Folke Ingman

Metal-Metal Oxide and Metal Oxide Electrodes as pH Sensors

The pH is one of the most important parameters for char-acterizing the chemical properties of an aqueous solution. The glass eiectrode is by far the most commonly used pH sensor. The determination of pH in special situations, e.g., in vivo applications where the fragility of the glass electrode is a draw-back, requires pH sensors that can easily be miniaturized and built into physically rugged sleeves. Also, for other applications where the volume of solution is very restricted miniaturization of pH sensors is very important. The glass electrode does not respond properly to pH in some corrosive environments (e.g., in hydrogen fluoride [HF] solutions). At present, among alternatives to the hydrogen-selective glass electrode group of sensors, potentiometric metal-metal oxide pH sensors respond to pH, ideally due to an equilibrium involving the metal and its oxide. In the case of metal oxide electrodes, the metal is not involved in the potential-determining reaction. This distinction is of course not clear in many cases because the mechanism giving the pH response is not always unequivocal.

Acid-base titrations by stepwise additions of equal volumes of titrant with special reference to automatic titrations—I: Theory, discussion of the gran functions, the hofstee method and two proposed methods for calculating equivalence volumes

The range of acid stability constants over which the Gran functions are applicable in evaluating acid-base titrations at normal concentrations is studied and found to be quite narrow. Alternatively, the titrations can be evaluated by using non-approximative methods based upon an equation that is derived without making the simplifying assumptions leading to the Gran functions. One such method, introduced by Hofstee, is discussed and the results of a study of the effects of a systematic error in the pH-determinations upon the determination of the equivalence volume are given. Two other non-approximative methods for evaluating acid-base titrations that are particularly suitable for use in automatic titrations of many similar samples are also described. These methods yield correct values of the equivalence volume regardless of a possible systematic error in the pH-values and are particularly useful for titrations of moderately strong and very weak acids (stability constants up to 10(10)-10(11) at 0.01M concentration) where the Gran functions cannot be used.

Titration of mixtures of acids of nearly equal strengths

Abstract Accurate and precise experimental methods that were developed for the purpose of determining stability constants in aqueous solutions are employed in titrations of acids which have nearly equal strengths. A computer program capable of handling five acids simultaneously has been designed to evaluate the titration data. The results show that it is possible to determine two acids which have stability constants differing by only about one logarithmic unit with an accuracy better than 1%. If the conditions are favourable, it is even possible to analyse a mixture of two acids with about 2% accuracy when the constants differ by as little as 0.2 logarithmic units.

Extraction rate in liquid-liquid segmented flow injection analysis

Abstract The extraction mechanism in liquid-liquid segmented flow-injection systems is investigated. A film is formed on the tubing wall by the phase with the highest affinity to the tubing material. This film surrounds the segments of the other phase. To a great extent the extraction takes place at the interface of this film. The extraction rate is influenced by the segment length, the inner diameter of the tubing and the flow velocity. Short segments, small inner diameter and high linear flow velocity lead to a high extraction rate. These findings indicate that miniaturization of the flow system will lead to faster extraction and to decreased sample-zone broadening.

An examination of the palladium/palladium oxide system and its utility for pH-sensing electrodes

Abstract Monocrystalline and polycrystalline palladium has been used for the electrochemical examination of the palladium—palladium oxide system in aqueous solutions. Two different methods were used to prepare the electrodes, namely a thermal method of oxidation at high temperature and an electrochemical method. Electrodes prepared by the electrochemical method did not have a good pH response. The properties of the thermally prepared electrodes depend on the temperature at which the oxidation was made. The optimum temperature is 750°C. The temperature range studied was 450–870°C. Electrodes prepared by the thermal method show an almost theoretical slope in the pH range 2.5–8.

Ion-pair extraction of Na+, K+ and Ca2+ with some organic counter-ions and dicyclohexyl-18-crown-6 as adduct-forming reagent

Extraction equilibria of ion-pairs formed by the Na(+), K(+) and Ca(2+) complexes of dicyclohexyl-18-crown-6 and beta- and gamma-dinitrophenol, picric acid and dipicrylamine have been studied in the two-phase system H(2)O/CH(2)Cl(2). Extraction constants and dissociation constants for the ion-pairs in the organic phase are given and show that the efficiency of extraction increases with the hydrophobicity of the counter-ion. At the same time, the selectivity decreases.

The analysis of aqueous solutions with ethanol-soluble reagents in a flow injection system

Abstract A spectrophotometric method for the determination of uranium is adapted to flow injection analysis. The reagent, 2(5-bromo-2-pyridylazo)-5-diethylaminophenol, forms a complex with uranyl ions, with an absorbance maximum at 578 nm. The detection limit of the procedure is 0.15 mg l −1 uranium and 60 samples can be analyzed per hour. Iron, calcium, ammonium, sulphate and fluoridedo not interfere; phosphate interferes even at moderate levels and carbonate interferes at high concentrations. Difficulties encountered in mixing the aqueous stream carrying the sample with the ethanol reagent stream initiated an investigation of different kinds of mixers in flow systems. A tight coil of teflon tubing proved to be the most efficient mixer, i.e., the one yielding the least baseline noise without excessive dispersion.

Phase Segmentation in Liquid-Liquid Extraction Continuous Flow Analysis

Abstract Several research groups have been actively designing flow systems for automatic or semiautomatic liquid-liquid extraction, most of them using air-segmented flow systems. The first application of liquid-liquid extraction in flow injection analysis (HA) was carried out simultaneously by Karlberg et al.1 and Bergamin et al.2 in 1978. Since then, a large number of analytical pmcedures have been devised in combination with all common detection systems.3–6

Effects of some complex-forming ligands on the potential of antimony pH-sensors.

The E vs. pH relationship for oriented monocrystalline antimony electrodes was studied in various ionic media, containing no buffer or one of several common buffer substances, and the stability of the electrode potential with time was investigated. Several makes of commercially available polycrystalline antimony electrodes were also studied. The results indicate that tris(hydroxymethyl)aminomethane may be used for calibration of antimony electrodes, but most other commonly used buffer substances, e.g., orthophosphate, yield erroneous results. Monocrystalline antimony electrodes are preferable to polycrystalline ones, especially when measurements are made in unstirred solutions. The long-term stability of monocrystalline electrodes is superior to that of polycrystalline ones.

Acid-base titrations by stepwise addition of equal volumes of titrant with special reference to automatic titrations—II: Theory of titration of mixtures of acids, polyprotic acids, acids in mixture with weak bases, and ampholytes

A general method for evaluating titration data for mixtures of acids and for acids in mixture with weak bases is presented. Procedures are given that do not require absolute [H]-data, i.e., relative [H]-data may be used. In most cases a very rough calibration of the electrode system is enough. Further, for simple systems, very approximate values of the stability constants are sufficient. As examples, the titration of the following are treated in some detail: a mixture of two acids, a diprotic acid, an acid in presence of its conjugate base, and an ampholyte.