In 1848 the methods, which have been proposed, were critically examined by Fehling, and from the date of his paper the determination of sugar by the copper method may be regarded as resting on a scientific basis.[75]
Since the date mentioned the principal improvements in the process have been in changing the composition of the copper solution in order to render it more stable, which has been accomplished by varying the proportions of copper sulfate, alkali and tartaric acid. For the better keeping of the solution the method of preserving the copper sulfate and the alkaline tartrates in separate flasks and only mixing them at the time of use has been found very efficacious.[76] For testing for the end of the reaction by means of an acetic acid solution of potassium ferrocyanid the filtering tube suggested by the author, the use of which will be described further on, has proved quite useful. Pavy has suggested that by the addition of ammonia to the copper solution the precipitated suboxid may be kept in solution and the end of the reaction thus easily distinguished by the disappearance of the blue color.[77] Allen has improved on this method by covering the hot mixture with a layer of paraffin oil whereby any oxidation of the suboxid is prevented.[78]
The introduction and development of the gravimetric process depending on securing the reduced copper oxid in a metallic state as developed by Allihn, Soxhlet, and others, completes the resumé of this brief sketch of the rise and development of the process.
114. Action of Alkaline Copper Solution on Dextrose.—The action to which dextrose and other reducing sugars are subjected in the presence of a hot alkaline copper solution is two-fold in its nature. In the first place there is an oxidation of the sugar which is transformed into tartronic, formic and oxalic acids, the two latter in very small quantities. At the same time another part of the sugar is attacked directly by the alkali and changed to complex products among which have been detected lactic, oxyphenic and oxalic acids, also two bodies isomeric with dioxyphenolpropionic acid. When the sugar is in large excess melassic and glucic acids have also been detected. The glucic acid may be regarded as being formed by simple dehydration but becomes at once resolved into pyrocatechin and gluconic acid according to the reaction C₁₂H₁₈O₉ = C₆H₆O₂ + C₅H₁₂O₇. The gluconic acid also is decomposed and gives birth to lactic and glyceric acids according to the formula C₆H₁₂O₇ = C₃H₆O₃ + C₃H₆O₄. The glyceric acid also in the presence of a base is changed into lactic and oxalic acids. Between lactic acid and pyrocatechin, existing in a free state, there is produced a double reciprocal etherification in virtue of which there arise two ethers isomeric with hydrocaffeic acid, C₉H₁₀O₄. One of these bodies is an acid and corresponds to the constitution
- CH₃
- /
- O ── CH
- / \
- C₆H₄ CO₂H (2)
- \
- OH (2)
and the other is of an alcoholic nature corresponding to the formula
- CO₂ ── CHOH ── CH₃ (1)
- /
- C₆H₄
- \
- OH₂ (2)
Of all these products only oxyphenic and lactic acids and their ethers and oxalic acid remain unchanged and they can be isolated. All the others are transformed in an acid state and they can only be detected by operating in the presence of metallic oxids capable of precipitating them at the time of their formation.[79]
115. Fehling’s Solution.—The copper solution which has been most used in the determination of reducing sugars is the one proposed by Fehling as a working modification of the original reagent used by Trommer.[80]
Following is the formula for the preparation of the fehling solution: