CD-4 for lichen spot testing?

CD-4 is very similar to p-phenylenediamine and has the same uses and toxicity (but it would be more stable) as it is more expensive there is no reason to change one’s habits in lichenology… (there are small quantities for sale on e-bay).

Alain

This is an interesting post. I can’t get hold of pd from anywhere and an alternative is of interest. Where do people get pd?

Thanks for any info!

Jon

It may be useful if I remind people of the two main ways of using Pd.

The best and preferred method is to drop a couple of drops of alcohol (meths, vodka or other strong ethanol you have to hand) on to a very few crystals of Pd (much less than a pinch of salt). This makes a very strong solution of Pd which can be transferred to the specimen using a cocktail stick, corner of a razor blade or fine glass rod. The traditional container in which to make the solution is a laboratory watch glass but I often use an old bottle top which can then be wrapped and discarded when finished with. You don’t have to trash the tiny amount of Pd in the ‘watch glass’ after the first use. It will soon dry and can then be kept covered for several more uses. Pd oxidises rapidly and becomes useless when in solution but oxidises slowly when dry. You will know when the crystals in the watch glass are getting too oxidised because they go dark (blackish) in colour. I suggest keeping the watch glass covered in between uses because the powdery (recrystallised) residue can disperse in air currents and, as well as the potential health risk with Pd dust in the air, any escapee dust can cause permanent dark stains on anything it comes into contact with. If you are at all house proud it is probably best to use Pd in a garage or shed.

All spot tests (K, C and Pd) are best made on dry material, using frugal spots of strong reagent and observed under a dissecting microscope. Of course we often want to conduct spot tests in the field and this is fine as long as we realise that the reactions may be weak or even missed altogether, especially if the specimen tested is at all damp or we clumsily dump an excessively large drop of reagent on it. The traditional way of using Pd in the field is Steiner’s solution, an aqueous solution of Pd made stable by the addition of sodium sulphite. Steiner’s solution has a deserved reputation for producing weak reactions. This is because Pd is sparingly soluble in water. I think I can claim to have made the simple innovation of a strong and stable solution, Powell’s alcoholic Steiner’s solution. Pd is readily soluble in alcohol, sodium sulphite readily soluble in water. I use something like vodka as a happy mixture of alcohol and water in which to create a strong, perhaps even saturated solution of both sodium sulphite and Pd. This creates a strong solution of Pd which can be in used in the field and usually lasts for several months. I only make up about 5 mils at a time, plenty for numerous tests.