curves were obtained using a BAS voltammograph (CV-27) using a 5-mM CdC12, 0.01 M NaC1 solution at a voltage sweep rate of 20 mV/sec. The current efficiency measurements were made based on the amount of charge utilized and weight of the deposit after depositing t hem on copper foil cathodes having an exposed surface area of one cm 2 using a 5-raM CdC12, 0.01 M NaC1 solution at a constant potential of - 1 . 2 V for 15 minutes. RESULTS A N D D I S C U S S I O N
Figure 1 shows the effect of different tetralkylammonium ions at a concentration of I x 10 3 M on the reduction of Cd(II) ions from a chloride solution. It is evident t h a t with the exception of TMAC the other t hree q u a t e r n a r y am m oni um halides decrease the limiting current density of the Cd(II) ion reduction. There is also a considerable am ount of shift in the hydrogen evolution potential with the increase in the chain length of the alkyl group of the tetralkyla m m o n i u m ions. It was thought t h a t the size of the alkyl chain of the q u a t e r n a r y a m m o n i u m halides is a deciding factor in determining the limiting current density of the Cd(II) ion reduction. Accordingly, because of the Metal Finishing
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Figure 1. Effect of various ion pairing quarternary ammonium compounds on the current voltage curves for the reduction of Cd(ll) ions in a 0.01-M sodium chloride solution; voltage sweep rate of 20 mV/sec. (a) 5omM CdCI 2, 0.01 M NaCI; (b) (a) + 1 × 10 -3 M TMAC; (c) (a) + 1 x 10 -3 M TBAC; (d) (a) + 1 x 10 -3 M TPAB; and (e) (a) + 1 × 10 -3 M THAB.
s m a l l e r size of t h e a l k y l chain, only T M A C is catalyzing t h e r e d u c t i o n of Cd(II) ions, a n d t h e r e m a i n ing t h r e e q u a t e r n a r y a m m o n i u m ions fail to exhibit s u c h a n effect. B u t a n e a r l i e r s t u d y h a s s h o w n t h a t t h e a d d i t i o n of i × 10 3 M of t e t r a b u t y l a m m o n i u m p e r c h l o r a t e (TBAP) is c a p a b l e of c a t a l y z i n g t h e elect r o d e p o s i t i o n of tin; is hence, in o r d e r to h a v e a b e t t e r u n d e r s t a n d i n g of this effect, c u r r e n t - v o l t a g e c u r v e s w e r e r e c o r d e d for t h e r e d u c t i o n of Cd(II) ions f r o m a chloride solution in t h e p r e s e n c e of T M A C (Fig. 2) a n d T B A C (Fig. 3) in t h e c o n c e n t r a t i o n r a n g e o f l × 10 -4 M to 1 × 10 2 M. I t is e v i d e n t f r o m F i g u r e s 2 a n d 3 t h a t t h e a d d i t i o n of T B A C did not c a t a l y z e t h e r e d u c t i o n of t h e CD(II) ions in t h e
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Potential, V vs SCE Figure 2. Effect of varying concentration of TMAC on the current voltage curves for the reduction of Cd(ll) ions in a 0.01-M sodium chloride solution; voltage sweep rate of 20 mV/sec. (a) 5-mM CdCI 2, 0.01 M NaCI; (b) (a) + 1 × 10 -4 M TMAC; (c) (a) + 1 x 10 -3 M TMAC; (d) (a) + 1 × 10 -2 M TMAC.
96
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Potential, V vs SCE Figure 3. Effect of varying concentrations of TBAC on the current voltage curves for the reduction of Cd(ll) ions in a 0.01 M sodium chloride solution; voltage sweep rate of 20 mV/sec. (a) 5-mM CdCI 2, 0.01 M NaCI; (b) (a) + 1 x 10 -4 M TBAC; (c) (a) + 1 x 10 -3 M TBAC; (d) (a) + 1 x 10 -2 M TBAC.
