Kunihiko Yokoi

Determination of titanium in sea water using catalytic cathodic stripping voltammetry

Abstract Titanium(IV) dissolved in sea water can be determined using adsorptive cathodic stripping voltammetry in the presence of mandelic acid. The method is improved by the addition of chlorate to the sample, which causes the peak current to increase by more than an order of magnitude owing to the reoxidation of the Ti(III) (mediated by the chlorate), which was produced at the electrode surface during the potential scan, to Ti(IV), where it again contributed to the reduction current. The sensitivity of the voltammetric technique was thus improved by a factor of 20, and the limit of detection was lowered to 7 pM with 60 s adsorption (ca. 1 pM with 600 s adsorption), sufficiently low to determine titanium in water of oceanic origin. The method was applied to the determination of titanium in the estuary of the River Mersey. The titanium concentration was found to vary between 0.2 and 0.3 nM (at salinities of 32–33) and 1.5 nM (at salinities of 1–4), revealing a conservative behaviour at salinities above 20 and some removal at low salinities.

Simultaneous determination of titanium and molybdenum in natural waters by catalytic cathodic stripping voltammetry

Abstract A procedure is set out by which titanium and molybdenum can be determined simultaneously in freshwater using cathodic stripping voltammetry preceded by adsorptive collection of their complexes with mandelic acid. The voltammetric procedure to determine molybdenum in natural waters is optimized. The optimal solution composition includes 4 mM mandelic acid, 45 chlorate, and a pH of 1.9. The reaction is characterized by a peak at −0.48 V vs. Ag/AgCl owing to the electrochemical reduction of M(VI)-mandelate to Mo(V), the re-oxidation of Mo(V) by the added chlorate to Mo(VI) and the re-reduction of the thus generated Mo(VI) to Mo(V) at the electrode surface. Thus the reduction current is catalytically enhanced by the chlorate. The limit of detection of the procedure is 1.4 pM with an adsorption time of 60 s using a 10 ml aliquot of sample; this analytical limit can be lowered further if required by extending the deposition time. Titanium (peak at −0.95 V) and molybdenum can be determined simultaneously at pH 3.1. Measurements in lake waters reveal subnanomolar levels of these elements. The data illustrate the extraordinary sensitivity of cathodic stripping voltammetry for certain single-electron reduction steps which can be enhanced catalytically.

Speciation of aluminium, chromium and titanium in the NW Mediterranean

Abstract The chemical speciation of aluminium, chromium and titanium was determined in samples from the NW Mediterranean using adsorptive cathodic stripping voltammetry (CSV). Total dissolved concentrations of aluminium and titanium were determined after UV-irradiation of acidified sample aliquots, whereas reactive concentrations were determined in frozen samples and acidified samples. The redox speciation of chromium was determined by measuring the concentration of chromium before and after reaction of the sample with an added chelating agent to mask reactive chromium(III). The total dissolved concentration of aluminium increased with sample depth from ∼60 to ∼150 nM in agreement with other data for the Mediterranean, whereas titanium had a concentration between 0.10 and 0.15 nM throughout the water column. On average 72% of aluminium and 67% of titanium were found to occur in a non-reactive form, either organically complexed or as inorganic colloidal matter. Part of the non-reactive fraction was released upon prolonged storage of acidified sample aliquots suggesting that this part could be ascribed to either inorganic colloidal matter or organic complexation. However, the remainder of the non-reactive fraction was released by UV-digestion of the organic matter indicating that this fraction was organically bound. The predominant oxidation state of chromium was chromium(VI). However, significant levels (up to 1.5 nM) of chromium(III) occurred in surface waters and at ∼ 200 m depth. Likely sources for this fraction include photochemical reduction in the surface waters and mineralisation of algal matter for the subsurface maximum.

