What is "spot test"

n. (context chemistry English) The addition of a single drop of a reagent to a small sample as a qualitative analytical test for the presence of a specific compound or class of compounds.

A spot test in lichenology is a spot analysis used to help identify lichens. It is performed by placing a drop of a chemical on different parts of the lichen and noting whether there is a color change associated with that chemical.

The most common chemicals ( reagents) used in lichen spot tests and abbreviations for their results are:

• K - A 10% solution of potassium hydroxide
• C - A strong solution of bleach ( sodium hypochlorite, NaClO), such as undiluted laundry bleach
• P - a solution of para-phenylenediamine, made by placing a drop of 70% isopropyl alcohol on a few para-phenylenediamine crystals, which lasts about an hour, or by other more long lasting means

These may be used individually, or in combination. The test results are then indicated with a "+" or "-" sign following the letter or combination of letters, followed by the color observed if "+". For example, "K+ orange, C-, P-" means the part of the lichen tested turned orange with the potassium hydroxide, and had no reaction to the C and P. "K-, C-, KC+ red, P-" means no reaction to K, C, or P, but turning red when both K and C are applied.