Voltage measuring device

E2, counter electrode with a constant potential A, current measuring device V, voltage measuring device. 

The overall cell potential, Ecell/ depends not only on the potentials of these two half-reactions but also on the boundary potential that develops across the thin glass membrane separating the reference HC1 solution from the test solution. Because the boundary potential depends linearly on the difference in the pH of the solutions on the two sides of the membrane, the pH of the test solution can be determined by measuring Ecell. The cell potential is measured with a pH meter, a voltage-measuring device that electronically converts Ecen to pH and displays the result in pH units. 

If very thin wires are required, use larger-diameter extension wires for regions where there is almost no temperature gradient (typically the room temperature leads that go to the voltage-measuring device). 

Two major types of end points find widespread use in neutralization titrations. The first is a visual end point based on indicators such as those described in Section 14A. The second is a potentiometric end point, in which the potential of a glass/calomel electrode system is determined with a voltage-measuring device. The measured potential is directly proportional to pH. Potentiometric end points are described in Section 21G. 

Although it is not difficult to measure relative half-cell potentials, it is impossible to determine absolute half-cell potentials because all voltage-measuring devices measure only differences in potential. To measure the potential of an electrode, one contact of a voltmeter is connected to the electrode in question. The other contact from the meter must then be brought into electrical contact with the solution in the electrode compartment via another conductor. This second contact, however, inevitably involves a solid/solution interface that acts as a second half-cell when the potential is measured. Thus, an absolute half-cell potential is not obtained. What we do obtain is the difference between the halfcell potential of interest and a half-cell made up of the second contact and the solution. 

A practical application of this circuit is the measurement of cell potentials. We simply connect the cell to the op amp input as shown in Figure 2 IF-7b, and we connect the output of the op amp to a digital voltmeter to measure the voltage. Modern op amps are nearly ideal voltage-measurement devices and are incorporated into most ion meters and pH meters to monitor high-resistance indicator electrodes with minimal error. 

If a decision has been made to use a thermocouple as a sensor, there is a wide range of voltage-measuring devices available from which to choose... 

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. 

While scopes are usually used as voltage measuring devices and therefore have high input impedances, there are probes available which, by clamping around a wire, measure the ac current. This can be useful in monitoring the current flowing in NMR transmitter coils. 

Fioure2.1. Schematic diagram of a simple galvanic electrochemical cell. W is a voltmeter or other voltage-measuring device. The arrows indicate the direction of the spontaneous flow of electrons. The + and — indicate the polarity of the cell as measured by a voltmeter. 

FIGURE 2.10 Circuit arrangement for a thermocouple showing the voltage-measuring device, the voltmeter, interrupting one of the thermocouple wires (a) and at the cold junction (b). 

Figure 11.4. Schematic diagram of Kim and Larter s oscillator (A) current supply, (B) voltage measuring device, (M) membrane, (I) chamber I, (II) chamber II.

Outer vessel contains liquid of lower density while the inner vessel contains liquid of higher density. The system is intrinsically unstable and is naturally maintained far away from equilibrium. Up and down flow of liquid occurs through the capillary, which is reflected by the oscillatory movement of the fluid in the inner vessel. When the electrodes in the two chambers are connected to a voltage-measuring device, oscillations in electric potentials are also observed for the cases when aqueous solutions of electrolyte-water system or aqueous solution of polar non-electrolytes are used in the system. This type of hydrodynamic instability is different from Benard instability or Taylor instability [29]. 

Polarization curves of protCHi exchange membrane fuel cells deviate from the simulated curve in Fig. 3a. Most significant is the low open-circuit potential of 0.9-1.0 V, as opposed to the reversible potential of approximately 1.2 V. The low open-circuit potential has a number of contributing factors, which include multiple reactions that set up a mixed potential, crossover of H2 or O2 through the membrane, and finite resistance effects of voltage measurement devices. These effects cannot be modeled with the overpotential method discussed here. Nonetheless, it is interesting to see how well the overpotential model can approximate a real curve. 

We must emphasize that no melhod can determine the absolute value of the potential of a single electrode. because all voltage-measuring devices determine only differences in potential. One conductor from such a device is connected to the electrode under study To measure a potential difference, however, the second conductor must make contact with the electrolyte solution of the half-cell under study. This second contact inevitably creates a solid-solution interface and hence acts as a second half-cell in which a chemical reaction must also lake place if charge is to flow. A potential is associated with this second reaction. Thus, we cannot measure the absolute value for the desired half-cell potential. Instead, we can measure only the difference between the potential of interest and the half-cell potential for the contact between the voltage-measuring device and the solution. 

Sensitivity is a ratio of the response of a measuring device (meter) to the magnitude of the measured quantity (volts, ohms, amperes, etc.). Voltage-measuring devices are rated in ohms per volt (i2 ). On any particular range, it is obtained by dividing the resistance of the instru-... 

A cell for measuring the conductance of an electrolyte sample must (a) transform the electronic current from the external circuit into an ionic current in the sample and (b) transmit the potential difference across the sample to the voltage-measuring device without introducing any additional potentials. 

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