Toshitaka Hori

A voltammetric study of Keggin-type heteropolymolybdate anions

Abstract The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo12O40]n− (XS, P, As, V, Si, Ge; n = 2–4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.

The structure of 18-molybdodisulphate(VI)(4–) ion in (NEt4)4S2Mo18O62·CH3CN

The structure of (NEt4)4S2Mo18,O62·CH3CN has been determined by the single-crystal X-ray method. The crystals are triclinic, space group P, with cell parameters a= 15.062(4), b= 23.296(4), c= 13.813(4)A, and α= 97.66(4), β= 115.98(2), γ= 84.63(4)°, and Z= 2. The structure model was refined by full-matrix least squares to R′= 0.063 for 1 1 700 reflections with Fo > 3σ(F). The existence of the [S2Mo18O62]4– heteropolyanion in the crystals has been confirmed. It is composed of two SO4 tetrahedra and eighteen MoO6 octahedra linked to give the Dawson α-[P2W18O62]6– structure. The anions are separated by NEt4+ cations and CH3CN solvate molecules.

A GEOCHEMICAL STUDY ON THE SPECIFIC DISTRIBUTION OF BARIUM IN LAKE BIWA, JAPAN

The vertical distribution of alkaline earth elements (AEEs) was measured in a mesotrophic lake, Lake Biwa, of Japan. By comparing the distribution profiles of the elements, it was noticed that the dissolved barium concentration markedly decreased in the bottom layer during the stagnation stage of the lake. This characteristic distribution disappeared in the circulation stage but appeared repeatedly in the succeeding stagnation stage. Such periodic variation of the barium distribution profile contrasts with the result that other AEEs were uniformly distributed without regard to depth or stage of lake water circulation. Adsorption of AEEs onto synthetic hydrous manganese oxide (HMO) was examined in lake water. The results suggest that the specific distribution of barium observed in Lake Biwa was caused by the selective adsorption of the element onto HMO, which is naturally produced at the bottom of the lake.

Distribution of dissolved organic carbon in lakes of different trophic types

The distributions of dissolved organic carbon (DOC) in the warm season were elucidated in ten lakes of different trophic types in Japan, Russia, and China. DOC showed similar vertical distributions in all the lakes in summer when thermal stratification occurred. DOC in the epilimnion was higher than the value of 0.8 mg C l−1 found in the hypolimnion. In three Japanese lakes, hypolimnion DOC was negatively correlated with apparent oxygen utilization (AOU), reflecting the net oxidation of DOC using the dissolved oxygen in lake water. The DOC : O2 ratios (0.115–0.179), calculated by the slopes of the regression lines of DOC versus AOU in hypolimnion water, were as low as those of deep-sea water, which indicates low bioavailability of lake water DOC for heterotrophic bacteria. DOC and conductivity did not correlate well except in two Japanese lakes: one showed a positive correlation and the other a negative correlation, indicating DOC loading from the inflowing rivers. Eutrophic lakes tended to have higher DOC values than meso- and oligotrophic lakes, and DOC values in the surface water negatively correlated with Secchi depths.

Air-segmented continuous-flow analysis for vanadium based on a catalytic reaction with Bindschedler's green leuco base

Abstract A method for the determination of vanadium at trace levels was investigated using air-segmented continuous-flow analysis based on a catalytic reaction with bindschedlers green leuco base. The detection limit for vanadium was 21 pM and the reproducubility at the 10 nM level was 0.6%. Vanadium in natural waters can be directly determined by this method.

Geochemical behavior of trace elements in Lake Biwa

A monthly survey of dissolved concentrations of various trace elements was performed in Lake Biwa. Particulate concentrations of the elements were also measured in early autumn and winter. Based on these results, the geochemical behaviors of trace elements are discussed. The redox-sensitive elements Mn and Fe showed characteristic vertical distribution profiles. Profiles of Mn changed drastically with the progression of the stagnation period. The dynamics of Ba were affected by the redox cycle of Mn. Dissolved V concentration showed a clear seasonal variation. In contrast, dissolved concentrations of Sr, Mo, Cu, Zn, and Ni were almost uniform, i.e., not dependent on the season or the depth. The distribution ratios of these elements between lake water and Mn nodules formed in the lake were calculated to assess their geochemical behaviors.

