Determination of folic acid by adsorptive stripping voltammetry at the static mercury drop electrode

Folic acid can be determined at nanomolar concentrations by controlled adsorptive accumulation of folic acid on a static mercury drop electrode held at ?0.3 V vs. Ag/AgCl followed by reduction of the surface species. In 0.1 M sulfuric acid, a cathodic scan gives peaks at ?0.47 v and ?0.75 V vs. Ag/Agcl; the latter peak provides greater sensitivity. Differential-pulse stripping is shown to be superior to normal-pulse and d.c. stripping. After a 5-min preconcentration, the detection limit is about 1 × 10^?10 M folic acid. The adsorptive stripping response is evaluated with respect to concentration dependence, preconcentration time and potential, solution acidity and the presence of gelatin and bromide. The relative standard deviation at the 5 × 10^?8 M level is 1.2%. This method is applied to the determination of folic acid in pharmaceutical tablets.     

Stripping voltammetry of manganese based on chelate adsorption at the hanging mercury drop electrode

A novel electrochemical stripping approach for the trace measurement of manganese is presented. The metal chelate with erichrome black T is adsorbed on a hanging mercury drop electrode, and the subsequent reduction current of the accumulated chelate is measured by voltammetry. Adsorptive preconcentration for 5 min results in a detection limit of 6 × 10^?10 M (32 l^?1). Cyclic voltammetry is used to characterize the redox and interfacial processes. Optimal experimental conditions include a 0.02 M piperazine-N,N′-bis(2-ethanesulfonic acid) solution (pH 12) containing 1 × 10^?6 M eriochrome black T, a preconcentration potential of ?0.80 V, and a linear potential scan. The response is linear up to 2.9 × 10^?7 M, and the relative standard deviation at 1.8 × 10^?7 M is 1.5%. The effects of possible interferences from metal ions or organic surfactants are evaluated.     

Trace metal speciation by adsorptive stripping voltammetry of metal chelates of solochrome violet RS

Adsorptive stripping voltammetry has become one of the most sensitive methods for trace metal determinations. The growing application of the method to natural water systems prompted an investigation into the fraction of the metal concentration that contributes to the adsorptive stripping response. Recent procedures for trace measurement of iron, titanium and gallium, based on chelation with solochrome violet RS (SVRS) are coupled to systematic ligand competition experiments. Tannic acid, EDTA, NTA, glycine, cysteine, carbonate and chloride ions are used as model natural ligands. It is shown that adsorptive stripping voltammetry measures the free ion and metal displaced from complexes by the “analytical” ligand. The exact fraction of the metal measured thus depends on the thermodynamic stability of the metal-SVRS chelate (compared to that of natural complexes), and on the relative concentrations of the competing ligands. The method offers possible distinction between metal complexes, based on their thermodynamic stabilities. The use ofa large excess of the “analytical” ligand can lead to measurement of the total metal content. Implications of these results relative to the use of this procedure for studying the speciation of trace elements in natural waters are discussed.     

Cathodic adsorptive stripping voltammetry of nickel complexed with hydroxynaphthol blue at a static mercury drop electrode

A new method is described for the determination of Ni based on the cathodic adsorptive stripping of Ni(II) complexed with hydroxynaphthol blue (HNB) at a static mercury drop electrode. Optimal conditions were found to be: accumulation potential -0.50 V (vs. Ag/AgCl); final potential -1.10 V; accumulation time 50 sec; scan rate 200 mV/sec; linear scan mode; filter 0.1 sec; supporting electrolyte acetic acid/acetate (0.25M, pH = 6.0) and concentration of HNB 3.3 x 10(-5)M. The response of the system was found to be linear in a range of Ni concentrations from 25 ppb to the detection limit. The detection limit was found to be 1.7 nM (0.10 ppb) with 2 mins of accumulation time. The effect of various potential interferences (including a variety of cations, anions and organic surfactants) were also studied. With the exception of Co, at less than equimolar concentrations no significant interferences were observed. Al was found to interfere at high concentrations with respect to Ni, but Al concentrations up to 1000 ppb may be masked by sodium citrate or sodium fluoride. The utility of the method is demonstrated by the recovery of Ni in a doped sample of commercial mineral water.     