c o n c e n t r a t i o n r a n g e s studied, a n d e v e n t h e addition of T M A C w a s found to d e c r e a s e t h e l i m i t i n g c u r r e n t d e n s i t y of the Cd(II) ion r e d u c t i o n a t a c o n c e n t r a t i o n of 1 × 10 2 M. T h e r e s u l t s of t h e c u r r e n t efficiency m e a s u r e m e n t s (Table I) f u r t h e r s u p p o r t t h e observ a t i o n s m a d e f r o m t h e c u r r e n t - v o l t a g e curves; T B A C did not c a t a l y z e t h e electrodeposition of cadm i u m in t h e c o n c e n t r a t i o n r a n g e s studied, a n d t h e a d d i t i o n of TMAC exhibits a c a t a l y z i n g effect only at c o n c e n t r a t i o n s below 1 × 10 3 M; hence, it is a p p a r e n t t h a t it is not only t h e size b u t also the concent r a t i o n of t h e a d d e d q u a t e r n a r y a m m o n i u m ions t h a t decides t h e i r c a t a l y z i n g effect. A l t h o u g h t h e c o n c e n t r a t i o n d e p e n d e n c e on t h e c a t a l y z i n g effect w a s realized earlier, in the case of t h e addition of Table I. Effect of Quaternary Ammonium Ions on the Current Efficiency of Cadmium Deposition from a 5-mM CdCIJ0.01 M NaCI Solution at a Constant Applied Potential of -1.2 V for 15 min. (Coppper cathodes: 1 cm 2)
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12.0
2.4.
System Studied Current 5-mM CdC12/0.01 M NaC1 5-mM CdC12/0.01 M NaC1 1 × 10 2 M T M A C 5-mM CdC12/0.01 M NaC1 1 × 10-3MTMAC 5-mM CdC12/0.01 M NaC1 1 × 10 2 M T M A C 5-mM CdCle/0.01 M NaC1 1 × 10 4 M T M A C 5-mM CdC1J0.01 M NaC1 1 × 10 ~ M TMAC 5-raM CdC12/0.01 M NaC1 1 × 10 - 2 M T M A C "Average of five determinations TMAC-Tetramethylammonium chloride TBAC-Tetrabutylammonium chloride
Efficiency, %" 26.0 42.4 33.0 20.8 23.2 18.6 10.4
Metal Finishing
TBAP during the electrodeposition of tin the reason for such a behavior was attributed due to the grouping of the q u a t e r n a r y a m m o n i u m ions with the tin ions. is It is interesting to recall the results on the effect of varying concentrations of t e t r a b u t y l a m m o n i u m ion on the limiting current density of the reduction of tin ions in the concentration range of i × 10 -9 M to 1 × 10 2 M; the limiting current density increases with increase in concentration of TBAP up to 1 × 10 -3 M and decreases thereafter; however, a similar addition of the t e t r a b u t y l a m m o n i u m ion does not seem to cause any catalyzing influence on the electrodeposition of cadmium even at a concentration of 1 × 10 -4 M. The added q u a t e r n a r y ammonium ions can exert an influence on the reduction of metal ions and other electroactive species in two possible ways: (1) by forming an ion pair with the negatively charged electroactive species, catalyzing their reduction rate; and (2) by blocking the surface of the electrode, inhibiting the reduction rate of metal ions. The increased rate of deposition of zinc, tin, and cadmium when they are present in the electrolyte as zincate, stannate, stannite, and cadmium cyanide complexes are proven examples of the effect of the first type whereas the corrosion-inhibiting effect of the q u a t e r n a r y ammonium ions reported elsewhere s-l~ and the noncatalyzing or inhibiting effect observed in the case of the electrodeposition of metals from positively charged complexes, such as the electrodeposition of copper from [Cu(ED)2] 2+ complex, etc., are examples of the latter type; hence, it is evident t h a t besides the concentration of quaternary a m m o n i u m ions and their size of the alkyl chain, the type of electroactive species present in the electrolyte and the concentration of the negatively charged electroactive species should also be considered when studying the influence of the q u a t e r n a r y ammonium ions on the electrodeposition of metals. In the electrolyte used for the present study the added cadmium chloride is expected to form at least three different chloro complexes, 2°'21 namely CdC1 +, CdC12, and CdC13-, of which only the latter one is capable of forming an ion pair with the added quat e r n a r y a m m o n i u m ions. As the concentration of CdC12 in the electrolyte used is 5 mM, the concentration of CdC13 will be very low. Since it is believed t h a t the added q u a t e r n a r y ammonium ions form a 1:1 ion pair complex with the negatively charged CdC13 , the excess concentration of the quat e r n a r y a m m o n i u m ions will tend to compete with the other positive and neutrally charged predomin a n t electroactive species, namely CdC1 ÷ and CdC12 for adsorption of the cathode. If this is the case then 98
