半胱氨酸盐酸盐

Electrochimica Acta 51 (2005) 133–145 The reduction of l-cystine hydrochlo ele illin de, 295 estern W anchest d Ele ity of S April 2 2005 Abstract The kinet ercury stationary m uction is the major mercu current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of −185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7× 10−9 m s−1. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation. © 2005 Else Keywords: l- 1. Introdu There is phy of l-cy acid media SCE with a being diffu chemically potential of RSSR + 2 where R = buffer solu the reductio RSSR(ads)+ ∗ Correspon E-mail ad 0013-4686/$ doi:10.1016/j vier Ltd. All rights reserved. Cysteine hydrochloride; l-Cystine hydrochloride; Mercury-plated copper; Rotating disc; Stationary mercury drop ction a significant body of literature on the polarogra- stine reduction in a variety of electrolytes [1]. In , l-cystine reduction commences at−0.1 V versus small pre-wave before the main wave, the latter sion controlled and both chemically and electro- irreversible. A limiting current plateau starts at a approximately −0.5 V versus SCE due to: H+ + 2e− = 2RSH (1) CH2(NH2·HCl)COOH, the pre-wave in pH 7.4 tion was attributed by Stankovich and Bard [2] to n of adsorbed l-cystine to form l-cysteine: 2H+ + 2e− = 2RSH (2) ding author. Tel.: +44 2380 598752; fax: +44 2380 598754. dress: f.c.walsh@soton.ac.uk (F.C. Walsh). Reaction (2) has been confused with mercury cysteinate formation in earlier studies in acid media, as noted in a review [1]. This situation has arisen due to the similar potentials for the pre-wave and the reversible oxidation of l-cysteine to mercury cysteinate: 2RSH + 2Hg− 2e− = (RS)2Hg2(ads)+ 2H+ (3) 2RSH + 2Hg− 2e− = 2RSHg(ads)+ 2H+ (4) Earlier studies have not, however, examined l-cystine hy- drochloride reduction under conditions typically found in the electrosynthesis of l-cysteine hydrochloride. Electrode kinetic studies have involved reactant concentrations much less than 0.4 mol m−3 to avoid problems with disulphide and thiol adsorption blocking the small electrode surface area available at a mercury drop electrode. This paper examines the reduction under conditions more closely resembling those in the electrosynthesis of l-cysteine hydrochloride and pro- vides detailed kinetic information obtained from a variety of – see front matter © 2005 Elsevier Ltd. All rights reserved. .electacta.2005.04.012 rotating disc mercury T.R. Ralph a,b,d, M.L. Hitchman a, J.P. M a Department of Pure and Applied Chemistry, University of Strathcly b Johnson Matthey Fuel Cells, Lydiard Fields, Great W c School of Chemical Engineering and Analytical Science, University of M ctrochemical Engineering Group, School of Engineering Sciences, Univers Received 15 February 2005; received in revised form 11 Available online 23 May ics of l-cystine hydrochloride reduction have been studied at a m ercury disc electrode (SMDE) in 0.1 mol dm−3 HCl at 298 K. The red side reaction. In contrast to steady state electrode kinetic studies at a ride at stationary and ctrodes gton c, F.C. Walsh d,∗ Cathedral Street, Glasgow G1 1XL, UK ay, Swindon SN5 8AT, UK er, Sackville Street, Manchester M60 1QD, UK outhampton, Highfield, Southampton SO17 1BJ, UK 005; accepted 15 April 2005 -plated copper rotating disc electrode (RDE) and at a of the disulphide is irreversible and hydrogen evolution ry drop electrode (which shows a well-defined limiting 134 T.R. Ralph et al. / Electrochimica Acta 51 (2005) 133–145 complementary techniques. A large, stationary mercury disc electrode (SMDE) has been employed to allow higher reac- tant concentrations to be examined using cyclic voltammetry. This was c rotating dis to be contr steady state in addition 2. Experim 2.1. Mercu Constru RDE cell h it consisted detachable was machin brass sprin the shaft. T larger than flow condi electrode a 99.99 wt% silicon; 1 p