Voltmeter high resistance

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

Transformer-rectifiers should have an ammeter to indicate the current and a high-resistance voltmeter to indicate the potential at the protection station. 

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

To measure structure/electrolyte potentials with electrolyte resistivities in excess of 2 kQ cm, a high-resistance potentiometer unit as shown in Fig. 10.43 or a potentiometric voltmeter as illustrated in Fig. 10.44 may be used. 

Valve voltmeters were widely used in the past, but have been replaced by transistor voltmeters. With instruments of this type it is possible to achieve an input resistance of 50 MQ or more, the current required to operate the instrument being of the order of 10" A. The early instruments had a tendency to zero drift on the lower ranges, but this has been overcome in the modern transistor types. Such instruments are most often used to make potential readings in extremely high-resistance electrolytes. The accuracy of such instruments is of the order of 2% full-scale deflection. It is necessary to ensure that both types are so designed that they do not respond to alternating currents. 

As their name suggests, these instruments are capable of carrying out a variety of measurements, e.g. structure/electrolyte potentials, current, resistivity and voltage. Most instruments of this type contain two meters in one case, one being a low-resistance millivolt/voltmeter and milliamp/ammeter, and the second a high-resistance voltmeter. 

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. 

The reference electrode is connected with the cell via a glass tube (Luggin capillary) filled with electrolyte, and the narrowed orifice of the tube is placed about 0.1 to 0.3 mm in front of the side of the working electrode that faces the counter electrode. The potential between this point and the surface of the working electrode is measured with a high resistance voltmeter that makes contact with the silver wire of... 

Generally, the best way of minimizing the current is to measure the emf with a voltmeter that itself has a huge internal resistance. For this reason, it is usual to see references given to high-resistance voltmeters. ... 

The problem will be approached as follows. We will assume that the cell is at equilibrium and the emf is measured by means of a high-resistance voltmeter. We will have waited for the voltmeter read-out to reach a steady value - in practice, this means that the read-out fluctuates by no more than, say, 2 mV or so. The potential of the reference electrode will be known, so we can determine accurately the electrode potential, Eq.r, of the half cell which contains the analyte of interest. 

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). 

The streaming current can be measured if the high- resistance voltmeter is replaced with a microammeter of low resistance compared with that of the plug. An alternating streaming current can... 

By means of sliding contacts the electrodes were connected to high resistance voltmeter V7-16 (4) or to a conductometer. The speed of rotation could be changed from 250 to 5000 rpm. The average experimental foam expansion ratio n0 was calculated from Eq. (4.32). 

A circuit for manual control of the working potential is shown in Fig. 854 it is made from components available in all laboratories. It includes, besides the cell, a high-resistance voltmeter or pH-meter, an ammeter, a coulometer, and a dc source. With the voltage adjuster the voltage across the cell is set to such a value that the potential difference between the working and the reference electrodes has the desired value the voltage between the anode and cathode is, as such, not important because it includes the potential drop caused by the ohmic resistance. 

Conductivity Measurements. Cell resistance measurements were made with a General Radio type 1650-A impedance bridge. It is equipped with an internal, 1000-cycle signal source and tuned null detector. For more sensitive balance at high resistances, a Hewlett Packard 400L vacuum tube voltmeter is used as an external null detector. 

Because of the high resistance of the glass membrane (10 to 100 MO), it is not practical to measure the emf directly. Instead, pH meters either use a direct-reading electronic voltmeter or electronically amplify the small current that flows through the cell and detect the voltage drop across a standard resistor potentiometrically. Both battery-operated and ac line-operated pH meters are available connnercially from such firms as Beckman Coulter, Thermo Orion, and Coming. Such pH meters are calibrated to read directly in pH units, have internal compensation for the temperature coefficient of emf, and have provision for scale adjustments. 

The pH meter is a specialized voltmeter that measnres the potential difference (in mV) between the sensing and reference electrode and converts it to a display of pH. To provide an accnrate measnrement of the voltage of an extremely high resistance electrode (10 [5], this speciahzed voltmeter mnst be designed with... 

The measurements of OCV, whieh are taken by high-resistance (>10 Q) electronic voltmeters to satisfy the requirements of i — 0, make it possible to determine the free energy, the entropy and the enthalpy of cell reactions, in addition to activity coefficients, equilibrium constants, and solubility products. 

Many designs of calomel electrodes exist, and several types are commercially available. Some of these are constructed for voltammetric work, whereas others are used in connection with glass electrodes. The latter type generally has a high resistance, which must be considered when the choice of voltmeter or potentiostat is made. 

Figure 18-4 Change in cell potential after passage of current until equilibrium is reached. In (a), the high-resistance voltmeter prevents any significant electron flow, and the full open circuit cell potential is measured. For the concentrations shown this is + 0.412 V. In (b), the voltmeter is replaced with a low-resistance current meter, and the cell discharges with time until eventually equilibrium is reached. In (c), after equilibrium is reached, the cell potential is again measured with a voltmeter and is found to be 0.000 V. The concentrations in the cell are now those at equilibrium as shown.

Numerous high-resistance, direct-reading digital voltmeters with internal resistances of >10" ohms are now on the market. These meters are commonly called pH meters but could more properly be referred to as plon meters or ion meters, since they are frequently used for the measurement of concentrations of other ions as well. A photograph of a typical pH meter is shown in Figure 21-18. 

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. 

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