Page 14 - A HANDBOOK OF ORGANIC ANALYSIS QUALITATIVE AND QUANTITATIVE

PRELIMINARY INVESTIGATION II

indicates the presence of a sulphide in the solution. Since
all organic sulphur compounds yield sodium sulphide by the
reducing action of sodium at high temperatures, this extremely
sensitive test may be taken as proof of the presence of
sulphur in the substance.
Should no sulphur or nitrogen be detected, a third portion
of the filtrate should be rendered acid with nitric acid, and
silver nitrate added. A precipitate of silver halide indicates
the presence of halogen in the substance. When sulphur or
nitrogen are present, this portion of the filtrate must, before
adding the silver nitrate, be boiled with excess of dilute
nitric acid for about :five minutes in order to remove the
hydrogen sulphide or cyanide from the solution. The silver
halides may be identified or separated. by the standard
methods, The presence of halogen may be confirmed by
heating a copper 'wire in an oxidising flame until the green
colour is no longer perceptible. A minute portion of the
substance is now placed on the wire, which is again held in
the flame ; should halogen be present in the substance, the
flame will again be coloured green, owing to the formation
of volatile copper halide. This test is extremely sensitive,
and care must be taken not to contaminate the wire with
laboratory dust or the fingers after its initial heating.
Phosphorus may be detected by either of the following
methods :--(1) A small quantity of the substance is heated
with a mixture of concentrated sulphuric and nitric acids
until a clear solution is obtained. On diluting with water,
and filtering if necessary, the solution is boiled with an excess of
ammonium nitrate, cooled, and ammonium molybdate solution
added to the mixture. On gentle warming a yellow precipitate
of ammonium phosphomolybdate indicates the presence of
phosphorus. (2) Small portions of the substance are carefully
added to a fused mixture of equal parts of potassium carbonate
and potassium nitrate in a nickel crucible, and the mixture
heated until effervescence ceases, The cooled melt is dis-
solved in excess of dilute nitric acid, and ammonium molyb-

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