The reactions of Mandelin involve both the formation of vanadium
coordination complexes and reduction of the Vanadyl ion (from V to IV). It is not a sign the reagent is going off, and can be seen with freshly made reagent. Several things can cause this, possibly also including the heat of the reaction and catalysis from certain metal impurities, or excipients. The complete reaction mechanisms with Mandelin are rather involved and I'm yet to see the full reaction sequences qualified in literature.
Marquis will often look brown as the reagent ages due to polymerisation of the formaldehyde, but also, as we know, Marquis reacts with many things and brown reaction products are frequently produced by these reactions.
If the use of reagents is to be taken to a next level, then a light table (backlit glass) can be employed which allows variations in the reaction product colours to be more accurately distinguished.
However, the best approach is to use the reagents in conjunction with TLC (thin layer chromatography). It's slightly more fiddly, but the overall procedure is easy. Typically, this involves:
- extraction; e.g. 5% acetic acid (not vinegar) in 95% lab grade ethanol (not metho)
- preparation of the TLC plate; drawing a line with a pencil and depositing a very small drop of the extract on the line*
- resolution by standing the plate in a mixed solvent solution** e.g. hexane/ethyl acetate mixture. This solvent mixture can use different proportions e.g. a 40:60 or 50:50 ratio, although different combinations may better suit if the mixture contains compounds with similar structures.
* A small capillary tube is made by heating a piece of borosilicate glass tubing and pulling so the melted glass forms a fine, hair thin point. It's then snapped and one piece is used to transfer the extract solution.
**The above solvent mixture is poured into a beaker or jar and the plate placed so that the solvent level is
below the pencil line. The plate is sat so that it leans against the top of the beaker.
After the compounds have separated, the plate is removed and dried. Often a developing or contrasting compound is then applied. This is where the reagents come in. Marquis is often the reagent of choice when unknowns are suspected amines or alkaloids, and this is
sprayed onto the plate. The associated reaction colours are then observed. UV light can often further improve resolution (which can also be applied to the light table method). Most TLC UV lights have both short and long wave band options, so a common fluorescent tube is not as good.
If more than one compound is present in the tested sample, ideally these will separate into specific spots, each displaying it's specific reaction colour with the reagent. If available, when preparing the plates, a reference pure sample of any suspected compound is included on the plate. If the unknown is one of the suspected compounds it will travel the same distance along the plate. There's also a simple equation known as the ratio of fronts (R
f value) which indicates the sample spot distance to the pencil line, over the solvent front distance to the line.
Some publications give R
f values for a wide range of drugs employing particular TLC conditions and solvent mixtures. For more info, see;
Thin Layer Chromatography
Thin Layer Chromatography - TLC
Clarke's Analysis of Drugs and Poisons (Note the free trial membership, highly recommended.)
TLC works well for most legal and OTC drugs, so the techniques can easily be learned without needing to use any illicit compounds.
WARNINGS: If intending to perform these techniques, please choose an appropriate environment i.e. not the kitchen or bathroom, and observe all relevant safety precautions, particularly if spraying reagents. DO NOT BREATHE ANY VAPOURS. Wear gloves and eye protection when spraying or handling the developed plates. Most importantly, only spray the reagents in a well ventilated area.
