Glass electrode potentiometer

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. 

A pH meter is a special type of millivolt potentiometer designed to measure the emf of a cell in which the electrolyte contains hydrogen ions. A glass electrode is employed as the measnring electrode, and a calomel electrode is nsed as the reference electrode. 

Because the cell potential is sensitive to the concentrations of the reactants and products involved in the cell reaction, measured potentials can be used, to determine the concentration of an ion. A pH meter is a familiar example of an instrument that measures concentration from an observed potential. The pH meter has three main components a standard electrode of known potential, a special glass electrode that changes potential depending on the concentration of H+ ion in the solution into which it is dipped, and a potentiometer that measures the potential between the two electrodes. The potentiometer reading is automatically converted electronically to a direct reading of the pH of the solution being tested. 

The potentiometer can be used for measurements of low-resistance circuits. The pH meter is a voltage measuring device designed for use with high-resistance glass electrodes and can be used with both low- and high-resistance circuits. Electrometers can also be used with high-resistance circuits. 

Figure 4-4 Hardware set up for NMR-controlled titrations NMR spectrometer, 2 PC, 3 burette, 4 glass electrode and potentiometer, 5 thermostat, 6 thermostated titration vessel, 7 stirrer, 8 pump, 9 NMR probe head.

Figure 3. Brown s potentiometer for measurement of hydrogen ion concentration using glass electrode. B and B - calomel electrodes A - glass electrode D - high voltage battery C - potentiometer GR - ground Q - quadrant electrometer. (Reproduced with permission from Ref.17 Copyright 1925 American Institute of Physics.)...

In 1928 two independent research groups — Partridge at New York University and Elder and Wright at the University of Illinois — applied the vacuum tube potentiometric circuit to the measurement of the hydrogen-ion concentration via the use of a glass electrode. H. M. Partridge submitted a paper to the Journal of the American Chemical Society on March 24, 1928, entitled "A Vacuum Tube Potentiometer for Rapid E.M.F. Measurements" (25.). This paper, published on January 8, 1929, was also presented on September 13, 1928, at the Fall Meeting of the American Chemical Society held in Swampscott, Massachusetts. 

To achieve this goal the authors described the circuit shown in Figure 6. The method of measurement was based on Morton s design, in which an unknown e.m.f. is measured by compensation with a calibrated potentiometer in the grid circuit (ZS.). In a later publication (22) Elder described in greater detail the pH measurement with the glass electrode and compared the results of the new vacuum tube instrument to the Lindemann cjuadrant electrometer. 

Figure 6. Vacuum tube potentiometer for pH measurement with the glass electrode. X- cell with glass and AgCl Cl electrodes P-potentiometer, G-1 and G-2 - high sensitivity galvanometers, B-1 and B-2 - 22.5 and 6 volt batteries. (Reproduced...

Wright, W. H. B.S. Thesis pH Measurement with the Glass Electrode and the Vacuum Tube Potentiometer, University of Illinois, 1929. 

Apparatus ultra-thermostat 250 ml 3-necked flask stirrer alkali-stable glass electrode with potentiometer 25 ml-burette. 

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. 

In general, the determination of pH using a pH meter is quite simple. The solution is placed in a small container. The glass and the reference electrode (or the combination electrode) are dipped into this solution so that the glass bulb is dipped completely. The potentiometer circuit is closed and adjusted till the null point is obtained. The pH can then directly be read off the display. Two general precautions must be taken ft) As stated above, the pH meter must be standardized before every measurement, and fii) when not in use, the reference and the glass electrode must be kept immersed in water so that the pH sensitive membrane does not become dehydrated. 

For titration of halides the solution is best buffered to about pH 5. The indicator electrode is a piece of silver wire of about 1 mm in diameter and about 3 cm long which has been curled into an open spiral the reference electrode may be a mercury-mercurous sulphate half-cell with a potassium sulphate bridge. An alternative reference electrode for use in titration of halides, and one which works well in practice and is convenient to use, is the glass electrode if a glass electrode is used in this way, however, the potentiometer must have a high-impedance, shielded input socket. The following practical details are suitable for halide titrations ... 

In the glass electrode method, the two solutions are separated by means of a thin glass membrane, and the potential difference of the electrodes is measured by means of a specially sensitive potentiometer. 

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