Potentiometry

Potentiometry (from lat. potentia - power, action, and Greek. metreo " measure") - a set of methods of research of systems based on measuring the electromotive force (EMF galvanic elements (chains), made up of carefully selected electrodes. Widely used in biochemical, physiological, clinical laboratories for determination of activity (concentration) of ions in solutions, pH, redox potential, bioelectric potentials for potentiometric titration and other
Measuring EMF chains make compensation method where at the time of measuring the current through the study chain fails, because it EMF compensated by external, oppositely directed by potential difference established at a certain period of resistance using a constant current source (usually the battery). The scheme of the device (potentiometer) for measuring EMF compensation method is given in Fig. 1. Shops resistance 3, 4 represent two identical sets of resistors, located in two semicircles connected by a conductor with Curbeam 11, that the rotation includes the electrical circuit at each store different resistance, but the total resistance on both remains constant. Switches 9 and 10 are used to alternately enable normal 7 and 8 analyzed elements. Resistance 2 is 1018,3 Ohms.
Installation for measuring EMF consists of the main circuit(1, 2, 3, 4, 5) and the side - with a normal element of Weston, with constant EMF, equal 1018,3 mV(2, 10, 6, 7,9), or with the test element(3, 11, 10, 6, 8, 9). For calibration shop resistance include normal element, closing its resistance 2, and pick up such resistance 5, in which the potential difference on each Om in the main circuit is equal to 1 mV. When this happens compensation EMF normal element and current in the side chain will not (that is determined by the galvanometer). Then include the analyzed item and seek his compensation by turning curbele without changing resistance 5. EMF analyzed item in mV is numerically equal to the reading of the store 3 in ohms, therefore they are usually indicated in mV.
To measure potential electrodes used reference electrodes, for example calomel or silver chloride, the potentials of which are permanent and measured relative to the standard hydrogen electrode, the potential of which is assumed equal to zero. Calomel electrode consists of mercury that accumulates suspension Hg2Cl2 in the solution КCl. On the border of mercury with a solution occurs potential eHg, the value of which depends on the concentration КCl. At 25oC potentials equal to 0.1 N. КCl 0,3337 In, for 1.0 N. КCl - 0,2801 In, for rich - Century 0,2412
To measure the potential of the investigational electrode constitute a chain:
where M-metal, MA - salt of this metal. Measure EDS E - chain and expect capacity by the formula eat = eHg - E or eat = eHg + E, if M is less active than mercury.
When measuring bioelectric potentials often used silver chloride reference electrode, consisting of silver wire coated with the layer of AgCl, immersed in a solution HCJ or КCl. Electrode solutions are usually connected by U-shaped tube, filled with jelly agar, rich КCl, sometimes KNO3 or NH4NO3.
Knowing eat, you can calculate the am activity of ions in solution, in relation to which the electrode is reversible. The dependence between potential and activity of ions is expressed by the formula Nernst, which for 25 degrees is:
eating is a normal electrode potential arising on the electrode when am = 1, n is the valency of this metal ion.
The pH of the solution is made by means of electrodes, the potential of which depends on the concentration of hydrogen ions. The three most common electrode: hydrogen, gingerangel and glass.
Hydrogen electrode. In this case, are of a hydrogen-calomel chain:
Measure EMF chain E and expect pH formula.
Hydrogen electrode cannot be used in the presence of strong oxidants and reductants, and "poisons"polluting platinum electrode (H2S,AgH3 and others).
Gingerangel electrode. Are hirikano-calomel chain, measure it EDS E, calculate the pH of the formula.
Gingerangel the electrode is not applicable in solutions with pH>8 or containing strong oxidants and reducing agents.
Glass electrode (Fig. 2) gives the most reliable results when measuring the pH of body fluids. It is applicable in a wide range of pH in the presence of oxidants and reducing agents. On both sides of the membrane glass is poured two solutions. One of them, standard, usually 0.1 N. Hcl, other, studied, with unknown pH. Both of the solution be connected with the reference electrodes (usually silver chloride and calomel). When measuring EMF obtained chain due to the large internal resistance of the glass membrane into the potentiometer, in addition to the salary installation, mount the power supply. Such devices are called pH-meters. Previously the device calibribu, measuring EMF several chains with a known pH, ions, and build the estimated schedule pH= f (EDS), which determine the pH.
Potentiometric titration produce for determining the total concentration of acids, alkalis, oxidizers or reducing agents salts and other Method is to measure EMF chain composed of the indicator electrode immersed in titrated solution, and a reference electrode. Indicator electrode is chosen so that its potential is concentration dependent titrated substances. During the titration of oxidants and reductants apply smooth platinum electrode.
Titration curve build in the coordinates of "EDS - V (volume of a solution of reagent)". The point of equivalence (see Titration) is determined by the abrupt change of potential in it. To calculate the concentration used the formula V1H1=V2Н2, where V and N - accordingly, the scope and normality titrated solution and solution add the reagent.

Fig. 1. The scheme of the potentiometer: 1 - battery; 2 - resistance; 3 and 4 shops resistors; 5 - rheostat; 6 - galvanometer; 7 - a normal element of Weston; 8 - the analyzed item (chain); 9 and 10 switches;
11 - kurbiel.
Fig. 2. Glass electrode: 1 - the glass membrane; 2 - the test solution; 3 - 0,1 n Hcl solution; 4 - silver chloride electrode; 5 - calomel electrode; in - solution КCl; 7 - suspension Hg2Cl2; 8 - mercury; 9 - platinum contact.