=108. General Principles.=—The methods for the chemical estimation of
sugars in common use depend on the reducing actions exerted on certain
metallic salts, whereby the metal itself or some oxid thereof, is
obtained. The reaction is either volumetric or the resulting oxid or
metal may be weighed. The common method is, therefore, resolved into
two distinct processes, and each of these is carried out in several
ways. Not all sugars have the faculty of exerting a reducing action
on highly oxidized metallic salts and the most common of them all,
_viz._, sucrose is practically without action. This sugar, however, by
simple hydrolysis, becomes reducing, but the two components into which
it is resolved by hydrolytic action do not reduce metallic salts in
the same proportion. Moreover, in all cases the reducing power of a
sugar solution is largely dependent on its degree of concentration, and
this factor must always be taken into consideration. Salts of copper
and mercury are most usually selected to measure the reducing power
of a sugar and in point of fact copper salts are almost universally
used. Copper sulfate and carbonate are the salts usually employed, and
of these the sulfate far more frequently, but after conversion into
tartrate. Practically, therefore, the study of the reducing action of
sugar as an analytical method will be confined almost exclusively to
the determination of its action on copper tartrate.
Direct gravimetric methods are also practiced to a limited extent in
the determination of sugars as in the use of the formation of sucrates
of the alkaline earths and of the combinations which certain sugars
form with phenylhydrazin. Within a few years this last named reaction
has assumed a marked degree of importance as an analytical method.
The most practical treatment of this section, therefore, for the
limited space which can be given it, will be the study of the reducing
action of sugars, both from a volumetric and gravimetric point of
view, followed by a description of the best approved methods of the
direct precipitation of sugars by such reagents as barium hydroxid and
phenylhydrazin.