Page 141 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SOLUTION OF THE SAMPLE 3.31

sodium carbonate, lithium metaborate and potassium pyrosulphate; nickel or
silver crucibles, for sodium hydroxide or potassium hydroxide; nickel, gold,
silver, or iron crucibles for sodium carbonate and/or sodium peroxide; nickel
crucibles for sodium carbonate and potassium nitrate (platinum is slightly
attacked).
For the preparation of samples for X-ray fluorescence spectroscopy, lithium
metaborate is the preferred flux because lithium does not give rise to interfering
X-ray emissions. The fusion may be carried out in platinum crucibles or in
crucibles made from specially prepared graphite: these graphite crucibles can
also be used for the vacuum fusion of metal samples for the analysis of occluded
gases.
To carry out the fusion, a layer of flux is placed at the bottom of the crucible,
and then an intimate mixture of the flux and the finely divided substance added;
the crucible should be not more than about half-full, and should, generally, be
kept covered during the whole process. The crucible is very gradually heated
at first, and the temperature slowly raised to the required temperature. The final
temperature should not be higher than is actually necessary; any possible further
attack of the flux upon the crucible is thus avoided. When the fusion, which
usually takes 30-60 minutes, has been completed, the crucible is grasped by
means of the crucible tongs and gently rotated and tilted so that the molten
material distributes itself around the walls of the container and solidifies there
as a thin layer. This procedure greatly facilitates the subsequent detachment
and solution of the fused mass. When cold, the crucible is placed in a casserole,
porcelain dish, platinum basin, or Pyrex beaker (according to the nature of the
flux) and covered with water. Acid is added, if necessary, the vessel is covered
with a clockglass, and the temperature is raised to 95-100 OC and maintained
until solution is achieved.
Many of the substances which require fusion treatment to render them soluble
will in fact dissolve in minera1 acids if the digestion with acid is carried out
under pressure, and consequently at higher temperatures than those normally
achieved. Such drastic treatment requires a container capable of withstanding
the requisite pressure, and also resistant to chemical attack: these conditions
are met in acid digestion vessels (bombs). These comprise a stainless-steel
pressure vessel (capacity 50 mL) with a screw-on lid and fitted with a Teflon
liner. They may be heated to 150-180°C and will withstand pressures of
80-90 atmospheres; under these conditions decomposition of refractory materials
may be accomplished in 45 minutes. Apart from the saving in time which is
achieved, and the fact that the use of expensive platinum ware is obviated, other
advantages of the method are that no losses can occur during the treatment,
and the resulting solution is free from the heavy loading of alkali metals which
follows the usual fusion procedures. A recent modification is the construction
of vessels made entirely of Teflon which can be heated in a microwave oven,
with even more rapid reaction times. A full discussion of decomposition
techniques is given in Ref. 13.
A decomposition procedure applicable to organic compounds containing
elements such as halogens, phosphorus or sulphur, consists in combustion of
the organic material in an atmosphere of oxygen; the inorganic constituents
are thus converted to forms which can be determined by titrimetric or
spectrophotometric procedures. The method was developed by Sch~niger'~*'~
and is usually referred to as the Schoniger Oxygen Flask Method. A number