pH Value, indicators, charge relations

pH Value, indicators, charge relations

pH Value:

The pH value is the negative logarithm to the base 10 of the hydrogen ion concentration.

Derivation:

In pure water, the concentration of hydrogen ions [H₃O⁺] is equal to the concentration of hydroxyl ions [OH^–], viz

[H₃O⁺] = [OH^-] = 10^-7 mol/L

At this equilibrium, the water has a neutral reaction. In practice, the minus sign is dropped, 10^-7 = pH 7.

According to the above equation, if the hydrogen ion concentration in an aqueous system is increased, the hydroxyl ion concentration becomes less, and the system has an acid reaction.

If the hydroxyl ion concentration is increased, the hydrogen ion concentration decreases accordingly, and the solution has an alkaline reaction.

pH = 1 ←	pH = 7	→ pH = 14
acid	neutral	alkaline

Determination of pH value:

1. Potentiometric (electrometric)

With a hydrogen electrode or glass electrode (measuring range pH 0 – pH 14)

2. Colorimetric

With indicators or indicator papers.

Common indicators

Indicator	Effective pH range	Color change	Concentration of solution
Thymol Blue	1.2 – 2.8	red – yellow	0.1% in 20% alcohol
Dimethyl Yellow	2.9 – 4.0	red – yellow	0.1% in 90% alcohol
Bromophenol Blue	3.0 – 4.6	yellow – blue	0.1% in 20% alcohol
Congo Red	3.0 – 5.0	blue – red	1% in water
Methyl Orange	3.2 – 4.4	red – yellow	0.1% in water
Bromocresol Green	3.8 – 5.4	yellow – blue	0.1% in 50% alcohol
Methyl Red	4.8 – 6.0	red – yellow	0.2 % in 90% alcohol
Litmus	5.0 – 8.0	red – blue	0.3 % in 90% alcohol
Bromocresol Purple	5.2 – 6.8	yellow – purple red	0.04 % in 90% alcohol
Bromothymol Blue	6.0 – 7.6	yellow – blue	0.1% in 20% alcohol
Neutral Red	6.8 – 8.0	red – yellowish orange	0.1 in 70% alcohol
Phenol Red	6.6 – 8.0	yellow – red	0.02 % in 90% alcohol
o-Cresol Red	7.0 – 8.8	yellow – purple red	0.1% in 20% alcohol
Tropaeoline 000	7.6 – 8.9	yellowish green – pink	1% in water
Phenolphthalein	8.2 –10.0	colorless – pink	1.0 % in 50% alcohol
Thymol Blue	8.0 – 9.6	yellow – blue	0.1% in 20% alcohol
Thymolphthalein	9.4 –10.6	colorless – blue	0.1% in 90% alcohol
Alizarin Yellow R	10.1 –12.0	yellow – red	0.1% in 50% alcohol
Tropaeoline O	11.3 –13.0	yellow – orange red	0.1% in water

Example: pH colour range of bromocresol green

Color: Yellow = pH 3.5 and lower

Yellow-green = pH 4.0

Green = pH 4.5

Blue-green = pH 5.0

Blue = pH 5.5 and higher