Voltmeter, high-impedance

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

By means of a resistance in the circuit the spontaneous corrosion reaction can be made to proceed at a predetermined rate, and the rate can be measured by means of an ammeter A. At the same time the potentials of the individual electrodes can be measured by means of a suitable reference electrode, a Luggin capillary and high-impedance voltmeters and Kj. At equilibrium there is no net transfer of charge (/ = A = 0). the e.m.f. of the cell is a maximum and equals the difference between the reversible potentials of the two electrodes... 

One of the more recent developments in potential measurement instruments has been their incorporation into what is best drescribed as data management devices . These units will not only display a potential reading but may be instructed to store this information for later retrieval and processing. The complete unit incorporates a high-impedance voltmeter with an integral microprocessor for data and code entries into a solid-state... 

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH. 

Perhaps the most important new approach to chemical measurements has been the use of sensors for oceanic chemistry. Sensors comprise a transducer and its supporting electronic instrumentation. The key feature of sensors is their ability to monitor the concentration of a particular analyte continuously, so that the dimension of time can be added to the traditional three dimensions of spatial measurements. An example of a sensor is a pH electrode, coupled with a high-impedance voltmeter and a means of standardization and temperature compensation in situ. In principle, such a sensor can monitor pH continuously for days at a time while transferring the data to a recorder or memory device. One can contemplate towing an array of sensors at various depths simultaneously, obtaining three-dimen-o tin us d ta t. i Dr v e th two- imensional data a ail-... 

Because of irreversibilities associated with electrode kinetics and concentration variations, the potential of an electrode is different from the equilibrium potential. This departure from equilibrium, known as the overpotential, can be measured with a reference electrode. So that significant overpotential at the reference electrode can be avoided, the reference electrode is usually connected to the working electrode through a high-impedance voltmeter. With this arrangement the reference electrode draws negligible current, and all of the overpotential can be attributed to the working electrode. 

Fig. 2.1. Schematic diagram of an electrochemical cell, showing the links to a high-impedance voltmeter with copper wire.

With the switch open, the potential measured by the high-impedence voltmeter would be -0.118 V. This value can be calculated from the Nernst equation. The reversible potential of the Pt wire in chamber A would be zero relative to NHE as it is itself an NHE, the reversible potential of the WE in chamber B would be... 

If the voltmeter did not have a high impedance (resistance) between its terminals, current would pass between the WE and RE, polarizing both and rendering the RE a useless standard for potential. ,02 = 1.23 - 0.059 pH = 1.05 V(NHE). 

The experimental apparatus normally consists of a drawn glass capillary microreference electrode that minimizes screening between anodic and cathodic sites, an apparatus for rastering the microreference electrode across the corroding electrode surface in a systematic manner, high-impedance voltmeters, and the necessary data recording devices. A representative schematic of the apparatus is shown in Fig. 49 (125). The sensitivity and resolution of the measurement depend on the diameter of the tip of the microreference electrode, the standoff distance... 

Potential noise is measured by collecting the potential versus time record between a noisy corroding electrode and a noiseless reference electrode using a high-impedance digital voltmeter (DVM). This is essentially a measurement of... 

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