pH and Indicators

The concentration of hydrogen ion is a measure of the acidity and the basicity of a solution. The concentration of hydrogen ion may be expressed in terms of the molarity of the acid or base solution; however, it is frequently more convenient to express the concentration as a function of the hydrogen ion concentration, pH. The pH of a solution may be defined as the exponent of the hydrogen ion concentration. This definition may be stated mathematically as:

pH = - log [H⁺] where [H⁺] is the molar hydrogen ion concentration.

The pH scale for water systems ranges from a value to 0 to 14 only. See the table that follows for the strength of the acid or base in water at 25º C.

pH range	[H⁺] range	Strength
0 to 2	1 E 0 to 1 E - 2	Strong acid
3 to 4	1 E – 3 to 1 E – 4	Moderate acid
5 to 6	1 E – 5 to 1 E – 6	Weak acid
7	1 E – 7	Neutral
8 to 9	1 E – 8 to 1 E – 9	Weak base
10 to 11	1 E – 10 to 1 E – 11	Moderate base
12 to 14	1 E – 12 to 1 E - 14	Strong base

Indicators have been developed in order to assist in the determination of the pH of a solution. These indicators are weak organic acids or bases which have the property of changing color in solution when the hydrogen ion concentration reaches a definite value. An acid indicator may be represented by the equation: HIn = H⁺ + In^-

The anion, In^-, represents a complex organic group which has changed its structure due to the loss of a hydrogen ion. The loss of the hydrogen ion is accompanied by a change in color. Since an indicator reaction is an equilibrium reaction, the addition of hydrogen ions would force the above reaction to the left and a color indicating an acid solution would result. The addition of hydroxide ions would cause the reaction to go to the right and a color associated to a basic solution would result.

The pH ranges of some indicators are given in the following table. With this table you can estimate the pH of a solution. Suppose phenolphthalein in introduced into a solution and the color of the solution becomes red. This red color indicates that the pH of the solution is 10.0 or higher. If indigo carmine is added to a new sample of the same solution and a blue color results, the pH will be narrowed to a range of 10.0 to 11.4, since the lower limit of color change for indigo carmine is blue. By using an additional indicator or indicators and a new sample of the solution, the pH of the solution can be narrowed to a small range.

Indicator	pH Range	Color Range
Methyl violet	0.0 – 1.6	Yellow to blue
Thymol blue	1.2 – 2.8	Red to yellow
Methyl orange	3.2 – 4.4	Red to yellow
Congo red	3.0 – 5.0	Blue to red
Methyl red	4.8 – 6.0	Red to yellow
Phenol red	6.6 – 8.0	Red to blue
Litmus	4.7 – 8.2	Red to blue
Cresol red	7.4 – 8.6	Yellow to red
Phenolphthalein	8. 2 – 10.0	Colorless to red
Thymolphthalein	9.4 – 10.6	Colorless to blue
Alizarin yellow R	10.1 – 12.0	Yellow to red
Indigo carmine	11.4 – 14.0	Blue to yellow