It has been fun researching this book and trying to figure out how to make things from scratch. Sometimes I have gained inspiration from
modern laboratory manuals, but more often than not, the projects have developed by trying to imagine what commonly-available materials could be used
to make something appropriate to each chapter; this chapter was one of the last ones to "come together." At first I imagined that I would make mauve
as Perkin had done, but the more I learned about Perkin's synthesis, the more discouraged I became. Perkin dissolved his aniline in aqueous sulfuric
acid. Oxidation of this solution with potassium dichromate produced an insoluble brown goo which would have to be filtered, and filtering goo is no
picnic. Once filtered, the goo would be extracted with ethanol, which would dissolve the dye. Perkin's yield was on the order of 5%, that is, 5% dye,
95% useless goo. Could caveman chemists be expected to make such a dye when even experts got such poor yields? Furthermore, chromium compounds pose a
health risk and a disposal problem I was not sure I wanted to impose on beginners. Finally, neither aniline, nor toluidine, nor potassium dichromate
is a household item. It seemed that there was much to be said against mauve as a project.Nevertheless, I wondered whether it might be possible to
substitute a household oxidizing agent for potassium dichromate. Wagner's Chemical Technology of 1872[1] reported that bleaching powder, calcium
hypochlorite, had been used as an alternative to potassium dichromate. I wondered whether sodium hypochlorite, modern laundry bleach, might also do
the trick.I also wondered whether the goo-filtering step might be avoided by performing the oxidation in ethanol rather than aqueous solution. I
expected that the goo might settle to the bottom of the container and the ethanolic dye might simply be poured off. My very first try using laundry
bleach in ethanol took my breath away; it resulted in a rich purple dye and no goo at all. Not only did the household chemical work, it apparently
worked better than any combination I had found in the chemical literature! But it was when I applied the dye to some silk cloth that I knew how Perkin
must have felt.To begin with, you need some aniline. If you buy aniline from a chemical supply it will likely be of higher purity than you need. Pure
aniline will produce the brown pseudomauveine which, while it is a perfectly good brown dye, is unlikely to have the visual impact of mauveine. To
make mauveine you need aniline contaminated with o-toluidine and p-toluidine. The optimum proportions are one mole aniline, two moles o-toluidine, and
one mole p-toluidine. Since these three compounds have similar molecular weights and densities, you can make it up as 1 gram of aniline, two of
o-toluidine, and one of p-toluidine. I will refer to this mixture as "aniline oil." The four grams just described will make a lot of dye; in a
classroom situation, this amount of aniline oil will be enough for about a hundred students.You will make two solutions. First measure one mL of white
household vinegar using a graduated pipette or a graduated cylinder. Place the vinegar into a small vial and add 1 drop of aniline oil to it. The oily
aniline will dissolve completely within a minute or two. Now measure 5 mL of ethanol into a graduated cylinder and add 10 drops of bleach to it. Add
the ethanol-bleach solution to the vinegar-aniline solution and put the cap on the vial. The combined solution should immediately turn brown as the
aromatic amines are oxidized. The brown color will turn blue in a few minutes and then purple in an hour or so. This vial of dye is sufficient for
dyeing a 12-inch silk handkerchief.
Dunn says that the aniline/o-toluidine/p-toluidine mole ratio should 1:2:1. This makes more sense as this is more like the ratio one would expect
when forming these amines from benzene and toluene in coal tar, as did Perkin.
Comments, suggestions, and questions are welcomed.
[Edited on 9-4-2014 by Magpie] |
