Evaluation of electrodes coated with metal hexacyanoferrate as amperometric sensors for nonelectroactive cations in flow systems

Two electrodes modified with either nickel or cupric hexacyanoferrate films were evaluated and compared as sensors for nonelectroactive cations in a flow-injection system. Both gave responses for group 1A and ammonium ions, but only the electrode modified with cupric hexacyanoferrate was sufficiently stable for use in flowing solutions. This electrode responded to K⁺, NH, Rb⁺, and Cs⁺ ions rather selectively. Within this group, the selectivity could be controlled from general to almost specific toward Cs⁺ by the potential at which the electrode was poised. The electrode was compatible with a mobile phase of dilute nitric acid commonly used in ion chromatography, and chromatographic detection limits of 2 × 10⁻⁷ M and linear responses over two decades were obtained. The electrode was applied to the ion chromatographic analysis of K⁺ and NH in urine and K⁺ in blood serum samples.     

Nitrate ion-selective sensor based on electrochemically prepared conducting polypyrrole films

Nitrate-doped polypyrrole (PPy) films on a glassy carbon substrate have been prepared electrochemically in aqueous, acetonitrile, and propylene carbonate solutions for use as nitrate sensors. Lithium nitrate, sodium nitrate, nitric acid, tetraethylammonium p-toluene sulfonate (TS), and tetradodecylammonium nitrate (TDN) were employed as electrolytes. The effect of dibutylphthalate (DBP) as a plasticizer on the sensitivity and lifetime of PPy film sensors was also investigated. A Nernstian behavior with a slope of 56.9 m V/decade over 0.1–7.4 × 10⁻⁵ M NO and a detection limit of 4.7 × 10⁻⁵ M were observed for the polymer sensor prepared in acetonitrile solution containing lithium nitrate and 15% plasticizer (DBP). A lifetime of more than 6 months for this PPy film electrode was obtained.     

Electrochemical behavior of NbV–EDTA and NbV–citrate systems in aqueous solutions

The electrochemical reduction of Nb^V using oxalatoniobic acid and ammonium oxooxalatoniobate salts was studied in aqueous solutions of citric acid and sodium ethylenediaminetetraacetate (EDTA), in a wide range of supporting electrolyte concentrations and pH. In EDTA two reduction processes were observed: Nb^V to Nb^IV, E = −1.000 V vs. Ag/AgCl at pH 4.50 and Nb^IV to Nb^IIIE = −1.400 V vs. Ag/AgCl (pH 4.50). In citric acid there was only one reduction process: (Nb^V to Nb^IV), E = −1.260 V vs. Ag/AgCl at pH 4.50. In both electrolytes a linear relationship was found between the diffusion current and the niobium concentration in the 1.0 × 10⁻⁵ to 5.0 × 10⁻³ M range. Using cyclic voltammetry, it waa observed that the charge transfer process in the Nb^V to Nb^IV process is reversible in EDTA and reversible–quasireversible in citric acid.     

Development of an ion-sensitive field effect transistor based on PVC membrane technology with improved long-term stability

An NH -ISFET sensor based on PVC membrane technology with improved long-term stability has been developed. As a new approach, the plasticizer (tetra-n-undecyl) 3,3′,4,4′ -benzhydroltetracarboxylate (ETH2112) was used in membrane preparation. Its lipophilic nature provides a restricted diffusion of the membrane components to the external solution and improves membrane adhesion to the gate area of the ISFET. The good performance of this plasticizer was confirmed by comparison with usual plasticizers applied in standard ISE technology. Moreover, the durability and stability of the sensor were enhanced by the application of a graphite-epoxy layer as an internal reference between the gate area and the PVC membrane. This composite layer permits the reduction of the optical sensitivity and improves the adherence of the PVC membrane to the ISFET surface. Furthermore, this composite layer acts as a plug, preventing the entrance of water upon the encapsulant-chip interface, thus protecting electrical connections from moisture. As a result, an NH -ISFET with a long-term stability of three months and a sensitivity of −58.7 ± 2.3 mV decade⁻¹ in a linear range of 10⁻⁵ −0.1 mol dm⁻³ has been developed. The application of this sensor to a continuous-flow system has confirmed the feasibility of the technological approach proposed.     

