In Chemistry and Metallurgy, a precipitated is the formation of a heterogeneous dispersed phase in a majority phase. The formation of a precipitate is the precipitation .

In chemistry, that in general indicates the formation of a solid crystal of a salt in a liquid. Out of metallurgy, that can moreover indicate the formation of a crystal of composition given within an alloy. In Meteorology, that indicates the formation of drops of Eau or crystals of Glace in the atmosphere, to see the article Précipitation .

Approaches in chemistry

Product of solubility

The presence of a stable precipitate results from a balance Thermodynamique between the liquid phase (solution) and the solid phase (salt).

Let us consider a solution, for example a aqueous Solution of Sodium chloride NaCl (cooking salt). If one puts a small quantity of salt in water, this salt dissolves. Starting from a quantity C_s given, which depends on the Température, salt does not dissolve any more: the solution is known as “saturated”.

If C_s grows with the temperature, one can have a dissolution of a precipitate by heating which will be reformed with the cooling of the solution. If C is the salt concentration, then:

C < C_s ( T ): salt is dissolved;

C > C_s ( T ): salt precipitates (saturation).

Precipitation is noted like a chemical equation:

Na⁺ + Cl^- ↔ (NaCl) _cr

where “Na⁺” and “Cl^-” indicate the dissociated species (dissolved salt) and “(NaCl) _cr” indicates crystallized salt. In general, one notes the dissolved species without index, and one places an arrow under the crystallized species:

As for any reaction of balance, one can define a Constante of balance K_s , called Produit solubility :

K_s ( T ) = · : (when there is precipitation)

and potential of this constant pK_s :

pK_s = - log ( K_s )

The use of the K_s is more general, because the ions Na⁺ and Cl^- can come from several different salts. If the concentrations are expressed in molar Fraction, then in this precise case one has until precipitation

C = = that is to say

K_s = C _S ²

In the general case, there are two Ion S.A. ^{m +} and B ^{N -} which can form a salt A _N B _m ; the product of solubility is written:

K_s = '' m '' + ^N · '' N '' - ^m

Mechanisms of precipitation

Various uses of precipitation in chemistry

The formation of a precipitate can characterize the presence of a species in solution in analytical Chimie .c' is also the means of separating a species present in a mixture, a metal ion to prepare metal or a species synthesized starting from the reactional mixture.

Approaches out of metallurgy

Out of metallurgy, and Science of the materials in general, one is interested in two phenomena:

• formation of the first crystals at the time of the Solidification;
• formation of a new solid phase whereas one is already in solid phase.

In both cases, the formation of this heterogeneous phase is done in two stages:

• germination (sometimes indicated by the Anglicism “nucleation”);
• growth.

In solid phase, after the growth of the precipitates, the latter can continue to evolve/move by coalescence. Lastly, there exists also a particular mode of precipitation, the decomposition spinodale.

Solidification

See also: Solidification

Germination

In the case of solidification, germination is done in general during cooling.

Let us take the case of a pure substance. For a Pression P given, below a Température T_f given (the Point melting, depend on the pressure), only the solid phase is stable, therefore in theory, the temperature remains stable with T _F ( P ) time that all the liquid is solidified (solidification releases from the Chaleur, the Latent heat of fusion , which compensates for the reduction in temperature).

In the facts, one attends Surfusion: the crystals create with T_f are dissouts by thermal agitation. It is necessary that the temperature drops sufficiently so that these microcrystals are stable; one can start germination by putting one inoculating (they are grains or interstitial atoms brought to voluntarily accelerate the process of germination and to return more alloy restistant and harder; one also speaks about impurtés) (of the grains of a different composition which does not found and which will form the starter of the crystals). These impurities improve the characteristics of formed alloy because they cause a higher formation, and in several place, germs which them will give rise to the grains.

Thermodynamically, the creation of a crystal releases from energy but also requires some because of creation of a solid-liquid interface (surface Tension); so that germination is done, it is necessary that this one is favorable thermodynamically, i.e. the energy released by the ordering of the atoms compensates for the surface tension.

Growth

Precipitation in solid phase

Precipitation in solid phase takes place within an alloy. In general, there is a majority metal M containing a minority quantity of alloy A. When the concentration in alloy element has is weak, one has a homogeneous phase made up of a solid Solution of has in M (see the article specific Défaut ).

When the concentration of has exceeds a value limits C _S ( T ), there is crystal formation of a stoechiometric alloy, in general ordered, M and of a: M _m A _has ( m and has being the stoechiometric proportions). By doing this, the alloy impoverished of is had, the process thus stops when the total concentration of has in the M remaining became lower than C _S ( T ). If has is also a metal, M _m A _has is called “Intermétallique”.

If the temperature is increased and that C _S ( T ) becomes higher than the concentration of has in the phase M, one then witnesses a dissolution of the precipitates, always in solid phase.

For example, a Steel or a cast iron is an alloy of Fer containing a small proportion of Carbone. Under certain conditions of temperature, it can be formed precipitates of Carbure Fe₃C (Cémentite) within steel or of the cast iron.

The phenomenon is in fact more complex because there are in general several alloy elements which influence the stability of the phases.

Germination

Precipitation in solid phase is also done during a slow cooling (a Trempe can prevent precipitation).

In the case of the Alloys of aluminum, locally, supersaturation in alloy element causes an ordering of the atoms which come to form a zone known as of “Guinier-Preston” or “zone G-P”. This zone of Guinier- Preston form the starter of germ: there is not yet really an ordered crystal. The movement of the atoms of the alloy element impoverished the mixture locally, and this impoverishment is compensated by a migration of the atoms, the diffusion. The zone of Guinier-Preston is thus fed in atoms of alloy element, and can thus be transformed into germ.

As in the case of solidification, the surface tension makes that germination takes place below the temperature of dissolution of the precipitate.

Growth

Coalescence

See too

Notes and references of the article

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