Precipitate

precipitate is the solid that is produced in a solution by the effect of crystallization or a chemical reaction . Said reaction can occur when an insoluble substance is formed in the solution due to a chemical reaction or because the solution has been supersaturated by some compound, that is, it does not accept more solute and since it cannot be dissolved, said solute forms the precipitate .

Summary

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  • 1 Training
    • 1 Solubility
  • 2 Pollution according to Kolthaff
  • 3 Types of precipitates
    • 1 Colloidal precipitate
    • 2 crystalline precipitate
  • 4 Mechanism
  • 5 External link
  • 6 Sources

Training

In most cases, the precipitate (the solid formed) falls to the bottom of the solution, although this depends on the density of the precipitate: if the precipitate is denser than the rest of the solution, it falls. If it is less dense, it floats, and if it has a similar density, it stays in suspension.

The effect of precipitation is very useful in many applications, both industrial and scientific, in which a chemical reaction produces solids that can then be collected by various methods, such as filtration , decantation or by a centrifugal process .

Solubility

Defining solubility as the amount of a substance that dissolves in a quantity of solvent at a specific temperature, leads to an identification of the compounds that will serve to form a saturated solution. And so distinguish them from a precipitate, which is defined as an insoluble substance that forms in a solution and separates from it.

The precipitates have a certain treatment to be separated, a procedure that begins with digestion, in this process the precipitates are brought into contact with their mother liquor. The digestion of most of the precipitates leads to their purification, as impurities remain in the solution during the recrystallization process. Thus, after digestion, a precipitate must be filtered, which comprises three phases:

  1. a) Decantation: It consists of passing through the filter as much liquid as possible, maintaining the precipitated solid practically without disturbing the glass where it was formed.

By doing so, the total filtration time is shortened and clogging of the pores of the filter medium with the precipitate is delayed.

  1. b) Washing: Process in which a small amount of the appropriate washing liquid is added to the precipitate, most of which must remain in the glass after the liquid in which it was formed has been poured onto the filter. The washing liquid must meet the following requirements.

one-. The solvent must have a wide temperature coefficient regarding the solubility of the substance.

2-. Impurities must be either hot insoluble or cold soluble

3-. Solvent volatility should be moderate.

4-. The solvent must be chemically inert with respect to the substance.

From a practical point of view, the success of the precipitation separations largely depends on the last stage of the phases, which is generally carried out by some filtration procedure.

  1. c) Transfer: The bulk of the precipitate is passed from the glass to the filter by means of suitable streams of the washing liquid, suitably directed.

In this last stage, the water will be combined with alcohol to discard the excess liquid. However, the contamination of the precipitates is an important aspect to consider in the processes of separation by precipitation, since due to the multiple forms of manifestation, it is difficult to have a totally pure precipitate.

Pollution according to Kolthaff

  • Co-precipitation: The fundamental precipitate and the pollutant originate at the same time, distinguishing between two types: by adsorption and by occlusion. By adsorption it consists of entrainment of impurities on the surface of the precipitate, while in occlusion, entrainment of the impurities is carried out inside the primary particles of the precipitate.
  • Post-precipitation: The fundamental precipitate was initially pure, with contamination occurring after the precipitation has ended. This phenomenon is favored with the time of agitation and with the elevation of the temperature, being able to reach an increase of 100% in the weight of the pure precipitate.

In crystallization the solid phase is formed by precipitation of the solution, when the crystal is constituted there is a great tendency to form an organized network of a single substance, leaving the impurities in the liquid phase.

The best crystals are obtained by slow cooling of the solution. The lower the final temperature the greater the performance. Slow cooling is generally achieved by simply leaving the flask idle without touching it. When you have crystals of the substance, from another source, it is generally possible to initiate crystallization by “seeding”, that is, adding some crystallites to the supersaturated solution.

Types of precipitates

Colloidal precipitate

It is made up of very small particles, which do not precipitate due to gravity, therefore, the solution has a cloudy appearance. These particles cannot be separated from the solvent by means of the filter paper, since, due to their small size, they cross the weft of the latter. This type of precipitate is formed if the supersaturation is large, since the nucleation rate is also large, and many nuclei are formed that grow little.

Crystalline precipitate

The particles that form the precipitate are large and the solution is transparent. This precipitate is formed if the supersaturation is small, because the speed is also small, and few nuclei are formed that grow a lot.

Mechanism

The formation of a precipitate, from its ions in solution , follows a mechanism consisting of two stages:

  1. a) Nucleation If we have a supersaturated solution, tiny particles are formed, but large enough to be considered a solid phase, called nuclei. This takes place by the union of a certain number of ions of opposite sign: The greater the supersaturation, the greater the speed …

 

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