Potentiometer

POTENTIOMETER, an apparatus used for measuring electrical potentials, i.e., electromotive forces (EMF). Voltmeters are usually used for this purpose. However, in some cases, such as when measuring the EMF of concentration cells, one must avoid the consumption of electrical energy during the measurement process, as this would affect the magnitude of the measured EMF at low current strengths. In these cases,

POTENTIOMETER

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voltmeters are not suitable, and the measurement must be performed using the Poggendorff-du Bois-Reymond compensation method, which is the basis for the operation of the potentiometer. The current under investigation is sent through the circuit IEV (Figure 1), and this circuit includes one or another instrument that registers the presence or absence of current (a null instrument). The potential difference between points B and V represents the sought-after EMF-Ex; this EMF, of course, does not change, regardless of where the movable contact B is located. A current, known to be stronger, is sent from the accumulator through the circuit AkDBV, and its potential drop occurs along the resistance DV. By moving the movable contact B, we change the part of the EMF of this circuit that passes through the section of the path BV, which is common to both circuits. Thus, it turns out that between points B and V there exist, as it were, two potential differences: one constant, Ex, originating from the first circuit, and the second—let us call it Ea—originating from the second circuit and changing depending on the position of point B. If Ex > Ea, then the current in the first circuit will usually flow in the direction of the arrows (if we denote the flow of electrons); if Ex < Ea, then the current in the first circuit will flow in the opposite direction, since the negative current from the accumulator will flow in the direction AnVIEZhB, and the element IE will represent the electrolyzed system. If Ex = Ea,

Figure 1. Device of a potentiometer with a voltmeter: Ak—accumulator; IE—element under investigation; Gl—galvanometer; V—voltmeter.

then there will be no current in the first circuit—the null instrument will show the absence of current between points B and Zh. Now, in order to find Ex, it is necessary to measure the Ea equal to it; this is done differently in various models of potentiometers. As already mentioned, potentiometers are often used for measuring the EMF of concentration cells, where the ultimate goal is not the measurement of the electromotive force of such a system itself, but the determination of the concentration of ions in it, for example, the concentration of H-ions (pH), and therefore potentiometers specifically intended for this are often called ionometers. It is clear that the measurement of electromotive forces can be carried out by the Poggendorff compensation method without any potentiometer, but by using assembled setups with a rheochord or resistance boxes (according to Michaelis) with a capillary electrometer or galvanometer as null instruments, switches, and many exposed wires. In principle, potentiometers represent this kind of setup, with the difference that in them all parts are firmly, compactly, and most conveniently connected once and for all for rapid measurement and enclosed in a box. When the lid of this box is removed, only a few knobs and contacts are visible on the board made of insulating material, and sometimes the measuring instruments enclosed therein. Thus, all critical parts of the compensation setup are protected from dust and dampness, and the entire setup becomes portable. There is a whole range of potentiometer models on the market intended for various purposes: physicochemical, biological-medical (pH measurement), technical (determination of the concentration of all kinds of ions and various electrometric titrations), etc. They differ from each other in the accuracy of determination, the method of reading, the method of verification, the resistance used, the null instruments, the layout scheme, etc. The following models are the most common and original. 1. Perhaps the simplest apparatus is one in which the EMF (Ea) is measured directly in the compensation position with the help of a sensitive voltmeter (Figure 1). 2. One can proceed somewhat differently, namely, for the compensation position, measure the current strength I in the second circuit with the help of a sensitive ammeter in amperes and the resistance R of the section DV in ohms (Figure 2). The product of these two values will give the sought-after EMF: IR = Ex. The two described methods of setup without a standard element are used in American potentiometers. However, they cannot be considered particularly accurate, firstly because it often happens that the magnetic system of the measuring instruments changes somewhat over time, and secondly because the scale of the instrument must be extremely finely divided in order to be able to calculate Ex with sufficient accuracy. They are used, especially the first setup, for electrical titrations. 3. There are models that are closest to ordinary electrometric compensation setups.

,-----=^£±-------wywv B AWWWWWWVVWVW BKWWWWWWWWWWVWl * ₊X_ ₊ J-Lj Lrt Figure 2. -o Figure 3. K* 9 °3 Figure 2. Device of a potentiometer with an ammeter: An—accumulator; IE—element under investigation; Gl—galvanometer; DV—resistance; A—ammeter. Figure 3. Device of a potentiometer with a rheochord, additional resistance, and a standard element: Ak—accumulator; IE—element under investigation; NE—standard element; Gl—null instrument; DV—rheochord 1018.7 mm; DS—additional resistance.

