Recent development of non-faradaic potentiometry

Non-faradaic potentiometry has been plagued by a great many fundamental errors and a lack of conceptualization. Of greatest concern is the second Nernst equation hiatus. Potentiometry may be generally classified as faradaic and non-faradaic. The former deals with the redox reactions using the Nernst equation to explain the potential origin. The latter deals with the non-redox reactions using the Boltzmann and modified Boltzmann equations to explain the origin of electrode potential. Redox faradaic potentiometry has been well described in the textbooks. However, non-faradaic potentiometry has been almost completely neglected in the literature. Many well-known electrodes, such as the pH glass electrode, common reference electrodes, and ion selective electrodes (ISE) have been mistakenly interpreted as redox reactions or ion exchange reactions. New theories and experimental results show their mechanisms to be non-faradaic in nature. Furthermore, the reaction mechanisms for ISE have been confused in textbooks with redox reactions and the Nernst equation. The ISE potentials originating from adsorption of ions or charged particles based on surface charge density will be explained using the double and counterion triple layers concept. The new counterion triple layer concept may be applied to the potential development of sensors. The reason for a new concept, theory, or mechanism is to better explain the phenomena. Examples will be given of how our new concept explains the capacitor, counterion triple layer, surface adsorbed layers interactions, and the interface structure. We will also discuss the new sensor development based on the new adsorption concept. For the first time a new type of Ag/AgCl reference electrode for non-faradaic potentiometry will be presented, one without a liquid junction and with a Pt wire instead of a salt bridge. They will help open up a new horizon for electrochemical sensor research and may be used under unusual conditions, such as high temperature and high pressure, stirring, etc.     

Enthalpies of Dilution of D-p-Hydroxyphenylglycine in Buffer Solutions at Different pH

The effect of pH on the dilution enthalpies of D-p-hydroxyphenylglycine in phosphate buffer solutions has been investigated by isothermal titration microcalorimetry at 298.15 K. The corresponding homogeneous enthalpic interaction coefficients have been calculated according to the excess enthalpy concept. The results show that the enthalpies of dilution of D-p-hydroxyphenylglycine in phosphate buffer solutions at different pH are all positive. The overall trend is that enthalpies of dilution become more positive with an increase of pH, but there is a minimum of the enthalpy of dilution at pH = 7.0. The enthalpic pair interaction coefficients, h ₂, all have negative values. The results are interpreted from the point of view of solute-solute and solute-solvent interactions involved in the solvent effects.     

Kinetic studies of the urease-catalyzed hydrolysis of urea in a buffer-free system

The kinetics of urea hydrolysis catalyzed by urease, mainly in the absence of buffers by use of the self-buffer effect of the products, was investigated. The effect of pH, temperature, and concentration of enzyme, substrate, product, salt ions, and buffers on the kinetic behavior of urease was examined. A kinetic model of a modified Michaelis-Menten form, incorporating substrate and product inhibition, pH dependence, and temperature effect, was developed to describe the reaction rate. Experimental data indicated that urease in a buffer-free solution was less susceptible to the inhibition of substrate product. The Michaelis constant keeps almost constant with the variation of pH and temperature, and increases with the addition of buffers and salts. The data also suggested that the noncompetitive pattern of the product inhibition, which is not significantly affected by temperature, increases gently with increasing pH. A Monod form rate expression was proposed to analyze the pH effect on the maximum rate. The proposed kinetic model was also examined by the long-time experiments in which pH, substrate, and product concentration varied obviously during the reaction course.     

果糖-水混合溶液中多组分电解质热力学

恒定混合溶液总离子强度I=1.0000 mol•kg－1,改变果糖-水混合溶液中果糖的质量分数w=2.5％、5.0％和7.5％的条件下,应用电动势方法测定下列无液体接界电池(A)和(B)在278.15、283.15、288.15、293.15、298.15、303.15、308.15、313.15、318.15 K等9个温度下的电动势: Pt, H2 (105 Pa)│HCl(m), C6H12O6(w), H2O(1－w)│AgCl-Ag (A) Pt, H2 (105 Pa)│HCl(mA), NaCl(mB), C6H12O6(w), H2O(1－w)│AgCl-Ag (B) 根据测得电池的电动势,计算出混合溶剂中AgCl-Ag电极的标准电极电势和HCl的标准迁移吉布斯自由能、迁移熵和迁移焓; 求出四元混合溶液中HCl的活度系数γA.结果表明在溶液中总离子强度I保持恒定,HCl的活度系数服从Harned规则,进一步讨论了混合物中HCl的介质效应.     

