Instant Notes: Analytical Chemistry

[ H 3 O++

The dissociation constant of the indicator, KIn, is given by

KIn=c(H 3 O+)¥ c(In-)/c(HIn)

For example, for methyl orange (Fig. 3):

Acid form, red [ Alkaline form, yellow + H+

As with buffers, the equilibrium and the concentration of hydrogen ions will

govern the ratio of color 2 to color 1.

In-

Colour 2

HIn

Colour 1

78 Section C – Analytical reactions in solution

• SO 3 NH N N(CH 3 ) 2 + = –SO 3 N N N(CH 3 ) 2 + H+

Red (pH<3) Yellow (pH>5)

Fig. 3. Structures of methyl orange at low and high pH (only one tautomeric form is shown).

Example 2

With the indicator bromocresol green, where KIn=1.6¥ 10 -^5 , or pKIn=4.8 and

color 1 (acid) is yellow, while color 2 (salt) is blue, a solution of pH 4.0 will give:

log [c(In-)/c(HIn)] =log [c(color 2)/c(color 1)] =pH -pKIn=-0.8

Therefore, c(color 2)/c(color 1) =0.16, which means about 14% blue, 86% yellow,

or visually a very yellowish green. When the pH is equal to the pKIn, there are

equal amounts of each form making a green color.

A wide range of indicators is available for titrations and other purposes and

these are discussed further in Topics C5 and C7.

This provides a useful and rapid method of estimatingpH by eye; for

example, using litmus paperwhich is red below about pH 6 and blue above

pH 8. Both wide range and narrow range indicator papers are available to

enable a rapid estimation of pH. However, to determine the pH accurately using

indicators, careful spectrometric comparison would be needed and this is a

time-consuming method that is rarely used.

The pH meter uses a reference electrode and a glass electrode with a high-

resistance voltmeter and affords a rapid and accurate method of measuring pH

(Fig. 4). The calomel reference electrode is decribed fully in Topic C3.

The glass electrode is an example of a membrane ion-selectiveelectrode and

is described in Topic C3. It responds to hydrogen ions:

E(glass) =E*+(RT/F) ln [a(H 3 O+)X]

Therefore the complete cell

Pt |Hg |Hg 2 Cl 2 (s) |KCl (sat, aq) ||Solution X |glass membrane |AgCl |Ag

has an e.m.f. at 25∞C equal to:

E (cell) =(E*-0.241) +( RT/F) ln [a(H 3 O+)X]