tin; <1 ppm A metho per RDE w ished to a m followed b ing cloths. distilled wa ter to remo ing was pe nitrate and viously deo electrode w sity of 50 m foil anode, ingress. Th tilled wate mirror finis BDH Anal constant cu Model 371 tated with a tor was cap 0 and 50 H 2.2. Statio The thre provided a based on a upper end. cal electrode area of 0.80 cm2. A platinum foil was placed around the rim to provide an approximately planar mercury surface. Electrical contact to the mercury was made via a um w ge. Electr ll of th ferenc ams w uggin ed the ce. Na ell. Th ectroly t. The e cath ompar urged nd fac A mu rical a ter ele near a E. Th in cap from onstan DE w rode c electr ce by a er tubi ured a cation t sepa durati rately electr ter ele ectrol r than port a lyte v re that l catho gated rricya ing cu educti e Lev 10−1 e ferr oxide, ombined with studies at a mercury-plated copper c electrode (RDE), which allowed mass transport olled and kinetic parameters to be extracted using voltammetry and controlled potential coulometry to cyclic voltammetry. ental details ry-plated copper electrode ction of the three electrode, three compartment as been considered in detail elsewhere [3]. Briefly, of a brass shaft in an epoxy resin shell with a bottom section containing the copper disc, which ed to give a tight, press fit into a PTFE sheath. A g provided electrical contact between the disc and he sheath diameter (2 cm) was close to 2.5 times that of the disc, to provide well-defined laminar tions [4]. The active electrode had a geometrical rea of 0.50 cm2. The copper (Goodfellow) was with 70 ppm silver; 2 ppm iron, lead, nickel and pm aluminium, bismuth, calcium, magnesium and chromium, manganese and sodium. d due to Daly et al. [5] was used to plate the cop- ith mercury. The copper electrode was wet pol- irror finish, initially on fine silicon carbide paper y 1, 0.1 and 0.05�m alumina, in turn, on polish- After rinsing with absolute alcohol and doubly ter, the electrode was ultrasonicated in pure wa- ve any remaining alumina particles. Electroplat- rformed from aqueous 0.15 mol dm−3 mercuric 0.12 mol dm−3 potassium cyanide (250 cm3) pre- xygenated with nitrogen for an hour. The copper as rotated at 30 Hz and a constant current den- A cm−2 was passed for 5 min using a platinum while nitrogen blanketed the cell to prevent air e mercury surface was rinsed with doubly dis- r then wiped with a paper tissue, resulting in a h. The electroplating solution was prepared from ar grade chemicals and doubly distilled water. A rrent density was supplied by an E G & G PARC potentiostat/galvanostat and the electrode was ro- n Oxford Electrodes assembly. The calibrated ro- able of providing rotation frequencies of between z to an accuracy of ±0.1 Hz. nary mercury disc electrode e electrode, three compartment glass cell, which n upward facing mercury working electrode, was vertical, glass syringe altered to give a cup at its The cup diameter of 1.01 cm gave a geometri- platin syrin 2.3. A the re mogr The L nicat surfa the c of el jacke ify th the c into p disc a line. omet coun Li SMD Lugg illary C per R elect ence surfa rubb ufact 324 ficien long accu after coun El rathe trans catho ensu smal vesti the fe limit ion r of th 6.3× for th hydr [6]. ire protruding into the cup and sealed within the ochemical cells e cells used a saturated calomel electrode (SCE) as e electrode. Steady state, linear and cyclic voltam- ere recorded at the mercury-plated copper RDE. capillary, manufactured from a syringe, commu- SCE (Ingold) to within less than 1 mm of the RDE fion 324 cationic ion-exchange membrane divided e catholyte compartment accommodated 250 cm3 te and was surrounded by a thermostated water large volume ensured the cell walls did not mod- olyte flow pattern at the disc. A Perspex top sealed tment mainly to prevent the dissolution of oxygen catholytes, although