A Gallery of Benzoic Acid Photomicrographs

(using polarized light illumination)

by Brian Johnston   (Canada)

This article is actually a second look at benzoic acid.  In an earlier article, I used phase-contrast illumination exclusively to produce a series of photomicrographs of the compound.  In the present article, I have used the slides prepared earlier to provide an alternative gallery of images using polarized light.  If you take a look at the earlier article, it should be noted that the same magnification was used for all images, whereas here, several magnifications were used.

Benzoic acid is a white crystalline solid with a melting temperature of about 122 degrees Celsius.  This low melting point makes it easy to produce a melt specimen by placing a few crystals on a slide, covering with a cover-glass, and heating gently over an alcohol lamp until the solid melts.  Slides prepared in this way cool to room temperature in about a minute.  It should be kept in mind that the MSDS safety document for the compound states: “May be harmful if swallowed. May act as an eye or respiratory irritant. May cause allergic respiratory or skin reaction.

Benzoic acid C6H5COOH is the simplest aromatic (based upon a benzene ring) carboxylic acid (containing the COOH group).  The structural formula and molecular shape, (produced using HyperChem Pro), can be seen below.

This compound is often used as an anti-microbial agent in products like cosmetics, toothpastes, mouthwashes and deodorants.  Fruit products, beverages and condiments may use benzoic acid as a preservative.  In such applications the quantity used, is of course very small, in order to reduce the harmful effects mentioned above.

In addition to crossed polars, a quarter wave plate and whole wave plate were used to change the colouration of the crystals.

Notice the parallel lines and bands that often occur in melt specimens of the compound.  (quarter-wave plate & whole-wave plate)

The different colouration shown in the image below resulted from the use of two quarter-wave plates.  This combination produces elliptically polarized light rather than the plane polarized light resulting from crossed polars alone.

In the two images that follow, note the small random crystal inclusions that have formed within the larger crystal structures. (quarter-wave plate & whole-wave plate)

In the photomicrograph that follows, tiny colourful crystals seem to bridge the gap between larger blue formations. (quarter-wave plate & whole-wave plate)

It takes a moment of study to determine that the two images below are of exactly the same field of view.  The use of plates, (sometimes called compensators), allows the photomicrographer to accentuate different details of a crystal field. (quarter-wave plate & whole-wave plate on the left and two quarter-wave plates on the right)

Another example follows.  Exactly the same field is shown in both images! (two quarter-wave plates  on the left and a quarter-wave plate & whole-wave plate on the right)

Many of the fields on the slides were rather chaotic. (quarter-wave plate & whole-wave plate)

Elliptically polarized light tends to give gray backgrounds.  (two quarter-wave plates)

Just to the right of center in the photomicrograph below, several straight streamers radiate out from the interface between two crystal structures. (quarter-wave plate & whole-wave plate)

Other similar formations are shown below. (quarter-wave plate & whole-wave plate on the left and two quarter-wave plates on the right)

The two images of the same field that are shown below are my favourite polarized light benzoic acid photomicrographs. (quarter-wave plate & whole-wave plate)  The whole-wave plate was rotated to produce the difference in colouration.  (Note: The first image in the article was produced by using Adobe Photoshop’s  “Invert (image)” command on the image on the left below.)

These strange flow patterns occurred at the edge of the cover-glass in one melt specimen.  Photoshop’s “Auto-level” command was used to increase the contrast in both images.

Most chemical compounds do not allow the use of phase-contrast illumination to produce photomicrographs.  Benzoic acid is a major exception to this, and I must confess that I prefer phase-contrast to polarized light images of this compound.

Photomicrographic 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 a polarizing condenser.  Crossed polars were used in all 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.

 All comments to the author Brian Johnston are welcomed.

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