Of these c?sia and rubidia are
rare, and lithia comparatively so. They are easily distinguished by
their spectra. They are characterised by the solubility of almost all
their salts in water, and, consequently, are found in the solutions from
which the earths and oxides of the metals have been separated by the
usual group re-agents.
The solution from which the other substances have been separated is
evaporated to dryness, and the product ignited to remove the ammonic
salts added for the purpose of separation. The residue contains the
alkali metals generally, as chlorides or sulphates. Before determining
the quantities of the particular alkali metals present, it is best to
convert them altogether, either into chloride or sulphate, and to take
the weight of the mixed salts. It is generally more convenient to weigh
them as chlorides. They are converted into this form, if none of the
stronger acids are present, by simply evaporating with an excess of
hydrochloric acid. Nitrates are converted into chlorides by this
treatment. When sulphates or phosphates are present, the substance is
dissolved in a little water, and the sulphuric or phosphoric acid
precipitated with a slight excess of acetate of lead in the presence of
alcohol. The solution is filtered, and the excess of lead precipitated
with sulphuretted hydrogen.
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