High input impedance

Operational amplifier. A linear, high-gain DC voltage or current amplifier with high input impedance, low output impedance, and the capability of producing a bipolar output from a bipolar input. 

Logarithmic scale for expressing acidity or alkalinity of water (7.0 to 0 indicates increasing acidity 7.0 to 14 indicates increasing alkalinity). Measured by means of a glass electrode/reference electrode pair immersed in the water sample under test. The potential difference depends upon the pH which is then displayed on a pH meter (high input impedance, millivoltmeter). 

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-... 

FIGURE 10.1 A schematic diagram for a typical electrode system for potentiometric pH measurements. A potential is established on the pH sensitive membrane-solution interface of a pH electrode that responds to the activity or concentration of hydrogen ions in the solution. The reference electrode has a very stable half-cell potential. The cell potential, which is proportional to the pH in the test solution, is measured using a high input impedance voltmeter between the pH electrode and the reference electrode. 

In order to allow the resistance measurement, the current injection is obtained in the circuit represented in fig. 3, by applying a voltage Vo- This current, due to the virtual ground condition determined by the circuit configuration (very high input impedance), will cross the feedback resistor Rf and determine an output voltage. In this example M and Yj (j=l,2,3) are the quantities ... 

When constituted of metals, thermopiles exhibit a very low noise, in particular only thermal noise if the voltage amplifier used for signal amplification has a very high input impedance. 

Very high input impedance, with a typical input current of a few pA. 

The pH meter is a specialized voltmeter that measures the potential difference (in mV) between the sensing and reference electrode and converts it to a display of pH. To provide an accurate measurement of the voltage of an extremely high resistance electrode (108 Q) [5], this specialized voltmeter must be designed with high input resistance or impedance characteristics (100 times that of the electrode used). Since the measurement potential difference per pH change is very small (59.16 mV/pH unit at 25°C), a reliable amplifier in the pH meter is also essential. It should be sufficiently sensitive to detect changes of at least 0.05 pH unit (or 3 mV). 

In this representation, Pt (and not Pt) has been written in at the right to show that a contact potential difference will arise where the platinum wire from the high-input impedance voltmeter (Fig. 7.14) contacts the copper electrode. The symbol //is used to indicate that the potential due to the junction between the solutions containing the H+ and Cu2+ has been minimized. 

These items of information can also be obtained by using a high-input-impedance voltmeter, which, when connected (e.g., across the cell),... 

In potentiometry, we measure the emf of a cell consisting of an indicator electrode and a reference electrode. For emf measurements, we generally use a pH/ mV meter of high input impedance. The potential of the reference electrode must be stable and reproducible. If there is a liquid junction between the indicator electrode and the reference electrode, we should take the liquid junction potential into account. 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

Any direct current source may be used as a current source (CS), in connection with a voltmeter with high input impedance, to control the applied potential at the working electrode. The applied potential can be measured via the reference electrode (R) using a voltmeter with a high input resistance (V). If a potentiostat is available, it will automatically control the working potential. 

Thus, the signal gain depends upon the ratio Rf/R. Figure 6.626 represents a buffer amplifier or voltage follower in which V0 = V,. This has the high input and low output impedances necessary to obviate the kind of inter-element loading problems illustrated in Section 6.11.6. For a more detailed treatment the reader is referred to Smith 881. 

A more satisfactory procedure involves a radioactive probe held a little above the water surface. The radioactive radiations ionise the air between the probe and the surface and ensure that they are at the same potential. The probe is connected to an electrometer having a very high input impedance which reads the surface potential. This procedure was pioneered by Guyot (70J and Frumkin [71J in the 1920s but has become much more convenient to use with the introduction of artificial radioactive isotopes and modern electronics. Americium 241 is particularly useful as the low energies of the a and y radiations produced only ionise the air in the immediate vicinity of the probe. 

Let us now return to the nonpolarized interface within the context of the working of the entire electrochemical cell, which we have to use in order to obtain useful information about concentration of fluoride ion, using (6.20). It is connected to a high-input impedance electrometer (e.gR > 10 2), so that current cannot pass through it. This ensures that the condition of zero current is satisfied. The general schematic of ISE is shown in Fig. 6.10a. In the usual cell notation we can write for the complete cell... 

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