Stripping voltammetry of thorium based on adsorptive accumulation

A sensitive stripping voltammetric procedure for quantifying thorium is described. The chelate of thorium with the azo dye Mordant Blue 9 is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated chelate is measured during a negative-going potential scan. Cyclic voltammetry is used to characterize the interfacial and redox behaviors. The effects of pH, dye concentration and accumulation potential are discussed. The detection limit is 4 × 10^?10 M (4-min accumulation), a linear current-concentration relationship is observed up to 1.3 × 10^?7 M, and the relative standard deviation (at the 6 × 10^?8 M level) is 3.1%. Possible interferences by trace metals and organic surfactants are investigated. Simultaneous quantitation of thorium and nickel is illustrated.     

Adsorptive stripping voltammetry of sex hormones at the static mercury drop electrode

Controlled adsorptive accumulation of testosterone, methyltestosterone and progesterone on the static mercury drop electrode provides the basis for direct stripping measurement of these compounds ar nanomolar concentrations. The adsorptive stripping behavior is evaluated with respect to preconcentration time and potential, stripping mode, concentration dependence, drop size and other variables. With 5-min accumulation, peak current enhancements of 45, 18 anal 12 are observed for 5 × 10^?8 M testosterone, progesterone and methyltestosterone, respectively, relative to direct pulse voltammetry. Detection limits are 1.6 × 10^?10 M for testosterone, 2 × 10^?10 M for progesterone and 3.3 × 10^?10 M for methyltestosterone with 15-min preconcentration. The relative standard deviation for 8 × 10^?8 M progesterone is 3.4% (n=8). Applicability to direct measurements of methyltestosterone in pharmaceutical formulations is assessed.     

Cathodic stripping voltammetry of copper-complexed reactive dyes at a hanging mercury drop electrode: Reactive violet 5

Reactive Violet 5 and its hydrolysis product, which is produced as a side product in the dyeing process, can be determined in an admixture at sub-ppb levels by cathodic stripping voltammetry because the potentials of their azo reduction peaks are separated sufficiently. For both dyes, at intermediate pH values the azo peak is preceded by a complexed -copper reduction peak at a less negative potential, which aids the identification of the dyes. The use of pH 6 EDTA buffer removes the complexed-copper peak, as does the use of an acidic buffer (pH < 3). This unusual use of EDTA as a pH buffer facilitates the determination of mixtures of the dye and its hydrolysis product.     

Determination of iron in high purity materials by adsorptive stripping voltammetry with solochrome violet

Stripping voltammetry peaks of an iron solochrome violet complex are shown to be strongly effected by anions present in the sample solution. Chloride and sulphate exhibit different dependencies of the peak height as a function of concentration. They have to be taken into account if the results of iron determinations are to be precise as well as accurate (i.e. the same as those obtained by an independent method). Graphite furnace atomic absorption spectrometry, after extraction of the acetylacetone complex, has been used here for comparison purposes.     

Indirect measurement of brucine by adsorptive stripping voltammetry at mercury electrode

After reaction with nitric acid, brucine can be transformed into cacotheline, and then measured indirectly by adsorptive stripping voltammetry. This method is based on the adsorptive accumulation of cacotheline at a hanging mercury drop electrode, followed by cathodic linear sweep voltammetry. The cathodic peak potential is about -0.35 V (vs. saturated Ag AgCl ). The detection limit of 2.0 x 10(-9) M is obtained under optimized conditions. The electrochemical behaviour of cacotheline and the mechanism of the electrode reactions are discussed.     

Adsorptive stripping voltammetry of chlordiazepoxide at the hanging mercury drop electrode

Controlled adsorptive accumulation at the hanging mercury drop electrode enables 0.8–11 × 10^?5 M chlordiazepoxide to be quantified by differential-pulse stripping voltammetry with accumulation times of 1–3 min. With 3-min accumulation, the peak current is enhanced 12-fold for 1.0 × 10^?7 M chlordiazepoxide compared to the current from differential pulse polarography. The detection limit is 0.9 × 10^?9 M for 4-min accumulation. The procedure is applied to spiked human serum after preseparation of the drug on a Sep-Pak C₁₈ cartridge.     

