Page 139 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

SOLUTION OF THE SAMPLE 3.31

the solution. If warming is necessary, it is usually best to carry out the dissolution
in a conical flask with a small funnel in the mouth; loss of liquid by spirting is
thus prevented and the escape of gas is not hindered. When using volatile
solvents, the flask should be fitted with a reflux condenser.
It may often be necessary to reduce the volume of the solution, or sometimes
to evaporate completely to dryness. Wide and shallow vessels are most suitable,
since a large surface is thus exposed and evaporation is thereby accelerated.
Shallow beakers of resistance glass, Pyrex evaporating dishes, porcelain basins
or casseroles, silica or platinum basins may be employed; the material selected
will depend upon the extent of attack of the hot liquid upon it and upon the
constituents being determined in the subsequent analysis. Evaporations should
be carried out on the steam bath or upon a low-temperature hot plate; slow
evaporation is preferable to vigorous boiling, since the latter may lead to some
mechanical loss in spite of the precautions to be mentioned below. During
evaporations, the vessel must be covered by a Pyrex clockglass of slightly larger
diameter than the vessel, and supported either on a large all-glass triangle or
upon three small U-rods of Pyrex glass hanging over the rim of the container.
Needless to Say, at the end of of the evaporation the sides of the vessel, the
lower side of the clockglass and the triangle and glass hooks (if employed)
should be rinsed with distilled water into the vessel.
For evaporation at the boiling point either a conical flask with a short Pyrex
funnel in the mouth or a round-bottomed flask inclined at an angle of about
45" may be employed; in the latter the drops of liquid, etc., thrown up by the
ebullition or by effervescence will be retained by striking the inside of the flask,
while gas and vapour will escape freely. When organic solvents are employed
the flask should be fitted with a 'swan-neck' tube and a condenser so that the
solvent is recovered.
Consideration must be given to the possibility of losses occurring during the
concentration procedure; for example, boric acid, halogen acids and nitric acid
are lost from boiling aqueous solutions.
Substances which are insoluble (or only slightly soluble) in water can often
be dissolved in an appropriate acid, but the possible loss of gaseous products
must be borne in mind. The evolution of carbon dioxide, hydrogen sulphide
and sulphur dioxide from carbonates, sulphides and sulphites respectively will
be immediately apparent, but less obvious are losses of boron and Silicon as
the corresponding fluorides during evaporations with hydrofluoric acid, or loss
of halogen by the treatment of halides with a strong oxidising agent such as
nitric acid.
Concentrated hydrochloric acid will dissolve many metals (generally those
situated above hydrogen in the electrochemical series), as well as many metallic
oxides. Hot concentrated nitric acid dissolves most metals, but antimony, tin
and tungsten are converted to slightly soluble acids thus providing a separation
of these elements from other components of alloys. Hot concentrated sulphuric
acid dissolves many substances and many organic materials are charred and
then oxidised by this treatment.
A mixture of hydrochloric and nitric acids (3:l by volume) known as
aqua regia is a very potent solvent largely due to its oxidising character, and
the addition of oxidants such as bromine or hydrogen peroxide frequently
increases the solvent action of acids.
Hydrofluoric acid is mainly used for the decomposition of silicates; excess