PH electrodes glass

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Membrane Potentials Ion-selective electrodes, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion. figure 11.10 shows a generic diagram for a potentiometric electrochemical cell equipped with an ion-selective electrode. The shorthand notation for this cell is... 

NO2 2NO2 + 3H2O NO3- + NO2- + 2H3O+ 0.02 M NaN02, 0.1 M KNO3 glass pH electrode... 

Another version of the urea electrode (Figure 11.17) immobilizes the enzyme in a polymer membrane formed directly on the tip of a glass pH electrode. In this case, the electrode s response is... 

Measurement of pH With the availability of inexpensive glass pH electrodes and pH meters, the determination of pH has become one of the most frequent quantitative analytical measurements. The potentiometric determination of pH, however, is not without complications, several of which are discussed in this section. 

Directions for preparing a potentiometric biosensor for penicillin are provided in this experiment. The enzyme penicillinase is immobilized in a polyacrylamide polymer formed on the surface of a glass pH electrode. The electrode shows a linear response to penicillin G over a concentration range of 10 M to 10 M. 

Mifflin and associates described a membrane electrode for the quantitative analysis of penicillin in which the enzyme penicillinase is immobilized in a polyacrylamide gel that is coated on a glass pH electrode. The following data were collected for a series of penicillin standards. 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

The glass pH electrode has been the most widely used tool for measurement of pH. Optical pH sensing is one of the most well established methods of pH determinations, which is based on measurements of the absorption spectmm of an indicator, either dissolved in the test solution or immobilized on a substrate. 

Penicillin can likewise be determined by using the enzyme penicillinase to destroy the penicillin with production of hydrogen ions which can be determined using a normal glass pH electrode. Many other organic materials can be determined by similar procedures.34,35... 

The user must be alert to some shortcomings of the glass pH electrode. For example, in solutions of pH 11 or more, the electrode shows a so-called alkaline error in which it responds also to changes in the level of alkali metal ions (particularly sodium) ... 

FIGURE 5-7 The alkaline and acid errors of several glass pH electrodes. A, Corning 015/H2SO4 B, Corning 015/HC1 C, Coming 015/1 M Na+ D, Beckman-GP/1 M Na+ E, L N BlackDot/lM Na+ F, Beckman E/1M Na+ G, Ross electrode. (Reproduced with permission from reference 16.)... 

Describe the source of errors in pH measurements using the glass pH electrode. 

Carbon dioxide devices were originally developed by Severinghaus and Bradley (59) to measure the partial pressure of carbon dioxide in blood. This electrode, still in use today (in various automated systems for blood gas analysis), consists of an ordinary glass pH electrode covered by a carbon dioxide membrane, usually silicone, with an electrolyte (sodium bicarbonate-sodium chloride) solution entrapped between them (Figure 6-17). When carbon dioxide from the outer sample diffuses through the semipermeable membrane, it lowers the pH of the inner solution ... 

Glass pH electrodes are simple to use and maintain. They respond selectively to hydronium ion concentration and provide accurate measurements of pH values between about 0 and 10. They can be small enough to be implanted into blood vessels or even inserted into individual living cells. In precision work, these electrodes are calibrated before each use, because their characteristics change somewhat with time and exposure to solutions. The electrode is dipped into a buffer solution of known pH, and the meter is electronically adjusted until it reads the correct value. 

Figure 3 is a schematic representation of a typical CO electrode. A KCI/HCOJ containing electrolyte solution is trapped within a nylon mesh spacer layer whose pH is monitored by a contacting conventional glass pH electrode. A CO permeable membrane isolates the electrolyte layer from the analyte phase. Currently available... 

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... 

E°)/6 = 1.371 V hence the voltage curve is high and far from symmetrical. If a glass pH electrode is used as the reference electrode the determination of the equivalence point becomes even more reproducible. 

Among the membrane electrodes, the glass pH electrode is by far the most important pH electrode in non-aqueous media in fact, there seem no real limitations to its use, if proper handling is adopted. 

The pH-metric method is an important reference method because it can be used to measure all pKas between 2 and 12, with or without a UV chromophore, provided that the sample can be dissolved in water or water/co-solvent over the pH range of interest. In this method, the sample solution is titrated with acid or base, the titration is monitored with a glass pH electrode, and the pKa is calculated from the... 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

FIGURE 10.2 A schematic diagram of a combination glass pH electrode. A thin glass bulb with an inner Ag/AgCI electrode responds to pH changes in the test solution. A second Ag/AgCI in an outer jacket with a liquid junction serves the reference electrode for potentiometric measurement. An attached temperature probe is used to compensate for temperature effects. 

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. 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

J.E. Pandolfino, S. Ghosh, Q. Zhang, M. Heath, T. Bombeck, and PJ. Kahrilas, Slimline vs. glass pH electrodes what degree of accuracy should we expect Alimentary Pharmacol. Therapeutics 23, 331-340 (2006). 

The most common potentiometry involves an instrument called a pH-Staf , in which a glass (pH) electrode follows reactions that either consume or produce protons. Since pH... 

Tor [7] developed a new method for the preparation of thin, uniform, self-mounted enzyme membrane, directly coating the surface of glass pH electrodes. The enzyme was dissolved in a solution containing synthetic prepolymers. The electrode was dipped in the solution, dried, and drained carefully. The backbone polymer was then cross-linked under controlled conditions to generate a thin enzyme membrane. The method was demonstrated and characterized by the determination of acetylcholine by an acetylcholine esterase electrode, urea by a urease electrode, and penicillin G by a penicillinase electrode. Linear response in a wide range of substrate concentrations and high storage and operational stability were recorded for all the enzymes tested. 

A 500-mL, four-necked, reaction flask, equipped with a mechanical stirrer, thermometer, and glass pH electrode combined with an automatic titrator (Note 1), is charged with sodium (meta)periodate (85.5 g, 0.4 mol) (Note 2) and water (200 mL). The suspension is cooled to 0°C in an ice bath and 3 N sodium hydroxide (about 133 mL, 0.4 mol) is added dropwise at a rate such that the temperature does not exceed 7°C. The final pH of the suspension is 5.5. The cooling bath is removed and finely powdered 5,6-0-isopropylidene-L-gulono-1,4-lactone (Note 3) (43.6 g, 0.2 mol) Is added in one portion. The temperature of the mixture is kept below 30°C (Note 4). The pH of the suspension is maintained at 5.5 during the course of the reaction by addition of aqueous 15% sodium carbonate (about 15 mL). The suspension is further stirred at room temperature for 30 min, saturated with sodium chloride (105 g), and filtered by suction using a Buchner funnel. The white solid (Note 5) is washed thoroughly with two, 50-mL portions of brine and the pH of the combined aqueous layers is adjusted to... 

---