What kind of connections fail in the reaction?

Precipitation reactions chemical reactions are called in which the starting materials (= the starting materials) are dissolved in the solvent and at least one product is insoluble or sparingly soluble in this solvent. The sparingly soluble product falls out, the precipitation becomes general Precipitation called. In reaction equations, the precipitation of a substance is indicated with a ↓ or an (s) for solid marked after the molecular formula of the substance.

Example reactions

The precipitation of lead sulphide precipitate from lead salt solution:

(A detection reaction for sulfide anions or lead (II) cations)

The precipitation of barium sulfate from barium chloride solution:

(A synthesis reaction for the production of "painter's white")

Examples of precipitation reactions of halogens; X = Cl, Br, I:

Application of precipitation reactions

The reaction of halide ions with silver nitrate forms sparingly soluble salts which are deposited on the bottom of the test tube. This precipitate shows that there are halide ions in a liquid.
By using specific precipitation reactions, it is possible to identify individual components of a solution. A classic example is the cation separation process, where precipitation reactions are used both for ion identification and for the precipitation of interfering ions.
In gravimetry, precipitation reactions are the basis of many content determinations and parts of volumetry (especially argentometry) work with precipitations.
In wastewater treatment, precipitation is the usual method to lower the phosphate concentration. Iron (II) sulfate, ferric chloride, aluminum chloride or polyaluminum chloride is added to this. In principle, all metal salts work in a similar way. However, every single metal salt has special advantages that have to be coordinated with local conditions, system construction, wastewater composition and the cleaning objective. A distinction is made between four methods of phosphate elimination, which differ in terms of the dosing points and have different effects on the cleaning objectives.
  1. Pre-precipitation: The dosing point is in the inlet of the primary treatment. Effect: The system is relieved. The use of 3 and higher valued metal salts is common here.
  2. Simultaneous precipitation: The dosing point is in the inlet of the biological stage or directly in the biological stage. Effect: Greatest efficiency of the precipitant used, most common phosphate precipitation, also suitable for the more cost-effective bivalent metal salts, which are used especially in large plants with corresponding quantities.
  3. Post-precipitation: The dosing point is located in the secondary clarification inflow or in the separate 3rd purification stage. The 3rd purification stage is hardly used nowadays because it is very expensive compared to simultaneous precipitation. Surprisingly, this phosphate purification stage is being rebuilt in new plants in other European countries. Effect: Targeted precipitation, which can be controlled by measuring phosphate in the biological stage. Only metal salts of 3 and higher values ​​are possible.
  4. Flocculation filtration: The dosing point is in the inlet of the filtration. Filtration systems are only available in a few large sewage treatment plants. There, the flock filtration is used to achieve the best results and to lower the discharge values ​​again. Usually this method is used when discharging into drinking water reservoirs such. B. on Lake Constance.
Interfering substances in an analyte solution are precipitated and thus removed. This process is also called reprecipitation and, in contrast to recrystallization, represents a chemical reaction. For example, a Carrez clarification removes proteins from a solution.

See also

Categories: Chemical Reaction | Separation process | Analytical chemistry