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Tin is usually said to be dyad in stannous compounds and a tetrad in stannic compounds, but in a compound like SnCl AmCl, is not tin really a tetrad? and yet it is a stannous compound, and gives a black precipitate with H S; so that valency does not necessarily go with the series.
The latter object is accomplished by oxidation to stannic chloride by means of mercuric chloride added in excess, as the mercuric salts have no effect upon ferrous iron or the bichromate. The reactions involved are: 2FeCl + SnCl > 2FeCl + SnCl SnCl + 2HgCl > SnCl + 2HgCl The mercurous chloride is precipitated.
It is essential that the solution should be cold and that the stannous chloride should not be present in great excess, otherwise a secondary reaction takes place, resulting in the reduction of the mercurous chloride to metallic mercury: SnCl + 2HgCl > SnCl + 2Hg. Heat at a temperature just below boiling until the undissolved residue is white or until solvent action has ceased.
Thus, tin may be trivalent, SnCl , or tetravalent, SnCl . Accordingly elements have been classed as monads, dyads, triads, etc.
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