Crystals
Preparing crystals on the slide
A wide variety of substances, both organic and inorganic, can be crystallized. Crystals hold important clues to the chemical makeup of the substance of which they are composed. Crystals are also beautiful. Because many crystals are invisible to the unaided eye, the microscope is a gateway to their study, which is called crystallography. For the deep study of crystals, a petrographic polarizing microscope is needed, but several tests can be done with an ordinary microscope. First, we need a way of forming crystals on a slide. This can be done in several ways.
Crystals from Evaporation
The advantage of forming crystals by evaporation is that their formation can be observed through the microscope.
In a watch glass, add the material to be crystallized to the solvent liquid (often water) until no more of the material will dissolve. It sometimes helps if the liquid is warm. Place a drop of the liquid on a slide. Very slowly warm the slide from below with a flame to evaporate some liquid. Crystals can be observed building at the edge of the drop. Begin observation under the microscope, and, as observation continues, occasionally stir the crystals from the edge into the drop to act as multiple seeds -- crystals from which more crystals can form.
As the drop begins to dry, it helps to recrystallize. Add a bit more solvent, but not enough to dissolve all of the crystals. This will allow longer observation of the separate crystals before they begin to grow together into larger structures.
Vitamin C (ascorbic acid) is an excellent substance for a first experiment with this technique. Tablets can be crushed and then dissolved, but vitamin C powder is easier to use and more pure. Viewed with polarized illumination at 40X to 100X, the substance produces wildly varied colors and patterns. Sugar also makes interesting crystals, but dries too slowly to be watched during crystallization. As noted in the section on staining above, a drop of almost any biological stain will form crystals. Epsom salts and Bromo Seltzer also make interesting crystals using this technique.
Crystals from precipitation
Reactive substances can be brought together to make crystals. In most cases, solids must be put into solution before appreciable reactions will take place. Very dilute concentrations of each substance usually result in the best crystals. Place a drop of each solution on the slide. Encourage the drops to slowly flow into one another. Uneven mixing will make it easier to distinguish crystals formed by evaporation from crystals formed by precipitation. If the drops are too close together they will mix too quickly and evenly. With some substances it helps to encourage the chemical reaction by warming the slide from below.
Remember that some reactions may release enough vapor to harm you or the objective. Work in a ventilated area and do not use powerful objectives with short focal lengths or oil immersion objectives when observing chemical reactions.
As a first experiment with this method, try mixing a weak ascorbic acid solution with a very weak sodium hydroxide solution.
Crystals from Sublimation
The figure shows one method of sublimating from slide to
slide.A coin is clipped between two slides with a clothes pin. The
coin serves as a spacer between the two slides. The thinnest coin
that will separate the slides and the material to be sublimated
should be used.
Material is sublimated from the bottom slide to the
top. The material is placed on the bottom slide near one of the
corners opposite the clothes pin. If the material is solid, simply
put a flattened pinch on the slide. If a solution is being used,
repeatedly place a drop of solution on the slide in the same place,
allowing each drop to dry before applying the next. The top slide
must be very clean. Apply heat with a small flame from below,
starting at a point away from the corner. This allows the material
to heat more slowly and form more perfect crystals on the upper
slide. The flame is slowly moved nearer the material until a
deposit forms on the top slide.
When a small deposit is formed, set the top slide aside and replace it with a new one. Repeat the process with a new slide until no more deposits are formed. Sublimation is similar to distillation in its ability to separate components of materials. Each successive top slide will have crystals sublimated from increasingly less volatile components of the material being used.
Take care not to scratch the condenser lens when viewing slides with specimens near the corner, and take precautions against toxicity when applying heat.
Try coffee and the contents of multivitamin capsules. You should be able to extract several kinds of crystals from each.
Crystals from Fusion
To form crystals from fusion, place a bit of dry material near the corner of a slide under a cover slip. Then apply heat from below and away from the corner. Increase the heat by moving the flame toward the corner until the material melts under the cover slip.
The contents of multivitamin capsules make an interesting fusion study. Different substances in the powder will melt and resolidify at different temperatures, creating complex patterns. Urea, citric acid, and dextrose also form interesting crystals.
Testing Refractive Index of Crystals
By trying a number of different mounting liquids, the approximate refractive index of a crystal can be determined. One can compare tables of the refractive indices of mounting liquids and refractive indices of crystals to help determine the identity of the crystal. A large collection of graduated mounting media are needed for exacting work. However, you can at least make rough estimates and eliminate many crystalline substances from consideration using commonly available media.
If you have a sample of a crystal, it may be possible to mix a mounting medium that has the crystal's exact refractive index. Using this medium you can test other crystals for this index. Some experimentation may be required to find a medium that does not dissolve the crystal.
Here is a list of useful refractive indices for commonly
available mounting media:
air 1.00
water 1.33
acetone 1.35
ethyl alcohol 1.36
kerosine 1.45
glycerin 1.46
olive oil 1.47
turpentine 1.475
castor oil 1.48
immersion oil 1.51
Canada balsam 1.53
clove oil 1.53
The Becke test is one way to determine whether a crystal's
refractive index is higher, lower, or the same as the medium that
surrounds it. The test is done in the following way: Set up
brightfield illumination and stop down the aperture diaphragm until
higher than usual contrast results, including some diffraction
artifacts. Find and focus on the edge of a crystal with a high
power objective. Slowly separate the objective from the specimen
until a bright border of light appears around the edge of the
crystal.
The figure shows how the bright border moves to the high index
medium as the objective is raised, to the low index medium as the
objective is lowered. If the crystal is invisible, the two mediums
are of almost exactly the same refractive index.
Oblique illumination can also be used to test the refractive
index of materials. Assume that we set up oblique illumination so
that the light strikes the specimen from below and to the north.
Focus on the specimen. Because the rays of light moving past the
objective cross, it will appear that the light is coming from the
south instead of the north.
The figure shows that if the crystal
has the higher index, it will appear to be lit on the north side of
the crystal and shadowed on the south; if the crystal has lower
refractive index, it will appear to be lit on the south side of the
crystal and shadowed on the north. This is because the facets of a
crystal of higher index than the mounting medium condense light;
the facets of a crystal of lower refractive index than
the mounting medium disperse light.
If the crystal is doubly refractive, polarizers must be used for the tests. Under crossed polarizers, rotate the crystal on the stage until it is darkest -- at extinction. Remove either the analyzer or the polarizer and perform the tests as usual. Then rotate either the crystal or the remaining polarizer 90 degrees and repeat the tests for the other refractive index. Different parts of the crystal may exhibit different sets of indices, so everything from observations of many crystals should be recorded. It helps to make a drawing the orientation of the crystals with each test.
Permanent mounts of crystals
Crystals are sometimes mounted dry; the difference between their refractive index and air is going to be larger than the difference between their refractive index and any permanent mounting medium. The crystal therefore seems to have a higher relief and is more easily seen in unpolarized light. A ring of mounting medium is placed around the crystals, and a cover glass is pressed into the medium. When there is no need for the crystal to show up under normal light, the crystal will last longer in a normal mount, assuming that the mounting liquid is not a solvent for the crystal and is not birefringent.
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