Malachite Green
Contents:
- RE: malachite green
by Ross Drewe <rdrewe/melbpc.org.au> (Mon, 29 Sep 1997)
by Ross Drewe <rdrewe/melbpc.org.au>
Date: Mon, 29 Sep 1997
[...]
This canard about the dye containing copper comes up regularly on the
Net, and I think it has several origins:
1) the mineral after which it was named is copper-containing;
2) in aquarium practice, I'm told there have been 'cure-all' mixtures
on the market which contain both the dye and copper, because copper can
be useful as a fungicide and molluscicide;
3) some particularly stupid people assume that the dye is the mineral,
powdered, without ever attempting to look it up;
4) gardeners are familiar with green copper oxychlorides and other
copper mixtures (eg, Bordeaux mixture) to control fungi, so they are
primed to believe that a fungicide with this name probably contains
copper.
Someone said recently on APD that it should be put into a FAQ - seems a
good idea.
Following are a few facts I've gleaned about this dye, in case they're
useful - a lot more could be added.
Name: Its hue is very similar to the mineral malachite green, and it
is often stated that the name was given for its colour - it seems
highly likely but I've never seen anyone add proof of the assertion. I
do not agree that it was a stupid name to give - it has been very
common to name dyes and pigments after some natural object with a
similar colour, especially in the early days of synthetic dyes. There
are numerous examples. The only problem is with people who don't ever
check their assumptions.
Synonyms: aniline green, benzaldehyde green, china green, and others.
For malachite green G: brilliant green.
Composition: Malachite green does NOT contain copper. It is chemically
well characterised - it belongs to the triphenylmethane family, as
below:
[ __ ]
[ / \__N(CH2)2 ]
[ __ / \__/ ] _
[ / \__C __ + ] Cl (Note: not all bonds shown correctly.
[ \__/ \ / \__N (CH2)2 ] Rings are benzene, not cyclohexane.)
[ \__/ ]
[ ]
More accurate diagrams can be looked up in numerous reference books
such as the Merck Index.
Dye is cationic, so it cannot form a copper salt. Conceivably it could
form an organometallic complex, but I haven't seen any mention of it.
Activity: (1) fungi (2) some Gram-positive bacteria (3) proliferative
kidney disease in salmonids (thought to be myxosporidial).
Usage: As mentioned: fungicide, topical bactericide, dye for yarns. In
aquarium use, most commonly as a bath (ie, added to the aquarium at 0.1
mg/L or less) but this is often ineffective because of the relatively
low concentration. Said to be toxic to some aquatic plants, although
clearly (from other APD messages) not to all. Far more effective when
used as a dip or paint for affected fish after netting and removal from
the aquarium, but length of time tolerated is very dependent on
concentration. Some levels I have successfully used are 0.04% paint, 1
mg/L for 10 mins dip, 0.5 mg/L for 1 hour dip, and quarter that for
tetras. Anaesthetising the fish first with MS222 or equivalent is a
useful way of reducing the stress from netting + toxic dye.
Cautions: (1) Tetras and scaleless fish more sensitive. (2) Some plants
sensitive (someone please help with more details!) (3) Teratogenic,
therefore not permitted on food fish. (4) Stains everything in sight,
including plastics.
Ross Drewe
rdrewe-at-melbpc.org.au
The Krib
Chemistry
The Krib