Interactions of dicarboxylic porphyrins with membranes in relation to their ionization state

The interactions of dicarboxylic porphyrins with membrane systems are discussed with particular emphasis on the effect of the charge of the porphyrin and the nature of the side-chains. The incorporation of hematoporphyrin or related dicarboxylic porphyrins within small unilamellar vesicles as membrane models is favored by a decrease of the pH in the range of physiological pH values. This effect might play an important role in the retention of porphyrins by tumors, which are more acidic than normal tissues. Kinetics studies also show that the partition of the porphyrin between the lipidic bilayer and the aqueous phase is governed by its release rate rather than by its incorporation rate.     

Fourier transform infrared spectroscopy study of the effect of pH on anion and cation adsorption onto poly(acrylo‐amidino diethylenediamine)

The role of hydrogen bonding in the chemistry of transition‐metal complexes remains a topic of intense scientific and technological interest. Poly(acrylo‐amidino diethylenediamine) was synthesized to study the effects of hydrogen bonding on complexes at different pHs. The polymer was synthesized through the coupling of diethylene triamine with polyacrylonitrile fiber in the presence of AlCl₃ · 6H₂O addition. The adsorption capacity of this polymer was 11.4 mequiv/g. The ions used for the adsorption test were CrO, PO, Cu²⁺, Ni²⁺, Fe²⁺, and Ag⁺. All experiments were confirmed with Fourier transform infrared. In the study of anion adsorption, at low pHs, only ionic bonds existed, whereas at high pHs, no bonds existed. However, in the middle pH region, both ionic bonds and hydrogen bonds formed between poly(acrylo‐amidino diethylenediamine) and the chromate ion or phosphate ion. When poly(acrylo‐amidino diethylenediamine) and metal ions (Cu²⁺, Ni²⁺, Fe²⁺, and Ag⁺) formed complexes, a hydrogen‐bonding effect was not observed with Fourier transform infrared. The quantity of metal ions adsorbed onto poly(acrylo‐amidino diethylenediamine) followed the order Ag⁺ > Cu²⁺ > Fe²⁺ > Ni²⁺. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2010–2018, 2004     

Solid-state pH nanoelectrode based on polyaniline thin film electrodeposited onto ion-beam etched carbon fiber

An ultramicro pH sensor has been constructed based on a thin polyaniline film that was electrochemically deposited onto a carbon fiber nanometer-size electrode. The substrate nanoelectrodes were fabricated using ion-beam conically etched carbon fibers with tip diameters ranging ca. from 100 to 500 nm. The polyaniline film was deposited from HCl solution containing the aniline monomer by cycling the potential between −0.2 and +1.0 V. The electromotive force (emf) signal between the pH sensitive polyaniline-coated nanoelectrode and an Ag/AgCl reference electrode was linear over the pH range of 2.0-12.5 with a slightly super-Nernstian slope of ca. −60 mV/pH unit. Response times ranged from several sec at pHs around 7 up to 2 min at pH 12.5. The proposed pH nanoelectrode displayed high ion selectivity with respect to K⁺, Na⁺, Ca²⁺, and Li⁺, with log K_H,M values around −12 and has a working lifetime of about 20 days. Key parameters important for the pH nanoelectrode performance, including polyaniline film preparation, selectivity, response time, temperature dependence, relative coating thickness, stability, and reproducibility, have been characterized and optimized. The performance of the pH nanoelectrode was examined by measuring the pH of several real samples including body fluids (serum, urine) and low ionic strength water samples (rain, deionized and tap water). The results agreed very well with those obtained by using commercial glass pH electrodes. The proposed pH nanoelectrode demonstrated attractive properties and seems particularly promising for use under physiological conditions.     

Analysis of basic compounds at high pH values by reversed-phase liquid chromatography

Reversed phase high performance liquid chromatography (RPLC) is currently the method of choice for the analysis of basic compounds. However, with traditional silica materials, secondary interactions between the analyte and residual silanols produce peak tailing which can negatively affect resolution, sensitivity, and reproducibility. In order to reduce these secondary interactions, which comprise ion exchange, hydrogen bonding, and London forces interactions, chromatographic analyses can be carried out at low or high pH values where silanol groups and basic compounds are mostly uncharged. The chromatographic behaviour of a particular bidentate stationary phase, Zorbax Extend C18, was studied with a set of basic and neutral compounds. Thanks to a higher chemical stability than traditional silica based supports, analyses were carried out with a high pH mobile phase, which represents a good alternative to the acidic mobile phases generally used to reduce ion exchange interactions. The performance of this bidentate stationary phase was also compared with that of other supports and it was proved that it is advantageous to work with high pH mobile phases when analyzing basic compounds.     

Sample-induced internal pH gradients in microcolumn liquid chromatography of proteins

Summary The technique of an internal pH gradient induced by the sample is applied to the separation of proteins by liquid chromatography. Compatibility of the method with microcolumns is demonstrated and examples of separations on different types of sorbents are given.