Page 140 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON hPPARATUS AND BASIC TECHNIQUES

hydrofluoric acid is removed by evaporation with sulphuric acid leaving a
residue of metallic sulphates. Complexes of fluoride ions with many metallic
cations are very stable and so the normal properties of the cation may not be
exhibited. It is therefore essential to ensure complete removal of fluoride, and
to achieve this, it may be necessary to repeat the evaporation with sulphuric
acid two or three times. Hydrofluoric acid must be handled with great care; it
causes serious and painful burns of the skin.
Perchloric acid attacks stainless steels and a number of iron alloys that do
not dissolve in other acids. Perchloric acid must be used with great care; the hot
concentrated acid gives explosive reactions with organic materials or easily
oxidised inorganic compounds, and it is recommended that if frequent reactions
and evaporations involving perchloric acid are to be performed, a fume cupboard
which is free from combustible organic materials should be used. A mixture of
perchloric and nitric acids is valuable as an oxidising solvent for many organic
materials to produce a solution of inorganic constituents of the sample. For
safety in such operations, the substance should be treated first with concentrated
nitric acid, the mixture heated, and then careful additions of small quantities
of perchloric acid can be made until the oxidation is complete. Even then, the
mixture should not be evaporated because the nitric acid evaporates first
allowing the perchloric acid to reach dangerously high concentrations. If a
mixture of nitric, perchloric and sulphuric acids (3:l: 1 by volume) is used, then
the perchloric acid is also evaporated leaving a sulphuric acid solution of the
components to be analysed. In this operation the organic part of the material
under investigation is destroyed and the process is referred to as 'wet ashing'.
Substances which are insoluble or only partially soluble in acids are brought
into solution by fusion with the appropriate reagent. The most commonly used
fusion reagents, or fluxes as they are called, are anhydrous sodium carbonate,
either alone or, less frequently, mixed with potassium nitrate or sodium peroxide;
potassium pyrosulphate, or sodium pyrosulphate; sodium peroxide; sodium
hydroxide or potassium hydroxide. Anhydrous lithium metaborate has found
favour as a flux, especially for materials containing silica;12 when the
resulting fused mass is dissolved in dilute acids, no separation of silica takes
place as it does when a sodium carbonate melt is similarly treated. Other
advantages claimed for lithium metaborate are the following.
1. No gases are evolved during the fusion or during the dissolution of the melt,
and hence there is no danger of losses due to spitting.
2. Fusions with lithium metaborate are usually quicker (15 minutes will often
suffice), and can be performed at a lower temperature than with other fluxes.
3. The loss of platinum from the crucible is less during a lithium metaborate
fusion than with a sodium carbonate fusion.
4. Many elements can be determined directly in the acid solution of the melt
without the need for tedious separations.
Naturally, the flux employed will depend upon the nature of the insoluble
substance. Thus acidic materials are attacked by basic fluxes (carbonates,
hydroxides, metaborates), whilst basic materials are attacked by acidic fluxes
(pyroborates, pyrosulphates, and acid fluorides). In some instances an oxidising
medium is useful, in which case sodium peroxide or sodium carbonate mixed
with sodium peroxide or potassium nitrate may be used. The vesse1 in which
fusion is effected must be carefully chosen; platinum crucibles are employed for