A pH meter is a piece of equipment which measures the concentration of hydrogen ions in a solution. The pH meter uses a probe called an electrode, and a dial with a pH scale. These pH meters are very expensive if highly accurate readings are required. A pH comparator kit is designed to calculate pH using indicator chemicals. The number of drops needed to change the color of the sample are used to calculate the pH of the rain.
Litmus paper uses indicator chemicals, which are combined with a special absorbent paper.
The final type of comparator kit uses chemicals, which are added to the rain sample.