Children like to test how long it takes for parents to get angry. 😊 A similar experiment can also be carried out with water: The ACID CAPACITY describes the amount of acid that water can absorb before the pH value changes. Starting at pH 4.3, the methyl orange indicator changes color from red to yellow. Well, what does that remind you of? Right, at the m-value and as luck would have it, the m-value is the old name of the acid capacity.