Potentiometry

The Potentiometry is an electrochemical analytical method of quantitative analysis while avoiding a current flow and uses the concentration dependence of the electromotive force (often electrochemical potential called).

In potentiometric titration ( dimensional analysis with potentiometric indication) an indicator electrode (e.g. pH electrode, redox electrode) is immersed in an analysis solution and the change in potential is tracked as a function of the added volume of a reagent solution. At the equivalence point the concentration and thus the electromotive force and the potential change strongly. With potentiometry, acid , base , redox , precipitation and complex formation titrations can be carried out; dissociation constants and solubility products can also be determined. Colored, cloudy solutions can be titrated potentiometrically. With pH electrodes , acid and base titrations can also be carried out in very dilute solutions (10 ⁻⁴ mol acid / base).

Potentiometry was introduced by Robert Behrend in 1893 .

Procedure

Potentiometric indicator electrodes such as glass electrodes for pH measurement and redox electrodes are readily available these days. With these electrodes, the pH value or redox potential can be read immediately.

In addition to these two electrodes, there are those that respond to the activity of one type of ion, the ion-selective electrodes . For example, with the silver / silver sulfide electrode, halide , sulfide , cyanide , thiocyanate or chromate ions can be determined by titration with a set silver nitrate solution. Ion concentrations of potassium , magnesium , calcium or nitrate can be determined with liquid membrane electrodes . Ion-selective electrodes can also be used for complexometric titrations. Ethylenediaminetetraacetic acid (EDTA) is mainly used as the titrant in complexometric determinations .

Potentiometry is used in two ways:

1. The indication procedure is a special form of titration . For this purpose, the change in the electromotive force is recorded during a titration and then the equivalence point is determined. This procedure is used whenever a normal titration cannot be carried out. This can be the case if there is no suitable indicator or the solution is so cloudy or colored that the point of change cannot be recognized.
2. The determination method is based on a concentration determination by measuring the associated electrochemical potential; the evaluation takes place on the basis of a calibration line created with the same measurement setup.

Experimental setup

Schematic representation

In the past, a galvanic cell was made up of two electrodes and the sample solution as the electrolyte. The difference between the potentials of the two electrodes is defined as the electromotive force EMF:

${\displaystyle EMK=|E_{1}-E_{2}|}$

An electrode, the reference electrode, of which the potential is known very precisely and is constant, is used for the construction. In general, an electrode of the 2nd type is used for this purpose. In practice these are usually a calomel electrode or a silver-silver chloride electrode . The voltage difference between the reference electrode and the measuring electrode immersed in the solution can be determined using a voltmeter. The choice of measuring electrode depends heavily on the type of titration. One used for that

• Redox titration a platinum electrode,
• complexometric determination either an ion-sensitive electrode or a modified silver electrode,
• Acid-base titration a glass electrode ,
• Argentometry a silver electrode.

The measuring electrode is immersed in the sample solution and must respond to the ion to be determined, that is, the potential of this electrode must be dependent on the concentration of the ion to be determined. This dependency is described by the Nernst equation . During the experiment, it must be ensured that the measurement is carried out without current, as otherwise the potentials would be falsified by electrolysis . A voltage compensation circuit was therefore used for the measurement .

evaluation

The evaluation is carried out by determining the equivalence point. The best way to recognize the equivalence point is to display the 1st derivative of the titration curve in the diagram. The maximum indicates the equivalence point. The volume of the titrated solution in ml is plotted on the x-axis and the measured potential in mV on the y-axis.

If you want to calculate a certain salary, you have to use two formulas:

1. ${\displaystyle n=c\cdot V\cdot f}$ With

   ${\displaystyle n}$= Amount of substance in mol

   ${\displaystyle c}$= Concentration in mol / l

   ${\displaystyle V}$ = Volume of the titration in l

   ${\displaystyle f}$= Titer factor ( if the titer factor is not known, set ) ${\displaystyle f=1}$

2. ${\displaystyle m=n\cdot M}$ With

   ${\displaystyle m}$ = Mass in g

   ${\displaystyle M}$ = molar mass in g / mol

literature

• Georg Schwedt: Analytische Chemie, p. 170 ff., Wiley-VCH-Verlagsgesellschaft, 2nd edition, Weinheim 2008, ISBN 978-3-527-31206-1
• Ullmann's Encyclopedia of Technical Chemistry, 3rd + 4th edition, keyword: electrochemical analysis methods