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Bare boron-doped diamond electrode (BDDE) was applied for investigation of deposition/stripping processes of silver ions in various supporting electrolytes. The highest signal was observed in 0.1 mol dm−3 HNO3. Detection of silver ions is also possible in 0.1 mol dm−3 Na2S2O3 where Ag+ is strongly complexed as [Ag(S2O3)2]3−. Though the sensitivity is 100 times lower, the potential of anodic dissolution of silver is significantly shifted towards the negative values. This shift might be useful for solving of some interferences which may occur in the detection process. After preliminary cyclic voltammetry, the analytical performance was studied by differential pulse anodic stripping voltammetry (DPASV) and square-wave anodic stripping voltammetry. Deposition potential of − 0.18 V vs. Ag/AgCl and deposition time of 240 s were selected as optimum DPASV parameters. The lowest detection limit of 2.0 × 10–10 mol dm−3 was achieved with DPASV in HNO3. Negligible effect of possible interferents on Ag response proved to be a good selectivity of method. The proposed method after validation was also applied to real samples analysis of some commercial products with complex matrix. The obtained results are statistically identical with data declared by manufacturer and gained by independent technique, making this method suitable for commercial product control.

This short review deals with the properties and significance of the determination of selenium, which is in trace amounts an essential element for animals and humans, but toxic at high concentrations. It may cause oxidative stress in cells, which leads to the chronic disease called selenosis. Several analytical techniques have been developed for its detection, but electroanalytical methods are advantageous due to simple sample preparation, speed of analysis and high sensitivity of measurements, especially in the case of stripping voltammetry very low detection limits even in picomoles per liter can be reached. A variety of working electrodes based on mercury, carbon, silver, platinum and gold materials were applied to the analysis of selenium in various samples. Only selenium in oxidation state + IV is electroactive therefore the most of voltammetric determinations are devoted to it. However, it is possible to detect also other forms of selenium by indirect electrochemistry approach.

Voltammetric behavior of omeprazole on chemically unmodified boron-doped diamond electrode was investigated. Omeprazole is possible to oxidize and reduce in various supporting electrolytes. Both processes are strongly dependent on pH value of the solution. Well defined oxidation and reduction signal of omeprazole was observed at high and low pH values of supporting electrolyte. Kinetics of charge transfer in the pH range from 3 to 10 in various supporting electrolytes was studied and characterized by the estimation of formal potentials (+1.07 to +0.89 V for oxidation and −0.95 to −1.18 V for omeprazole reduction), charge transfer coefficients (0.52–0.33 for oxidation and 0.19–0.17 for reduction) together with standard heterogeneous rate constants. The analytical performance was studied with linear sweep voltammetry, square-wave voltammetry and differential pulse voltammetry. The lowest detection limit of 9.1 × 10 −8 mol L −1 for oxidation of omeprazole was achieved by SWV technique. The proposed simple, sensitive and cheap method was validated and applied for pharmaceutical formulations and human urine as real samples.

Very simple and sensitive electroanalytical technique based on synergistic combination of reaction electrochemistry (specificity) and bare boron-doped diamond electrode (sensitivity) for the detection of fluorides in drinking water was developed as variant based on dynamic electrochemistry to ISE analysis. It is based on the formation of electroinactive fluoride complexes with Fe(III) and Ce(IV) ions decreasing such diffusion current of free metal on boron-doped diamond electrode. Due to low background signal of boron-doped diamond electrode reasonably low detection limits of the order of 10 −6 mol L −1 for linear sweep voltammetric method using formation of [FeF 6 ] 3− and 10 −7 mol L −1 in a square-wave variant of this technique have been achieved. This is approximately 1–2 orders lower than in the case of platinum comb-shaped interdigitated microelectrode array. Linear sweep voltammetric method based on [CeF 6 ] 2− complex formation has lower sensitivity and may be suitable for samples with higher content of fluoride and not to analysis of drinking water.

