Glass electrodes response

Fig. 5 pH response of a membrane containing DOBP (solid circles), compared with a glass electrode response (open circles). 

An electrode for measuring urea has been described (Gll), consisting of a thin film of urease, immobilized in acrylamide gel, on the surface of a glass electrode responsive to NH. Conditions are carefully selected to ensure stability of the enzyme, and the potential developed is proportional to the logarithm of the urea concentration. Blood glucose and lactate have been determined with a membrane electrode in which the enzyme (glucose oxidase or lactate dehydrogenase) is trapped in a porous or jellied layer at the membrane surface (W20). 

In addition to the glass electrode responsive to hydrogen ions, other ion selective electrodes are now available. These electrodes were first developed around 1964, and E. Pungor was associated with some of the early developments. A great many ions can now be estimated in this way, and among the more widely used electrodes are those responsive to potassium, calcium, fluoride and nitrate ions. 

Haber and Klemensiewicz also made an important contribution to the knowledge of the effect of glass composition and pretreatment on glass electrode response. 

The pH meter is basically an electronic voltmeter (or potentiometer) designed for use with a glass electrode system. It is composed of (i) a reference electrode, (ii) a glass electrode responsive to the pH of the solution surrounding it, and (Hi) an electrometer, which is a device capable of measuring very small differences in electrical potentials in a circuit of extremely high resistance. 

A glass electrode response test can use the same buffer solutions as employed for the span test. After standardization of the pH meter in pH 9.18 buffer, the reference electrode tip is immersed in pH 4.01 buffer to preequilibrate for a period of 5 minutes. This eliminates any response time due to the reference electrode. After this time period, rinse the glass electrode with pH 4.01 buffer and immerse the bulb in the same buffer solution with the reference electrode. Record the pH value versus time or observe the reading after 10 seconds. The reading after the 10-second period should be 98% of the final reading that is, the meter should read 4.11 or less within the 10 seconds. If the electrode fails this test, rejuvenation may help to increase its response. Response time for electrodes is discussed in detail in Section 5.3. 

L. M. Mukherjee and D. P. Boden, Electrochim. Acta 17, 965 (1972). Glass electrode response to lithium ion activity in propylene carbonate solutions. The effects of the ion K+, NH4+ and (Et)4N+ were studied at an ionic strength of 0.25M. 

Immersion electrodes are the most common glass electrodes. These are roughly cylindrical and consist of a barrel or stem of inert glass that is sealed at the lower end to a tip, which is often hemispherical, of special pH-responsive glass. The tip is completely immersed in the solution during measurements. Miniature and microelectrodes are also used widely, particularly in physiological studies. Capillary electrodes permit the use of small samples and provide protection from exposure to air during the measurements, eg, for the determination of blood pH. This type of electrode may be provided with a water jacket for temperature control. 

Specific-Ion Electrodes In addition to the pH glass electrode specific for hydrogen ions, a number of electrodes that are selective for the measurement of other ions have been developed. This selectivity is obtained through the composition of the electrode membrane (glass, polymer, or liquid-liquid) and the composition of the elec trode. Tbese electrodes are subject to interference from other ions, and the response is a function of the total ionic strength of the solution. However, electrodes have been designed to be highly selective for specific ions, and when properly used, these provide valuable process measurements. 

Assuming that the glass electrode shows an ideal hydrogen electrode response, the emf of the cell still depends on the magnitude of the liquid junction potential j and the activity coefficients y of the ionic species ... 

If the preference for hydrogen ion exchange shown by lime-soda glasses can be reduced, then other cations will become involved in the ion exchange process and the possibility of an electrode responsive to metallic ions such as sodium and potassium exists. The required effect can be achieved by the introduction of aluminium oxide into the glass, and as shown in Table 15.2, this approach has led to new glass electrodes of great importance to the analyst. 

The construction of these electrodes is exactly similar to that already described for the pH responsive glass electrode. They must of course be used in conjunction with a reference electrode and for this purpose a silver-silver chloride electrode is usually preferred. A double junction reference electrode is often used. The electrode response to the activity of the appropriate cation is given by the usual Nernst equation ... 

The urease is incorporated into a polyacrylamide gel which is allowed to set on the bulb of the glass electrode and may be held in position by nylon gauze. Preferably, the urease can be chemically immobilised on to bovine serum albumin or even on to nylon. When the electrode is inserted into a solution containing urea, ammonium ions are produced, diffuse through the gel and cause a response by the ammonium ion probe ... 

Glass electrodes are responsive to univalent cations. The selectivity for these cations is achieved by varying the composition of a thin ion-sensitive glass membrane. 

Stainless steel microelectrodes were prepared by sheathing 100-pm diameter stainless steel wire in glass. Tips were polished on a precision diamond wheel. Electrode response to variation in cathodic depolarizer concentration was confirmed by exposure to solution containing up to 6 mM HjOj. The increase in for the electrode was comparable to the change observed for sample coupons exposed to the same H2O2 concentrations. The electrode was conditioned by exposure to the influent reactor solution for several hours before measuring E, within the biofouling deposits. 

Most suitable would be the use of a perfectly NH4+ ion-selective glass electrode however, a disadvantage of this type of enzyme electrode is the time required for the establishment of equilibrium (several minutes) moreover, the normal Nernst response of 59 mV per decade (at 25° C) is practically never reached. Nevertheless, in biochemical investigations these electrodes offer special possibilities, especially because they can also be used in the reverse way as an enzyme-sensing electrode, i.e., by testing an enzyme with a substrate layer around the bulb of the glass electrode. 

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. 

---