Potentiometer Circuits

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]. 

Potentiometers Measuring the potential of an electrochemical cell under conditions of zero current is accomplished using a potentiometer. A schematic diagram of a manual potentiometer is shown in Figure 11.2. The current in the upper half of the circuit is... 

Figure 8-5 shows the main circuit diagram of a potential control rectifier provided with magnetic amplifiers (transducers). The chosen potential is set at the nominal value with a potentiometer. The actual potential is compared with this value, which corresponds to the voltage between a reference electrode and the protected object. 

The coil (wound on a light metal former) can be suspended by a fine strip of phosphor bronze between the pole pieces. Attached to this suspension is a small mirror which reflects on to a scale a beam of light which is focused upon it. An instrument of this kind is known as a D Arsonval galvanometer and is used in potentiometer circuits and various methods of measurement of resistance. 

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. 

If it is desired to use the biamperometric method for detecting the end point, then the calomel electrode and also the silver rod (if used) must be removed and replaced by two platinum plates 1.25 cm x 1.25 cm. The potentiometer (or pH meter) used to measure the e.m.f. must also be removed, and one of the indicator electrodes is then joined to a sensitive galvanometer fitted with a variable shunt. The indicator circuit is completed through a potential divider... 

This can be accomplished by applying an electrical potential in the external circuit in such a manner that an emf occurs in opposition to that of the galvanic cell. The opposing emf is varied by means of a potentiometer until the current flow from the cell is essentially zero. Under these conditions, the cell may very well approach reversibility. This is readily tested by changing the direction of the current and allowing an infinitesimally small current flow in the opposite direction. If the cell is reversible, the cell reaction will proceed in the reverse direction with the same efficiency as in the forward direction. For a reversible reaction... 

If two zinc electrodes are set up in opposition to one another as in Figure 6.12 (A), the difference of potential between them, measured by a potentiometer or voltmeter, is zero. If an infinitesimally small external emf is applied to the electrodes so that A is positive and B is negative, a very small current flows round the circuit, and Zn atoms pass from A into solution as Zn2+ ions, and Zn2+ ions leave the solution and are deposited as Zn atoms on B. If the small emf is reversed so that B is positive and A is negative, the current flows in the opposite direction, and zinc is dissolved from B and deposited on A. An electrode such as the zinc electrode, which reacts thus to an infinitesimal applied emf, is known as a reversible electrode. The hydrogen electrode described earlier is a reversible electrode. If two molar hydrogen electrodes are set up in opposition to one another, Figure 6.12 (B), the... 

Hence, by applying a bias emf corresponding to 2.3026RT/F pH, or by offsetting the mechanical zero of the potentiometer by pH units, the zero shift is compensated for any temperature within the range (about 20° C) over which pH is fairly constant. Modern instruments often include the possibility of varying the pH, bias (to allow the use of different electrode systems) by incorporating the isopotential bias into the meter circuits. 

Fig. 5.15 Basic circuit for the electrode potential measurement during current flow A is working (indicator, test), B, auxiliary and R, reference electrode connected by means of the Luggin capillary (arrow) and P, potentiometer. 

Figure 4.5 A potentiometer circuit The voltage from the test circuit is balanced against a known voltage by means of a variable resistance using a galvanometer to indicate the position at which no current flows in either direction.

Contemporary potentiometers contain circuits with high internal resistances. The resistance of the electrode membranes thus creates no serious problem, but the leads to the electrodes should always be shielded and the high-resistance systems should be placed in a Faraday cage otherwise the measurement is subject to noise. 

This circuit gives the user the option to use a DC current or to use a pulsed current as a load (useful for checking load-switching transients of power supplies). The 15 V input signal is dropped to 10 V by a 5 V zener in series. Switch A selects between the simple potentiometer voltage divider circuit (limited to 5 V maximum by the divider resistors)... 

Fig. 5.40 Circuits for conductivity measurements with two-electrode cell (a) and four-electrode cell (b). In (a), S AC voltage source D detector I, II, III bridge elements. In (b), S constant-current source POT potentiometer Rs variable resistor C and C electrodes for current flow P and P electrodes for voltage measurement.

The voltage for a complete reaction is the difference between the potentials of the two half-reactions E = E+ — E, where E+ is the potential of the half-cell connected to the positive terminal of the potentiometer and E is the potential of the half-cell connected to the negative terminal. The potential of each half-reaction is given by the Nemst equation E = E° — (0.059 16/n) log Q (at 25°C), where each reaction is written as a reduction and Q is the reaction quotient. The reaction quotient has the same form as the equilibrium constant, but it is evaluated with concentrations existing at the time of interest. Electrons flow through the circuit from the electrode with the more negative potential to the electrode with the more positive potential. 

Figure 17-1 shows how we might conduct the experiment. The potentiometer measures the voltage applied by the power source between the two electrodes. The ammeter measures the current flowing through the circuit. 

Fig. 2.4 The Poggendorff potentiometer a circuit which permits the emf of a ceil to be backed off so that the spontaneous cell reaction can be allowed to proceed, to be exactly halted (galvanometer at zero) or to be reversed...

Figure 6-3 Device for measurement of electrode potentials. The electrode reactions are indicated below each half-cell. The maximum electrical work that can be done by such a cell on its surroundings is - AG = nEF, where E = V2-V1as measured by a potentiometer. If A is reduced to AH2 by H2/ electrons will flow through an external circuit as indicated. A will be reduced in the right-hand cell. H2 will be oxidized to H+ in the left-hand cell. Protons will flow through the gel bridge from left to right as one of the current carriers in the internal circuit.

The signal Vu, for compensation of iRu, must be proportional to this quantity and identical in phase. A convenient source of Vu is the output of the current follower, OA3, of Figure 7.1 A. Adjustment of the magnitude is provided by a potentiometer so that some fraction F of iRu is compensated. Because of the wide range of resistor values that may be used in the circuit, Vu will not generally be equal to F(iRu), but will be scaled according to the equation... 

The temperature of a gas oil product flowing through a pipe is monitored using a chromel/alumel thermocouple. The measurement junction is inserted into the pipe and the reference junction is placed in the plant control room where the temperature is 20°C. The emf at the thermocouple junction is found to be 6.2 mV by means of a potentiometer connected into the thermocouple circuit adjacent to the reference junction. Find the measured temperature of the gas oil. 

---