Direct spectrophotometric determination of sulphate ion based on the formation of a blue molybdosulphate complex

In an aqueous medium containing 50%V/V acetone, sulphate ion reacts with a mixed reagent consisting of MoV-MoVI-HCl to give a blue molybdosulphate complex, the amount formed being proportional to the sulphate ion present. Using this colour forming reaction, sulphate ion could be determined spectrophotometrically, and the limit of detection (signal to noise ratio = 3) was 3.1 × 10–6M. When combined with an extraction-concentration of the blue complex, the proposed method gave a lower limit of detection, viz., 1.0 × 10–6M. Phosphate, arsenate and germanate ions interfered owing to the formation of similar blue complexes at concentrations 1.0 × 10–5M, but the presence of 1.0 × 10–4M silicate ion and 1.0 × 10–2M chloride ion caused no significant interference. Sulphate in natural waters, particularly saline waters, was determined and the results showed good agreement with those obtained by ion chromatography or inductively coupled plasma atomic emission spectrometry.

RAMAN STUDIES ON THE FORMATION OF 12-MOLYBDOPYROPHOSPHATE

Abstract The formation of a yellow 12-molybdopyrophosphate complex was ascertained in both solid and solution states. A Raman spectroscopic study revealed the spontaneous transformation of the parent 18-molybdopyrophosphate anion, [(P2O7)Mo18O54]4−, via the 15-molybdopyrophosphate anion, [H6(P2O7)Mo15O48]4−, into the 12-molybdopyrophosphate anion, [H12(P2O7)Mo12O42]4− in the CH3CN–water system. In contrast, 12-molybdopyrophosphate was the only heteropoly species formed in aqueous solution. It turned out that the spectrophotometric determination of pyrophosphate with the Fiske–Subbarow reagent and the Rucena-Conde and Prat reagent was based on the formation of the mixed-valence blue species of 12-molybdopyrophosphate.

Crystal structure of sodium tris(tetramethylammonium) pentamolybdodisulfate(VI) tetrahydrate, Na[NMe4]3S2Mo5O23.4H2O

The structure of Na[NMe4]3S2Mo5O23·4H2O has been determined by the single-crystal X-ray method: monoclinic, space group P2/c, a= 29.25(2), b= 12.85(1), c= 18.24(1)A, and β= 123.89(2)° and Z= 6. The structure was refined by full-matrix least squares to R′= 0.0678 for 5532 reflections. The existence of the Strandberg-type heteropolyion [S2Mo5O23]4–1 in the crystals has been confirmed. The geometry of 1 was compared systematically with various analogues containing SIVO32–, PO43–, MePO32– or NH3C2H4PO3– as the central polyhedron. The UV spectrum of the salts Na[NMe4]3S2Mo5O23·4H2O and [NMe4]4S2Mo5O23 in neat MeCN, hence the kernel heteropolyion 1 showed absorption maxima at λ= 213 and 243 nm (Iµ= 3.2 × 104 and 3.3 × 104 dm3 mol–1 cm–1). Spectroscopic measurements also indicate that 1 and [Mo6O19]2– form simultaneously in aqueous–MeCN media containing MoVI and H2SO4.

Formation and conversion of yellow heteropoly complexes in a Mo(VI)Se(IV), Te(IV) system

Abstract In acidic solution containing CH 3 COCH 3 or CH 3 CN, Mo(VI) reacted directly with SeO 3 2− to form the previously unknown heteropoly anion, [(SeO 3 ) 2 Mo 15 O 45 ] 4− , which was subsequently transformed into the [(SeO 3 ) 2 Mo 12 O 36 ] 4− anion. Similar formation and transformation processes were also found for a Mo(VI)TeO 3 2− HClCH 3 COCH 3 or CH 3 CN system. Their formation conditions were elucidated by a combined Raman and voltammetric study. The [(SeO 3 ) 2 Mo 12 O 36 ] 4− and [(TeO 3 ) 2 Mo 12 O 36 ] 4− anions were also formed in aqueous acidic solution containing Mo(VI) and SeO 3 2− or TeO 3 2− .