Argentometry
The argentometry is a method for the quantitative determination of certain types of ions. Depending on the technology, a distinction can be made between different processes.
Argentometry is based on the low solubility of some silver compounds in water. This applies to the compounds silver chloride , silver bromide , silver iodide , silver cyanide and silver thiocyanate , which precipitate quantitatively from aqueous solutions. Fluoride is the only halide that can not be determined using argentometry, since silver (I) fluoride is easily soluble in water. Since the determination is carried out by titration , argentometry can be referred to as precipitation titration .
General procedure
For the determination of chloride , bromide , iodide , cyanide or thiocyanate, silver nitrate solutions with a precisely known concentration are used as standard solutions for titration . If, on the other hand, the silver content of a solution is to be determined, ammonium thiocyanate is used as the titer .
The general reaction for the precipitation of a halide is:
Special procedures
Depending on the reaction conditions and indicators used, a distinction can be made between different processes, each of which is named after the person who discovered it.
The titration according to Mohr can be applied for chloride and bromide. It uses potassium chromate as an indicator; at the equivalence point, silver chromate precipitates as a red-brown precipitate.
The Volhard titration can be used for all possible ions. Iron (III) ions, which form a red complex with thiocyanate, serve as indicators . Only silver can be determined directly, the others are determined by back titration .
The Fajans titration is again a direct determination of halide ions with silver nitrate as the titer. Since after the equivalence point the precipitate is positively charged by additional silver, it can then adsorb the eosin or fluorescein used as an indicator - depending on the ion to be titrated - with a change in color.
The Liebig titration can only be used for cyanide. This initially forms a soluble complex with the silver, then the silver cyanide precipitates at the equivalence point and shows this.
Potentiometry
Argentometry can also be carried out potentiometrically . The potential difference between a sheet of silver and a solution containing silver salts is measured. A calomel electrode is used as the reference electrode , but it must not be directly immersed in the solution because of the diffusion of interfering potassium chloride . The calomel electrode is therefore immersed in an ammonium nitrate solution, which is connected to the measurement solution via a bridge.
literature
Jander, Blasius, Strähle: Introduction to the inorganic-chemical practical course. 14th edition. Hirzel, Stuttgart 1995, ISBN 978-3-7776-0672-9 .