Voltammetric determination of dissolved iron and its speciation in freshwater

Analytical methods were developed to determine the concentration of total dissolved iron and its chemical speciation in freshwater using cathodic stripping voltammetry (CSV) with 1-nitroso-2-naphthol (NN) at pH 8.1. The concentrations of total dissolved iron in river water that iron concentration was certified and in natural water samples from Lake Kasumigaura were determined successfully. The natural iron ligand concentration and the conditional stability constant were determined by ligand competition between NN and the natural ligands present in the sample. In the water samples from Lake Kasumigaura, the concentrations of total dissolved iron and natural ligand were 47.8 ± 4.4 nM and 80.0 ± 19.6 nM and the conditional stability constant (K′FeL) was 1025.9±0.4 M−1 (n = 3). The value of K′FeL was greater than any reported K′FeL for seawater. More than 99.9% of the dissolved iron existed as organic species due to the very high value of the conditional stability constant. The inorganic iron concentration calculated from these results was 10−13.4 M, indicating that the inorganic iron level in Lake Kasumigaura was similar to that in the open ocean and therefore that iron can be a limiting factor for algal growth in Lake Kasumigaura. This is the first report of the complexation of iron(III) and inorganic iron levels in lake water determined by CSV.

Direct determination of trace concentrations of lead in fresh water samples by adsorptive cathodic stripping voltammetry of a lead : Calcein Blue complex

Abstract A very sensitive cathodic stripping voltammetric method for the determination of lead, with adsorptive collection of complexes with Calcein Blue (8-[ N , N -bis(carboxymethyl)aminomethyl]-4-methylumbelliferone) on to a hanging mercury drop electrode is developed. After accumulation of the complex, the electrode potential is scanned in a negative direction with differential pulse modulation and the reduction current for the lead complex is measured at −0.5 V. Under optimum conditions (300 nM Calcein Blue, pH 6.5, adsorption at −0.15 V), a 3σ limit of detection of 0.04 nM after 1 min collection was obtained. Interference from iron can be decreased by the addition of citrate ion. The method is successfully applied to standard river water SLRS-2 and various fresh water samples.

Studies on the structure of molybdenum(VI) in acidic solution by xanes and exafs

Abstract X-ray absorption spectra for molybdenum(VI) in acidic solutions were measured and their XANES and EXAFS were analysed. The cationic monomer MoO22+ is present in HClO4, H2SO4 and HNO3. In HCl the coordination of chloride ions to MoO22+ was observed. Diethyl ether extracts MoO2Cl2 selectively from a HCl solution of molybdenum(VI).

Effective UV photolytic decomposition of organic compounds with a low-pressure mercury lamp as pretreatment for voltammetric analysis of trace metals

The effectivity of UV irradiation with low and high pressure mercury (L-Hg and H-Hg) lamps on the decomposition of organic compounds in aqueous solutions, as pretreatment for the voltammetric determination of trace metals, is compared. The photolytic decomposition with the L-Hg lamp was much faster than with the H-Hg lamp. The higher efficiency of the L-Hg lamp is caused by its greater light intensity at short wavelengths. Interferences of organic compounds on the voltammetric determination of nickel and indium were eliminated successfully by ∼90 min irradiation with the L-Hg lamp. Humic acid and organic interference with the voltammetric determination of nickel in natural river water were successfully eliminated. The decomposition using the L-Hg lamp can be carried out without added oxidizing reagents and at room temperature, thus eliminating loss of water samples by evaporation at higher temperature.

Studies on the structure of the paramagnetic μ-oxo dimer of molybdenum(V) in hydrochloric acid solutions by exafs

Abstract X-ray absorption spectra for molybdenum(V) in hydrochloric acid solutions were measured and their XANES and EXAFS were analysed. The μ-oxo dimer of Mov was detected in 4.5–6.0 M HCl solutions. The Mo: in the dimer is 2.94–3.06 A and the Mo ;OMo angle is ca 104–111°.

Dimerization of electrogenerated Mo(V) in sulphuric acid solutions

Abstract The electrochemical behaviour of molybdenum(VI) in sulphuric acid solutions was investigated by cyclic voltammetry. In the reduction of Mo(VI) to Mo(III) a dimerization reaction of Mo(V) is involved; the rate constant for the reaction was estimated to be 2.79×102 M−1 s−1 and the activation energy was ca. 35 kJ mol−1 in 0.1 M H2O4. Oxidation of the monomer and dimer Mo(V) species take place at −0.31 and +0.18 V (vs. SCE), respectively.

Differential pulse and alternating current polarographic study of Mo(VI) in sulfuric acid solutions

Abstract The electrochemical behavior of Mo(VI) was studied by differential pulse and alternating current polarography. The effect of the addition of ionic surfactants on the first three peaks corresponding to the reduction of Mo(VI) to Mo(V) indicates that the Mo(VI) complexes giving the first two peaks and the thrid, are anionic and cationic respectively. A tensammetric wave was observed in the DP and ac polarograms of Mo(VI).