Very simple and fast electroanalytical method for the detection Sb(III) on chemically unmodified boron-doped diamond electrode (BDDE) has been developed. Voltammetric behavior of antimony was investigated in various acidic supporting electrolytes and the most suitable medium for the determination of Sb(III) on bare BDDE has been 6 mol L−1 HClO4 solution. The analytical performance was studied with differential pulse anodic stripping voltammetry (DPASV) with optimized conditions (deposition potential −1 V vs. Ag/ AgCl and deposition time 240 s). An analysis of possible effects due to the presence of other metal ions (especially As(III)) in the solution was eliminated using NaH2PO4 as supporting electrolyte with addition EDTA as selective complexing agent for Sb(III). Speciation of antimony was also investigated. The detection limit of this analytical strategy achieved value of 1.08 × 10−7 mol L−1. The proposed method was validated and applied for natural water from former antimony mines as real samples.

A simple and sensitive square-wave voltammetric method for the determination of adrenaline on unmodified boron-doped diamond film electrode was developed. Adrenaline exhibited the quasi-reversible behavior with oxidation peak on the forward scan at + 0.75 V and smaller reduction peak on the reverse scan at − 0.10 V vs. Ag/AgCl electrode in 0.5 M HClO4. The effect of supporting electrolyte, pH and scan rate on the current response of adrenaline was examined to select the optimum experimental conditions. At optimized square-wave voltammetric parameters (amplitude of 100 mV, frequency of 50 Hz and step potential of 5 mV), the linear concentration range from 0.7 to 60 μM (R2 = 0.998, number of measurements n = 6), the excellent repeatability (relative standard deviation of 3.5% for n = 50) and the detection limit of 0.21 μM were achieved without any chemical modification and pretreatment of working electrode. The practical application of method was demonstrated in the determination of adrenaline in spiked human urine samples with satisfactory recoveries (98 to 102%).

An electrochemical behavior of methamphetamine on a self-assembled boron-doped diamond electrode was studied by cyclic voltammetry. Methamphetamine yielded a single irreversible oxidation peak at a higher positive potential of +1.23 V (vs. Ag/AgCl/3 M KCl electrode) in Britton-Robinson buffer solution at pH 10. The nature of the electrode reaction was found to be diffusion controlled with minor contribution of adsorption. The differential pulse voltammetric methodology was proposed for the sensitive determination of methamphetamine in a linear concentration range from 0.07 to 80 μM with a detection limit of 0.05 μM (7.46 μg L−1) and a good repeatability (relative standard deviation of 2.8% at 20 μM concentration level, n = 6). The effect of interfering agents (common urinary compounds) was appeared to be negligible confirming a favorable selectivity of method. The practical usefulness of method was demonstrated in the assessment of methamphetamine content in model human urine samples with good accuracy (recoveries varied from 93.4% to 97.6%). The proposed procedure could represent a suitable alternative to other analytical methods for the determination of low concentration levels of methamphetamine in biological samples. It may enable methamphetamine excreted in urine of drug addicted people to be assayed sensitively and selectively.

An unmodified boron-doped diamond film electrode was used for the first time as a sensitive and selective electrochemical sensor for the determination of codeine by the use of differential pulse voltammetry. Codeine provided a single well-defined oxidation peak at +1.0 V vs . Ag/AgCl in Britton-Robinson buffer solution at pH 7.0. Using the optimal differential pulse voltammetric conditions (modulation amplitude of 50 mV, modulation time of 40 ms and scan rate of 50 mV s −1 ), the detection limit of 0.08 μM, the linear response of peak current on codeine concentration in the range from 0.1 to 60 μM ( R 2 = 0.998, n = 6) and relative standard deviation of 0.9% at 10 μM concentration level ( n = 10) were achieved without any electrode surface modification. The influence of potential interfering agents on the current response was also studied and the results indicated that the proposed method was sufficiently selective. The method was successfully applied in the determination of codeine in real samples including pharmaceutical tablets and human urine with results similar to those declared by manufacturer and obtained by reference high-performance liquid chromatography method, respectively. The typical benefits of the method may be summarized as: rapidity (20 determinations per hour), sensitivity and selectivity, low cost and elaborateness, simplicity, wide linear concentration range, low detection limit and excellent repeatability. It might also represent the competitive alternative to the existing analytical methods in monitoring of drug abuse.

