Principle of Qualitative and Quantitative Analysis

Solubility product

When salts like AgCl, PbSO₄,BaSO₄etc when dissolved in water are sparingly soluble and the there exist between the undissolved ionic solid and the ions of the dissolved salt.

For example, AgCl \(\rightleftharpoons\) Ag⁺+ C

According to law of chemical equilibrium,

K = \(\frac{[Ag⁺][Cl^-]}{[AgCl}\)\

[Ag⁺] [Cl^-] = K \(\times\) [AgCl]

Here, [AgCl] = constant

∴ [Ag⁺] [Cl^-] = K \(\times\) constant = K_sp

Where K_sp= solubility product. [Ag⁺] [Cl^-] = ionic product

Generally, A_xB_y\(\rightleftharpoons\) xA⁺+ yB^-

The solubility product is K_sp= [A⁺]^x[B^-]^y

Solubility product of a binary electrolyte, like AgCl, BaSO₄, or CuS, may be defined as the product of the concentration of ions in its saturated solution. Soubility product of a salt depend on temperature so it changes with temperature.

Solubility product of a salt is equal to the product of the concentration of its ions, each raised to the appropriate power, for example,

BaCO₃\(\rightleftharpoons\) Ba⁺⁺+ CO₃^-- K_sp= [Ba²⁺] [CO_3^2-]

Importance of solubility product in qualitative analysis

Solubility product can be useful in predicting the occurrence of precipitation when two solutions containing ions are mixed. For example, if two different solutions containing silver and chloride ions or not can be predicted on the basis of solubility product and ionic product. If the ionic product is greater than solubility product then the precipitation takes place. This is called solubility product principle and is helpful in group separation of the metal ion in the qualitative salt analysis.

Common ion effect

When ammonium hydroxide which is a weak electrolyte is added in solution it ionizes partially and establishes equilibrium as given in the following reaction.

NH₄OH \(\rightleftharpoons\) NH₄⁺ + OH^-

If ammonium chloride is added to above solution, which is the strong electrolyte, it ionizes almost completely and gives NH₄⁺ and Cl^- ions.

NH₄Cl \(\rightleftharpoons\) NH₄⁺ +Cl^-

NH₄Cl introduces large amount of NH₄⁺ ions. This shifts the equilibrium towards the left i.e., ammonium ions and hydroxide ions will combine to give undissociated molecules of ammonium hydroxide. This will ultimately suppress the ionization of ammonium hydroxide. This is known as common ion effect.

The common ion effect may be defined as the suppression of the degree of ionization of weak electrolyte by the addition of another strong electrolyte containing the common ion.

Importance of common ion effect in qualitative analysis

1. The selective precipitation in the qualitative analysis of cations is achieved through common ion effect. For example, metal ions such as Pb⁺⁺, Cu⁺⁺,Cd⁺⁺ etc of group II are precipitate out in the form of their sulphides by an addition of HCl which gives H⁺ as common ion and the concentration of S^—ion is suppressed due to common ion effect.
2. In the gravimetric analysis gravimetric analysis, common ion effect plays an important role. The precipitate of gravimetric has low solubility in a precipitating medium which is important for gravimetric estimation. Therefore the solubility of a precipitate can be decreased by the use of common ion effect. This can be done by adding a small amount of the precipitation agent having a common ion.

The role of solubility product in group separation

Group I

The metal cations such as Pb²⁺, Ag⁺ and Hg⁺ belongs to group I. These cations are precipitated in their chloride forms (PbCl₂, AgCl and Hg₂Cl₂) by the addition of HCl. This happens due to the low solubility product of the cations of group I.

Group II

The cations of group II are Hg²⁺, Pb²⁺,Bi³⁺, Cu²⁺, Cd²⁺, As³⁺, Sb⁺⁺⁺ and Sn⁺⁺

These are precipitated as their sulphide by passing H₂S through the solution containing HCl

Cu²⁺ + H₂S → CuS + 2H⁺

Functions of HCl: HCl being a strong electrolyte dissociates completely and suppresses the degree of ionization of H₂S (a weak electrolyte) by common ion effect, as explained below

HCl is a strong acid and furnished a large number of hydrogen ions are:

HCl → H⁺ + Cl^-

H₂S→ 2 H⁺ + S^2-

Here the ion H⁺ is present in both the equilibrium and common ion effect operates. This results in the suppression in the dissociation of H₂S and the concentration of sulphide ions is reduced very much in the solution. However, this reduced concentration is enough to precipitate the cations of group II. This occurs due to low solubility product of the sulphides of cation of group II which can be easily cross by ionic products.