In them, a resistance wire with divisions—a rheochord 1018.7 mm long—is used as resistance; in addition, an additional resistance is included in the second circuit, and in the first, instead of the element under investigation, a standard element can be included (Figure 3). Having sent a current from the standard element (voltage 1018.7 mV) into the first circuit along the path IEZhZD V, the compensation position is reached with the help of the additional resistance DS. Now each millimeter of the rheochord corresponds to a potential drop in the second circuit of 1 mV. Having switched off the standard element, a current from the element under investigation is sent into the first circuit along the path IEZhZhKV, and a new compensation position is found by moving the slider B. The sought-after EMF-Ex, expressed in millivolts, is equal to the number of mm of the rheochord on the segment BV. Thus, it is easy to read the EMF with an accuracy of 1/3 mV. Among the individual models, one can mention: the Luer ionometer (H. Luers) from the Lautenschläger firm (F. & M. Lautenschlager), which is essentially an electrometric setup assembled on one board (Figure 4),

Figure 4. Luer ionometer.

suitable more for technical purposes, the Emsländer potentiometer (Emsländer) No. 9 «14» from the Kohler firm (F. Kohler); the Lautenschläger micro-ionometer, an instrument in which the resistance wire of 1018.7 ohms is wound in the form of a spiral (there is no classic slider), and the capillary electrometer can be replaced by a galvanometer—whereby a very portable apparatus is obtained (Figure 5).

Figure 5. Lautenschläger micro-ionometer. The 4th group combines models in which, instead of a rheochord, there are rheostats with rotary knobs (crank rheostat). Usually, they contain 1 rheostat for 1,000 or 1,100 ohms (Figure 6, MZ) with 10 divisions of 100 ohms each and a resistance spiral for 100 ohms (IO); thus, any resistance from 0 to 1,100 or even 1,200 ohms can be included in the first circuit. These potentiometers are equipped with an additional resistance DS, galvanometers as a null instrument, and standard elements. The method of working with them is analogous to that just described for group 3. Only in this case, when compensating the standard element, a resistance of 1018.7 ohms must be included in the first circuit, then 1 ohm of resistance turns out to correspond to 1 mV. The sought-after EMF Ex, expressed in millivolts, is equal to the number of ohms of resistance located in the line to the galvanometer, on the segment ZhK. One can mention the following models: the potentiometer manufactured by the Main Chamber of Weights and Measures in Leningrad; the poten-

Figure 6. Device of the Mislowitzer potentiometer: Ak—accumulator; IE—element under investigation; NE—standard element; Gl—galvanometer; MZ—rheostat with divisions of 100 ohms; IO—resistance wire; DS—additional resistance; BS—ballast resistance-

Figure 7. Mislowitzer potentiometer.

tiometer of Mislowitzer (Mislowitzer), manufactured by the Leitz firm (Figures 6 and 7) with a special galvanometer for it, which, by including a large ballast resistance of 100,000 ohms in front of it, located in the potentiometer box (Figure 6, BS), is converted from a null instrument into a voltmeter, and then can serve instead of a standard element for a quick check of the EMF passing through the entire resistance (1,100 mV);

is available in many scientific-medical laboratories. The Trun and

Figure 8. Trun and Tödt potentiometer.

The Theda (Thrun, Todt) model by Leitz. In this model, both the galvanometer and the normal element are built into one case with the P. itself (fig. 8). The P. of the American firm Leeds and Northrup in Philadelphia, distinguished by the accuracy of EMF reading (up to 0.01 mV). Suitable for physicochemical research. -5. Finally, mention should be made of another recent construction of the ionometer - the Stato-Ionometer by Wulf - by the firm Lautenschläger, which is based on a vacuum tube voltmeter. The potential to be measured is connected to the grid and to the negative end of the filament of the electron tube (fig. 9). When the filament is heated, a current flows from k to A, supplied by battery BA; the strength of this current can be measured by an ammeter A; it turns out to depend on the voltage on the grid. Thus, the EMF of the element under investigation can be determined from the strength of the anode current and read directly on a correspondingly graduated measuring instrument. It should be added that in this method, as little current is taken from the current source of interest as in the compensation method. This method allows for the direct registration of pH on a rotating drum (fig. 10). Such

pH curves are very convenient for electrometric titration. The accuracy of the latest construction of this instrument is not lower than that of other potentiometer models. Lit - Walter O., Methods for determining the concentration of hydrogen ions, L., 1932; Mislovitzer E., Determination of the concentration of hydrogen ions in liquids, L., 1932.

Figure 9. Circuit diagram of a vacuum tube potentiometer: E - electron tube; a - anode; k - filaments; r - grid; IE - element under investigation; A - ammeter; BN - filament battery; BA - anode battery.

Figure 10. Wulf's statioionometer.

pH curves very convenient for electrometric titration. The accuracy of the latest construction of this instrument is not lower than that of other potentiometer models. Lit - Walter O., Methods for determining the concentration of hydrogen ions, L., 1932; Mislovitzer E., Determination of the concentration of hydrogen ions in liquids, L., 1932.