Precipitation reaction

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Precipitation reactions are chemical reactions in which the reactants are dissolved in the solvent and at least one product of the reaction is insoluble or sparingly soluble in this solvent or the solution becomes supersaturated by cooling . The product with poor solubility precipitates out, the precipitate is generally called a precipitate . In reaction equations, the precipitation of a substance is marked with a ↓ or an (s) for solid after the chemical formula of the substance.

Illustration of the precipitation reaction


Precipitants are substances or mixtures of substances that cause dissolved substances to precipitate into insoluble solids ( precipitation ).

As a precipitant in the cation separation of analytical chemistry , z. As hydrochloric acid , hydrogen sulfide , ammonium sulfide and ammonium carbonate , as they insoluble heavy metal chlorides , - sulfide and - carbonate form.

Precipitants play a very special role in wastewater treatment (see also precipitation , flocculation , flocculants ) to remove sulphides or phosphates .

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 to produce "painter's white")

Examples of precipitation reactions of halogens; :

Application of precipitation reactions

By using specific precipitation reactions it is possible to identify individual components of a solution. A classic example is the cation separation process , in which 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 ( phosphorus elimination ). To this end, iron (II) sulfate , iron chloride , aluminum chloride , sodium aluminate or polyaluminium chloride added. In principle, all metal salts work similarly. 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 metal salts of three or more values ​​is common here.
  2. Simultaneous precipitation : The dosing point is located 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. Reprecipitation : The dosing point is located in the secondary clarification inlet or in the separate 3rd purification stage. The third purification stage is rarely used nowadays, as 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 that can be controlled by measuring phosphate in the biological stage. Only metal salts of 3 and higher values ​​are possible.
  4. Flocculation filtration : The metering point is located in the inlet of the filtration. Filter 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 then also called reprecipitation and, in contrast to recrystallization, represents a chemical reaction. For example, a Carrez clarification removes proteins from a solution.
Precipitation reactions are an industrially used process for the production of solids, such as. B. pigments or other functional particles. Many iron color pigments are produced using a precipitation reaction. The control of the particle size produced by means of precipitation is a problem in the case of rapid precipitation reactions, since the uniform mixing of the starting materials cannot be guaranteed on an industrial scale in a short time. High shear mixing processes, e.g. B. Ultrasound. Brief application of ultrasonic waves to a supersaturated solution can initiate the formation of particle nuclei. Since the ultrasound treatment is only carried out in the initial phase, the subsequent particle growth takes place unhindered, which leads to larger particles. Continuous sonication of the supersaturated solution leads to small particles, since the continuous ultrasound generates many nuclei, which leads to the growth of many small particles. Pulsed ultrasound is the application of ultrasound at certain intervals. A precisely controlled supply of energy by means of ultrasound makes it possible to influence particle growth in order to obtain a tailor-made particle size.

Web links

Individual evidence

  1. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 389-390.
  2. Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 2: Cm-G. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1981, ISBN 3-440-04512-9 , p. 1232.
  3. Ultrasound-assisted crystallization and precipitation .