An indicator gives different colours in an acidic and a basic medium. Thus, we can check whether a given substance is acid or base or neutral with the help of indicator.

`=>` Litmus, red cabbage leaves, turmeric (haldi) coloured petals of some flowers such as hydrangea, petunia and geranium etc. are natural acid-base indicators. Methyl orange, phenolphthalein etc. are synthetic indicators.

`=>` Universal indicator is a mixture of several indicators. It shows different colours at different concentrations of hydrogen ions. We can estimate the strength of acids and bases by using universal indicators.

(a) Alizarin : It is yellow at pH 5.5 and red at pH 6.8.

(b) Cochineal : It is yellow in acidic solution, and deep violet in alkaline solution.

(c) Turmeric : Turmeric solution stays yellow in the presence of acids and changes to purple brown in the presence of bases. Dry turmeric paper is bright yellow and changes to red in the presence of bases.

(d) Curcumin : It is yellow at pH 7.4 and red at pH 8.6.

(e) Red cabbage : Red cabbage juice indicator is red in acid solution, purple in neutral solutions and greenish yellow in basic solutions. Cabbage paper turns green in the presence of bases and pink to red in the presence of acids.

(f) Phenolphthalein : Phenolphthalein is purple in very basic solutions and colourless in acidic solutions.

Generally pH of an aqueous solution decreases on addition of a small amount of `HCl` because of the increase in the concentration of `H^+` ions. On the other hand, if a small amount of `NaOH` is added, the pH of the solution increases. However, there are some solutions which resist the change in pH on addition of small amount of strong acid or alkali. Such solutions are called buffer solutions.

`=>` The solution which maintains its pH or reserve acidic or basic nature even upon addition of a small amount of acid or a base is called buffer solution.

`=>` These solutions have a definite pH value.

`=>` In biological systems, buffer system of carbonic acid and sodium bicarbonate is found in our blood. It maintains the pH of our blood (about 7.4).

`=>` In industries, buffer solutions are used for alcoholic fermentation (pH 5 to 6.5), tanning of leather, electroplating, sugar and paper industries.

Synthetic buffers are of two types

Acidic buffer : Acidic buffer solution contains equirnolar quantities of a weak acid and its salt with strong base. For example, acetic acid (`CH_3COOH`) and sodium acetate (`CH_3COONa`). A solution containing equimolar quantities of acetic acid and sodium acetate maintains its pH value around 4.74.

Basic buffer : Basic buffer solution contains equimolar quantities of a weak base and its salt with a strong acid. For example, ammonium hydroxide (`NH_4OH`) and ammonium chloride (`NH_4Cl`). A solution containing equimolar quantities of ammonium hydroxide and
ammonium chloride maintains its pH value around 9.25.