Electrodes pH-sensitive

Antimony electrode (or antimony-antimony oxide electrode) -> pH-sensitive electrodes... 

According to the authors the developed solid-state CPRE exhibits sufficient stability and reproducibility in acid-base, sodium chloride, and potassium chloride solutions. According to the presented results, performance of such CPREs were exceptionally good, even better than performance of the conventional Ag/AgQ electrode. pH sensitivity of CPRE was very small and was almost unchanged within 600 days of dry storage. Unfortunately, despite encouraging informatimi presented, the reports do not provide sufficient data to be reproduced experimentally. 

In summary, using three kinds of techniques for measuring proton kinetics — pH electrode, pH sensitive dyes, and the shape of the R2 component of the ERP — it was found that the formation of Meta II is intimately and kineticaUy associated with the uptake of a proton. Arnis and Hofmann showed that in some unusual circumstances, proton uptake can lag behind Meta II formation. Moreover, titration data for the equilibrium between Meta 1 and Meta 11, along with the titration of the fast photocurrent due to proton uptake associated with Meta 11 formation, proved that the pK of the group binding the proton is approximately 6.4 in 150 mM salt. 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

Electrochemical detection has been achieved in a number of ways. The change in pH has been sensed with a traditional glass pH electrode antimony electrode or amperometrically via the pH sensitive oxidation of hydrazine... 

Glass electrode [see Fig 2.10 (1)J. The pH glass electrode, as the most important representative of the glass electrodes, will be the first subject to be treated, and especially in its application to aqueous solutions. Attached to the stem of high-resistance glass, the electrode proper consists of a pH-sensitive glass bulb that acts as a membrane between an inner reference electrolyte and an outer... 

With respect to pH sensitivity and an adequate speed of response (time constant r = RC where R is the resistance of the measuring circuit and C the capacitance of the electrode), a certain degree of superficial swelling is needed however, the gel layer thus formed should remain thin in order to minimize the solubility of the glass and to guarantee sufficient durability of the electrode. In this respect lithium barium silicates offer an attractive compromise32. 

Since many new substances of interest are very poorly soluble in water, the assessment of the pKa in aqueous solution can be difficult and problematic. Potentiometry can be a quick technique for such assessment, provided the solubility of the substance is at least 100 pM. (Solutions as dilute as 10 pM can still be analyzed, but special attention must be given to electrode calibration, and ambient carbon dioxide must be excluded.) If the substance is soluble to only 1-10 pM and possesses a pH-sensitive UV chromophore, then spectrophotometry can be applied. CE methods may also be useful since very small sample quantities are required, and detection methods are generally quite sensitive. 

Background currents of all NO electrodes are sensitive to changes of temperature and pH. Depending on type of electrodes, the effect may be more or less. Clark type NO electrodes are very sensitive to temperature change. The temperature induced response... 

FIGURE 10.1 A schematic diagram for a typical electrode system for potentiometric pH measurements. A potential is established on the pH sensitive membrane-solution interface of a pH electrode that responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The cell potential, which is proportional to the pH in the test solution, is measured using a high input impedance voltmeter between the pH electrode and the reference electrode. 

The principle of pH electrode sensing mechanisms which are based on glass or polymer membranes is well investigated and understood. Common to all potentiometric ion selective sensors, a pH sensitive membrane is the key component for a sensing mechanism. When the pH sensitive membrane separates the internal standard solution with a constant pH from the test solution, the potential difference E across the membrane is determined by the Nemst equation ... 

Selectivity is one of the most important characteristics of an electrode, as it often determines whether a reliable measurement in the sample is possible or not. In practice, most pH sensitive membranes will also respond slightly to some interfering ions. As can be seen in Eq. (5), the potential of such a membrane is governed mainly by the activity of the hydrogen ion and also by the concentration of other interfering ions. To improve selectivity, advanced membrane compositions or protection membranes with size-exclusion or ion-exchange properties are often utilized. 

Although glass pH electrodes are, in general, simple to use and available at a reasonable cost, they are limited by the potential problems of glass breakage [65] and miniaturization difficulties [60, 66], One of the alternative approaches to preparation of non-glass pH sensors is to use polymer-based pH sensitive membranes to replace solid glass membranes. 

Depending on the fabrication techniques and deposition parameters, the pH sensitive slope of IrOx electrodes varies from near-Nemstian (about 59 mV/pH) to super-Nemstian (about 70mV/pH or higher). Since the compounds in the oxide layers are possibly mixed in stoichiometry and oxidation states, most reported iridium oxide reactions use x, y in the chemical formulas, such as lr203 xH20 and IrOx(OH)y. Such mixed oxidation states in IrOx compounds may induce more H+ ion transfer per electron, which has been attributed to causing super-Nerstian pH responses [41],... 

Reference electrodes provide a standard for the electrochemical measurements. For potentiometric sensors, an accurate and stable reference electrode that acts as a halfcell in the measurement circuit is critical to providing a stable reference potential and for measuring the change in potential difference across the pH sensitive membrane as the pH concentration changes. This is especially important in clinical applications such as pH measurements in the blood, heart, and brain, where the relevant physiological pH range is restricted to a very small range, usually less than one unit. 

Rao et al. [140] described a study using a commercial pH sensitive radio-telemetry capsule (RTC) to evaluate small bowel and colonic transit time in athletes with gastrointestinal symptoms. The RTC (type 7006 Remote Control Systems, London, UK) consists of a glass electrode with an integral reference cap and battery. RF transmissions from the capsule are detected by a solid-state receiver worn on the belt of the patient. The recorder samples the pH from the capsule at 6 second intervals for a period of 24 hours. They used pH changes as an indication of the pH capsule s movement. A sharp rise in pH from around pH 2 to pH 6 indicates that the capsule has moved into the duodenum from the stomach. Then the pH progressively rises to a plateau around pH 8, which indicates that the capsule has moved into the terminal ileum. Another commercially available wireless pH sensor (Bravo) from Medtronic Inc. has been used to measure esophageal pH for a period of 48 hours and it will be discussed later. 

Simultaneous and continuous measurements of extracellular pH, potassium K+, and lactate in an ischemic heart were carried out to study lactic acid production, intracellular acidification, and cellular K+ loss and their quantitative relationships [6, 7], The pH sensor was fabricated on a flexible kapton substrate and the pH sensitive iridium oxide layer was electrodeposited on a planar platinum electrode. Antimony-based pH electrodes have also been used for the measurement of myocardial pH in addition to their application in esophageal acid reflux detection. 

The electrodes used in the above studies were double-barreled glass pH sensitive microelectrodes, and the spatial retinal pH profile was recorded by withdrawing the microelectrode tip at a rate of 1 //m/s or lOOpm/step across the retina in vivo or in vitro. In a typical retina pH profile (Fig. 10.9), measured in cat retina by the microelectrode, started from the choroids (Ph = 7.41, at distance Ojum). The pH steadily decreased to a minimum value (a maximum [H+] concentration) in the proximal portion of the outer nuclear layer (pH = 7.14 at —140 jum), then increased to —7.28 (at —310 pm) at the vitreous retinal border. The peak [H+] concentration in this layer indicated that a net production of proton occurred across the avascular outer retina [76],... 

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