Typically an acidified version of Lugols solution is used to preserve most algal samples. Acid Lugols solution is prepared as above but with the addition of 100 ml of glacial acetic acid (CH3COOH).
Acid Lugol’s solution is good for all phytoplankton but not for coccolithophorids as the acid may dissolve the coccoliths. If coccolithophorids need to be preserved with the coccoliths intact, a parallel sub-sample should be fixed with alkaline Lugol’s solution.
Alkaline Lugols solution is prepared as above substituting the 100 ml of glacial acetic acid for 50 g of sodium acetate (CH3COONa). It is best prepared by first dissolving the sodium acetate in 50 ml of the water first.
The amount of acid Lugol’s solution which should be added to an algae sample depends on the concentration of algae in the sample, but typically 0.3 ml to 1.0 ml of acid Lugol’s solution per 100 ml of sample is sufficient with the larger volume of Lugol’s being used for particularly dense algal samples.
A good rule of thumb is to add the Lugol’s to the sample a few drops at a time until the colour of the water in the sample is that of weak tea or light beer (as illustrated below) allowing a little time between each addition for the Lugol’s to be absorbed into the algal cells.