The determination of some 2-thiobarbiturates by cathodic stripping voltammetry

Pulse polarography and cyclic voltammetry are employed in studies of the electrochemical behaviour of 5-ethyl-5'-(l-methylbutyl)-2-thiobarbituric acid (I), l-methyl-5-ethyl-5'-(l-methylpropyl)-2-thiobarbituric acid (II) and l,3-dimethyl-5-ethyl-5'-p-chlorophenyl)-2-thiobarbituric acid (III) in the pH range 4–12. All three compounds show anodic and cathodic waves or peaks in this pH range. Compounds (I) and (II) are oxidized at mercury indicator electrodes to produce mercury salts which can adsorb thereon and are thus amenable to cathodic stripping voltammetric analysis (c.s.v.) down to concentrations of the order of 10^-6 M, which is superior to the sensitivities obtained by differential pulse polarography (d.p.p.) based on a reduction peak. Compound (III) oxidizes to produce sulphur which is subsequently plated as HgS. Again the sensitivity of the c.s.v. method is of the order of lO^-6 M and analytically superior to d.p.p. The optimum pH for the three determinations is 8. The determination of (II) in the presence of its oxygenated analogue and metabolite, phemitone, and the effect of chloride ions are reported.     

A polarographic and voltammetric study of some primary,secondary and tertiary thioamides of pharmaceutical importance

The d.c. polarographic, and cyclic and cathodic stripping voltammetric behaviour of some primary, secondary and tertiary thioamides based on tetrahydroquinoline, is described. Catalytic reduction occurs in all cases; the primary and secondary thioamides undergo anodic oxidation and cathodic stripping, with the formation of mercury(II) sulphide. Tertiary thioamides are not amenable to anodic electrolysis/stripping. Mechanisms for these oxidation and reduction processes are postulated. Differential pulse polarography has a limit of detection of 5 × 10^?7 M. Cathodic stripping voltammetry can be applied with a detection limit of 2 × 10^?8 M.     

Determination of pseudouridine at submicromolar concentrations by cathodic stripping voltammetry at a mercury electrode

Pseudouridine (5-ribosyluracil), uridine (N,1-ribosyluracil), deoxyuridine (N,1-deoxyribosyluracil) and uracil are investigated by means of d.c. polarography and by differential and normal pulse polarography. Pseudouridine, which is known to be a cancer marker, yields anodic polarographic currents in the pH range 7–11, whereas uridine and deoxyuridine are inactive under the same conditions. The polarographic response of pseudouridine obtained is due to the formation of a sparingly soluble mercury compound. Pseudouridine can be determined by differential pulse polarography in the concentration range 2–6 × 10^?6 M and by differential-pulse cathodic stripping voltammetry at concentrations two orders of magnitude lower. Small excesses of uridine, deoxyuridine or proteins do not interfere with the determination.     

Polarographic determination of hydroxylamines: application to analysis of photographic processing solutions

Procedures for the determination of hydroxylamine and N,N-diethylhydroxyIamine (DEHA), based on anodic polarographic waves, are described. The importance of using a strongly alkaline supporting electrolyte and of complete removal of dissolved oxygen is illustrated. With rapid alternating current (a.c.) polarography, 3 × 10^-6 M hydroxylamine and 4 × 10^-5 M DEHA can be detected. Detection limits with the differential pulse technique are approximately tenfold lower. In a practical application, rapid a.c. polarography is shown to be suitable for the direct determination of hydroxylamine and DEHA in photographic processing solutions. The only pretreatment of samples is dilution with a strongly alkaline supporting electrolyte. Possible interferences from other constituents of the processing solutions are avoided by using the standard addition method.     

Polarographic study of uracil derivatives

Eighteen uracil derivatives were studied by d.c. polarography, differential-pulse (d.p.)polarography and d.p. cathodic stripping voltammetry. In a borax buffer at ca. pH 7.6, uracil, thymine and derivatives such as 5-halouracils, 5-trifluoromethyl-,5-aza-,5-acetyl-, 5-formyl- and 5-vinyl-uracil produced well-defined peaks at potentials between 0 V and ca. 160 mV vs. silver/ silver chloride (satd. KCl). The peaks are ascribed to the formation of sparingly soluble mercury salts. For the other derivatives tested (e.g., 5-nitro- and 5-ethynyl-uracil and 6-substituted uracils), the peaks were less well-defined and in some cases the polarographic curves were very complex. 2-Thiouracil produced a single peak at ca. ?400 mV, but only at pH 12.2. The shapes, heights and potentials of the peaks depended on the kind and position of the substituent on the pyrimidine ring. Rectilinear relationships of peak current vs. concentrations were found for most compounds (10^?5-10^?4 M) by d.p. polarography; d.c.polarography was not tested quantitatively. For 5-fluorouracil and 5-vinyluracil, linear calibrations were found for concentrations of 0.5–5 × 10^?7 M by d.p. cathodic stripping voltametry. Interference studies showed that small amounts of chloride and phosphate did not interfere but 5-fluorodeoxyuridine, which did not itself produce a peak, and proteins interfered seriously.     

