Universal indicator papers and pH scale. This paper is used to measure the strength of an acid or an alkali. When the paper is dipped into a solution, it changes colour depending on the solution's pH, which can be read from the scale at bottom. Red is most acidic (pH 1), pale green is neutral (pH 7), and indigo is most alkaline (pH 11). (The univeral indicator scale is from 1 to 11, while the full pH scale is from 1 to 14.) pH is a logarithmic measure of hydrogen ion concentration. Acid solutions have a high concentration of hydrogen ions, while alkali solutions have a low concentration of hydrogen ions.