Anodic stripping voltammetry of germanium at the hanging mercury drop electrode

The Ge(IV)—Ge(0) system was investigated by cyclic and stripping voltammetry at HMDE in acidic pyrogallol medium and in phosphate, borate and carbonate buffers. It was found that germanium electrodeposited from dilute Ge(IV) solutions dissolved anodically forming two peaks corresponding to the oxidation of the unstable homogeneous and stable heterogeneous amalgams. Both peaks can be exploited analytically for the determination of traces of germanium but due to the complex nature of the germanium amalgam the sensitivity and reproducibility of the determinations are lower compared to the results obtained for metals well-soluble in mercury.     

Cathodic adsorptive stripping voltammetric behaviour of guanine in the presence of copper at the static mercury drop electrode

The cathodic adsorptive electrochemical behavior of guanine in the presence of some metal ions at the static mercury drop electrode was investigated. A 1.0×10⁻³ mol l⁻¹ NaOH or a 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 solutions were used as supporting electrolytes. The reduction peak potential for guanine was found to be around −0.15 V, which is very close to the mercury reduction wave. A new peak appears at −0.60 V in the presence of copper or at −1.05 V in the presence of zinc. A square wave voltammetric procedure for electroanalytical determination of guanine in 2.0×10⁻² mol l⁻¹ Hepes buffer at pH 8.0 containing 1.6×10⁻⁵ mol l⁻¹of copper ions, was developed. An accumulation potential of −0.15 V during 270 s for the prior adsorption of guanine at the electrode surface was used. The response of the system was found to be linear in the range of guanine concentration from 6.62×10⁻⁸ to 1.32×10⁻⁷ mol l⁻¹ and the detection limit was 7.0×10⁻⁹ mol l⁻¹. The influence of DNA bases such as adenine, cytosine and thymine was also examined. Cyclic voltammetry was used to characterize the interfacial and redox mechanism.     

Phase-selective a.c. adsorptive stripping voltammetry of lumazine on a hanging mercury drop electrode

The electrochemical behavior of lumazine (LMZ), an important antibacterial agent, has been studied at the hanging mercury drop electrode (HMDE). The nature of the process taking place at the HMDE was clarified. Its adsorption behavior at HMDE has been studied by using a.c and cyclic voltammetry (CV). Both the molecule and its reduced product appeared to be adsorbed at the surface of the electrode. Controlled adsorptive accumulation of LMZ on the HMDE provides the basis for the direct stripping measurement of that compound in the subnanomolar concentration level. Experimental and instrumental parameters for the quantitative determination were optimized. Phase-selective a.c voltammetry provided the best signal and gave a detection limit of 0.15 μg L^–1 (9.0 × 10^–10 mol/L) LMZ in aqueous solution. Molecules or ions which may interfere were studied.     

Phase-selective a.c. adsorptive stripping voltammetry of lumazine on a hanging mercury drop electrode

The electrochemical behavior of lumazine (LMZ), an important antibacterial agent, has been studied at the hanging mercury drop electrode (HMDE). The nature of the process taking place at the HMDE was clarified. Its adsorption behavior at HMDE has been studied by using a.c and cyclic voltammetry (CV). Both the molecule and its reduced product appeared to be adsorbed at the surface of the electrode. Controlled adsorptive accumulation of LMZ on the HMDE provides the basis for the direct stripping measurement of that compound in the subnanomolar concentration level. Experimental and instrumental parameters for the quantitative determination were optimized. Phase-selective a.c voltammetry provided the best signal and gave a detection limit of 0.15 microg L(-1) (9.0 x 10(-10) mol/L) LMZ in aqueous solution. Molecules or ions which may interfere were studied.     

Polarographic behaviour of some s-triazine herbicides and their determination by adsorptive stripping voltammetry at the hanging mercury drop electrode

The electrochemical behaviour of six triazine herbicides (atrazine, terbutylazine, desmetryne, prometryne, terbutryne and methoprotryne) was studied by fast scan differential pulse voltammetry (FSDPV) at a static mercury drop electrode and by constant potential coulometry. The adsorption of these compounds at the mercury electrode was employed for their determination by adsorptive stripping voltammetry (AdSV) in Britton-Robinson buffers. The detection limits calculated from regression analyses ranged between 0.1 and 0.9 g/l. Preconcentration (solid phase extraction on a C18 column) was used for the determination of very low contents in water samples. The recovery was 97–99.6%.     

