Measurement techniques
Sample application
Liquid samples
It is recommended that liquid samples be transferred to the prism surface using a pipette rather
than a stirring rod or pouring directly from a beaker. After taking up the sample, any drips adhering
to the outside of the pipette should be wiped off then discharge a few drops from the pipette
directly onto the prism surface and close the prism box. This is of considerable importance when
taking concentration measurements since thin films adhering to a stirring rod and exposed to the
atmosphere can evaporate solvent rapidly when moved through the air, giving rise to errors in
measurement.
Solid Samples
These are applied in the same manner as the test piece using a contact liquid. A surface must be
prepared, polished as flat as possible and placed on the prism surface with the hinged prism opened
out of the way. If the solid has an index higher than 1.65, methylene iodide can be used as a contact
liquid (B&S code 10-61) in place of monobromonaphthalene which can only be used up to this limit.
Thin Films
Results may be obtained on most thin films but here a technique must be evolved, determined by
the material and conditions.
Direct Application (Reflection mode: High Accuracy models only)
Soft plastics and rubbery materials may be cured in a press between thin sheets of aluminium foil
and reduced to a thickness of about 0.25mm. After preparation ensure that the prism surface is
clean, strip off the foil on one side of the film and apply the exposed surface directly to the prism
using no contact liquid.
Indirect Application (Reflection mode: High Accuracy models only)
Resins and other low melting point solids are best prepared by melting them onto a thin glass
substrate (B&S code 10-59). After hardening, the substrate should be placed on the prism surface
with a contact liquid with the coated surface uppermost. Two borderlines will appear, one due to
the sample, the other due to the substrate which may be previously found and ignored. It is
essential that the refractive index of the substrate should be greater than that of the sample.
Dark samples (
Reflection mode:
High Accuracy models only)
With certain materials of a non-transparent nature, such as thick oils, tar, marzipan etc., too much
light may be absorbed in the sample film or be so scattered that definition is lost. In these cases, the
trouble can generally be overcome by using reflection mode.
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