it aided centralisation of the ilitated attachment of a thermometer and purging ch larger platinum foil auxiliary electrode of ge- rea 4.7 cm2 ensured that oxygen evolution at the ctrode was not rate controlling. nd cyclic voltammograms were recorded at the e design was similar to the RDE cell. A two-piece illary was used to allow easy removal of the cap- the cell. t potential coulometry at the mercury-plated cop- as performed in a glass cell having a working ompartment of 25 cm3. The SCE (Ingold) refer- ode was connected to within 1 mm from the RDE Luggin capillary formed from a length of silicone ng attached to a glass end section, which was man- s an integral part of the catholyte chamber. Nafion ic ion-exchange membrane divided the cell. Ef- ration was particularly important because of the on of the coulometry experiments and the need to determine the reactant and product concentrations olysis. The geometrical area of the platinum foil ctrode was 1.8 cm2. ysis times were minimised by using an RDE, a stationary electrode, to provide efficient mass nd to maximise the electrode area (0.50 cm2) to olume ratio (0.50 cm2/25 cm3 = 0.02 cm−1). To RDE laminar flow theory [4] was obeyed in the lyte compartment for the range of rotation rates in- measurements were made at a platinum RDE with nide/ferrocyanide ion redox couple. Well-defined rrent plateaux were obtained for ferricyanide on and ferrocyanide ion oxidation. Application ich equation produced diffusion coefficients of 0 m2 s−1 for ferricyanide and 7.1× 10−10 m2 s−1 ocyanide ion in aqueous 1.0 mol dm−3 potassium at 298 K, in close agreement with the literature T.R. Ralph et al. / Electrochimica Acta 51 (2005) 133–145 135 2.4. Chemicals and solutions BDH Analar grade l-cystine and l-cysteine were used as supplied. M or 2.0 mol system was ionic streng 2.5. Voltam reduction The me maintained quired for initial pote SCE in aqu versus SCE Steady mercury-pl tential from to the initi ≤2 mV s−1 ton 2000 X multimeter the potentio Linear s the mercur the electro 25 and 500 a Houston All volt cated other mostated u the cell wit with nitrog blanketed w ingress. A PARC delivering tiostat/galv tial sweep sweep gen maximum ing and ref current den being made 2.6. Coulo the mercur l-Cystin coulometry electrode w and the ele sis, the cha Electrolyte trolysis and a steady stream of the gas was passed over the catholyte during measurements to prevent air ingress. At the end of an experiment, voltammograms were recorded the ca and l rmanc of the . n E G at 100 igital C emp hrom UV d 211 p us mo ltaneo signal h enab ly det ard sa Measu lution llenka d usin d pH esults Mercu ere is d elec mplo he me film, c per am rs obs ing am ed to d mercu mmet s used e elec at a fre for th i-fluo as me d with roppin 0.062 ost experiments were performed in aqueous 0.1 dm−3 hydrochloric acid. The effect of pH on the investigated using aqueous solutions of constant th and pH [7]. mograms of l-cystine hydrochloride rcury-plated copper RDE and the SMDE were at a potential at least 50 mV negative of that re- calomel formation and corrosion of copper. The ntial chosen was approximately −0.2 V versus eous 0.1 mol dm−3 hydrochloric acid and−0.25 V in aqueous 2.0 mol dm−3 hydrochloric acid. state voltammograms were recorded at the ated copper RDE by sweeping the electrode po- the initial to the final potential limit then back al potential (to check for hysteresis) at a rate of . The current output was monitored with a Hous- Y chart recorder or with a Keithly 178 digital connected across the current output terminals of stat. weep and cyclic voltammograms were recorded at y-plated copper RDE and the SMDE by sweeping de potential linearly at sweep rates of between mV s−1, and recording the current response on 2000 XY chart recorder. ammograms were recorded at 298 K, unless indi- wise, and at least in triplicate. The cells