Universal indicator pH scale. The pH scale measures the strength of an acid or alkali. When universal indicator is added to a solution, it changes colour depending on the solution's pH. The tubes contain solutions of pH 0 to 14 from left to right. Acids increase in strength from pH 6 to pH 0, and alkalis increase in strength from pH 8 to pH 14. The green tube (centre) is neutral, pH 7. For a colour-enhanced version of this image, see A500/427.