A simple, sensitive and selective square-wave voltammetry method for simultaneous determination of paracetamol and penicillin V on a bare (unmodified) boron-doped diamond electrode has been developed. The good potential separation of about 0.35 V between the oxidation peak potentials of both drugs present in mixture was found. It was found by cyclic voltammetry that paracetamol gave quasireversible wave and penicillin V provided irreversible oxidation peak. The effect of supporting electrolyte, pH and scan rate on voltammetric response of both drugs was studied to select the optimum experimental conditions. The optimal conditions for quantitative simultaneous determination were obtained in acetate buffer solution at pH 5.0. The oxidation peak of paracetamol and penicillin V showed a systematic increase in peak currents with increase of their concentration. The calibration curves for the simultaneous determination of paracetamol and penicillin V exhibited the good linear responses within the concentration range from 0.4 to 100 μM for both drugs. The detection limit was established to 0.21 and 0.32 μM for paracetamol and penicillin V, respectively. The method proved the good sensitivity, repeatability (RSD of 1.5 and 2.1% for mixture solution of 10 μM PCM and PEN) and selectivity when influence of interferents commonly existing in human urine was negligible. The practical analytical utility of proposed method was demonstrated by simultaneous determination of paracetamol and penicillin V in human urine samples, with results similar to those obtained using a high-performance liquid chromatography method as an independent method.

A novel technique based on dynamic electrochemistry for the detection of fluoride ions was developed. It is based on its strong complexation with ferric ion. Formed fluoroferric complex is cathodically inactive at the potential of the reduction of free ferric aquo ion. The voltammetric and amperometric response of platinum comb-shaped interdigitated microelectrode array is decreased after fluoride addition. This decrease serves for the quantification of fluoride ions added to the solution. The detection limit of 4.5 × 10−5 mol dm−3 was achieved when one of the segments of interdigitated microelectrode array (IDA) was used as an indicating electrode. The detection limit is about one order of magnitude lower than in the case of conventional platinum macroelectrode. In comparison with ISE electrodes this method is faster and also avoiding large error resulting from the antilogarithmization of ISE Nerstian response. The method was applied to the analysis of toothpaste.

The paper describes electrochemical properties of NO(en)(H2O)Cr(CN)2 and the polarographic investigation of its acid hydrolysis. Stoichiometry (1:1) of NO(en)(H2O)Cr(CN)2 reactions with Hg2+ and Ag+ was determined using potentiometric method. Adducts formed by these reactions were investigated polarographically and linkage isomerisation of bridging cyano ligands has been found to be involved in their formation. Decomposition kinetics of the adduct NO(en)(H2O)Cr(NC)2Hg2+ was studied, factors affecting it discussed and found kinetics data compared with those determined for similar cyano complexes.

The reaction of trans -Cr(NH 3 ) 4 (NCS)(CN) + with Hg 2+ develops in three stages: (1) immediate CrNCSHg bonding; (2) formation of CrNCHg bridges at a measurable rate, occurring with cyano-isocyano rearrangement about Cr(III); (3) slower Hg(II)-promoted aquation of cyanide. The processes were studied by absorption and emission spectrophotometry, as well as by polarography and potentiometry. The identified species participating in this sequence are: (1) [ trans -(NC)Cr(NH 3 ) 4 (NCS)] 2 Hg 4+ and trans -Cr(NH 3 ) 4 (CN)(NCSHg) 3+ ; (2) [ trans -(SCN)Cr(NH 3 ) 4 (NC)] 2 Hg 4+ ; (3) trans -Cr(NH 3 ) 4 (H 2 O)(NCS) 2+ . The quotients for the consecutive equilibria of step (1), involving soft-acid/soft-base HgS interactions, are estimated as K 1 = (4±1) × 10 6 M −2 and K 2 = 50±40 at 25°C and μ = 0.10 M (NaClO 4 ). Under the same conditions, the pseudo-first-order rate constant for NC→NC linkage isomerization, measured by the disappearance of the phosphorescence of Cr(III) in process (2), is k 1 = (7.5±0.1) × 10 −3 s −1 . Aquation (3) is conducted to proceed in two steps, whereby Hg(II) causes successive detachment of cyanide from the two CrNCHg bridges, with rate constants k 1 = (5.9±0.7) × 10 −4 s −1 and k 2 = (1.8±0.2) × 10 −4 s −1 at 25°C and μ = 0.10 M. The trans configuration of the Cr(III) units is completely retained throughout all reactions. The absorption data place the Hg 2+ -bound ligands in the spectrochemical series in the order: NH 3 ∼NCHg + >NCSHg + >NCS − .