the adsorbed quaternary ammonium ions will inhibit the reactions occurring at the surface of the electrode, the reduction of the Cd(II) ions, and water or hydronium ions leading to the respective reaction of the deposition of cadmium and the hydrogen evolution. Moreover, such an adsorbed quaternary ammonium ion, depending upon the bulkiness of its alkyl group, is also capable of affecting the transport of the electroactive species approaching towards the electrode surface. CONCLUSIONS
The effect of four different quaternary ammonium ions, which differ mainly with respect to their alkyl chain length, on the electrodeposition of cadmium was studied. When added at a concentration of i × 10 3 M, with the exception of TMAC, the other three quaternary ammonium ions exhibit an inhibiting effect. When studied for varying concentrations in the range of I × 10 -4 M to 1 × 10 2 M, TBAC fails to show any catalyzing influence for the entire concentration range studied and even TMAC showed an inhibiting effect at concentrations above I × 10 3 M. The concentration of the negatively charged electroactive species present in the system is a deciding factor in determining whether the added quaternary ammonium ion will exert a catalyzing or inhibiting effect on the electrodeposition of t h a t metal. The bulkiness of the alkyl chain of the quaternary ammonium ions can affect the transport of the electroactive species approaching the electrode surface. From the study it is valid to conclude t h a t the addition of quaternary ammonium ions can catalyze the electrodeposition of metals provided (1) the size of the alkyl chain is relatively small; and (2) their concentration is in excess of what is required for forming a 1:1 ion pair complex with the negatively charged electroactive species present in the system. ACKNOWLEDGEMENT
Financial assistance from the Robert A. Welch Foundation of Houston is gratefully acknowledged. REFERENCES
1. Franklin, T.C., Surface and Coatings Technology, 30(4):415; 1987 2. Franklin, T.C., Plating and Surface Finsihing, 81(4): 62; 1994 3. Franklin, T.C. and T.S.N. Sankara Narayanan, Journal of the Electrochemical Society, 143(10):2759; 1996 4. Franklin, T.C. et al., Journal of the Electrochemical Society, 144(9):3064; 1997 5. Franklin, T.C. et al., Journal of the Electrochemical Society, 145(3):801; 1998 Metal Finishing
6. Starks, C.M. and C. Liotta, "Phase Transfer Catalysis: Principles and Techniques," Academic Press, N e w York; 1978 7. Dehmlow, E.V. and S.S. Dehmlow, "Phase Transfer Catalysis," 2nd Ed., Verlage Chemic, Weinheim, Germany; 1983 8. Schweinberg, D.P. and V. Ashworth, Corrosion Science, 28:539:1988 9. Frignani, A. et al., Corrosion Science, 28:539; 1988 10. Zvauya, R. and J.L. Dawson, Journal of Applied Electrochemistry, 24:943; 1994 11. Vasudevan, T. et al., Corrosion Science, 37:1235; 1995 12. Wotring, V.J. et al., Analytical Chemistry, 62:1506; 1990 13. Katsu, Takashi, Analytical Chemistry, 65:176; 1993 14. Ma, S-C. et al., Analytical Chemistry, 65:2078; 1993 15. Afifi, S. et al., Journal of the Electrochemical Society, 134:2169; 1987 16. Franklin, T.C. and T. Jimbo, Journal of the Electrochemical Society, 134:2169; 1987 17. Tanaka, K. et al., Journal of Chemical Society, Perkin Transactions, 2:1571; 1995 18. Franklin, T.C. et al., Journal of the Electrochemical Society, 133:893; 1986 19. Franklin, T.C. et al., Journal of the Electrochemical Society, 135:1638; 1988 20. Sillen, L.G. and A.E. Martell, "Stability Constants for Metal-ion Complexes," 2nd Ed., The Chemical Society, London; 1964 21. Sillen, L.G. and A.E. Martell, "Stability Constants for Metal-ion Complexes," Supplement No. 1, Part II, The Chemical Society, London; 1971 MF
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