Contacting voltmeters

FIGURE 3-23 Voltage divider circuit in high voltage probe connected to voltmeter. 

A contacting high voltage probe connected to a multimeter or similar laboratory instrument is fundamentally different from that of a an electrostatic 

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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. 

Often it is necessary in designing a cathodic-protection system to know the conductivity of a protective coating (e.g. bitumen enamel) on a structure. This measurement is usually carried out by finding the resistance between an electrode of known area placed in contact with the coating and the structure itself. The electrode placed on the structure can be either of thin metal foil or, preferably, of material such as flannel soaked in weak acidic solution. The resistance between the pad and the metal is measured by means of either a resistivity meter, as previously described, or a battery with a voltmeter and an ammeter or microammeter. Generally speaking, in field work where such measurements have to be made, a resistivity meter is preferable. 

In practice, the voltage of a battery is measured when its two ends are connected to the two terminals of a voltmeter, one contact secured to the positive terminal of the battery and the other at the negative. But a voltmeter is a device to measure differences in potential, so we start to see how the voltage cited on a battery label is simply the difference in potential between the two poles of the battery. 

Most electrodes are metallic. Sometimes the metal of an electrode can also be one component part of a redox couple. Good examples include metallic iron, copper, zinc, lead or tin. A tin electrode forms a couple when in contact with tin(IV) ions, etc. Such electrodes are called redox electrodes (or non-passive). In effect, a redox electrode has two roles first, it acts as a reagent and, secondly, it measures the energy of the redox couple of which it forms one part when connected to a voltmeter. 

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... 

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. 

The best technique for lower resistances is the four-contact method shown in Figure 8.6. The leads and contacts that measure the voltage across the resistor, caused by the current through the resistor, are not the same leads and contacts that supply the current. Since the current does not appear in the voltmeter leads or contacts, no error due to an iR voltage in the voltmeter contacts will occur. 

Consider the redox titration of 120.0 mL of 0.100 M FeS04 with 0.120 M K Cr Oy at 25°C, assuming that the pH of the solution is maintained at 2.00 with a suitable buffer. The solution is in contact with a platinum electrode and constitutes one half-cell of an electrochemical cell. The other half-cell is a standard hydrogen electrode. The two half-cells are connected with a wire and a salt bridge, and the progress of the titration is monitored by measuring the cell potential with a voltmeter. 

Interfacial potential differences are not directly observable. The usual way of measuring a potential difference between two points is to bring the two leads of a voltmeter into contact with them. It s simple enough to touch one lead of the meter to a metallic electrode, but there is no way you can connect the other lead to the solution side of the interfacial region without introducing a second second electrode with its own interfacial potential, so you would be measuring the sum of two potential differences. Thus single electrode potentials, as they are commonly known, are not directly observable. 

Note the potentials of the graphite and the aluminum alloy that you determined. If these two are connected with an electrical contact, their potentials should move toward each other. Further, since the solution is relatively conductive, and assuming that the electrical lead connecting them was highly conductive, they would come to the same potential. Therefore connect the leads of the two electrodes together and connect them both to the positive (or V) lead of the voltmeter. Measure the potential of this galvanic couple relative to one of the reference electrodes and confirm that the couple potential does indeed rest somewhere in between the corrosion potentials of the two materials. 

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). 

Any effect of contact resistance is then avoided, providing the contact resistance is much smaller than the input resistance of the voltmeter. 

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. 

Figure 22-8 Apparatus for controlled-potential electrolysis. The digital voltmeter monitors the potential between the working and the reference electrode. The voltage applied between the working and the counter electrode is varied by adjusting contact C on the potentiometer to maintain the working electrode (cathode in this example) at a constant potential versus a reference electrode. The current in the reference...

A half-cell contains the oxidized and reduced forms of an element, or other more complex species, in contact with one another. A common kind of half-cell consists of a piece of metal (the electrode) immersed in a solution of its ions. Consider two such half-cells in separate beakers (Figure 21-6). The electrodes are connected by a wire. A voltmeter can be inserted into the circuit to measure the potential difference between the two electrodes, or an ammeter can be inserted to measure the current flow. The electric current is the result of the spontaneous redox reaction that occurs. We measure the potential of the cell. 

This cannot be measured because there are other metal-metal and metal-semiconductor contacts in the measuring circuit, including those of the voltmeter, and the sum of all contact potentials is zero. As can easily be deduced from Fig. 2.3, the barrier height e5 at the metal-semiconductor contact is given by... 

To measure the surface resistivity of paper, a variant of a four-point probe method proposed by Cronch 15 was used. This approach has proven to be reliable, avoiding the effects of contact resistance by employing an electrostatic voltmeter (utilizing contactless probes) to measure the surface potential of paper subject to a constant current. 

Potentiometry requires a reference electrode, a working electrode and a potentialmeasuring instrument, e.g. voltmeter, otherwise known as a potentiometer. The test solution must be in direct contact with the working electrode, which is sometimes referred to as the chemical sensor as it is sensing the output of a chemical reaction. The reference electrode can also be placed in the test solution or can be brought into contact with the test solution via a salt bridge. The measured potential can be related to the concentration of the species being measured and this approach is called direct potentiometry. 

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