were ther- sing a Townson and Mercer water bath linked to h silicone tubing. Electrolytes were deoxygenated en for an hour prior to measurements and the cell ith the gas during measurements to prevent air Model 371 potentiostat/galvanostat (capable of 1 A at 30 V maximum) or a Model 363 poten- anostat (7 A at 20 V maximum) was used. Poten- rates were controlled using a Thompson DRG 16 erator. Current interrupt studies showed that the uncompensated ohmic drop between the work- erence electrodes was approximately 10 mV at a sity of 150 A m−2, the majority of measurements at much lower current densities. metry of l-cystine hydrochloride reduction at y-plated copper RDE e hydrochloride reduction was examined using . Immediately upon contacting the electrolyte the as set at the desired potential for the electrolysis ctrode rotation rate was fixed. During electroly- rge passed and electrolysis time was recorded. s were deoxygenated with nitrogen prior to elec- and ride perfo tails [3,8] A (1 A 179 d HPL uid C array with taneo simu Low whic readi stand 2.7. So a Ga mine Ingol 3. R 3.1. Th plate was e that t cury a cop autho derly form as a volta ment Th ined value to sem ide w accor at a d tion: jL = tholyte was analysed for l-cystine hydrochlo- -cysteine hydrochloride concentrations by high e liquid chromatography (HPLC). Further de- HPLC measurements are available elsewhere & G PARC Model 173 potentiostat/galvanostat V maximum) with an E G & G PARC Model coulometer as a plug in accessory was used. The loyed a Hewlett Packard HP 1090 Series M Liq- atograph system with autoinjector, 1040A diode etector and computer control. The diode array, hotodiodes reading every 10 ms, allowed simul- nitoring of different detection wavelengths and us recording of chromatographs and UV spectra. to noise ratios were possible by signal averaging led low absorbances to be used. Peak purity was ermined by comparing UV spectra with those for mples. rement of electrolyte properties viscosity was determined by standard U-tubes in mp viscometer bath. Solution density was deter- g density bottles. A Corning 150 pH/ion meter and electrode were used to measure solution pH. and discussion ry-plated copper RDE always some concern over the purity of mercury- trode surfaces. Both Daly et al. [5], whose method yed in this study and Yoshida [9] have suggested rcury-plated electrode surface consists of a mer- ontaining not more than 3 ppm copper, on top of algam. After approximately 24 h, however, both erved dissolution of the mercury film in the un- algam. Three experiments were, therefore, per- etermine whether the surface behaved essentially ry film, or an amalgam, on the timescale of the ry (<30 min) and the coulometry (≤8 h) experi- in this study. trochemical reduction of fluorescein was exam- sh mercury-plated copper RDE surface. The E1/2 e reversible, one electron reduction of fluorescein rescein in aqueous 0.1 mol dm−3 sodium hydrox- asured as −1.175 V versus SCE. This is in close an E1/2 value of−1.168 V versus SCE measured g mercury electrode (DME) [5]. The Levich equa- zFD2/3ν−1/6cbω1/2 (5) 136 T.R. Ralph et al. / Electrochimica Acta 51 (2005) 133–145 was used to measure the diffusion coefficient of fluores- cein at the RDE. Here, jL is the limiting current density (A m−2), z the number of electrons transferred to the fluo- rescein molecule (z= 1), D the diffusion coefficient of flu- orescein (m2 s−1), v the kinematic viscosity of the elec- trolyte (1.08× 10−6 m2 s−1 [5]), cb the bulk concentration of fluorescein (5 mol m−3) and ω is the rotation rate of the electrode (rad s−1). A linear plot of jL versus ω1/2 en- abled the diffusion coefficient of fluorescein to be measured as 3.0± 0.2× 10−10 m2 s−1 in close agreement with the value of 3.2× 10−10 m2 s−1 found using a channel electrode [10]. While the fluorescein studies provided confidence in the