Potentiometer vacuum tube

It is quite impossible to determine the absolute potential difference across a sin e met /solution interface, and the potential must be evaluated indirectly from the e.m.f. of a cell comprising the interface under consideration and another electrified interface. The e.m.f. of the cell can be determined readily by a suitable measuring device such as a potentiometer, vacuum-tube voltmeter or an electrometer, which are capable of measuring the e.m.f. with the minimum passage of electrical charge. This is essential since if a significant current is allowed to pass, the electrodes (electrified interfaces) become polarised and the e.m.f. will be less than the equilibrium value. Consider the determination of the interfacial potential at the surface of a zinc electrode in equilibrium with Zn ions in solution. In order to determine the potential it is necessary to couple it with another electrode, and for the purpose of this discussion the equilibrium between ions in solution and gas will be chosen, i.e. the reversible hydrogen electrode in which the equilibrium between and H takes place at a platinised-platinum surface. The spontaneous cell reaction will be... 

Fig. 62. Vacuum-tube potentiometer for cells of high resistance...

Power amplification is produced by four tetrode vacuum tubes connected in pushpull-parallel as a Class AB2 amplifier. Control of the master audio oscillator signal amplitude is provided by a potentiometer on the master control panel. The output voltage of the equipment may be reduced to zero even when full voltage is applied to the plates of the power tetrodes. 

The history of the measurement of hydrogen-ion concentration parallels not only progress in understanding of the acid-base concept but also the development of suitable instrumentation for indication. This brief survey explores the early beginnings, the development of the potentiometer, standard cells, and null point devices. These led to the vacuum tube voltmeter and the design of the first commercial pH meter. 

Determination of pH with a Vacuum Tube Potentiometer and Glass F.lect rode... 

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. 

After most of this work had been completed (August 12th) the authors learned that H.M. Partridge has carried out similar measurements of glass cell potentials with a vacuum tube potentiometer, at New York University. 

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. 

The device described herewith is somewhat more convenient than most vacuum tube potentiometers for chemical measurements in that valves do not have to be calibrated and it is not essential that their characteristics remain constant over long periods. It is considerably more rapid than the average potentiometer in that only one adjustment is necessary for the measurement. 

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...

Figure 7. Diagram of a vacuum tube potentiometer for rapid E.M.F. measurement using tetrode and triode. (Reproduced from Ref. 25 Copyright 1929 American Chemical Society)...

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

The latter can be regarded as the indicator electrode. The two beakers would be connected by a salt bridge (see p. 24) and the potential established between the two half-cells measured by a potentiometer or a vacuum tube voltmeter. It is important to obtain the polarity of the electrodes by noting which of the electrodes must be connected to the positive and negative terminals of the instruments in order to obtain meaningful readings. This is predictable, however, from theory. The unknown pH is calculated by application of the Nernst Equation to the indicator electrode in the following manner ... 

Figure 8.4l. Apparatus for the parallel recording of DTA. T, TG. DTG. TGT- and DTGT curves (86). 1. compressed test piece 2. compressed reference substance 2. furnace 4. silica bell 5, inlet tube for carrier gas 6. tube for eas extraction 7. silica tube S. suka tube with stirrup-shaped end 9. thermoelement 10. diaphragms 11. light cell 12- lamps l3. optical slit 14. magnet l5. coil 16. galvanometer 17. photographic paper 18. damns transformer 19, absorber 20. electrodes 2l, amplifier 22. vacuum pump 23. automatic burette 24. potentiometer 25. servomotor.

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