Contribution à l'électrochimie des thiols et disulfures : Partie IV. Polarographies d.c., a.c. et impulsionnelle différentielle de la cystéamine et de la cystamine

The electrochemical behaviour of the cysteamine—cystamine system was investigated by the d.c.a.c. and d.p.p. techniques, as functions of concentration and pH. Adsorption phenomens were observed, and the cystéamine anodic and the cystamine cathodic waves were found to be diffusion-controlled. Separation of these two sulfur amino acids was achieved; 6 mg l^-1 of cysteamine HCl can be determined in the presence of 11 mg l^-1 of cystamine dihydrochloride. Detection limits are 5 ·1O^-7 M for cysteamine and 1 · 10^-6M for cystamine; as little as 0.012 mg of the thiol and 0.23 mg of the disulfide per litre can be determined. The polarographic behaviour of the cystamine—cysteamine system resembles that described previously for the cysteine— cystine system.     

Evaluation of derivative normal pulse polarography for use in metal speciation studies

A simple inexpensive microcomputer-orientated approach for obtaining pseudo-derivative polarograms is demonstrated. The method, called derivative normal pulse polarography (d.n.p.p.), produces better signal-to-noise ratios, and therefore lower detection limits, than differential pulse polarography (d.p.p.). The foremost advantage of d.n.p.p. is its relative freedom from influences of homogeneous chemical kinetics which makes it an excellent method for metal speciation studies. Response curve shapes for Pb(II) between 2 × 10^-4 and 8 × 10^-4 M exhibit excellent agreement with theory. Titration of EDTA with 0.01 M Cu(II) and 0.01 M Cd(II) using d.n.p.p. for detection exhibited recoveries of 98.0 and 99.5%, respectively.     

Polarographic behaviour and hydrolysis of midazolam and its metabolites

The electrochemical behaviour of midazolam [7-chloro-5-(o-fluorophenyl)-3H-(2′- methyllimidazo) [1,5-a]-benzodiazepine was studied by polarography and cyclic voltammetry. The irreversible two-electron were is not strongly affected by the imidazole ring or the 5-o-fluorophenyl substituent, but the latter increases the rate of the hydrolysis in acidic media. Kinetic parameters are evaluated for midazolam and three of its hydroxylated metabolites. The hydrolysis is a first-order reaction initially but becomes second order. The 3-hydroxy matabolites are more easily hydrolyzed than midazolam. Midazolam (10^?4–10^?7 M) can be quanitified by using differential-pulse polarography; the detection limit is 6 × 10^?8 M.     

Cathode ray polarography and differential pulse polarography of the catalytic molybdenum(VI) wave

The optimum conditions of electrolyte composition and pH were studied for the cathode ray polarography (c.r.p.) of the catalytic molybdenum(VI) wave; 1–2 M potassium nitrate at pH 1.6–2.2 is optimum, the detection limit is 3 × 10^-9 M (0.3 ppb) molybdenum(VI). Equal molar concentrations of foreign electroactive substances do not interfere at the potential of the molybdenum wave and 500-fold amounts of these can be tolerated if their c.r.p. peaks are separated from the molybdenum wave by 0.1 or 0.2 V. Similar conditions were used for the differential pulse polarography of the catalytic wave. The detection limit is 2 × 10^-8 M (2 ppb). being limited in part by lead impurities which contribute to the background.     

An electroanalytical study of the anticancer drug dacarbazine

The electrochemical behaviour of dacarbazine [5-(3,3-dimethyl-1-triazenyl) imidazole-4-carboxamide; DTIC] was investigated by Tast and differential pulse polarography (d.p.p.) at the dropping mercury electrode, by cyclic and differential pulse voltammetry at the hanging mercury drop electrode and by anodic voltammetry at the glassy carbon electrode. Calibration graphs were obtained for 2×10^?8?2×10^?5 M DTIC by d.p.p., for 5×10^?9?1×10^?5 M by adsorptive stripping voltammetry ar a hanging mercury drop electrode, and for 1?10×10^?5 M by high-performance liquid chromatography with oxidative amperometric detection at a glassy carbon electrode. The methods are compared and applied to determine DTIC added to blood serum after a simple clean-up procedure.     

