Page 183 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

HAZARDS IN SAMPLING 5.6

figures will only possess real significance if the material is not appreciably
hygroscopic and no chemical changes, other than the loss of water, take place
on drying.
In a course of systematic quantitative analysis, such as that with which we
are chiefly concerned in the present book, the unknowns supplied for analysis
are usually portions of carefully analysed samples which have been finely ground
until uniform.
It should be borne in mind that although it is possible to generalise on
sampling procedures, al1 industries have their own established methods for
obtaining a record of the quantity and/or quality of their products. The sampling
procedures for tobacco leaves will obviously differ from those used for bales of
Cotton or for coal. But although the types of samples differ considerably the
actual analytical methods used later are of general application.

5.5 CRUSHING AND GRlNDlNG
If the material is hard (e.g. a sample of rock), it is first broken into small pieces
on a hard steel plate with a hardened hammer. The loss of fragments is prevented
by covering the plate with a steel ring, or in some other manner. The small
lumps may be broken in a 'percussion' mortar (also known as a 'diamond'
mortar) (Fig. 5.2). The mortar and pestle are constructed entirely of hard tool
steel. One or two small pieces are placed in the mortar, and the pestle inserted
into position; the latter is struck lightly with a hammer until the pieces have
been reduced to a coarse powder. The whole of the hard substance may be
treated in this manner. The coarse powder is then ground in an agate mortar
in small quantities at a time. A mortar of mullite is claimed to be superior to
one of agate: mullite is a homogeneous ceramic material that is harder, more
resistant to abrasion, and less porous than agate. A synthetic sapphire mortar
and pestle (composed essentially of a specially prepared form of pure aluminium
oxide) is marketed; it is extremely hard (comparable with tungsten carbide)
and will grind materials not readily reduced in ceramic or metal mortars.
Mechanical (motor-driven) mortars are available commercially.

Fig. 5.2

5.6 HAZARDS IN SAMPLING
The handling of many materials is fraught with hazards1° and this is no less
so when sampling materials in preparation for chemical analysis. The sampler
must always Wear adequate protective clothing and if possible have detailed
prior knowledge of the material being sampled. When dangers from toxicity