Vertically arranged interdigitated array (IDA) electrodes with a continuous Pt-film basis and a 0.5 μm thick insulation layer in the middle separating the upper Pt-IDA electrodes (26 pairs of 400 μm long fingers, 5 μm wide with 5 μm gaps) from the Pt-basis were applied for Fe trace analysis in ultrapure spectral carbon. The testing with spiked samples as well as comparison of results with those obtained by independent graphite-furnace atomic absorption spectrometry (AAS) proved the reliability of the proposed sample preparation procedure and IDA signal generation in the range of Fe content 1 × 10 −5 to 1.5 × 10 −4 wt.%. The method can be considered as a less expensive alternative to the AAS determination of Fe pollutants in this ultrapure material.

Amperometric determination of electroactive species not requiring calibration described in this work is based on separation of linear and spherical parts of limiting diffusion current of a determined substance to the hemispherical Hg-semimicroelectrode (radius 37.5 μm). Charges transfered by these two components of current during experiment are comparable for semimicroelectrodes and the determination error is acceptably low (< 10%) for concentration interval 50–500 μmol dm−3. The calibrationless amperometric determination was verified with cadmium and lead. A new way of calculating optimum value of electrode radius to minimize the determination error is also presented.

Polarography and spectrophotometry reveal various interactions between trans-Cr(NH3)4(CN)2+ and Hg2+ in acidic aqueous solution. There is evidence for formation of two trinuclear adducts with Cr:Hg proportions of 2:1 and 1:2, accompanied by linkage isomerization (CrNCHg) of the cyanide bridging groups. These processes are followed by Hg2+-induced aquation of CN−. The six Cr(III) species occurring in the reaction sequence are identified by their half-wave reduction potentials. The trans configuration of the final aquation product, Cr(NH3)4(H2O)23+, indicates complete stereoretention during all the reactions, consistent with the observed stoichiometry of CrHg association which differs from that of a cis-dicyano analogue. The linkage isomerization of trans-Cr(NH3)4(CN)2+ is much faster than that of trans-Cr(NH3)4(H2O)(CN)2+, while the rates of the respective Hg2+-promoted aquation reactions are the same, denoting trans effects of CN− in the former process but not in the latter.

Oxidation rate of methanol, ethanol and 2-propanol by Os(VIII) in 1 M KOH was determined by evaluating anodic voltammograms of Os(VI) in the presence of the aliphatic alcohols. The proposed method is based on the evaluation of the change of voltammogram shape by catalytic process and on the assumption of its approximation by fractionally integrated voltammograms. This assumption has been confirmed empirically for k'f /a in the interval of . The rate constants from this interval could be determined with sufficient accuracy. The method is suitable for evaluation of reaction rates slow and moderately fast catalytic reactions. It was compared with other evaluation methods from the literature.

Simple, sensitive and selective differential pulse voltammetric method for determination of penicillin V on a bare (unmodified) boron-doped diamond electrode has been developed. Penicillin V provided highly reproducible and well-defined irreversible oxidation peak at very positive potential of + 1.6 V (vs. Ag/AgCl). The optimum experimental conditions for oxidation of penicillin V were achieved in acetate buffer solution (pH 4.0). The modulation amplitude of 0.1 V, modulation time of 0.05 s and scan rate of 0.05 V s − 1 were selected as optimum instrumental parameters for differential pulse voltammetry. Linear response of peak current on the concentration in the range from 0.5 to 40 μM with coefficient of determination of 0.999, good repeatability (RSD of 1.5%) and detection limit of 0.25 μM were observed without any chemical modifications and electrochemical surface pretreatment. The effect of possible interferents such as stearic acid, glucose, urea, uric acid and ascorbic acid appeared to be negligible which evidently proved the good selectivity of method. The practical analytical utility of proposed method was demonstrated by determination of penicillin V in pharmaceutical formulations (tablets) and human urine samples with satisfactory recoveries (from 98 to 101% for tablets and 97 to 103% for human urine).