Square-wave voltammetry of the o-catechol-Ge(IV) catalytic system after adsorptive preconcentration at a hanging mercury drop electrode

Square-wave (SW) voltammetry in connection with a hanging mercury drop electrode has been applied for studying the reduction of Ge(IV) catalyzed by o-catechol after adsorptive preconcentration. Acetate buffer solutions with low and high analytical concentrations of o-catechol [with respect to that of Ge(IV)] have been used. Dependences of SW catalytic peaks on accumulation conditions have been attributed to the formation of the catalytic complexes via a direct-adsorption mechanism. The nature of the electrocatalytic process (involving an apparently quasi-reversible electrode reaction with an adsorbed reactant and a diffusing product) allows us to explain complex dependences of catalytic peaks on SW-parameters. Adequate conditions for determination of o-catechol and Ge(IV) by means of SW catalytic adsorptive voltammetry have been established.     

Indirect determination of hexavalent chromium ion in complex matrices by adsorptive stripping voltammetry at a mercury electrode

A sensitive method for the determination of chromium ion(VI) in complex matrices such as crude oil and sludge is presented based on the decreasing effect of Cr(VI) on cathodic adsorptive stripping peak height of Cu-adenine complex. Under the optimum experimental conditions (pH 7.5 Britton-Robinson buffer, 5 × 10⁻⁵ M copper, 8 × 10⁻⁶ M adenine and accumulation potential −250 mV versus Ag/AgCl), a linear decrease of the peak current of Cu-adenine was observed, when the chromium(VI) concentration was increased from 5 μg L⁻¹ to 120 μg L⁻¹. Detection limit of 2 μg L⁻¹ was achieved for 120 s accumulation time. The relative standard deviations (R.S.D., %) were 1.8% and 4% for chromium(VI) concentrations of 18 μg L⁻¹ and 100 μg L⁻¹, respectively. The method was applied to the determination of chromium(VI) in the presence of high levels of chromium(III), in various real samples such as crude oil, crude oil tank button sludge, waste water and tap water samples. Effects of foreign ions and surfactants on the voltammetric peak and the influences of instrumental and analytical parameters were investigated in detail. The accuracy of the results was checked by ICP and/or AA.     

Cathodic stripping voltammetry of 2-thioorotic acid at the hanging mercury drop electrode

Controlled adsorptive accumulation of 2-thioorotic acid (6-carboxy-2-thiouracil) on the hanging mercury drop electrode provides the basis for the direct stripping measurement of that compound in the nanomolar concentration level. Differential pulse voltammetry, following 3 min preconcentration, yields a detection limit of 5.0×10^-10 M 2-thioorotic acid. The cathodic stripping response is evaluated with respect to experimental parameters such as preconcentration time and potential, bulk concentration and others. Best results are obtained using a 0.001 M NaOH electrolyte.Two different methods of cathodic stripping voltammetry can be proposed for the determination of 2-thioorotic acid and the reproducibility of these methods is studied.     

Adsorptive stripping voltammetry of Tartrazine at the hanging mercury drop electrode in soft drinks

Adsorptive stripping voltammetry was used for the determination of trace amounts of the dye Tartrazine (E-102) by square-wave (SWS) and differential pulse techniques (DPS). Its adsorptive voltammetric behaviour was investigated at different pH media. NH₄Cl/NH₃ buffer solution was chosen as the most suitable, taking into account the sensitivity and definition of the reduction peaks obtained. The effects of the experimental parameters on the determination are discussed. Standard deviations of 3.3% and 2.6% were obtained by SWS and DPS for 100 and 50 μg/L Tartrazine solutions, respectively (n = 10). Both methods were applied to determine the dye in several commercial soft drinks, containing very small amounts of it. Measurements were made directly in the commercial samples. A comparison of the results obtained by the proposed voltammetric methods with those of an HPLC method was also made. Good correlations between the voltammetric results and the values supplied by the manufacturer were found, whereas recoveries of the same order of magnitude were obtained by the HPLC method. Received: 23 May 1996 / Revised: 5 July 1996 / Accepted: 10 July 1996