Flexi Says: Certain compounds, called indicators, change color when acids or bases come into contact with them, so they can be used to detect pH of solutions. pH is the negative logarithm of the hydrogen ion concentration in solution. The pH value can also be measured using electrochemical measuring systems and litmus papers. Litmus is placed on small strips of paper that may be red or blue. If you place a few drops of a base on a strip of red litmus paper, the paper will turn blue.