Ammonium sulphate is one of many compounds used to prepare agricultural fertilizers.  It is also used as a component in nutrient mixtures for bacterial cultures in wastewater treatment.  In addition, this salt is a component of fire-extinguishing powders.

Historically, this compound was one of the first types of “synthetic nitrogen” fertilizer manufactured and used in large quantities.  Its popularity was due to the fact that it was a by-product of the town gas works in many countries.

The white crystals have a moderately high melting temperature – about 235 ^oC.  This would normally discourage me from attempting to prepare a melt specimen, but in this case, I persevered.  A small quantity of the solid was placed on a microscope slide, covered with a cover-glass, and heated gently over an alcohol lamp.  As soon as the solid melted, and had formed a thin liquid film, the slide was removed from the heat and allowed to cool slowly.

Note:    The MSDS safety document concerning ammonium sulphate states:

            Contact with strong oxidizers may cause fire or explosion.
            Harmful if swallowed.
            Eye, skin and respiratory irritant.

Under the conditions described above, many long, thin rectangular crystals form that have rounded ends.  Elliptically polarized light was used to produce the gray background in the following image. (Crossed polars + two lambda/4 plates)

The two images below show approximately the same area on the slide.  The first uses dark-ground illumination to delineate the edges of the crystals, whereas the second uses elliptically polarized light. (Crossed polars + two lambda/4 plates)

If a phase-contrast condenser is combined with an ordinary, (non-phase) objective, the image produced has a distinctly three-dimensional character.

The same type of area, visualized by the use of a dark-ground condenser, looks like those shown in the two images below.

A phase-contrast condenser combined with a proper phase objective (PHACO) shows interesting structural details.  Note that the three images that follow are of a higher magnification than the others in the article.

Elliptically polarized light was used to illuminate the section of slide shown below at low magnification. (Crossed polars + two lambda/4 plates)

Higher magnification images of a similar field follow. (Left image: Crossed polars + two lambda/4 plates) – Right image: (Crossed polars + lambda/4 plate + lambda plate)   Note: The first image in the article is the same as the first image below, but it had Photoshop’s “Invert (colour)” command used on it.)

Still higher magnification reveals the following. (Crossed polars + lambda/4 plate + lambda plate)

Ordinary plane-polarized light provides the ultimate contrast between colourful crystals and background.  (Crossed polars)

This common industrial chemical’s high melting temperature makes the preparation of melt specimens particularly difficult, but I feel that the results are worth the effort.


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 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.