The startling yellow chemical in this photo is the ferricyanide anion, [Fe(CN)6]3−. Ferricyanide has a colourful history, rising to prominence for its use in the dark Prussian Blue dye when combined with iron. More recently, its electroactivity has become important in a range of applications.
In this experiment, the ferricyanide is being used to mediate, or enable, bacteria to respire in the absence of oxygen. Rather than use oxygen, the bacteria chemically reduce the ferricyanide to a similar but colourless compound, ferrocyanide. The ferrocyanide can be easily measured using a microelectrode, providing a rapid, accurate determination of the respiration rate of the bacteria. For this specific application, the test is done to determine the toxicity of different pesticide types, but other examples of applications include testing for food contamination, bacterial fuel cells and in the control of wastewater treatment plants.