The reaction of Cr(NH 3 ) 5 (CN) 2+ with Hg 2+ was investigated in aqueous medium by various electrochemical techniques, as well as by absorption and emission spectrophotometry. Evidence was found for formation in solution of a stable 2:1 adduct, the polarographic and spectral properties of which indicate linkage isomerization (CrNCHg) of the cyano bridging group. In addition, two solids containing 4:1 and 2:1 Cr:Hg adduct species were isolated and characterized. The kinetics of adduct formation and subsequent Hg 2+ promoted CN − aquation were studied by monitoring the phosphorescence of cyanopentaamminechromium(III) end by polarography, respectively. The former reaction obeys the rate law: −d[Cr]/d t = k 1 [Cr]+ k 2 [Cr] 2 , with k 1 independent of the [Hg 2+ ]:[Cr] ratio in the 5–700 range, and k 2 decreasing as this ratio increases. The aquation rate is independent of both [Hg 2+ ] and [H 3 O + ]. The factors affecting the two reaction stages are briefly discussed.

Chemical reactions coupled with the electrode process lead to the change of shape and/or height of the measured voltammogram, thus complicating its application in quantitative analysis. This article describes a fractional differintegration procedure which corrects voltammograms for interference by catalytic reactions while simultaneously providing a value for the catalytic rate constants. A peocedure is proposed which utilizes the change of the shape of a reversible charge transfer voltammogram caused by slow and moderately fast catalytic reaction to the elimination of its influence on the analytical signal. It is based upon computer symmetrization of the voltammogram by fractional differentiation of the optimized order and the following correction of the signal. The procedure also enables an advantageous determination of the catalytic reaction rate constant. Since the height of the symmetrized signal is more sensitive to the catalytic process than the height of the original voltammogram, it is of special importance for lower rate constant values. Both elimination of catalytic influence and the determination of rate constant were tested with the Ti(III) NH2OH catalytic system.

A simple and sensitive method is proposed applying a gold fibre microelectrode for the DPASV determination of Hg and Cu traces in an ultrapure spectral carbon. The procedure allows simultaneous determination of both elements at the purity level lower than 1 . 10-6 wt.%. Two different sample preparation techniques -ashing in stream of oxygen, and elution with a mixture of concentrated HCl and HNO3 - were applied for releasing the metals spread in carbon matrix. Testing of the digestion techniques with synthetic Hg and Cu spiked samples proved both to be satisfactory. The proposed determination has a sensitivity high enough to verify the required highest purity level of the spectral carbon as concerns Hg and Cu traces.

Preparation of the [Cr(H2O)2CN (en) NO]+ ion in solution by acid hydrolysis of [CrH2O(CN)2 (en) NO] including its ion exchange separation is described. The polarographic properties of [Cr(H2O)2CN (en) NO]+ as well as the kinetics of its acid hydrolysis were studied. Reactions of this complex with Hg(II) and Ag(I) were investigated and binuclear and trinuclear adducts were found to form. Linkage isomerization of the bridging cyanide in such adducts is deduced from their electrochemical properties. The spontaneous isomerization of the unstable isomer with the Cr-NC bond to the stable isomer with the Cr-CN bond was followed after its liberation by a competitive complexation of Hg(II) with excess iodide. The kinetics of decomposition of the adducts with Hg(II) was measured to examine the influence of H+ and Hg(II) concentrations.

