A Gallery of Cobaltous Nitrate Photomicrographs


(using polarized light & dark-ground illumination)

by Brian Johnston   (Canada)

Cobaltous nitrate hexahydrate, (or cobalt II nitrate hexahydrate as it should be referred to nowadays), is most commonly used to supply cobalt ions Co+2 (aq) in water based solutions.  These ions, (atoms with an electrical charge), are used as a catalyst in order to speed up the rate of many chemical reactions.  The hexahydrate term refers to the 6H2O in the formula.  Six water molecules are associated with each Co(NO3)2 group in the crystal lattice.

The red crystalline powder has the extremely low melting temperature of about 56 oC, and this makes it possible to prepare a melt specimen by placing a small quantity of the solid on a microscope slide, covering with a cover-glass, and heating gently over an alcohol lamp.  As soon as the solid melts and has formed a thin liquid film, the slide is removed from the heat and allowed to cool slowly.

Note:    The MSDS safety document for the compound states:

            Strong oxidizer - incompatible with reducing agents.
            Harmful if swallowed or inhaled. Respiratory and eye irritant.
            The substance decomposes on heating producing toxic gases, including nitrogen oxides.

Occasionally, large almost perfect crystals form amongst the general matrix of smaller crystals.  The two images that follow made use of elliptically polarized light.  (Crossed polars + two lambda/4 plates)  One of the plates was rotated in order to produce the differences seen in the two images.

Sometimes the larger crystals are far from perfectly formed, as the right hand image below demonstrates. (Crossed polars + two lambda/4 plates)

If the polarizing condenser is replaced by a dark-ground condenser, the crystal edges are highlighted.  This produces a dramatically different view of the structures.

Two photomicrographs of the same crystal field can be seen below.  Elliptically polarized light was used to form the left image, while plane polarized light was used in the right image. (Left: Crossed polars + two lambda/4 plates - Right: crossed polars)

Low magnification barely resolves individual crystals. (crossed polars)

A higher magnification reveals a mosaic of individual crystals in the field. (crossed polarsNote: The first image in the article is the same as the first image below, but it had Photoshop’sInvert (colour)” command used on it.)

These mosaic-like patterns can be striking in both form and colour.  (First image: crossed polars  – Second image: Crossed polars + two lambda/4 plates  –  Third image: Crossed polars + lambda/4 plate + lambda plate)

Here is another example of the colour transformations made possible by rotating one of the plates. (Crossed polars + rotated lambda/4 plate +  lambda plate)

The gaps between the (blue) mosaic pattern are filled with random crystalline “garbage”. (Crossed polars + two lambda/4 plates)

Cobaltous nitrate normally forms long, thin crystals (monoclinic) if they are allowed to grow freely without the constraints imposed by melt specimen preparation. (Crossed polars + lambda/4 plate +  lambda plate)

Notice the interesting detail on the light gray bridge feature at the center of the image below. (Crossed polars + two lambda/4 plates)

This same feature is shown at the top of the two images that follow.  Notice how the illumination can accentuate or de-accentuate particular details.  (First image: Crossed polars + two lambda/4 plates  –  Second image: Crossed polars)

As a chemistry teacher, I have often used cobaltous nitrate as a catalyst in senior chemistry experiments.  Not only does the aqueous solution have an attractive pink colour, but when the solid is melted and recrystallized,  the resulting formations are photogenic as well.

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 two illumination techniques: dark-ground, 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.

 All comments to the author Brian Johnston are welcomed.

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