A polarographic study of some N-oxygenated products of N-ethyl-β-methoxy-β-(3'-trifluoromethylphenyl)-ethylamine (sk and f 40652a)

The polarographic behaviour of N-hydroxy-β-methoxy-β-(3'-trifluoromethylpbenyl)-ethylamine, N-ethyl-N-hydroxy-β-methoxy-β-(3'-trifluoromethylphenyl)ethylamine and (3-methoxy-β-(3'-trifluoromethylphenyl)acetaldoxime has been studied over the pH range 0—14. The hydroxylamines gave rise to anodic and cathodic behaviour whereas the oxime gave only a cathodic wave. The mechanism of the oxidation and reduction processes was investigated by d.c. polarography and preparative micro-coulometry. The optimum pH values for analytical purposes were 7, 8 and 4 for the two hydroxylamines and the oxime, respectively. The polarographic behaviour of a mixture of the three compounds was studied and the determination of traces of such compounds by differential pulse polarography is discussed.     

Etude Polarographique De La Marcellomycine

Abstract

Polarographic study of marcellomycine.

d.c., a.c. and d.p. polarography and cyclic voltammetric techniques were used to investigate the electrochemical properties of marcellomycine. These are found to be quite similar to those of aclacinomycine or carminomycine, the main reversible two-electron wave being attributed tot he reduction of the quinonic structure. The molecule undergoes a degradation process starting in acidic medium and giving rise to the formation of two small irreversible waves at more negative potentials. This process is time, illumination and pH dependent. Important adsorption phenomena interfere in the investigated pH range but are lowered in very acidic medium. Quantitative measurements have been carried out at pH 1 and 6 in a 10^?3 to 10^?5 M concentration range using d.c. polarography. d.p. technique allows determinations in a restricted 10^?5 to 10^?4 M range. A detection limit of 1 · 10^?5 M has been fixed at pH 1.     

Electrochemical study and polarographic determination of ranitidine

In the polarographic reduction of ranitidine, an H₂-antagonist of histamine, three waves are observed; their half-wave potentials and limiting currents depend strongly on the pH of the solution. The first and second waves are due to reduction of teh protonated, CHNO₂H⁺, and unprotonated, CHNO₂, nitroethene group of ranitidine, respectively; the origin of the third wave is unknown. The characteristics of the second and third waves are studied in acetic acid/acetate buffer at pH 5.5; the first wave does not appear at this pH. The second wave (E_{$\text{1}\text{2}$} = ?0.90 V, vs. Ag/AgCl) is useful for determining ranitidine in the range 2.4–4.9 × 10^?4 M by direct current polarography and in the range 2.5 × 10^?7?2.05 × 10^?5 M by differential pulse polarography.     

Voltammetric on-line analysis for some sulphur-containing drugs

Some 2-thiobarbituric acid and thioamide drugs are determined voltammetrically in a flow-through cell. Both direct oxidation in the d.c. mode and cathodic stripping on a mercury-coated glassy carbon electrode are examined for the determination of these drugs. The thiobarbiturie acids can be determined by both methods; the linear range covers 1–2 orders of magnitude with a detection limit in the range 10^-3–10^-6 M depending on the method used and the compound. The thioamides do not plate satisfactorily on the mercury film when operated in the on-line mode and can only be determined by the d.c. anodic oxidation method; the linear range covers at least two orders of magnitude with a detection limit of 5×10^-8–5×10^-7 M depending on the compound.     

Polarographic reduction of 2,4,6-trinitrobenzene-1-sulphonic acid and some 2,4,6-trinitrophenyl-amino acid derivatives

The rapid d.c. polarography of 2,4,6-trinitrobenzene-1-sulphonic acid and its derivatives with serine (I), threonine (II), glycine (III) and histidine (IV) revealed a 3-wave reduction and a marked pH dependence of the reduction potential. The polarographic waves of the derivatives (2.5 × 10^?4 M) showed appreciable changes when sulphite ions were present, with the development of a new wave at more negative potential in ?0.01 M sulphite solutions at pH 7.0. The E_1/2 values of these waves in pH 7.0 supporting electrolyte were: (I) ?1000; (II) ?1007; (III) ?1021; (IV) ?949 mV (vs. Ag/AgCl, sat. KCl). These waves were used to determine the amino acids investigated (1–4 × 10^?4 M) in the presence of excess of 2,4,6-trinitrobenzene-1-sulphonic acid, with good precision (2%).     

