Universal indicator papers. 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. The pale green paper is neutral (pH 7), whereas the orange and red papers are increasingly acidic (approximately pH 4 and pH 2 respectively). The dark green and violet papers are increasingly alkaline, or basic, (approximately pH 8-9 and pH 11 respectively). 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.