22.3.

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.

Is your dye a single compound or a mixture of compounds? To answer this question we'll use a technique invented by Otto Unverdorben for precisely this purpose: paper chromatography. You've probably seen water-soluble ink bleed when a piece of paper gets wet. If the ink is a mixture and if the mixture's components have different solubilities, then the more soluble components bleed faster than the less soluble ones. As the water spreads out on the paper, the more soluble ink components separate from the less soluble ones. This is the main idea behind chromatography. Since our dye is not soluble in water we'll use a solution of vinegar and ethanol as a solvent.

To make your chromatograms you'll need a piece of filter paper (90 mm in diameter), a pencil, a glass capillary tube (1 mm inside diameter, open both ends), a glass jar with a lid, and a solvent consisting of 75% white vinegar and 25% ethanol. Begin by folding the filter paper lengthwise into quarters and snipping off the ends with a pair of scissors. The resulting paper will be able to stand up, as shown in Figure 22-5(L). With a pencil[2] make a dot 1 cm from the bottom of the paper. Your goal is to place as much dye onto the smallest spot possible. To this end, dip the capillary tube into your dye; dye will climb into the tube by capillary action. Practice using the capillary tube by touching it briefly to a piece of paper towel. Some of the dye will be transferred to the towel, forming a spot. Practice making the smallest possible spots. Once you have some confidence, draw up some more dye into the capillary tube and spot the paper towel until a 1 cm column of dye remains in the tube. You will now deliver that 1 cm column of dye onto the pencil dot on your filter paper, as shown in Figure 22-5(R). Place the first spot onto the pencil mark and count to thirty, allowing the spot to dry. Place a second spot on top of the first and count to thirty. Continue to build this spot until you have delivered the entire 1 cm column of dye onto the filter paper. If you do a good job this "young" dye spot will be no more than 5 mm in diameter. If you mess up, just spot another paper until you get it right. You will now develop your chromatogram using a solvent composed of vinegar and ethanol.

Figure 22-5. Folding and Spotting the Chromatogram

Mix 9 mL of white vinegar and 3 mL of ethanol in a glass jar large enough to contain your filter paper. Screw the lid onto the jar and shake it to thoroughly wet the walls of the jar. We want the air in the jar to be saturated with solvent so that the solvent doesn't evaporate as it climbs the paper. Open the jar and place your filter paper inside, standing it on the end with the spot and taking care that the paper doesn't touch the walls of the jar. Then replace the lid. The vinegar/ethanol solvent will climb the filter paper, taking the dye with it. Your chromatogram will take about an hour to develop, as shown in Figure 22-6(L).

Figure 22-6. Developing and Interpreting the Chromatogram

When the solvent has climbed to within 5 mm of the top, remove your filter paper from the jar and draw a pencil line where the solvent stops. Your chromatogram will look something like Figure 22-6(R), which is shown in color on the back cover of this book . The chromatogram on the right is for a dye which has been allowed to mature for a week. At least four substances are evident in these chromatograms. Spot a was present in the young dye but not in the mature one. It represents a precursor, a combination of two or perhaps three aromatic amines. Spots b and c overlap but represent at least two different mauve dyes with slightly different colors. Spot d is soluble in the highly ethanolic dye solvent, but not in the less ethanolic chromatographic solvent. Consequently, it has not budged from the original spot.

Chemists characterize chromatographic spots by comparing the distance traveled by each one to that traveled by the solvent. Measure the distance from the original pencil dot to the center of each spot and to the solvent line. Divide the distance for each spot by that for the solvent to get what is called the Rf value (ratio of fronts). Spot a in the chromatogram shown has an Rf value of 56/65 = 0.86. Spot d has an Rf of 0.

By the time your chromatogram develops, your dye is mature enough to dye your silk handkerchief.[3] Push the entire handkerchief into your vial of dye and once it has taken up the dye, fish it out again. Spread it out on a few sheets of newspaper and allow it to dry. Be careful not to drip dye where it ought not to be. Once it's completely dry, you can wash your handkerchief with soap and water. Wow, is that a pretty color!

ImportantQuality Assurance
 

Your dye is not so great if the color isn't fast. If the color hasn't washed out, tape your mauve handkerchief and your chromatogram into your notebook as proof of your dye-making prowess. Make a table of Rf values for your dye spots and explain how many substances were present in your young dye.

 

In this way was the World created. From this there will be amazing applications because this is the pattern.

 The Emerald Tablet of Hermes Trismegistos

Notes

[1]

Reference [29], p. 577.

[2]

A mark made with a pen would bleed.

[3]

If you wish, you can hold back some of your dye and repeat the chromatography after it has matured for a week.