Page 23 - Chemistry - XI

Determination of pH Values
3 Determination of pH Values

The ionic equilibrium of pure water may be represented as

HO ( )l H OH
2 ( aq) ( aq)

or 2HO ()l HO ( aq) OH ( aq) (This is because H does not have independent existence.)
+
3
2
[H O ] [OH ]
–
+
Equilibrium constant for this equation may be written as K = 3
[H O]
2
2
If due to dissolution, the concentration of H O increases, the solution is said to be acidic and if the
+
3
concentration of OH increase, then solution is alkaline.
–
The pH of a solution is defi ned mathematically as the negative logarithm of hydrogen ion concentration or
hydronium ion concentration.
pH = – log[H O ] or pH = – log [H ].
+
+
3
If pH = 7, solution is neutral, pH < 7 solution is acidic, pH > 7 solution is basic.
Strong acids ionise completely and hence we assume that H concentration of a solution is equal to the
+
total acid concentration. Weak acids do not ionise completely in aqueous solution and the calculation of pH
is based on the equation of the dissociation constant of weak acid.

EXPERIMENT - 10

Objective
To determine the pH of some fruit juices, acid, base, and salts with dilutions.

Principle/Theory
Red cabbage juice is a good universal indicator. pH papers of wide pH range are also useful to get the
complete picture of pH. A solution of a mixture of dyes which can measure pH value from zero to 14 is known

as universal indicator.

Universal Indicator as Solution
pH Colour
Chart of Diff erent 3.0 Red
Colour Shades 5.0 Orange red

5.5 Deep yellow
6.0 Light-yellow
7.0-7.5 Greenish-yellow
8.0 Green
9.5 Blue

Fig. 10.1 10.0 Light violet
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