Masashi Hojo

Arsenic speciation including 'hidden' arsenic in natural waters

Recent studies indicate the existence in natural waters of ‘hidden’ arsenic which had previously been undetected by the hydride generation technique. A speciation method for arsenic species has been developed in which hidden arsenic was classified into two fractions by their lability to the photochemical degradation procedure: the ultraviolet-labile fraction and the ultravioletresistant fraction. The ultraviolet-labile fraction was the major fraction of hidden arsenic and comprised 15‐45% and 4‐26% of the total arsenic in Uranouchi Inlet and Lake Biwa (Japan), respectively. The highest concentration of the ultraviolet-resistant fraction was observed in Uranouchi Inlet during the summer, in which dimethylarsinic acid increased in the water column. We discuss the hidden arsenic fraction as the key to explaining arsenic speciation in natural waters. Copyright # 1999 John Wiley & Sons, Ltd.

Spectrophotometric, polarographic and conductometric evidence for triple ion formation from benzenesulfonate and diphenyl phosphate salts in protophobic aprotic solvents

Abstract The formation of triple cations (C 6 H 5 SO − 3 (Li + 2 ) in acetonitrile was shown by re-resolution of C 6 H 5 SO 3 Li precipitate with the addition of a large excess of LiClO 4 to tetraethylammonium benzenesolfonate (Et 4 N C 6 H 5 SO 3 ). The conductivities of tributylammonium benzenesulfonate in benzonitrile and nitrobenzene were explained by symmetrical triple ion formation (M 2 X + and MX − 2 ) in addition to ion pair formation (MX), while in acetonitrile by ion pair formation alone. Tributylammonium diphenyl phosphate ( n -Bu 3 NH(PhO) 2 PO 2 ) formed triple ions even in propylene carbonate with the high dielectric constant (e r = 64.4 at 25°C). The present study and a recalculation of previous work have shown that the association of R 3 NH + X − (R = Et or n -Bu) in benzonitrile increases in the following order: NO − 3 6 H 5 SO − 3 p -toluenesulfonate 3 SO − 3 , while the K a values by the Shedlovsky analysis increase in the opposite order.

Spectroscopic and voltammetric studies on the formation of Keggin-type V(V)-substituted tungstoarsenate(V) and -phosphate(V) complexes in aqueous and aqueous-organic solutions

Abstract The tetra-n-butylammonium salts of [AsVxW12−xO40](3+x)− (x=1–2) were isolated from an acidic aqueous-CH3CN solution and characterized by elemental analysis, FT-IR, UV–Vis and cyclic voltammetry. The formation conditions of Keggin-type V(V)-substituted tungstoarsenate(V) and -phosphate (V) complexes were elucidated by cyclic voltammetry, Raman spectroscopy, and 31P NMR in aqueous and aqueous-organic solutions. It was found that water-miscible organic solvents such as acetonitrile and acetone, affected the conversion processes of [XW12O40]3− into [XVxW12−xO40](3+x)− (X=P or As). It was suggested that these results were due to the stabilities of [XW12O40]3− in aqueous and aqueous-organic solution.

Hydrogen bonding in alcoholic beverages (distilled spirits) and water-ethanol mixtures.

The hydrogen-bonding properties of water-ethanol of alcoholic beverages and water-ethanol mixtures of the corresponding ethanol contents were examined on the basis of OH proton NMR chemical shifts and the Raman OH stretching spectra of water and ethanol. Japanese shochu, an unaged distilled spirit of 25% (v/v) alcoholic content made from various grains, was provided for the samples; it is a high-purity spirit as it contains only small amounts of dissolved components, like typical vodka, gin, and white rum. The hydrogen-bonding structure in shochu containing some acids was found to be different from that of the water-ethanol mixture with corresponding ethanol content. It was concluded that, by the presence of small amounts of organic acids, the water-ethanol hydrogen-bonding structure was strengthened, at the same time, the proton exchange between water and ethanol molecules was promoted in shochu, compared with the water-ethanol mixture. The NMR chemical shifts of fruit cocktail drinks suggested that the hydrogen bonding of water-ethanol in the solution was developed by organic acids and (poly)phenols from fruit juices.

