Glass electrode membrane type

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

Glass electrodes are used for the analysis of hydrogen ions various other types of ion-selective electrodes are used for the other ions. Electrodes with ion-selective solvent membranes have become very popular. These electrodes are made in the form of thin glass capillaries (about 1 rm in diameter), which in the lower part contain a small volume of a liquid that is immiscible with water the remainder of the capillary is filled with electrolyte solution (e.g., 3M KCl). 

Two aqueous phases separated by a liquid membrane, EM, of nitrobenzene, NB, were layered in a glass tube, which was equipped with Pt counterelectrodes in W1 and W2 and reference electrodes in three phases as in Eq. (1). Reference electrodes set in W1 and W2 were Ag/AgCl electrodes, SSE, and those in LM were two tetraphenylborate ion selective electrodes [26,27], TPhBE, of liquid membrane type. The membrane current, /wi-w2 was applied using two Pt electrodes. The membrane potential, AFwi-wi recorded as the potential of SSE in W2 vs. that in W1. When a constant current of 25 /aA cm was applied from W1 to W2 in the cell given as Eq. (1), the oscillation of membrane potential was observed as shown in curve 1 of Fig. 1. The oscillation of AFwi-wi continued for 40 to 60 min, and finally settled at ca. —0.40 V. 

Fig. 2.10. Membrane electrode types. 1 Glass electrode 2, 3 and 4 crystal membrane electrodes. 

Heterogeneous liquid membrane electrodes. This type, which has become of considerable practical importance, consists of a liquid ion-exchange layer or a complex-forming layer within a hydrophobic porous membrane of plastic (PTFE, PVC, etc.), sintered glass or filtering textile (glass-fibre, etc.). The construction of such an electrode is depicted in Fig. 2.12. 

This type of membrane consists of a water-insoluble solid or glassy electrolyte. One ionic sort in this electrolyte is bound in the membrane structure, while the other, usually but not always the determinand ion, is mobile in the membrane (see Section 2.6). The theory of these ion-selective electrodes will be explained using the glass electrode as an example this is the oldest and best known sensor in the whole field of ion-selective electrodes. 

Myocardial tissue pH measurement has been used in the studies of different approaches to myocardial protection in various cardiac operations. A needle type glass membrane miniature electrode has been used in studies for pH-guided myocardial management [127], As described in the previous section, this electrode was also adapted to measure brain pH [132], The electrode has a right-angled glass electrode... 

At the turn of the century, considerable attempts were being made to find suitable membrane models. These models fall into two groups compact, usually liquid ( oil ) and soUd membranes [10, 33, 62, 75] and porous membranes [9]. At the very beginning of the study of compact membranes, the glass electrode was discovered [ 18, 34], whose membrane represented the first observation of marked selectivity for a particular type of ion, here the hydrogen ion. It is interesting that this first ion-selective electrode remains the best and most widely used of all such electrodes. 

L. C. Clark first suggested in 1956 that the test solution be separated from an amperometric oxygen sensor by a hydrophobic porous membrane, permeable only for gases (for a review of the Clark electrode see [88]). The first potentiometric sensor of this type was the Severinghaus CO2 electrode [150], with a glass electrode placed in a dilute solution of sodium hydrogenocarbonate as the internal sensor (see fig. 4.10). As an equilibrium pressure of CO2, corresponding to the CO2 concentration in the test solution, is established in the... 

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

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. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

Membrane electrode The membrane electrode is a thin phase through which charge can be transported (by ion migration) so that electrochemical equilibrium can be maintained for at least one type of ion across the electrode. For example, pH is often measured in a solution (X) by means of a glass electrode between an HC1 solution of known... 

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. 

In the field of electroanalysis the problem of the operative mechanism of glass electrodes may be mentioned. Thin membranes of several types of glass act as perfect Nemst-type electrodes for protons or other ions over many decades of concentration. The basic question is whether a glass electrode reacts to an ion i because 1 is charge-determining or because a liquid junction potential is set up with the transport dominated by i. In the former case Nernst s law for the electrode potential, 11.5.5.1], applies, in the latter [1.6.7.8] is valid, with t = = 1... 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

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