Electrometer amplifier

An example of the experimental setup for the measuring of extracellular action and resting potentials is shown in Fig. 4. All electrochemical measurements can be conducted at constant temperature inside a Faraday cage mounted on a vibration-stabilized table in a laboratory (Fig. 4). Ag/AgCl electrodes were connected to a voltmeter/pFl meter [Cole Palmer Microcomputer pFl-vision Model 05669-20, Fig. 4(a)] with high input impedance or a programmable electrometer/amplifier [Keithley-2000/20, Keithley-6517, or Keithley-6514, Fig. 4(b)]. An IBM-compatible microcompu-... 

Each detector must have its own electrometer-amplifier, if one is needed for the particular detector, thereby flaking it possible to adjust the sensitivity of each detector as needed or desired. 

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. 

For measurement of redox couples, a frequently overlooked but convenient reference electrode is a conventional glass pH electrode (assuming that the sample solution system contains a constant level of acidity). Such an electrode provides an extremely inert and stable reference potential that is completely indifferent to most redox species. However, the glass electrode requires the use of an electrometer amplifier such as that contained in pH meters. 

Electrometer amplifier— An electronic amplifier with an extremely high -> input impedance (Rln > 1014 Q). The device allows measurements of electrical voltages (potentials) at practically zero current. Early devices employed specially designed and selected vacuum tubes (electrometer tubes) operated in a mode with very low grid current. The development of field effect transistors of various types allowed the application of solid-state devices. Electrometer amplifiers are employed in - pH meters (and generally in so-called pi meters, where I stands for ion), all types of instruments for po-tentiometric measurements and in the reference electrode input of -> potentiostats. Because of the high input impedance electrometer amplifiers are sensitive towards electric interferences, consequently some potentiostats have their -> reference electrode input circuitry (essentially an electrometer amplifier) mounted in a separate housing to be attached as close as possible to the reference electrode in order to minimize external interference. 

The value depends on the voltage and Ri (the input impedance). In an ideal case (- electrometer amplifier, -> voltage follower) Ri is very large resulting in an input... 

Fig. 4.4. Block diagram of optical system for electronic absorption spectrophotometry. L, L, biconvex silica lenses M, Mi, multiple reflexion cell (eight traversals shown) N, deuterium arc P, Hakuto R106 photomultiplier cell Q, wavelength drum and marker (R) S, recorder T, Vibron electrometer amplifier, U, grating. (After ref. 60.)...

Figure 7.31. Transient response for modified Bak Electrometer amplifier. Signal level Approximately 5 V peak-to-peak.

EMF measurements with an ion-selective electrode require very little external power for the needed electrometer amplifier. Another advantage is the easily attainable miniaturization of the measuring electrode and amplifier. The latter can be battery-operated, making field measurements quite easy. This attribute promotes their use as detectors for environmental protection. The relative accuracy, within 1 to 10% depending on the interferences present, in most cases completely suffices for trend analyses in the field (i.e. investigations of land, air and plants). In other cases they can render valuable service by allowing fast preliminary analyses far easier than any other analytical technique. 

Finally, it should be mentioned that with these electrodes it is difficult to reproduce the results of other workers. In some cases long lasting potential drifts arise, in others the active compound dissolves out of the PVC matrix. Because of the undefined and unknown potential-creating process at the metal/PVC phase boundary, one should work with electrometer amplifiers showing the highest input impedance (> 10 " 12) and lowest current to avoid any polarization of the unknown redox reaction at the metal/PVC interface. 

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