Triple-ion formation and acid–base strength of ions in protophobic aprotic solvents

The formation of symmetrical triple ions [2M++ X–⇌ M2X+, (K2); M++ 2X–⇌ MX–2, (K3); K2=K3] and the quadrupole [M2X++ X–⇌ M2X2, (K4); M++ MX–2⇌ M2X2, (K5); K4=K5], in addition to ion-pair formation [M++ X–⇌ MX, (K1)] from various uni–univalent salts were examined by means of conductometry in acetonitrile, benzonitrile and propylene carbonate. The salts are made from bases (B) and acids (HX) of varying strength. For diethylcyclohexylammonium chloride [(0.4–6.0)× 10–3 mol dm–3 in benzonitrile], the calculated molar conductivities (Λ/S cm2 mol–1) were fitted to the observed ones within 0.36% of the standard deviation of the relative error, considering the symmetrical formation of the triple ions. In the higher concentration range [(0.4–12.0)× 10–2 mol dm–3] of the salt a minimum was observed in the relation between Λ and C1/2. The observed minimum could be reproduced in terms of the large formation constants of the triple ions and the quadrupole complex by computer simulations. However, a larger formation constant of the quadrupole above the critical value caused the minimum to disappear. Weaker basicities of amines tended to give higher formation constants of the triple ion; however a levelling-off was observed below pKBH+= 18 in acetonitrile. On the other hand, the formation constants for the ion pair and the triple ions from salts with different anion basicities (from perchlorate to 3,5-dinitrobenzoate) were proportional to the acidities of the corresponding acids.

Voltammetric and Raman spectroscopic study on the formation of Keggin-type V(V)-substituted molybdoarsenate complexes in aqueous and aqueous-organic solution

Abstract The formation and conversion processes of V(V)-substituted molybdoarsenate complexes with the Keggin structure were studied by cyclic voltammetry and Raman spectroscopy. It was found that water-miscible organic solvents such as acetonitrile, acetone, ethanol, 1,4-dioxane and so on played an important role in order to form [AsVMo 11 O 40 ] 4− and [AsV 2 Mo 10 O 40 ] 5− complexes stably. These complexes were isolated as tetrabutylammonium salts and characterized by IR and Raman spectrometry and cyclic voltammetry. In addition, the V(V)-substituted molybdoarsenate complexes were shown to catalyze the electrochemical reduction of free V(V) ions to V(IV) ions in acidic aqueous–CH 3 CN solution. The formation conditions of V(V)-substituted molybdoarsenate complexes were also corresponded with those of V(V)-substituted molybdophosphate complexes in both aqueous solution and aqueous–CH 3 CN solution.

Conductometric identification of triple-ion and quadrupole formation by the coordination forces from lithium trifluoroacetate and lithium pentafluoropropionate in protophobic aprotic solvents

The molar conductivities (Λ/S cm2 mol–1) of LiCF3CO2 and LiC2F5CO2 in acetonitrile, benzonitrile, nitromethane or propylene carbonate have been explained in terms of symmetrical triple-ion formation (2M++ X–⇄ M2X+, K′a, 2 and M++ 2X–⇄ MX–2, K′a, 3; K′a, 2=K′a, 3) and quadrupole formation (M2X++ X–⇄ M2X2, K′a, 4 or M++ MX–2⇄ M2X2, K′a, 5; K′a, 4=K′a, 5) in addition to the ion pair formation (M++ X–⇄ MX, K′a, 1) in the concentration range (0.4–6.0)× 10–3 mol dm–3. Surprisingly, a great enhancement in quadrupole formation for LiCF3CO2 and LiC2F5CO2 was observed in propylene carbonate with the highest relative permittivity (Iµr= 64.4 at 25 °C) of all the solvents. For trifluoroacetate, the limiting molar conductivity (Λo= 72.65) given by the Shedlovsky analysis [(0.4–4.0)× 10–3 mol dm–3] was much larger than that [Λo, calc= 26.37] calculated by Kohlrauschs additivity law with strong electrolytes. Lithium pentafluoropropionate gave a similar excess in the Λo value. Computer simulations showed an increase in the Shedlovsky Λo value with increase in the quadrupole formation constant. At the same time, the apparent association constant (M++ X–⇄ MX, Ka) calculated by Shedlovsky analysis was 10 times larger than the ion-pair formation constant (K′a, 1) in propylene carbonate (owing to strong quadrupole formation) and was much smaller than the K′a, 1 value in the other solvents (mainly owing to strong triple-ion formation). A distinct triple-ion formation from tributylammonium trifluoroacetate or tributylammonium pentafluoropropionate was observed in benzonitrile. Causes of the failure in the Shedlovsky analysis have been discussed from the standpoint of higher-ion aggregates.