An analytical application of chronoamperometry for determination of electroactive species not requiring standardization is described in this paper. It is based upon the separation of current component limited by linear diffusion from that limited by cylindrical contribution to the total flux of determined substance to the cylindrical (wire) Pt-semimicroelectrode (radius 0.02 mm and length 15 mm). The absolute method of determination was verified with o-dianizidine, ferrocyanide, ascorbic acid and dopamine. The optimalization of radius of the cylindrical electrode to minimize the determination error is also presented.

Electrochemical properties of [Cr(CN2)(en)2]+ are described. Reactions of this complex with Hg(II) and Ag(I) were studied. Their stoichiometry (1 : 1) was determined using potentiometric and amperometric titration. Different pulse voltammetry (DPV) was applied to identify their products - dinuclear adducts. It was concluded on the bases of DPV experiments that linkage isomerization of bridding cyano ligand occurs in the process of adducts formation. The decomposition (aquation) of these adducts was followed and the influence of H+ and Hg2+ concentration on its rate was measured and discussed. The paper also deals with a spontaneous isomerization of unstable CN linkage isomers Cr(NC)2 and Cr(CN)(CN) to the stable isomer Cr(NC)2.

Electrochemical properties of [Cr(CN 2 )(en) 2 ] + are described. Reactions of this complex with Hg(II) and Ag(I) were studied. Their stoichiometry (1 : 1) was determined using potentiometric and amperometric titration. Different pulse voltammetry (DPV) was applied to identify their products - dinuclear adducts. It was concluded on the bases of DPV experiments that linkage isomerization of bridding cyano ligand occurs in the process of adducts formation. The decomposition (aquation) of these adducts was followed and the influence of H + and Hg 2+ concentration on its rate was measured and discussed. The paper also deals with a spontaneous isomerization of unstable CN linkage isomers Cr(NC) 2 and Cr(CN)(CN) to the stable isomer Cr(NC) 2 .

The possibility of calibrationless chronoamperometric determination is described using a pair of individually addressable and diffusion layers interacting segments of interdigitated microelectrode array (IDA). It utilizes dual voltammetric mode where the first segment is polarized with potential corresponding to the limiting current of determined species electrode reaction and the second segment is polarized with potential corresponding to the opposite electrode reaction limiting current. Time at which the current of the collector segment reaches one half of the steady state is hyperbolically dependent on the diffusion coefficient of analyte. The determination of diffusion coefficient allows direct calculation of bulk concentration avoiding calibration with a standard solution. The equipment for measuring of fast response of IDA arrays in dual mode has been developed using a bipotentiostat connected with A/D transducer. It allows less than 1 ms sampling period for ultrafast registration of chronoamperogram. The method was tested and validated with [Fe(CN)6]4−, [Ru(NH3)6]Cl3, and ferrocene model samples using various types of IDA arrays.

A simple and sensitive electroanalytical method was developed for microvolume trace elements determination. Commercially available interdigitated microelectrodes (IDA) from ALS Japan Inc. based on a thin carbon film technology were used. Anodic stripping voltammetry with collection at IDA was applied. Carbon IDA microbands were coated with a pre-deposited mercury film for better performance. The method with this sensor enables analysis of some heavy metals in total sample volumes as small as 0.05 cm3. The method was applied to Pb in blood samples determination by a standard addition method.

A gold-fibre microelectrode is proposed for the determination of mercury in high-purity gallium arsenide by stripping analysis. The precision and accuracy were tested using mercury-spiked gallium arsenide samples and were found to be satisfactory for mercury contents higher than approximately 10 ppb. Both the precision and accuracy exceed those of the method applying a compact gold rotating disc electrode. De-aeration of the sample solution and its stirring during accumulation are unnecessary in the analyses with gold-fibre microelectrodes.

