Acids dissociate in aqueous solution to produce H+(aq.) ions and an anion, known as conjugate base of that acid.
HA →H+ + A-
Some acids dissociate completely while some other partially. The distingction made between strong and weak acids is based on the degree of dissociation of the acid. A strong acid dissociates essentially completely into hydrogen ion and its conjugate base. As a result there is virtually no molecules of undissociated acid in solution. HCl, H2SO4, HNO3 and HClO4 – these four acids are strong acids.
Weak acids, on the other hand, does not dissociate completely in a solution. At equilibrium, there is a finite concentration of each component.
HA `hArr` H+ + A-
The relative strength of a weak acid can be expressed either in terms of the degree of dissociation or in terms of equilibrium constant (Ka) value. In the latter case, the constant is often referred to as acid dissociation constant. Higher the Ka value, stronger is the acid. Some weak acids are stronger when compared with others. Thus, carbonic acid is stronger than lactic acid or boric acid.
On this background, a method for distinguishing between weak and strong acids can be designed. Take two acid-base indicators, phenolphthalein and methyl orange. Titrate a strong acid and a weak acid against a strong base solution (say, 0.1M NaOH) using these indicators one by one and note the titre value in each case. The strong acid is one which gives the same titre value for both the indicators. For the weak acid, the methyl orange color change will be much earlier, thus giving two differtent titre values for two indicators.``