Polarographic study on the interaction between alkali metal cations and acyclic polyamines in acetonitrile

Abstract Two anodic waves (E 1 2 = −0.50 and −0.60 V vs. Ag/0.1 M AgClO4MeCN) were produced by triethylenetetramine (Trien N4) in acetonitrile containing 0.1 M Et4NClO4 as the supporting electrolyte. The waves were attributed to the following reaction: Hg + Trien ⇆ 1 2 [Hg2 (trien)2]2+ + e− ⇆ [Hg(trien)]2+ + e− Tetraethylenepentamine (Tetren, N5) also gave two anodic waves; however, the second wave (the more positive one) was extremely irreversible. The second wave from diethylenetriamine (Dien, N3) appeared at a much more positive potential. With 0.1 M LiClO4 and NaClO4, the two anodic waves of trien were combined into a single wave (two-electrone process) at −0.40 and −0.49 V, respectively. The complex formation constants between alkali metal cations and trien (1:1) were obtained not only from the positive shift of the anodic wave from trien but also from the positive shift of the cathodic wave of [Hg(trien)]2+ in the prsence of a large exces of Li+ or Na+. The negative shift of the cathodic wave of Na+ in the presence of trien gave the same complex formation constant (log K = 2.7) as those obtained by the above two methods. Dien and even ethylenediamine (En, N2) interacted with Li+ (log K = 5.4 for [Li(en)2]+).

Specific interactions between anions and cations in protophobic aprotic solvents. Triple-ion and higher aggregate formation from lithium and tributylammonium thiocyanates

Triple-ion and quadrupole formation in addition to ion-pair formation from lithium and tributylammonium thiocyanates has been examined by means of conductometry, in several protophobic aprotic solvents: nitrobenzene, benzonitrile, acetonitrile, nitromethane and propylene carbonate. The formation constants of the ion pair (M++ SCN–⇌ MSCN, Ka1; where M+= Li+ or n-Bu3NH+), symmetrical triple ions [2M++ SCN–⇌(M+)2SCN–, Ka2; M++ 2 SCN–⇌ M+(SCN–)2, Ka3; Ka2=Ka3] and the quadrupole [(M+)2SCN–+ SCN–⇌(MSCN)2, Ka4; M++ M+(SCN–)2⇌(MSCN)2, Ka5=(Ka5) were evaluated after correction of the activity coefficients of the ions. Effects of the ratio ΛT/Λ0 on the conductivities were examined, where ΛT and Λ0 are the molar conductivities at infinite dilution of the triple ions and simple ions, respectively. A remarkable enhancement of the triple-ion formation from LiSCN was observed in nitromethane: Ka′1= 5.5 × 104, Ka′2=Ka′3= 8.0 × 107 and ΛT/Λ0= 0.4 (the effects of the ionic atmosphere and viscosity changes are taken into account). At higher salt concentrations, quadrupole formation was observed, which caused the disappearance of the minimum in the Λ–C1/2 curve. In contrast, the degree of triple-ion formation from n-Bu3NHSCN was much smaller than that from LiSCN and quadrupole formation was not observed in all the solvents. The formation of (Li+)2SCN– was suggested by spectrophotometry. Both the donor and acceptor numbers of the solvents were concerned with the formation of higher ion aggregates.

Role of triple ion formation in the acid–base reaction between tropolone and triethylamine in acetonitrile

The acceleration of the proton-transfer reaction between tropolone (2-hydroxycyclohepta-2,4,6-trien-1-one) and triethylamine by the addition of various salts in acetonitrile was examined by UV–VIS spectroscopy. The addition of lithium perchlorate to the tropolone–triethylamine solution caused the formation of a cationic ‘triple ion’, C7H5O2–(Li+)2(λmax= 393 nm): C7H5O2H–NEt3+ 2 Li+⇄ C7H5O2–(Li+)2+ Et3NH+; the ‘free’ tropolonate ion (Bu4NC7H5O2) gave a band at 414 nm in acetonitrile. The effects of Na+ on the reaction were much smaller than those of Li+. Alkaline-earth metal ions (M2+= Mg2+, Ca2+, Sr2+ and Ba2+) promoted the deprotonation of tropolone by forming the species C7H5O2–(M2+). On the other hand, the addition of Et4NCl to the tropolone–triethylamine mixture produced the ‘free’ tropolonate ion (λmax= 414 nm): C7H5O2H–NEt3+ 2 Cl–⇄ C7H5O2–+ Et3NH+(Cl–)2. The decrease in the amount of C7H5O2– formed with Et4NBr or Et4Nl was attributed to a decrease in the formation constants of the anionic ‘triple ions’[Et3NH++ 2 X–⇄ Et3NH+(X–)2]. Thus, ‘the salt effects’ upon the deprotonation of the weak acid by the amine were explained quantitatively by the ‘coordination’ reactions and not merely by ion-pair exchange reactions.