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Precipitation Reaction

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One of the simplest and oldest solution phase synthetic methods are the precipitation reaction. A precipitation reaction occurs when a solution originally dissolved species, produces a solid, which generally is denser and falls to the bottom of the reaction vessel.

When a solution containing soluble ionic compounds are mixed, the hydrated ions intermingle, whether or not one of the three common exchange reactions - precipitation reactions acid-base reactions occur depends on the identity of the ions in solution.

What is Precipitation?

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Precipitation is similar to crystallization except that the driving force in the latter is solubility, whereas in the former it is a chemical reaction. In either case the solid produced is stable and insoluble in a given liquid phase solvent if the solvation energy is lower in strength than the cohesive or binding energy between the particles of the solid. There are three types of solution phase chemical reactions double exchange, redox and acid-base reactions. When the product of any of these solution phase chemical reactions is an insoluble product, it is called a precipitation reaction.
Often with precipitation reactions the starting materials are limited to whatever salts are soluble in the solvent of choice. For water systems this is often limited to metal salts of halides, nitrates and some sulfates and phosphates. Halides in particular chlorides have a pronounced effect on precipitation reactions.

Types 

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Silver nitrate and sodium chloride are both salts. They dissolve in water to generate the ionic species. A precipitation reaction is possible based on solubility guidelines. NaNO3 is soluble in water whereas AgCl is not. So we conclude this as precipitation reaction.

Ag+(aq) + Cl-(aq) $\rightarrow$ AgCl(S)

The magnificent stone formation in Mammoth Caves in Kentucky or Carlsbad Caverns in New Mexico is made from calcium carbonate (CaCO3) which is formed by the reaction.

CaCl2(aq) + Na2CO3(aq) $\rightarrow$ CaCO3(s) + 2NaCl(aq)

This reaction is an example of a precipitation reaction in which two soluble reactants produce an insoluble product. The calcium and carbonate ions are so strongly attracted to each other that water molecules cannot hold them apart.

Some of the types of precipitation are
  • Selective precipitation
  • Gel precipitation
  • Fractional precipitation
  • Quantitative precipitation

Properties of Precipitates

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Some Properties has been describe below:-
  • The precipitates are usually sparingly soluble and the formation of precipitates occurs relatively under high supersaturation conditions.
  • If precipitation occurs rapidly it is not affected by the presence of solute crystalline material and thus does not involve secondary nucleation.
  • If the precipitation occurs because of the presence of high supersaturation, nucleation plays a major role in the precipitation processes.
  • Supersaturation necessary for promoting precipitation frequently results from a chemical reaction indeed precipitation is sometimes referred to as reactive crystallization.
  • The chemical reaction may involve rapid mixing of concentrated chemical reactants and are generally very fast.
  • In most practical applications, the properties of the precipitated particles are of key importance and great attention must be paid to "particle design" or "crystal engineering".

Applications and Examples

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Precipitation reaction can take place in a liquid medium or in semisolid medium such as gels, agarose or polyacrylamide gels.

Applications

Precipitation reaction is use for
  • Detection of silver Ag (more sensitive)
  • Identification of microbial components in infective tissues.
  • Detection of Ab (less sensitive)
Precipitation (examples) can be performed as
  • Ring test - Ascolis thermoprecipitation test.
  • Immunodiffusion test, for example Agar gel diffusion test for diagnosis of various microbial infections, radial immunodiffusion test for quantitation of Ags, Abs, proteins etc, in the serum and other body fluo-ids.
  • Immunoelectrophoresis, counter current immunoelectrophoresis, rocket immunoelectrophoresis for diagnosis of infectious diseases and quantition of proteins, Ags, Abs etc, in serum and other body fluids.

Solubility Rules

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Chemist have discovered regularities in the solubilities of ionic substances. These patterns can be summarized in the following guidelines that identify ionic substances soluble in water.
  1. Salts that contain the following cations are soluble: NH4+ and Group I metal cations.
  2. Salts that contain the following anions are soluble: ClO4-, NO3-, Cl-, Br-, I-, SO42-, HSO4- and CH3CO2- (acetate).
  3. Most salts not covered by guidelines 1 or 2 are insoluble.
  4. There are several exceptions to guideline 2: AgX, PbX2, Hg2X2 (where X is Cl, Br or I that is any Group 17 element except F) and MSO4 (where M = Ca, Sr, Ba or Pb) are insoluble.
  5. A few compounds not covered by guidelines 1 and 2 are nevertheless soluble: Ba(OH)2 and MS (where M is any of these Group 2 elements: Mg, Ca and Ba) are soluble.

Precipitate Colors

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If a small fragment of the reagent (potassium iodide) is applied to a drop of 1.5 solution containing the element, the following color precipitates occurred.
  1. Antimony - Solution turns yellow. Color very faint in 0.1 percent Sb solution.
  2. Arsenic - Yellow amorphous precipitate.
  3. Bismuth - Solution turns yellow.
  4. Copper - Light yellow amorphous precipitate.
  5. Lead - Greenish yellow hexagons.
  6. Selenium - Chocolate brown amorphous precipitate
  7. Tellurium - Chocolate brown precipitate.
  8. Mercury - Black precipitate.
  9. Zinc - White precipitate.
  10. Gold - Yellow amorphous ring surrounds KI fragment.

Selective Precipitation

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Ions can be separated from each based on the solubilities of their salts. Selective precipitants may sometimes be found for the removal of radioactive constituents even in the presence of large amounts of inert salts. Therefore, precipitation methods should be considered for decontaminating systems with high solids content.

Selective Cs (cesium) and Sr (stronsium) precipitation methods are useful in trying to separate these long lived elements from the rest of the fission products in order to permit more rapid disposal of the short lived residue. The usual technique for narrowing size distribution include sedimentation of colloidal particles, electrophoresis or size selective precipitation. Traditionally tuning of size distribution at the diameter of the particles below 10nm is performed by a size selective precipitation procedure which is based on size dependent solubility of colloidal particles in solvents of different polarity.

Size selective precipitation is known to have very high separation efficiency but it usually requires a huge amounts of sample and several cycles of treatment. Recently another approach for separation of nanoparticles by size selective absorption of nanocrystals by mesoporous molecular sieves was suggested.

Precipitation Reaction Practice Problems

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Some of the problems based on precipitation reactions are given below.

Solved Examples

Question 1: Which of the following are precipitation reactions?

2HCl(aq) + Ba(OH)2(aq) $\rightarrow$ BaCl2(aq) + 2H2O(l)
2HCl(aq) + Pb(NO3)2(aq) $\rightarrow$ PbCl2(s) + 2HNO3(aq)
2HCl(aq) + Pb(OH)2(s) $\rightarrow$ PbCl2(s) + 2H2O(l)
Solution:
Only the second reaction is a precipitation reaction. In a precipitation reaction, one of the products must be a solid and all of the reactants must be dissolved in water. In the first equation none of the products is a solid, and in the third reactions one of the reactants is a solid.

Question 2: When a solution of AgNO3 is mixed with a solution of Na3PO4, solid Ag3PO4 forms. Write the net ionic equation for this precipitation reaction.
Solution:
The product is Ag3PO4 which contains Ag+ and PO43- ions. Therefore the net ionic equation must be the reaction of Ag+ and PO43- to form Ag3PO4.

Ag+(aq) + PO43-(aq) $\rightarrow$ Ag3PO4(s)

Balance the equation. It takes three Ag+ ions and one PO43- ion to make Ag3PO4, so the balanced equation is 

3Ag+(aq) + PO43-(aq) $\rightarrow$ Ag3PO4(s)