Instant Notes: Analytical Chemistry

To compare the formation constants at similar pH conditions the equilibrium

may be written:

K’MY=KMYa 4 =c(MY(n-4)+)/(c(Mn+). cL)

Example

The formation constant KMYfor magnesium-EDTA is 5¥ 108. Should pH 5, or

pH 10 be used to titrate Mg?

At pH 5, for Mg:

K’MY= 5 ¥ 108 ¥ 3.5¥ 10 -^7 =1.75¥ 102

At pH 10, for Mg:

K’MY= 5 ¥ 108 ¥ 3.5¥ 10 -^1 =1.75¥ 108

Therefore, magnesium could be titrated only at pH 10, as the value of the equi-

librium constant is too small below this.

The fact that EDTA forms a number of ring systems adds considerably to the

stability of the complex. This is called the chelate effect.

Solubility The formation of insoluble compounds by reaction between two soluble species
is discussed in detail in Topic C8, where the amount of an insoluble product is
measured gravimetrically (Table 2).
The solubility equilibrium is described by the solubility product, Ksp:

Mn+(solvated) +An-(solvated) =MA(solid) +solvent

Ksp=a(Mn+). a(An-)

(Note: This equilibrium constant is written in the inverse way to most others,

and thus a very small solubility product is desirable for complete reaction. The

activities of the pure solid MA and of the solvent are taken as 1.)

There are some general rules governing which cations will form precipitates

with which anions. Nitrates and perchlorates are generally soluble.

Various other reagents are also useful for gravimetric analysis, for example

dimethylglyoxime (a) for nickel and oxine (b) for aluminum and magnesium

(Fig. 2(a) and (b)).

If an excess of a precipitating ion is present, in order to keep the equilibrium

constant unchanged, the concentration of the other ion must decrease. This is

called the common ion effect. Occasionally, complex formation may occur, so

that the precipitate formed redissolves when excess reagent is added.

Since equilibrium constants are thermodynamic quantities, they should really

be written in terms of activities and the activity coefficient, depending on the

ionic strength taken into account.

C6 – Complexation, solubility and redox equilibria 87

Table 2. Ions forming insoluble products (Ksp~< 10 -^5 )

Anions Cations

Cl-, Br-, I- Ag+, Pb^2 +, Cu+, Hg 22 +

CO 32 - Ca^2 +, Sr^2 +, Ba^2 +, Mg^2 +

SO 42 - Ca^2 +, Ba^2 +, Pb^2 +

OH- Al^3 +, Co^3 +, Cr^3 +, Fe^3 +, Mg^2 +, Mn,^2 +Ni^2 +, Zn^2 +

S^2 - Ag+, Cd^2 +, Cu^2 +, Fe^2 +, Hg^2 +, Mn^2 +, Pb^2 +, Zn^2 +