pH indicators are a very simple, accurate and reliable method of measuring
Indicators work on the principle that
color produced by the particular dye used in the indicator formulation is
dependant on the pH of the solution (Fig 10.3). Experience shows that if
aiming to adjust pH to 5.5 (orange) then an accuracy of +/- 0.2 is
Note that pool and aquarium pH indicators are not usually suitable for
nutrient solutions because they do not operate below pH 6 i.e. their color
does not vary for pH’s below 6.
Obtaining pH readings with indicators
Step 1. Sample preparation: Stir the nutrient reservoir thoroughly
before sampling, especially after
Up or pH Down is used. Ensure the sampling container is clean by
rinsing a few times with the nutrient solution.
Step 2. Add a drop of the indicator, mix, then compare the solution
color with those on the colored reference chart (Fig 10.3).
Step 3. If the pH is not between 5.0 and 6.5,
adjust the pH of the nutrient reservoir.