How is a universal indicator obtained? How is the wide range of pH of solution tested by it?

As explained in the chapter: 'We can do this by making use of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution.' Thus a universal indicator is not a single dye but a carefully prepared combination of several acid-base indicators (each of which changes colour over a different, narrow pH range), blended so that together they cover the whole pH scale.

Testing the wide range of pH: A scale for measuring hydrogen ion concentration, called the pH scale, has been developed, running generally from 0 (very acidic) to 14 (very alkaline), with pH 7 being neutral. 'Generally paper impregnated with the universal indicator is used for measuring pH.' When a drop of the solution to be tested is placed on this universal indicator paper (or the paper is dipped into the solution), the paper develops a distinct colour depending on the hydrogen ion concentration of that solution. This developed colour is then matched against a standard reference colour chart (as shown in Fig. 2.7 of the chapter) that maps each colour to an approximate pH value on the 0–14 scale. In this way, a single universal indicator can approximately indicate any pH value across the entire range, rather than being restricted to detecting only 'acidic' or 'basic' the way litmus does — 'Higher the hydronium ion concentration, lower is the pH value,' and as pH increases from 7 to 14 it shows an increase in $OH^-$ ion concentration.