Making KBr pellets

Generally it is easy to make good KBr pellets if the evacuable die is used correctly, however some faults may occur for a variety of reasons. The faults described are for pure KBr or other halide salts that do not contaminate the sample.

When the sample is added to the halide salt the clarity of the disc will depend on the quantity and type of sample. Usually 0.1%-2% of weight to weight of sample to KBr is perfectly adequate. The overall quality of the pellet depends on the quality of KBr (or other halide salt) used which should always be spectroscopic grade purity.

Sample pellet not clear

Sample is damp, contaminated KBr or insufficient pressure when compacting

Pellet is clear, but has opaque spots

Powder not uniformly flat in the die, leaving large particles that do not vitrify when pressed. Sieve out large particles and re-press

Sample is cloudy

Insufficient evacuation time or leaky seals; Check the seals on the die, and increase evacuation time

Pellet is clear at first, but rapidly becomes cloudy

Damp powder or damp atmosphere; dry the KBr powder, check seals, and increase evacuation time. It is essential that the sample is thoroughly blended with the KBr; Blending can be achieved by using a pestle and mortar or a grinding mill. If the pellet breaks apart, then possibly not enough total mixture is being used in the die, and under compression it has become too thin and fragile. Excess moisture in the KBr and insufficient compaction can also cause problems. A 13mm die with a total sample/ KBr mix of 400mg, and a load of 6-7 tons will produce a disc of approximately 1mm thickness