pHh-dependent permselectivity of the fibrinogen-modified carbon paste electrode

The modification of carbon paste matrices with fibrinogen is reported. The effect of the pH of the solution on the CV peak currents of positively or negatively charged redox analytes was examined at the fibrinogen-modified carbon paste electrode. In the presence of the coating, pH-dependent selectivity in electrochemical detection of charged species was demonstrated depending on the sign of the supported charge. Above the isoelectric pH attributed to the immobilized protein (5.5), the current response of anionic redox probes [Fe(CN)/Fe(CN)] was impeded while the response was almost totally restored below this pH. Opposite trends were observed with the Ru(NH₃)/Ru(NH₃) cationic redox analytes.     

Rate constants for the reactions of OH radicals with CH3OCF2CF3, CH3OCF2CF2CF3, and CH3OCF(CF3)2

The rate constants for the reactions of OH radicals with CH₃OCF₂CF₃, CH₃OCF₂CF₂CF₃, and CH₃OCF(CF₃)₂ have been measured over the temperature range 250–430 K. Kinetic measurements have been carried out using the flash photolysis, laser photolysis, and discharge flow methods combined respectively with the laser induced fluorescence technique. The influence of impurities in the samples was investigated by using gas‐chromatography. The following Arrhenius expressions were determined: k(CH₃OCF₂CF₃) = (1.90) × 10⁻¹² exp[−(1510 ± 120)/T], k(CH₃OCF₂CF₂CF₃) = (2.06) × 10⁻¹² exp[−(1540 ± 80)/T], and k(CH₃OCF(CF₃)₂) = (1.94) × 10⁻¹² exp[−(1450 ± 70)/T] cm³ molecule⁻¹ s⁻¹. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 846–853, 1999     

Chain-length dependent termination in pulsed-laser polymerization, 5. The evaluation of the rate coefficient of bimolecular termination kt for the reference system styrene in bulk at 25°C

Following earlier suggestions the values for the rate coefficient of chain termination k_t in the bulk polymerization of styrene at 25°C were formally calculated (a) from the second moment of the chainlength distribution (CLD) and (b) from the rate equation for laser-initiated pseudostationary polymerization (both expressions originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation k_p. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of k_t vs. the number-average degree of polymerization of the chains at the moment of their termination yielded exponents b of 0.16–0.18 in the power-law k_t = A · P_n ^−b, A ranging from 2.3 × 10⁸ to 2.7 × 10⁸ L · mol⁻¹ · s⁻¹. These data are only slightly affected if termination is not assumed to occur by recombination only and a small contribution of disproportionation is allowed for.     

Chain-length dependent termination in pulsed-laser polymerization, 6. The evaluation of the rate coefficient of bimolecular termination kt for the reference system methyl methacrylate in bulk at 25°C

The values for the rate coefficient of chain termination k_t in the bulk polymerization of methyl methacrylate at 25°C were formally calculated (i) from the second moment of the chain-length distribution and (ii) from the rate equation for laser-initiated pseudostationary polymerization (both expressions were originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation k_p. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of k_t vs. the average degree of polymerization of the chains at the moment of their termination v′ yielded exponents b of 0.16–0.17 in the power-law k _t = A · v′^−b, A ranging from 1.1 × 10⁸ to 1.3 × 10⁸ (L · mol⁻¹ · s⁻¹). A 70% contribution of disproportionation to overall termination has been assumed in the calculations.     

Les thio-complexes du thallium en solution aqueuse

The solubility of thallium sulfide in 1M NaClO₄ at 25° ranges between 10⁵M as a function of pH and total sulfide concentration. Soluble thallium was analysed by anodic stripping using the hanging mercury drop electrode. The following complexes have been identified and their stability constants calculated: Tl₂(HS)⁺, Tl(HS), Tl₂(OH) (HS) and Tl₂(OH)₂(HS).     

