Conductometry

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Conductometry or conductivity measurement is a chemical-physical analysis method that uses the electrolytic conductivity of a liquid sample to determine its dissociated constituents and their concentration .

Conductometer for determining the ash content (corresponds to the mineral content) by means of conductivity measurement; right conductivity measuring cell ( Zucker-Museum , Berlin)

Procedure

Essentially, a distinction is made between two different conductometry methods.

  • In the determination process , the conductivity is measured and the concentration is determined using calibration curves.
  • In the indication method , the change in conductivity is measured during a titration , and conductometry is used to determine the end point of the titration. This type of conductometry is important in many cases of analytical practice. Such conductivity titrations can be acid-base titrations or precipitation titrations. The great advantages of conductometry are the simplicity of the procedure and the possibility of exact conductometric titrations even with very dilute solutions (10 −4 molar). Furthermore, no chemical indicators are required.

Conductivity measurement

schematic illustration of a measuring cell

The conductivity measurement is based on the determination of the ohmic resistance of the analysis solution or its reciprocal value, the electrical conductivity (in Siemens S = Ω −1 ). It uses alternating current . Two inert electrodes are immersed in the solution to be measured in a parallel or coaxial arrangement. The resistance measurement uses a Vienna bridge or a complex electronic circuit contained in modern devices. The resistance of a solution decreases when the electrode area of ​​the measuring device increases or the distance between the electrodes decreases. Adjusted devices are already set to standard areas and distances. With some devices you can read the electrical conductivity of a solution directly ; otherwise the conductivity of the solution is obtained by multiplying the conductivity by the cell constant.

The electrolytic conductivity is a device-independent variable that is only specific to the solution; it is influenced by the value , mobility and particle density of the ions in aqueous solutions.

Measuring device

Special conductivity measuring devices, which are also called conductometers, are usually used for measurement . Typically these are provided with a thermometer and the measuring frequency can be set or at least switched. The measured variable is often given in S / cm (Siemens per centimeter) (example: highly pure water 0.05 to 0.1 µS / cm, tap water 0.3 to 1 mS / cm, sea water about 50 mS / cm). The electrical structure of the measuring devices is similar to that of the Wheatston measuring bridge , so it is a very precise resistance measurement .

There are already very inexpensive conductivity meters on the market, the price of which is around € 35. These devices usually show the conductivity of the solution in μS / cm (2000–9999 μS / cm). These devices are therefore only suitable for a range up to a maximum of 0.1 mol / L table salt and are therefore not suitable for concentrated salt solutions. Either the aqueous solution has to be diluted with distilled water (1/10 or 1/100) or special devices (price from 130 €, up to 100 mS / cm) have to be used. These modern devices are quite accurate, there is even temperature compensation . In doing so, they use a temperature sensor and an electronic program to convert the measured conductivity value to 25 ° C. Identical conductivity values ​​are thus obtained at both 20 ° C and 29 ° C, so that the values ​​can be more easily compared with conductivity information from reference works. However, the user should check the temperature compensation beforehand. Slight deviations are possible. By measuring the conductivity of a known calibration solution (e.g. potassium chloride solution) at a precisely set temperature, the accuracy of the device can be checked and - in the case of slight deviations - sometimes readjusted.

The temperature of the electrolyte solution must not fluctuate during the measurement - if there is no temperature compensation in the device. A temperature change of 1 ° C means a change in conductivity of approx. 2%. With temperature compensation, the deviation is usually less than 0.2% per degree Celsius.

Conductivity of electrolyte solutions

The electrolytic conductivity of a solution depends on the

Kohlrausch's square root law , Ostwald's law of dilution , and the Debye-Hückel theory , see molar conductivity, are helpful for the conductometric determination of the concentration of ionically uniform substances (e.g. the acetic acid concentration of household vinegar ) by conductometric measurement .

Applications of the direct determination method without titration

  • Control of the purity of solutions or purified water ( distilled water , demineralized water , especially ultrapure water , which should have a conductance of 0.055 µS / cm or a specific resistance of 18.2 MΩcm at 25 ° C)
  • Determination of the concentration (acid bath, saline solution) with calibration curves, tables, e.g. B. Solutions of regeneration salt
  • Determination of the total concentration of all electrolytes (water analysis, medicine)
  • Analysis of a substance based on typical substance properties (comparison with tables)

Conductometric titration

Titration curve with added acid

Conductivity titration or condumetric titration (also referred to as conductometry for short) is a titration method in which the ohmic resistance of the analysis solution or its reciprocal value, the electrical conductivity, is measured using alternating current after each addition of reagent. In the conductometric titration, ions with a very high conductivity (e.g. protons , hydroxide ions - see limit conductivity ) are neutralized by adding ions with the opposite charge. Acid protons, for example, are converted into water and salt by the hydroxide ions of a sodium hydroxide solution. Water and salt have a lower conductivity than a pure acid. This is noticeable in a decrease in conductivity. As soon as all protons have been converted to water during a titration, the conductivity rises sharply again with further addition of caustic, since ions with high conductivity (hydroxide ions) are now again present. If the titration is carried out with standardized normal solutions, the increase in conductivity shows the end point of the titration and the amount of ions (e.g. acidity) of a sample.

schematic experimental setup

The measured values ​​are plotted graphically (conductivity via reagent addition). The measuring points in the immediate vicinity of the equivalence point are often not on the straight line, so the titration point is determined by extrapolation . It is even possible to titrate a weaker and a stronger acid together, since the respective amount of substance results from the break points of the two different slopes of the straight line.

Use of conductometric titration

The most important types of content determination by conductometric titration are

Because protons and hydroxide ions have a very high conductivity (more precisely, limit conductivity ), acid-base titrations are particularly popular in conductometric titrations. Titrations can also be carried out in non-aqueous solutions (e.g. amines, amides, aniline derivatives in anhydrous acetic acid with adjusted perchloric acid).

history

The first conductometer was developed by Friedrich Wilhelm Georg Kohlrausch . In contrast to direct current measurements, alternating current avoids the overvoltage of the electrodes, so that the conductivity of the electrolyte solution can be determined without measurement deviation. Conductometry was introduced into analytics by IM Kolthoff, G. Jander and O. Pfundt.

literature

  • Ullmann's Encyclopedia of Technical Chemistry, 3rd + 4th edition, keyword: electrochemical analysis methods
  • Udo R. Kunze: Fundamentals of quantitative analysis, Georg Thieme Verlag, Stuttgart 1980, pp. 179-181, ISBN 3-13-585801-4

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