The electrochemical reactions of bond isomers cis-NO(H 2 O) 4 Cr-CN + and cis-NO(H 2 O) 4 Cr-NC + and of their complexes with Hg 2+ are described. The reaction schemes proposed are based chiefly on polarographic and coulometric measurements. The measurement of the potentials of the reversible reduction of Hg(II) complexed in {NO(H 2 O) 4 Cr-NC} 2 Hg 4+ was used for the calculation of the formation constant of this complex from NO(H 2 O) 4 Cr-NC + and Hg 2+ , K = 1.43 X 10 21 M −2 at 25°C and an ionic strength of 1 M (ClO − 4 , Na + ), and of the acid-base dissociation constant of NO(H 2 O) 4 Cr-NCH 2+ , K a = 2.75 X 10 −2 M for the same conditions. An analysis of the limiting diffusion current of reduction of the complexed Hg(II) yielded the value of the equilibrium constant for the reaction of Hg 2+ with {NO(H 2 O) 4 Cr-NC} 2 Hg 4+ , Q = 7 ± 4 for 25°C and an ionic strength of 1 M (ClO − 4 , Na + , H + ).

The paper describes the conditions for the preparation of Cr(CN)(H2O)4NO+, its isolation in solution and its identification on the basis of the analysis of the components, spectrophotometric and elctrochemical properties and the kinetics of acid hydrolysis. The reactions of Cr(CN)(H2O)4NO+ with Ag+ and Hg2+ ions were studied from the point of view of the stoichiometry and equilibrium. The products of these reactions were characterized by the electronic absorption spectra and sorption on ion-exchangers. On the basis of the product properties, the course of the reactions and of analogous reactions, the structures characterized by groups CrNCHg and CrNCAg were proposed. The kinetics of the decomposition reactions of the Cr(CN)(H2O)4NO+ adducts with Ag+ and Hg2+, giving Cr(H2O)5NO2+ and Hg(CN)2, are described and the factors affecting them are discussed.

The equilibrium of the CN bond isomers of NO(H 2 O) 4 Cr(CN) + was studied polarographically using the fast formation of the NO(H 2 O) 4 CrNC + complex with Hg 2+ . The isomerization equilibrium constant. Q i = [NO(H 2 O) 4 CrCN + ]/[NO(H 2 O) 4 CrNC + ] = (2.6 ± 0.1) X 10 2 , and the deprotonation equilibium constant, K a = [NO(H 2 O) 4 CrNC + [H + ]/ [NO(H 2 O) 4 CrNCH 2+ ] = (2.5 ± 0.2) X 10 −2 M, (both values at 25 °C and an ionic strength of 1 M ), obtained from these mesurements are in a good agreement with the values obtained by other methods. The electrochemical reactions of {NO(H 2 O) 4 CrNC} 2 Ag 3+ were described and explained from the viewpoint of the stochiometry and the mechanim of the formation reaction. The schemes proposed are based on voltammetric and coulometric measurements. From an analysis of the polarographic limiting diffussion current for the reduction of complexes Ag(I), the value of the equilibrium constant, Q = [NO(H 2 O) 4 CrNCAg 2+ ] 2 /[Ag + ][{NO(H 2 O) 4 CrNC} 2 Ag 3+ ] = (0.12 ± 0.08), was obtained for 25 °C and an ionic strength of 1 M .

It has been found from a kinetic and spectrophotometric study that the formation of {NO[H2O}4CrNCHg3+ involves two steps: 1) rapid equilibrium association of NO(H2O)4CrCN+ with Hg2+_(the equilibrium constant K = (7.2 ± 0.8) X 102 M−1 for 278 K, an ionic strength of 1M and [H+] = 0.1 M); 2) the rate-determining step, independent of [Hg2+], with a rate constant of k = 6.0 X 10−2 s−1 for 278 K, an ionic strength of 1 M and [Hg+ = 0.1 M. The spectrophotometric measurements extrapolated to the instant of mixing of the reactants verify the formation of the associate and enable the calculation of the equilibrium constant in good agreement with the value obtained kinetically, as well at the construction of the intermediate visible spectrum. It has been found that the rate of the NO(H2O)4CrNC+ isomerization to the more stable NO(H2O)4CrCN+ is pH-dependent, as the isomerization rate constant for the reactant protonated form, kp, is given by kp The two reactions studied are discussed from the point of view of the common mechanistic features and are compared with analogous reactions of cyanochromium(III) complexes that do not contain the NO ligand.