Syndiospecific polymerization of styrene. II. Monocyclopentadienyltributoxy titanium/methylaluminoxane catalyst

Syndiospecific polymerization of styrene was catalyzed by monocyclopentadienyltributoxy titanium/methylaluminoxane [CpTi (OBu)₃/MAO]. The atactic and syndiotactic polystyrenes were separated by extracting the former with refluxing 2-butanone. The activity and syndiospecificity of the catalyst were affected by changes in catalyst concentration and composition, polymerization temperature, and monomer concentration. Extremely high activity of 5 × 10⁷ g PS (mol Ti mol S h)^?1 with 99% yield of the syndiotactic product were achieved. The concentration of active species, [C^*], has been determined by radiolabeling. The amount of the syndiospecific and nonspecific catalytic species, [C] and [C] respectively, correspond to 79 and 13% of the CpTi(OBu)₃. The rate constants of propagation for C and C at 45°C are 10.8 and 2.0 (M s)^?1, respectively, the corresponding rate constants for chain transfer to MAO are 6.2 × 10^?4 and 4.3 × 10^?4s^?1. There was no deactivation of the catalytic species during a batch polymerization. The rate constant of chain transfer with monomer is 6.7 × 10^?2 (M s)^?1; the spontaneous β-hydride transfer rate constant is 4.7 × 10^?2 s^?1. The polymerization activity and stereospecificity of the catalyst are highest at 45°C, both decreasing with either higher or lower temperature. The stereoregular polymer have broad MW distributions, M?_w/M?_n = 2.8–5.7, and up to three crystalline modifications. The T_m of the s-PS polymerized at 0–90°C decreased from 261.8 to 241°C indicating thermally activated monomer insertion errors. The styrene polymerization behaviors were essentially insensitive to the dielectric constant of the medium.     

Gas phase rate coefficients and activation energies for the reaction of butanal and 2‐methyl‐propanal with nitrate radicals

Rate coefficients have been determined for the reaction of butanal and 2‐methyl‐propanal with NO₃ using relative and absolute methods. The relative measurements were accomplished by using a static reactor with long‐path FTIR spectroscopy as the analytical tool. The absolute measurements were made using fast‐flow–discharge technique with detection of NO₃ by optical absorption. The resulting average coefficients from the relative rate experiments were k = (1.0 ± 0.1) × 10⁻¹⁴ and k = (1.2 ± 0.2) × 10⁻¹⁴ (cm³ molecule⁻¹ s⁻¹) for butanal and 2‐methyl‐propanal, respectively. The results from the absolute measurements indicated secondary reactions involving NO₃ radicals and the primary formed acyl radicals. The prospect of secondary reactions was investigated by means of mathematical modeling. Calculations indicated that the unwanted NO₃ radical reactions could be suppressed by introducing molecular oxygen into the flow tube. The rate coefficients from the absolute rate experiments with oxygen added were and k = (1.2 ± 0.1) × 10⁻¹⁴ and = (0.9 ± 0.1) × 10⁻¹⁴ (cm³ molecule⁻¹ s⁻¹) for butanal and 2‐methyl‐propanal. The temperature dependence of the reactions was studied in the range between 263 and 364 K. Activation energies for the reactions were determined to 12 ± 2 kJ mole⁻¹ and 14 ± 1 kJ mole⁻¹ for butanal and 2‐methyl‐propanal, respectively. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 294–303, 2000     

Ultradrawing of high-molecular-weight polyethylene cast from solution. II. Influence of initial polymer concentration

Ultradrawing of films of high-molecular-weight polyethylene (M?_w = 1.5 × 10⁶) produced by gelation crystallization from solution is discussed. The influence of the initial polymer volume fraction (?) on the maximum draw ratio (λ_max) of the dried films is examined in the temperature region from 90–130°C. The results can be described very well by the relation λ_max = λ ?^?1/2 where λ is the (temperature-dependent) maximum draw ratio of the melt-crystallized film. An attempt is made to discuss the marked influence of the initial polymer volume fraction on λ_max in terms of the deformation of a network with entanglements acting as semipermanent crosslinks.     

Über hexagonale Perowskite mit Kationenfehlstellen. XXIV. Rhomboedrische 9 L-Stapelvarianten in den Systemen Ba3W M □ O9−x/2□x/2 mit MV = Nb,Ta

On Hexagonal Perovskites with Cationic Vacancies. XXIV. Rhombohedral 9 L Stacking Polytypes in the Systems Ba₃W M □O_9?x/2□_x?2 with M^V = Nb, Ta In the system Ba₃WNb□O_9?x/2□_x/2 stacking polytypes of rhombohedral 9 L type (sequence (hhc)₃; space group R3 m) can be prepared with ～1/3 ? × ? 2. For x = 2(Ba₃Nb₂□O₈□) two modifications are formed. In the corresponding Ta system the phase with is reduced to a smaller region with x ? 1/3.     

