Nickel II nitrate is commonly used in the electroplating industry as a supply of nickel ions that are reduced to shiny nickel metal atoms on the surface of “nickel-plated” utensils.  Older chemists would have used the archaic “nickelous nitrate” name to refer to the compound.  It is usually supplied as nickel II nitrate hexahydrate crystals having a transparent green colour.  (The hexahydrate term means that six water molecules are associated with each nickel nitrate molecule in the crystal lattice.)

The compound has a very low melting temperature, 57 degrees Celsius, and therefore makes a good melt specimen.  Keep in mind however, that it is a very strong oxidizer, and contact with other materials may cause fire or explosion.  It is harmful if swallowed or inhaled, and may cause contact dermatitis.  Nickel II nitrate is also thought to cause cancer when a person is exposed over a long period.

Although a structural formula and molecular shape can be drawn for such a compound, the results are misleading.  Nickel II nitrate forms crystals, with the nickel ions and nitrate ions arranged in a rigid three-dimensional lattice.  An example crystal lattice is shown below.  (“HyperChem” was used to produce the first two images, and “CrystalMaker” generated the third.)

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.

Nickel II nitrate melt specimens tend to be difficult to photograph since the best sections tend to be rather thick.  The human eye, (and brain), do a wonderful job of coping with a variety of focus planes.  Cameras do not!  The auto-focus, (which I use constantly when taking photomicrographs), sometimes chooses the wrong plane to focus on.  Experience with a particular camera usually provides a way to “trick” the mechanism to focus on the desired plane by using a high contrast feature in the plane as the focus point.

The two polarized light images below show typical fields.  Careful examination reveals tiny “perfect” parallelogram-shaped crystals sprinkled over both crystalscapes.

A higher magnification reveals that the “perfect” crystals are actually quite “imperfect”.  Corners are sometimes missing, and bubbles often form within the growing crystal.

Other areas of a slide may contain intricately mottled sections like the one below.  (Both images are of exactly the same field.)  Compensators were used to alter the colours.

Most areas contain circular, or elongated “specks”.  The right image shows a magnified picture of an area in the left image.  Again, compensators were used to produce a different colouration.

The transmitted light image on the left is rather boring when compared to a polarized light image of the identical field shown on the right.

Three images of similar areas are shown below.

Dark-ground illumination can be used to show a different perspective of the thicker sections of the melt.  A polarized light image, (using compensators), of the same field is shown on the right.

Dark-ground illumination provides a clear view of the “perfect” parallelogram-shaped crystals in a particular field.

The thicker areas of the melt also provide unusual views with phase contrast illumination.

Nickel II nitrate hexahydrate, in my very humble opinion, is not one of the “stars” of the crystal universe.  It requires more effort, and produces less photogenic images than most other compounds that I have photomicrographed.