Instant Notes: Analytical Chemistry

but additional terms are often added to the Debye-Hückel equation to compen-

sate for the change in the activity.

The pX notation The concentration of species in solution may range from very small to large. For
example in a saturated aqueous solution of silver chloride, the concentration of
silver ions is about 10-^5 M, while for concentrated hydrochloric acid the
concentration of hydrogen and chloride ions is about 10 M. For convenience,
a logarithmic scale is often used:

pX =-log (X)

where X is the concentration of the species, or a related quantity. Thus, for the

examples above, pAg =5 in saturated aqueous silver chloride and pH =-1 in

concentrated HCl.

Since equilibrium constants are derived from activities or concentrations as

noted below, this notation is also used for them:

pK =-log (K)

Most reactions will eventually reach equilibrium. That is, the concentrations of

reactants and products change no further, since the rates of the forward and

reverse reactions are the same.

From the above arguments concerning solutions, and from the laws of

thermodynamics, any equilibrium in solution involving species D, F, U and V:

D +F [U +V

will have an equilibrium constant , KT, at a particular temperature Tgiven by:

KT=(aU. aV)/(aD. aF)

where the activities are the values at equilibrium. It should be noted that KT

changes with temperature. The larger the equilibrium constant, the greater will

be the ratio of products to reactants at equilibrium.

There are many types of equilibria that occur in solution, but for the impor-

tant analytical conditions of ionic equilibria in aqueous solution, four examples

are very important.

(i) Acid and base dissociation.In aqueous solution, strong electrolytes (e.g.,

NaCl, HNO 3 , NaOH) exist in their ionic forms all the time. However, weak

electrolytes exhibit dissociation equilibria. Forethanoic acid, for example:

HOOCCH 3 +H 2 O [H 3 O++CH 3 COO-

Ka=(aH. aA)/(aHA. aW) =1.75¥ 10 -^5

where HA, W, H and A represent each of the species in the above

equilibrium. In dilute solutions the activity of the water aW is close to 1.

For ammonia:

NH 3 +H 2 O [NH 4 ++OH-

Kb =(aNH4+. aOH-)/(aNH 3. aW) =1.76¥ 10 -^5

Waterbehaves in a similar way:

2 H 2 O [H 3 O++OH-

KW=(aH 3 O+.aOH-) = 10 -^14

Equilibria in
solution

58 Section C – Analytical reactions in solution