Kinetic study of azidopentaammine cobalt(III) complex as photoinitiator and termination agent of N‐vinylpyrrolidone radical polymerization

N‐Vinylpyrrolidone polymerization photoinitiated at 365 and 546 nm by azidopentaammine cobalt(III) {[Co(NH₃)₅N₃]²⁺} was investigated at room temperature in an argon atmosphere. By excitation into the ligand to metal charge transfer (LMCT), the cobalt complex showed an efficient photoredox process leading to the formation of a cobalt(II) and an azide radical (N, Φ_photoredox = 0.24). The same process was found to occur by excitation into the ligand field band with a low but not negligible quantum yield (Φ_photoredox = 0.016). Two different domains were clearly present when the plot of the rate of polymerization as a function of the cobalt(III) complex was studied; for [Co(III)] < 2.0 × 10⁻⁴ M, the termination step mainly involved a mutual annihilation of growing radicals whereas an oxidative termination was present in the range of 2.0 × 10⁻⁴ M < [Co(III)] < 1.0 × 10⁻³ M. Within the former domain the rate of polymerization (R_p ) varied with the first power of the monomer concentration and with the square root of the absorbed light intensity while for the latter domain the R_p was proportional to the monomer concentration and absorbed light intensity. Further investigations using the viscosity‐average molecular weight data allowed us to corroborate the proposed polymerization mechanism. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3997–4005, 2000     

Substituent Effects on the C-C Bond Strength, 15[1] Geminal Substituent Effects, 7[2]. Thermochemistry and Thermal Decomposition of Alkyl-substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)₃C⁻ anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol⁻¹, ΔS ( 10c ) = 58· 5 J mol⁻¹ K⁻¹]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol⁻¹), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol⁻¹), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol⁻¹), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol⁻¹), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol⁻¹), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol⁻¹) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol⁻¹ was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol⁻¹) and α,α-dicyanoalkyl (27 kJ mol⁻¹) radicals, which were recalculated from bond homolysis measurements^[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).     

Lactone polymers. II. Hydrodynamic properties and unperturbed dimensions of poly-ε-caprolactone

Poly-ε-caprolactone prepared by a dibutylzinc-catalyzed bulk polymerization process was fractionated, and the solution properties of the fractions were studied in benzene and in dimethylformamide. In these solvents at 30°C the Mark-Houwink relations were [η] = 9.94 × 10^?5 M and [η] = 1.91 × 10^?4 M , respectively. The value of K_Θ was found to vary from 1.1 to 1.2 × 10^?3 when determined by three known extrapolation techniques. Poly-ε-caprolactone chains appear to be quite flexible in solution, and the steric hindrance parameter σ had the low value of 1.37. Root-mean-square end-to-end dimensions were approximated from the experimental data and calculated from the Debye-Bueche and the Kirkwood-Riseman theories.     

Nonlinear dynamics in the oxidations of substituted thioureas I: The reaction of 4‐methyl‐3‐thiosemicarbazide with acidic iodate [1]

The substituted thiourea, 4‐methyl‐3‐thiosemicarbazide, was oxidized by iodate in acidic medium. In high acid concentrations and in stoichiometric excess of iodate, the reaction displays an induction period followed by the formation of aqueous iodine. In stoichiometric excess of methylthiosemicarbazide and high acid concentration, the reaction shows a transient formation of aqueous iodine. The stoichiometry of the reaction is: 4IO + 3CH₃NHC(S)NHNH₂ + 3H₂O → 4I⁻ + 3SO + 3CH₃NHC(O)NHNH₂ + 6H⁺ (A). Iodine formation is due to the Dushman reaction that produces iodine from iodide formed from the reduction of iodate: IO + 5I⁻ + 6H⁺ → 3I₂(aq) + 3H₂O (B). Transient iodine formation is due to the efficient acid catalysis of the Dushman reaction. The iodine produced in process B is consumed by the methylthiosemicarbazide substrate. The direct reaction of iodine and methylthiosemicarbazide was also studied. It has a stoichiometry of 4I₂(aq) + CH₃NHC(S)NHNH₂ + 5H₂O → 8I⁻ + SO + CH₃NHC(O)NHNH₂ + 10H⁺ (C). The reaction exhibits autoinhibition by iodide and acid. Inhibition by I⁻ is due to the formation of the triiodide species, I, and inhibition by acid is due to the protonation of the sulfur center that deactivates it to further electrophilic attack. In excess iodate conditions, the stoichiometry of the reaction is 8IO + 5CH₃NHC(S)NHNH₂ + H₂O → 4I₂ + 5SO + 5CH₃NHC(O)NHNH₂ + 2H⁺ (D) that is a linear combination of processes A and B. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 193–203, 2000     

