The co-precipitation It is the contamination of an insoluble substance that carries dissolved solutes from the liquid medium. Here the word 'contamination' is applied for those cases where soluble solutes precipitated by an insoluble support are undesirable; but when they are not, an alternative analytical or synthetic method is in hand.
On the other hand, the insoluble support is the precipitated substance. This can carry the soluble solute inside (absorption) or on its surface (adsorption). How you do it will completely change the physicochemical properties of the resulting solid..
Although the concept of co-precipitation may seem a bit confusing, it is more common than you might think. Why? Because, more than simple contaminated solids, solid solutions of complex structures and rich in invaluable components are formed. The land from which plants are nourished are examples of co-precipitation.
Likewise, minerals, ceramics, clays, and impurities in ice are also the product of this phenomenon. Otherwise, soils would lose a large part of their essential elements, minerals would not be as they are currently known, and there would be no important method for the synthesis of new materials..
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To better understand the idea of co-precipitation, the following example is offered.
Above (top image) there are two containers with water, of which one contains dissolved NaCl. NaCl is a highly water soluble salt, but the sizes of the white dots are exaggerated for explanatory purposes. Each white point will be small aggregates of NaCl in a solution on the verge of saturation.
A mixture of sodium sulphide, NatwoS, and silver nitrate, AgNO3, will precipitate an insoluble black solid of silver sulfide, AgS:
NatwoS + AgNO3 => AgS + NaNO3
As can be seen in the first container of water, a black solid (black sphere) precipitates. However, this solid in the container with dissolved NaCl carries particles of this salt (black sphere with white dots). NaCl is soluble in water, but when AgS precipitates, it is adsorbed on the black surface.
It is then said that the NaCl coprecipitated on the AgS. If the black solid were analyzed, NaCl micro crystals could be seen on the surface.
However, these crystals could also be inside the AgS, so the solid would “turn” grayish (white + black = gray).
The black sphere with white dots, and the gray sphere, demonstrate that a soluble solute can co-precipitate in different ways..
In the first, it does so superficially, adsorbed on the insoluble support (AgS in the previous example); while in the second, it does so internally, "changing" the black color of the precipitate.
Can you get other types of solids? That is, a sphere with black and white phases, that is, of AgS and NaCl (together with NaNO3 which also coprecipitates). It is here where the ingenuity of the synthesis of new solids and materials arises.
However, going back to the starting point, basically the soluble solute coprecipitates generating different types of solids. The types of coprecipitation and the solids that result from them will be mentioned below..
We speak of inclusion when in the crystal lattice, one of the ions can be replaced by one of the co-precipitated soluble substance.
For example, if NaCl had coprecipitated through inclusion, the Na ions+ would have taken the place of the Ag+ in a section of the crystal arrangement.
However, of all the types of co-precipitation, this is the least likely; since, for it to happen, the ionic radii must be very similar. Returning to the gray sphere of the image, the inclusion would come to be represented by one of lighter grayish tones.
As just mentioned, the inclusion occurs in crystalline solids, and to obtain them, one must have mastery of the chemistry of the solutions and several factors (T, pH, stirring time, molar ratios, etc.).
In occlusion, the ions are trapped within the crystal lattice but without replacing any ion in the array. For example, occluded NaCl crystals can form within AgS. Graphically, it could be visualized as a white crystal surrounded by black crystals.
This type of co-precipitation is one of the most common, and thanks to it, there is the synthesis of new crystalline solids. Occluded particles cannot be removed with simple washing. To do this, it would be necessary to recrystallize the whole assembly, that is, the insoluble support.
Both inclusion and occlusion are absorption processes given in crystalline structures.
On adsorption, the coprecipitated solid lies on the surface of the insoluble support. The size of the particles of this support defines the type of solid obtained.
If they are small, a coagulated solid will be obtained, from which it is easy to remove impurities; but if they are very small, the solid will absorb copious amounts of water and will be gelatinous.
Returning to the black sphere with white dots, the NaCl crystals coprecipitated on the AgS can be washed with distilled water. So on until the AgS is purified, which can then be heated to evaporate all the water.
What are the applications of co-precipitation? Some of them are the following:
-It allows to quantify soluble substances that are not easily precipitated from the medium. Thus, through an insoluble support, it carries, for example, radioactive isotopes, such as francium, for further study and analysis..
-By coprecipitating ions in gelatinous solids, you are purifying the liquid medium. Occlusion is even more desirable in these cases, since the impurity will not be able to escape to the outside.
-Coprecipitation makes it possible to incorporate substances into solids during their formation. If the solid is a polymer, then it will absorb soluble solutes that will then co-precipitate inside, giving it new properties. If it is cellulose, for example, cobalt (or other metal) could be co-precipitated within it.
-In addition to all of the above, coprecipitation is one of the key methods for the synthesis of nanoparticles on an insoluble support. Thanks to this, bionanomaterials and magnetite nanoparticles have been synthesized, among many others..
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