Estimation of pH - pH Paper
pH paper is made by coating a piece of paper with acid-base indicators like those described previously. pH paper comes in two basic types:
1. litmus type
A single indicator is coated on a piece of paper. A drop of solution is touched to the paper and the results interpreted just as if an acid/base indicator were used.
2. wide range/short range
Usually this is a combination of indicators coated on a piece of paper. A drop of solution is touched to the paper and the resulting color is compared to a color chart supplied with the paper. Wide range paper spans the normal pH range (1-14) with the resulting pH value being good to about ± 1 pH unit. Short range paper covers a much narrower range (a couple of pH units) and gives a pH value good to about ± 0.1 units. Wide range paper is generally used first to find the approximate pH and then short range paper is used.