Melzer's reagent

The use of iodine-containing solutions as an aid to describing and identifying fungi dates back to the mid-19th century. and takes its name from its inventor, the mycologist Václav Melzer, who modified an older chloral hydrate-containing IKI solution developed by botanist Arthur Meyer. In Melzer and Jaroslav Zvára's 1927 Russula monograph, they refer to the solution as "jodkalichloral" (potassium iodide-chloral). ==Composition==

Melzer's reagent is an aqueous solution of chloral hydrate, potassium iodide, and iodine. Václav Melzer's original formulation was given as a solution of 20 g water, 1.5 g potassium iodide, and 0.5 g iodine, with a portion of this mixture combined as needed in a 1:1 ratio with chloral hydrate. == Related reagents ==

Melzer's reagent is part of a class of iodine/potassium iodide (IKI)-containing reagents used in biology. Lugol's iodine is another such formula. Václav Melzer himself describes using a zinc chloride-iodine solution to highlight the basidiospore ornamentation in Russula, before abandoning it in favor of his newly-formulated reagent, which he notes yielded a stronger reaction. Chloral hydrate-iodine solutions have been used in botany since the 1820s to examine starch granules in chloroplasts and other structures, In phycology, the von Stosch staining protocol, developed by , is a common technique for staining the theca of dinoflagellates using a chloral hydrate-hydroiodic acid-iodine solution. In response to difficulties obtaining chloral hydrate, scientists at Rutgers formulated a proprietary compound called Visikol as a replacement for chloral hydrate in formulations for microscopy. A 2019 paper described an experiment demonstrating that Visikol/IKI behaves differently to Melzer’s reagent in several key situations, noting it should not be recommended as a viable substitute. ==Reactions==

Melzer's is used by exposing fungal tissue or cells to the reagent, typically in a microscope slide preparation, and looking for any of three color reactions: • Amyloid or Melzer's-positive reaction, in which the material reacts blue to black. • Pseudoamyloid or dextrinoid reaction, in which the material reacts brown to reddish brown. • Inamyloid or Melzer's-negative, in which the tissues do not change color, or react faintly yellow-brown. Melzer's reactions are typically almost immediate, though in some cases the reaction may take up to 20 minutes to develop. An experiment in which spores from 35 species of basidiomycetes were tested for reactions to both Melzer's and Lugol's showed that spores in a large percentage of the species tested display very different reactions between the two reagents. These varied from being weakly or non-reactive in Lugols, to giving iodine-positive reactions in Lugol's but not in Melzer's, to even giving dextrinoid reactions in Lugol's while giving amyloid reactions in Melzer's. Melzer's degrades into a cloudy precipitate when combined with alkaline solutions, hence it cannot be used in combination or in direct series with such common mycological reagents such as potassium hydroxide or ammonium hydroxide solutions. When potassium hydroxide is used as a pretreatment, the alkalinity must be first neutralized before adding Melzer's. ==References==