quality of the mercury-plated surface, a key feature of mer- cury in the studies of l-cystine hydrochloride reduction is a high hydrogen overpotential. Consequently, the ca- thodic potential limit for hydrogen evolution from aque- ous 0.1 mol dm−3 hydrochloric acid was measured. At a fresh mercury-plated copper surface, a current density of 0.010 A m− close to th potential w [11]. It is like optimum ac gen overpo [9] noted th hydrogen Around 15 per base m off during is estimate The l-c evolution a There is str cury [11]. amined at that compa hydrochlor Fig. 1 show Fig. 1. Cyclic aqueous 0.1 m – an SMDE. trolyte deoxy reduction of dissolved l-cystine hydrochloride to l-cysteine hydrochloride: RSSR.2HC + − in aqueous mercury-pl potential a electrodes. In terms the current creased fro versus SCE grams for did not cha timescale o the plated s After a peri concentrati ydrog lectrod Contr ury-pl . Aqu itially bove 1 ere in ls mo hieve r ls, ho encies f adso ous st nce o ury el tive el t disul densiti ffects parate onstan 50% nor th be d due to Deter tine h sing 1 ol dm DE, nt fo react 2 at −1.0 V versus SCE was recorded. This is e value of 0.008 A m−2 measured at the same ith a hanging mercury drop electrode (HMDE) ly that the mercury-plated electrode represents the hievable by this method, in terms of a high hydro- tential for a moderate mercury loading. Yoshida at above 0.1 mg cm−2 of deposited mercury, the overpotential of the electrode is not improved. 0 mg cm−2 of mercury is deposited on the cop- etal in this study; although some of this is wiped the preparation, a value in excess of 0.1 mg cm−2 d to have remained on the surface. ystine/l-cysteine system differs from hydrogen nd fluorescein reduction in one important respect. ong interaction between the amino acids and mer- Consequently, the disulphide reduction was ex- an SMDE of similar area to the RDE, to check rable performance for the reduction of l-cystine ide was observed in the cyclic voltammograms. s the main wave in cyclic voltammograms for the voltammograms of 10 mol m−3 l-cystine hydrochloride in ol dm−3 HCl at – a mercury-plated copper disc electrode and Potential sweep rate: 100 mV s−1. Temperature: 298 K. Elec- genated with N2. the h the e 3.2. merc 3.2.1 In bly a acid w tentia to ac tentia effici fect o Previ evide merc nega A rent tion e to se ing c below ther z could acid 3.3. l-cys U 0.1 m per R evide trode l + 2H + 2e = 2RSH.HCl (6) 0.1 mol dm−3 hydrochloric acid, at both the ated copper RDE and the SMDE. Both the peak nd the peak current are very similar at the two of the longer time-scale of the coulometry studies, density for hydrogen evolution is only slightly in- m 0.010 to 0.013 A m−2 at a potential of −1.0 V . Also, the main wave in the cyclic voltammo- reduction of dissolved l-cystine hydrochloride nge after 8 h. The evidence suggests that, on the f both voltammetry and coulometry experiments, urface is a mercury film rather than an amalgam. od of 8 h, there may be some increase in the copper on of the film but this effect does not greatly alter en overpotential or the electrocatalytic activity of e for disulphide reduction. olled potential coulometry at the ated copper RDE eous 2.0mol dm−3 hydrochloric acid , l-cystine hydrochloride concentrations apprecia- 0 mol m−3 in aqueous 2.0 mol dm−3 hydrochloric vestigated at the mercury-plated copper RDE. Po- re negative than−1.2 V versus SCE were required easonable rates of reduction. At such negative po- wever, rates of reduction were variable and current no better than 80%. This is most probably an ef- rption of the amino acids at the mercury surface. udies [11] at a much smaller area DME showed f the disulphide and thiol adsorption blocking the ectrode surface and moving the reduction to more ectrode potentials. phide concentration of 10 mol m−3 and below, cur- es moved to less negative potentials and