Conductometry

Conductometry or Leitfähigkeitstitration is a chemical-physical analysis method which exploits the electrical conductivity of a liquid sample to determine their dissociated ingredients and their concentration. It can form acid - base titration or precipitation titrations are performed. Conductometry than titration very popular because there are no chemical indicators needed. Great benefits of Conductometry are the simplicity of the procedure and the possibility of precise conductimetric titrations with very dilute solutions ( 10-4 molar).

Principle

In conductometric titration ions with a very high conductivity (e.g., proton, hydroxide - see boundary conductivity) of opposite charge neutralized by the addition of ions. Acid protons are converted, for example, a sodium hydroxide solution of hydroxide ions to form water and salt. Water and salt have a lower conductivity than pure acid. This manifests itself in a decrease in conductivity. Once all protons have converted to water at a titration, the conductivity increases with further addition of alkali strongly again, because once again ions with high conductivity ( hydroxide ions ) are present. When performing the titration of standardized normal solutions can be identified by the increase in conductivity, and the titration amount of ions (e.g. acidity ) of a sample.

Conductometry or Leitfähigkeitstitration is a method in which after each Reagenzzusatz the ohmic resistance of the solution for analysis or its inverse, the electrical conductivity ( in Siemens S = Ω -1 ) by means of alternating current is measured. This dive two equal inert electrodes in the solution to be measured. The resistance determination is performed by means of a Wheatstone bridge circuit or a requirement of modern complex electronic circuit devices. The resistance of the conductivity measurement of a solution is lower when the electrode area of the measuring device is increased or the distance between the electrodes is reduced. Calibrated devices are already set to default surfaces and distances. With these devices you can directly the specific conductivity ( ĩS / cm ) off a solution. The specific conductivity is a device-independent size and is influenced only by the solution, that is, the type and number of ions in aqueous solutions. By multiplying the specific conductivity of 1 cm we obtain the conductance of the solution.

In commerce there are already very inexpensive conductivity meters whose price is about 35 €. These devices show the specific conductivity of the solution is usually in ĩS / cm (2000-9999 ĩS / cm). These devices are thus only suitable for a range up to a maximum of 0.1 mol / L sodium chloride. For concentrated salt solutions to these instruments, however, are not suitable. Either the aqueous solution with distilled water (1/ 10 or 1/100 ) should be diluted or have special equipment ( Price from 130 € up to 100 mS / cm ) can be used. These modern devices are very accurate, there is even a temperature compensation. In compensation, a temperature sensor and an electronic program always refers to the conductivity value at 25 ° C. Both at 20 ° C as at 29 ° C is thus obtained, identical conductivity values ​​, so that the values ​​can be easily compared with conductivity from reference information. The user should check the temperature compensation, however, in advance. Slight deviations are possible. By measuring the conductivity of a known calibration solution (eg potassium chloride solution) at exactly the set temperature, the accuracy of the instrument can be checked and - are sometimes calibrated - with slight variations.

During the measurement the temperature of the electrolyte solution must not fluctuate - if no temperature compensation device is present. A temperature change of 1 ° C denotes a conductance change of about 2%. With temperature compensation, the difference is usually less than 0.2 % per degree.

Because protons and hydroxide ions have a very high conductivity (exact boundary conductivity ), acid- base titrations, conductometric titrations at are particularly popular. Titrations can be carried out in non-aqueous solutions (eg, amines, amides, aniline derivatives in anhydrous acetic acid with perchloric acid set ). Also in precipitation (eg halides by means of a solution of silver nitrate or sulfate by means of a barium chloride or barium or Mercaptide or thiols by means of a HgCl2 solution) and Komplexbildungstitrationen the conductometric titration can be applied.

The measured values ​​are plotted (conductivity: ordinate against Reagenszusatz in ml ) The measurement points often lie in the immediate vicinity of the equivalence point not on the line, the titration is therefore determined by extrapolation. It is even possible, a weaker and a stronger acid to titrate together, as the respective amount of substance from the inflection points of the two different slopes straight results.

The conductivity of an electrolyte solution is dependent on the

• Concentration (linear)
• Ion charge
• Limiting equivalent conductivity of ion

Helpful for the conductometric determination of the concentration of ionic single substances (eg acetic acid concentration of household vinegar ) by conductometric measurement are the carbon intoxication square-root law, the Ostwald dilution law, the Debye- Hückel theory, the molar conductivity.

Method

The basic distinction between two different methods.

• When determining method, the conductivity is measured and the concentration on the basis of calibration curves.
• The indication method, the conductivity change is measured during a titration

Experimental setup

Most special instruments are used to measure. Typically, these are provided with a thermometer, and the measuring frequency is set or at least reversible. The measured values ​​are given in S / cm (Siemens per cm ) (Example: high-purity water 0.05 ĩS / cm to 0.1 ĩS / cm, tap water 300 ĩS / cm to 1 mS / cm, sea water is about 50 mS / cm ). The internal structure of the measuring instruments is similar to the Wheatstone measuring bridge, so it is a very accurate resistance measurement.

Application of the conductometric

Typical application areas are:

• Control of the purity of solutions (example: demineralized water)
• Content determined by conductometric titration (eg, acid / base, precipitation)
• Determination of the concentration ( acid bath salt solution) with calibration curves, tables
• Determination of the total of all electrolytes ( water analysis, medicine)
• Analysis of a substance on the basis of typical material properties (comparison with tables)

History

The first conductivity meter was developed by Friedrich Wilhelm Georg Kohlrausch. Unlike direct current measurements, the overvoltage of the electrode is eliminated by alternating current, so that the pure conductivity of the electrolyte solution can be determined. Conductometry was introduced by IM Kolthoff, G. Jander and O. Pfundt in the analysis.