Blackcurrant juice indicator in an alkali solution (image 2 of 2). Changes in colour (here from purple, the original colour of the juice, to green) are a common way to test the pH of a solution. Alkali and acid are the two parts of the pH scale, indicating the hydrogen ion concentration in a solution relative to that found in water. Alkalis (pH 8-14) contain fewer free hydrogen ions than water, while acids (pH 1-6) contain more. Water (pH 7) is considered neutral. See A500/520 for blackcurrant juice in an acidic solution.