K₃Cr(C₂O₄)₃·3H₂O

This is potassium tris(oxalato)chromate (III), synthesized from potassium dichromate, oxalic acid and potassium oxalate. A very similar complex, potassium bis(oxalato)chromate (III) exists, which can be synthesized from oxalic acid and potassium dichromate without the potassium oxalate.

Potassium tris(oxalato)chromate (III) is a remarkable compound, the color of which is very hard to observe accurately. The observed color of this compound strongly depends on the light, under which it is viewed.

Below follow two pictures of the same sample, but with different ambient light. Look at the difference in color! The left picture is made with light, which resembles daylight quite a lot, the right picture is made with plain fluorescent tube light. Under fluorescent light, the compound looks dark purple/black. When the compound is viewed under tungsten light, then it looks gray/black (no picture here).

This remarkable property is even stronger for solutions of this compound. A solution of this compound can seem to have any colors, ranging from a blue-like color to deep red/purple, again strongly depending on the light under which the liquid is viewed. Below follow two pictures. The left picture shows a solution of the compound, made with light, very much like daylight. The right picture shows the same test tube, but now the picture was made with plain fluorescent light.