of the sample for the complete oxidation-reducing pro-
cess between the Cu-complex and the reducing sugars
(see reaction 2). According to the existing literature,
the duration of this reaction is 3 min under direct heat-
ing of the sample. Such heating, however, contains a
certain risk of loss of part of the analysis sample due to
sprinkling of the solution during the boiling.
Therefore, we heated the sample in a water bath
and determined that the results after 5min heating in
the water bath are identical to those obtained with 3
min direct heating of the sample.
The high alkalinity of the sample after adding
the solutions of Fehling (pH 10-11) made necessary the
utilization of a more concentrated acetic acid buffer
2 mol l-1. This creates suitable conditions for the comple-
tion of the complexometric reaction between Cu 2+ and
EDTA (reaction 3). By means of experiments, we proved
that 8 ml 2 mol l -1 acetate buffer are sufficient for this
purpose.
The volume of the solutions of Fehling I and
II through which Cu 2+ is added to the sample was
also determined. 25 ml volume of both solutions is
sufficient in order for part of Cu 2+ to be able to react
with the reducing sugars and at the same time enough
surplus of Cu 2+ to remain in the solution. This sur-
plus is titrated by approximately 20 30 ml solution
of EDTA.
After determing the optimal conditions of the
analysis we investigated the ratio Creducing sugars= f (VS),
where Creducing sugars is the concentration of the reducing
sugars in the sample, VS is the volume of the sample.
The graphical representation of the function is given in
Fig. 1 The strictly linear relationship Creducing sugars= f (VS),
passing through the origin of the coordinate system
proves the quantitative realization of reactions (1), (2)
and (3) under the specified conditions.