Page 142 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES
of reviews of the procedures have been p~blished'~*'~ giving considerable
details of al1 aspects of the subject.
In outline the procedure consists of carefully weighing about 5-10 mg of
sample on to a shaped piece of paper (Fig. 3.1 1c) which is folded in such a way
that the tail (wick) is free. This is then placed in a platinum basket or carrier
suspended from the ground-glass stopper of a 500 mL or 1 litre flask. The flask,
containing a few millilitres of absorbing solution (e.g. aqueous sodium hydroxide),
is filled with oxygen and then sealed with the stopper with the platinum basket
attached.

Fig. 3.11 Conventional flasks for microdeterminations: (a) air leak design; (6) stopper
design; (c) filter paper for wrapping sample. Reproduced by permission from A. M. B.
Macdonald, in Advances in Analytical Chemistry and Instrumentation, C. N. Reilly (Ed.),
Vol. 4, Interscience, New York, 1965, p. 75.

The wick of the sample paper can either be ignited before the stopper is
placed in the flask neck, or better still ignited by remote electrical control, or
by an infrared lamp. In any case combustion is rapid and usually complete
within 5-10 seconds. After standing for a few minutes until any combustion
cloud has disappeared, the flask is shaken for 2-3 minutes to ensure that
complete absorption has taken place. The solution can then be treated by a
method appropriate to the element being determined.
Organic sulphur is converted to sulphur trioxide and sulphur dioxide by the
combustion, absorbed in hydrogen peroxide, and the sulphur determined as
sulphate.
The combustion products of organic halides are usually absorbed in sodium
hydroxide containing some hydrogen peroxide. The resulting solutions may be
analysed by a range of available procedures. For chlorides the method most
commonly used is that of argentimetric potentiometric titration18 (see Section
15.20), whilst for bromides a mercurimetric titrationlg is comparable with the
argentimetric method.
Phosphorus from organophosphorus compounds, which are combusted to
give mainly orthophosphate, can be absorbed by either sulphuric acid or nitric
acid and readily determined spectrophotometrically either by the molybdenum
blue method or as the phosphovanadomolybdate (Section 17.39).
Procedures have also been devised for the determination of metallic
constituents. Thus, mercury is absorbed in nitric acid and titrated with sodium
diethyldithiocarbamate, whilst zinc is absorbed in hydrochloric acid and
determined by an EDTA titration (see Section 10.65).