L'action de l'ammoniac sur l'hydroxyde de cadmium et la stabilité des complexes en milieu aqueux

The solubility of precipitated Cd(OH)₂ was determined at 25°C in 1 M NaClO₄, as a function of pH and of the ammonia content of the solutions. Formation constants were obtained for the following hydroxo, ammine and hydroxo-ammine complexes: CdOH⁺, Cd(OH)₂, Cd(OH), CdNH, Cd(NH₃), Cd(NH₃), Cd(NH₃) and Cd(OH)₂NH₃. The solubility product of the hydroxide was also calculated. The presence of polynuclear species was investigated by titrimetric determinations of the hydrogen ion concentration at constant metal concentration.     

Nitridsulfidchloride der Lanthanide: II. Der Formeltyp M6N3S4Cl (M = La?Nd)

Nitride Sulfide Chlorides of the Lanthanides. II. The Composition M₆N₃S₄Cl (M = La? Nd) The oxidation of the “light” lanthanides (M = La? Nd) with sulfur and NaN₃ in the presence of the chlorides MCl₃ yields chlorine-poor nitride sulfide chlorides with the composition M₆N₃S₄Cl when appropriate molar ratios of the reactants are used. Additional NaCl as a flux secures complete and fast reactions (7 d) at 850°C in evacuated silica vessels as well as single-crystalline products (red-brown needles). The crystal structure was determined from X-ray single crystal data for the limiting representatives La₆N₃S₄Cl (orthorhombic, Pnma (no. 62), Z = 4, a = 1159.7(4), b = 410.95(7), c = 2756.8(9)pm, R = 0.030, R_w = 0.027) and Nd₆N₃S₄Cl (a = 1137.1(3), b = 399.34(6), c = 2687.6(9)pm, R = 0.034, R_w = 0.033). Guinier powder data revealed the cerium and praseodymium analogues to be isotypic. The crystal structure exhibits two different chains of connected [NM₄] tetrahedra which are commensurate in translation. Six crystallographically different M³⁺ are present, two of them (M1 and M2) build up the chain [(N1)(M1) · (M2)]³⁺ together with (N1)^3? by cis-edge connection of tetrahedra. The four remainders (M3? M6) arrange as pairs [N₂M₆] of edge-shared [NM₄] tetrahedra with (N2)^3? and (N3)^3? which are further connected via four vertices to form the [(M5)(N-2){(M3)_(1+1)/(1+1)(M4)_(1+1)/(1+1))}^e(N3)(M6)]⁶⁺ double chain. Bundled along [010] like a closest packing of rods, both types of chains are held together by five crystallographically different but by X-ray diffraction indistinguishable anions S^2? (S1? S4) and Cl^? adjusting the charge balance in a molar ratio of 4:1.     

Über geordnete Perowskite mit Kationenfehlstellen. XI. Verbindungen vom Typ ABB□1/4WVIO6 ≙ ABIIB□WO24 mit AII,BII = Ba,Sr

On Ordered Perovskites with Cationic Vacancies. XI. Compounds of Type A B B □_1/4W^VIO₆ ? A B^IIB □W O₂₄ with A^II, B^II = Ba, Sr Depending on the ionic radii of the two and three valent cations in the perovskites of type ABB □_1/4W^VIO₆ ?; AB^IIB □WO₂₄ order disorder phenomena are present. The results of the x-ray and vibrational spectroscopic investigations as well as the diffuse reflectance spectra and the visible photoluminescence are reported.