The compound known as resorcinol is much utilized in the chemical industry.  It is an unusually useful “intermediate” in the production of more complex chemicals.  Derivatives of resorcinol are found in the light screening agents that protect many plastics from degradation due to sunlight exposure, and in the dyes used to colour our fabrics.  The primers used to detonate explosives, flame retardants, and adhesives used in the manufacture of tires for passenger cars and trucks, all make use of resorcinol.

Aromatic compounds, of which resorcinol is a member, are based on a benzene ring structure.  As can be seen from the following illustrations of the structural formula, and molecular shape, resorcinol is a fairly simple organic (carbon containing) molecule.  Alternative names for the compound are 1,3-dihydroxybenzene and m-benzenediol.  (HyperChem Pro was used to produce the illustrations.)

In the lab, resorcinol is supplied as a white (or slightly pinkish) crystalline solid with high solubility in water and a melting temperature of about 110 degrees Celsius.  The crystals turn faintly brown with exposure to air and light.  Although it would have been easy to produce an “evaporation specimen” for study under the polarizing microscope, I chose instead to prepare a “melt specimen”.

A couple of crystals were placed on a microscope slide and covered with a cover-glass.  An alcohol lamp was used to heat the slide very gently, until the crystals melted and formed a thin layer between slide and cover-glass.  When the melt solidified, it was examined under the microscope.

Note:

The MSDS safety document for resorcinol describes the very unpleasant consequences of a failure to handle the compound carefully.

“Danger! may be fatal if swallowed. Harmful if inhaled or absorbed through skin. May cause methemoglobinemia*. Affects cardiovascular system, central nervous system, blood, spleen, liver and kidneys. Causes severe irritation to skin and eyes. Causes irritation to respiratory tract. May cause allergic skin reaction.”

* Methemoglobinemia is a condition in which the iron in the hemoglobin molecule (the red blood pigment) is defective, making it unable to carry oxygen effectively to the tissues.

My slides were all prepared in a fume hood in my chemistry classroom.  I do not recommend producing melt specimens of this compound in the home environment!

The first image in the article, and the one below, show typical crossed-polar images.  The gray areas are thinner sections of the crystal layer formed during re-solidification of the melt.

Feathery structures often form as the melt solidifies.  Note that the image on the right is a higher magnification view of an area near the top centre of the left image.  The third image uses a compensator, (lambda/4 plate) to produce an alternative colouration.

Many of the crystals formed contain tiny black areas, probably produced by overheating the slide and causing tiny bubbles (voids) to form that look black between crossed-polars.

The tiny voids seem to form along fault-lines in the crystals.  This can be seen more clearly as the magnification increases in the three images below.

At a very high (relatively speaking) magnification, the crystal layer can be seen to be literally “peppered” with imperfections.  (Right image)

The following two images illustrate another phenomenon seen with melt specimens.  Over long time periods (several months to years), some of the solid compound can sublime directly to a gas.  This leaves voids having a different appearance than those mentioned previously.  (The background in the right-hand image is gray rather than black due to the fact that two lambda/4 compensators were used to produce elliptically-polarized light, rather than the “normal” plane-polarized variety.)

By using a combination of lambda and lambda/4 compensators, it is possible to completely change the appearance of a particular field on a slide.  The lambda/4 compensator was rotated to produce the two images below.

Near the edge of the cover-glass, a rather amorphous field of small crystals often forms.  (This ring at the edge of the cover-glass may cool more quickly, as it has not been directly heated by the flame of the alcohol lamp.  Thus there is insufficient time for larger crystals to grow.)

Higher magnifications reveal details in the amorphous areas.

The five images that follow show areas at the very edge of the cover-glass.  The first, fourth, and fifth images, (those with an orange background), were produced using ordinary transmitted light illumination.  Note however, that the “auto-level” command in Photoshop was used to produce the strange effect.  (Some purists may object to the “computer processing” of images, but the technique sometimes results in striking images such as the first one.)

As mentioned in earlier articles, I often ring the cover-glass on crystal slides with finger-nail polish.  In some cases, the solvent in the polish dissolves the crystals at the very edge of the cover-glass.  Over a period of time, as the solvent evaporates, crystals re-form in new and interesting ways.  The perfect way to study these crystals is to use phase-contrast illumination.  Typical fields are shown below.  (Note that the contrast has been increased in Photoshop.)

This same technique has been used to obtain the following images of fields, slightly farther from the edge of the cover-glass.  (The bright colours in several of the images are produced by interference phenomena, as a side-effect of the technique.)

Finally, several dark-ground illumination images of resorcinol can be seen below.

Although resorcinol is an unpleasant compound to work with, it often produces an interesting array of crystal structures to investigate.

Equipment

The images in the article were photographed using a Nikon Coolpix 4500 camera attached to a Leitz SM-Pol polarizing microscope.  Images were produced using several illumination techniques: dark-ground, phase contrast and polarized light.  Crossed polars were used in all polarized light images.  Compensators, ( lambda and lambda/4 plates ), were utilized to alter the appearance in some cases.  A 2.5x, 6.3x, 16x or 25x flat-field objective formed the original image and a 10x Periplan eyepiece projected the image to the camera lens.