Group IIIA

The cations of group IIIA (Fe³⁺, Cr³⁺ and Al³⁺) are precipitated as their hydroxides by adding NH₄OH in the presence of NH₄Cl. NH₄Cl being strong electrolyte ionizes completely and suppresses the degree of dissociation of NH₄OH by common ion effect as explain below.

NH₄Cl→ NH₄⁺+ Cl^-

NH₄OH → NH₄⁺+ OH^-

Due to presence, of common ions in both the equilibria, common ion effect operates. Therefore, the concentration of hydroxide ions is reduced very much in the solution. However, this reduced concentration is enough to precipitate the cations of group IIIA. This occurs due to low solubility product of the hydroxides of cation of group IIIA which can be easily crossed by ionic products.

Group IIIB

The cations of group IIIB (Ni²⁺, Mn²⁺and Zn²⁺) are precipitated as their sulphides by passing H₂S through their solution containing ammonium hydroxide. NH₄OH increases the ionisation of H₂S and thereby increases the concentration of sulphide ions as given below.

NH₄OH \(\rightleftharpoons\) NH₄⁺+ OH^-

In case of H₂S,

H₂S \(\rightleftharpoons\) 2H⁺+ S^2-

The hydroxide ions from equilibrium reaction combine with H⁺ions from equilibrium reaction to form undissociated molecules as given below

H⁺+ OH^-→ H₂O

Consequently, more H₂S molecules dissociate and in the process concentration of sulphide ions increases. The increase in the concentration of sulphide ions results in an increase in the ionic products of group IIIB cations. Then the precipitation of sulphides of the cations of group IIIB occurs when the ionic products ge tcrossed by solubility products.

Group IV

The cations of group IV (Ba²⁺, Sr²⁺ and Ca²⁺are precipitated as their carbonates by adding (NH₄)₂CO₃to their solution containing NH₄Cl and NH₄OH.

Function ofNH₄Cl

NH₄Cl, a strong electrolyte produces large amount of NH₄⁺ions as:

NH₄Cl \(\rightleftharpoons\) NH₄⁺+ Cl^-

(NH₄)₂CO₃(a weak electrolyte) ionises as:

(NH₄)₂CO₃ \(\rightleftharpoons\) 2NH₄⁺+ CO₃^2-

Due to the presence of NH₄⁺ions in both equilibria, common ion effect operates. Therefore the concentration of carbonate ions is reduced in the solution. However, this reduced concentration of carbonate ions is sufficient to precipitate the cations of group IV as their solubility product are low and ionic products cross their solubility product.

Function of NH₄OH:

NH₄(HCO₃) + NH₄OH→ (NH₄)₂CO₃+ H₂O

In case bicarbonate ions are present in solution, then bicarbonates of Ba²⁺, Sr²⁺and Ca²⁺are formed which are soluble in water. Consequently, the quantity of the precipitate formed in group IV decreases very much.

Group V

The cations of group V are Mg²⁺, Na⁺, K⁺ and NH₄⁺. These cations are tested independently as there is no group reagent for them.

General principle of Gravimetric Analysis

A general principle of gravimetric method of analysis is based on a chemical reaction between analyte and reagent

aA + rR→ AaRr

'a' molecules of analyte (A) is made to react with r molecules reagent (R). The product formed AaRr can be weighed after drying of it or it can be ignited to another compound of known composition and weighted.

The reagent is always added in an excess amount to decrease the solubility of the precipitate.

General principle of Volumetric Analysis

A general principle of Volumetric method of analysis is based on following chemical reaction

aA + tT→ Products

where A = analyte

a = number of moles of analyte

T = Titrant (the reagent)

t = mole of titrant necessary to neutralize 'a' mole of analyte

chemguide. n.d. <http://www.chemguide.co.uk/physical/ksp/introduction.html>.

chemwiki. n.d. <http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Equilibria/Solubilty/Common_Ion_Effect>.

Moti Kaji Sthapit, DR. Raja Ram Pradhananga. Foundations of Chemistry. Vol. 1. Taleju Prakashan, n.d. 3 vols.