Voltammetric Determination of Adrenaline Using a Poly(1‐Methylpyrrole) Modified Glassy Carbon Electrode

A voltammetric method using a poly(1‐methylpyrrole) modified glassy carbon electrode was developed for the quantification of adrenaline. The modified electrode exhibited stable and sensitive current responses towards adrenaline. Compared with a bare GCE, the modified electrode exhibits a remarkable shift of the oxidation potentials of adrenaline in the cathodic direction and a drastic enhancement of the anodic current response. The separation between anodic and cathodic peak potentials (ΔEp) for adrenaline is 30 mV in 0.1 M phosphate buffer solution (PBS) at pH 4.0 at modified glassy carbon electrodes. The linear current response was obtained in the range of 7.5 × 10^?7 to 2.0 × 10^?4 M with a detection limit of 1.68 × 10^?7 M for adrenaline by square wave voltammetry. The poly(1‐methypyrrole)/GCE was also effective to simultaneously determine adrenaline, ascorbic acid and uric acid in a mixture and resolved the overlapping anodic peaks of these three species into three well‐defined voltammetric peaks in cyclic voltammetry. The modified electrode has been successfully applied for the determination of adrenaline in pharmaceuticals. The proposed method showed excellent stability and reproducibility.     

Determination of total phthalate esters in wastewaters by differential pulse polarography

A reliable procedure for the determination of total phthalate esters as phthalic acid in environmental samples is based on differential pulse polarography (d.p.p.). The phthalate esters are extracted from the sample water with hexane; concentrated sulphuric acid/hexane partitioning provides effective removal of organic interferences. The individual phthalate esters are hydrolyzed by refluxing with 10 M potassium hydroxide to phthalic acid, which is extracted with ethyl acetate followed by evaporation of the extract. This procedure gives recoveries of 83–90%. The residue is dissolved in 0.1 M acetic acid/0.1 M potassium chloride for d.p.p. The otpimal conditions for polarography are discussed. The calibration graphs are linear over the range 2 × 10^?6–1 × 10^?4 M and the detection limit for phthalic acid is 5 × 10^?7 M. The method was successfully applied to determine total phthalate esters over the range 0.3–30 μg l^?1 in crude and treated wastewaters.     

Differential pulse polarographic determination of cephalosporins and their degradation products

The differential pulse polarography (d.p.p.) of several cephalosporins — cephalothin, cephalosporin C, cephaloridine, cephalonium, cefuroxime, cephoxazole, cephalexin, cephradine, desacetylcephalosporin C, 7-aminocephalosporanic acid and 7-aminodesacetoxy-cephalosporanic acid — and some degradation products has been studied. Cephalexin, cephradine and 7-aminodesacetoxycephalosporanic acid, which contain an unsubstituted 3-methyl group but no reducible group, do not give d.p.p. peaks at the dropping mercury electrode, whereas certain of their degradation products do. The other cephalosporins, in which the 3-methyl group is substituted, give a d.p.p. peak at about—1 V (pH 2–4). Although these peaks are near the cathodic limit under the conditions used, these cephalosporins can be determined down to about 0.1 μg ml^-1. Cephaloridine exhibits two d.p.p. peaks in this region, and can be determined. Cefuroxime, in addition to the peak at —IV, has a larger peak at —0.45 V (pH 2). Cephalonium shows a large well-defined peak at —0.72 V (pH 3) and two overlapping peaks. D.p.p. data for several degradation products including the diketopiperazines formed from cephalexin and cephradine are reported.     

Cathodic and Adsorptive Stripping Votammetry of Naftazone

Abstract

Naftazone (1,2-naphthoquinone-2-semicarbazone) undergoes a reversible two-electron transfer in both acidic and alkaline solutions and also gives rise at pH > 7 to an anodic wave attributed to the formation of a mercury derivative. Cathodic stripping voltammetry is proposed to determine the compound down to 5 × 10^?9 M after accumulation of its mercury salt formed at -0.05V in a 0.05M sodium hydroxide solution. These results have been compared with those obtained by performing an adsorptive collection of the drug in a pH 3 sodium perchlorate solution. Concentrations ranging from 1 × 10^?7 to 2 × 10^?7M can be easily investigated, the detection limit being 7 × 10^?11M. The influence of several operational parameters has also been considered.     

Investigations on Bioanalytical Chemistry Part I. On Differential Pulse Voltammetry of Bilirubin

Abstract

The electrochemical behaviour of the bilirubin in many kinds of supporting electrolytes on a glassy carbon electrode (GCE) and a hanging mercury drop electrode (HMDE) was investigated by means of anodic or cathodic differential pulse voltammetry. The influences of different methods of pre-treatment of the glassy carbon electrode was also discussed. In Na₂B₄.O₇-KH₂PO₄ buffer solution, the linear range was 2×10^?9-1×10^?9 mol/l and the detection limit was 3.3×10^?9 mol/l by anodic differential pulse voltammetry at GCE. A linear relationship holds between the peak current and the concentration of bilirubin in a concentration range of 1×10^?9-4×10^?7 mol/l with good precision and accuracy, and the limit of detection was 2×10^?10 mol/l, when cathodic differential pulse adsorption voltammetry at HMDE was used.