c., is boiled, and titrated in the manner
described.
Lead salts must be absent in the titration, and so must strontia and
lime. Ferrous salts should be peroxidised by means of permanganate or
chlorate of potash. Other salts do not interfere.
MAGNESIA.
Magnesia, the oxide of magnesium (MgO) occurs in nature in the rare
mineral periclase (MgO); and hydrated, as brucite (MgH_{2}O_{2}). As
carbonate it occurs in large quantity as magnesite (MgCO_{3}), which is
the chief source of magnesia. Mixed with carbonate of lime, it forms
magnesian limestone and dolomite. It is present in larger or smaller
quantity in most silicates; and the minerals, serpentine, talc, steatite
and meerschaum are essentially hydrated silicates of magnesia. Soluble
magnesian salts occur in many natural waters; more especially the
sulphate and the chloride. Kieserite (MgSO_{4}.H_{2}O) occurs in
quantity at Stassfurt, and is used in the manufacture of Epsom salts.
~Detection.~--Magnesia is best detected in the wet way. Its compounds
give no colour to the flame, and the only characteristic dry reaction is
its yielding a pink mass when ignited before the blowpipe (after
treatment with a solution of cobalt nitrate). In solution, it is
recognised by giving no precipitate with ammonia or ammonic carbonate in
the presence of ammonic chloride, and by giving a white crystalline
precipitate on adding sodium phosphate or arsenate to the ammoniacal
solution.
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