PH glass membrane electrodes

The potential of an ion-selective electrode in the presence of a single ion follows an equation similar to Equation 13.38 for the pH glass membrane electrode ... 

Oyama and Hirokawa [804] described a potentiometric pH sensor that used a film of poly(l-amino pyrene) (P(lAPyre)) in which no internal standard solution, as is required in conventional glass pH electrodes, was used. The pH detection was based on the sensor functioning as an ion-sensitive field effect transistor (ISFET). The sensor had high ion selectivity with respect to Na, and Ca, and insensitivity to O2, much like a conventional pH glass-membrane electrode. Response time was in the 30 to 60 sec region, with excellent linearity of response in the pH 4.0 to 10.0 region. 

Since pH glass membrane electrodes are used in a variety of applications, manufacturers of pH electrodes have devoted significant effort in designing combination pH electrodes (containing pH and reference electrode in a single body) in many shapes and sizes, for use in NMR test tubes, with flat surfaces for paper and cheese pH measurements, pressure resistant electrodes for reactor applications, and a wide variety of laboratory pH electrodes. Beyond the type, size and shape of the pH sensitive glass, an essential component of the combination pH electrode is the reference electrode. Depending on the application of the pH electrode, one can... 

Replacing Na20 and CaO with Li20 and BaO extends the useful pH range of glass membrane electrodes to pH levels greater than 12. 

A second complication in measuring pH results from uncertainties in the relationship between potential and activity. For a glass membrane electrode, the cell potential, Ex, for a solution of unknown pH is given as... 

The following is a schematic of an electrochemical cell consisting of an AgCl-coated Ag wire reference electrode and a pH-sensing glass membrane electrode. The bars represent phase boundaries. 

New polymer membrane-based ISEs for nitrate and carbonate exhibit detection limits and selectivities that may be applicable for ocean measurements. In addition, a number of these ISEs can be used as internal transducers for the design of useful potentiometric gas sensors. For example, dissolved C02 can be detected potentiometrically by using either a glass membrane electrode or a polymer-based carbonate ISE, in conjunction with an appropriate reference electrode, behind an outer gas permeable membrane. Novel differential pC02 sensors based on two polymer membrane-type pH sensors have also been developed recently. 

One of the most important and extensively used indicator electrode systems is the glass-membrane electrode that is used to monitor hydronium ion activity. Although developed in 1909, it did not become popular until reliable electrometer amplifiers were developed in the 1930s. When the outside surface of the glass membrane is exposed to an ionic solution, a response for the hydronium ion activity meets with the Nicholsky equation, which is similar to the Nernst expression. In view of the importance and widespread use of the hydronium or pH electrode, this system is discussed in a separate chapter. 

This chapter examines various probes for pH measurement such as ion-selective and glass-membrane electrodes as well as simultaneous cellulose removal and bleaching of textiles with enzymes. 

Glass-membrane electrodes, such as pH electrodes. In this type of electrode, a glass body acts as a membrane and shows affinity for different... 

Both cells are equipped with gas-permeable membranes that allow for nearly specific gas transfer from the sample into a thin indicator layer (buffer) of a cell that is in contact with the electrochemical sensing electrode. For the C02-GSS, the indicator layer is a flat pH glass membrane, while for the 02-CSS it is a cathode made of platinum or gold. 

Thus, online measurements of composition are usually limited to some overall property. A typical example is pH, defined as the absolute value of the logarithm of the molar concentration (or, more exactly, activity) of hydrogen ion pH can be measured by exploiting the electric potential established between two proper electrodes immersed in the sample fluid, usually a glass membrane electrode and a reference electrode [15], Notwithstanding the temperature dependence and the alkaline error (at high pH, a marked sensitivity to the effect of Na+ and of other monovalent... 

Usually, rather than using a hydrogen gas electrode, a glass membrane electrode is used for the measurement. As discussed in Sec. 8, the potential across such a membrane can be proportional to the difference in pH s of the solutions on each side of the membrane. One design for a membrane-type pH electrode, which incorporates a Ag/AgCl reference electrode in a tube concentric to the membrane electrode, is shown in Fig. 6. The electrode is immersed in the solution whose pH is to be measured, with the solution level above the porous plug. 

Glass membrane electrodes are employed to measure pH and Na, and as an internal transducer for PCO2 sensors. The H" " response of thin glass membranes was first demonstrated in 1906 by Cremer. In the 1930s, practical appHcation of this... 

Describe the source of pH dependence in a glass membrane electrode. 

Boundary potential, E, The resultant of two potentials that develop at the surfaces of a glass membrane electrode. Bronsted-Lowry acids and bases An acid of this type is defined as a proton donor and a base as a proton acceptor the loss of a proton by an acid results in the formation of a species that is a potential proton acceptor, or conjugate base of the parent acid. Buffer capacity The number of moles of strong acid (or strong base) needed to alter the pH of 1.00 L of a buffer solution by 1.00 unit. 

Microelectrodes based on closed-end pH glass membranes were the first to be described in the literature (Cl). Hinke (H2) fiuther developed glass membrane microelectrodes with tip diameters of 10 im for the measurement of sodium and potassium. Various designs and approaches to the fiibri-cation of these all-glass microelectrodes have been taken over the years, and specific fabrication procedures may be found in the literature (H3, K3). Similarly, solid-state type electrodes based on pressed pellets of Ag2S with tip diameters on the order of 100 fim have been reported for the determination of Ag" ", S , I, Cl , Cu +, Br , etc. (C13). We have already discussed that such solid-state electrodes can foul when applied for direct measurements in biological systems, so the fabrication of these will not be discussed here. 

A glass membrane electrode was used to measure the pH of the silicate solutions. Adsorption of the smaller alkali metal cations, K, Na, and Li, caused an underestimation of the pH. The pH error reached several pH units for the most alkaline Li silicate samples. On the other hand, the errors for Na silicates were around 0.5 pH unit and the errors for larger cations were relatively small. Corrections to the measured pH were made by comparison of the electrode response for the silicate solutions with that for alkali metal hydroxide solutions of known concentration. Since the hydroxide content of the base solutions is known, the interference from cation adsorption was calculated and added to the pH measured for the silicate solutions (16). 

Discuss the mechanism of the glass membrane electrode response for pH measurements. 

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