Part 18

One hundred cubic centimeters of the mercuric cyanid solution prepared as directed above, will be completely reduced by

202 milligrams of dextrose, 200 ” ” invert sugar, 198 ” ” levulose, 308 ” ” maltose, 311 ” ” lactose.

By reason of the unpleasant odor of the boiling mercuric cyanid when in presence of a reducing agent, the process should be conducted in a well ventilated fume chamber. With a little practice the process is capable of rapid execution, and gives reasonably accurate results.

=111. Sachsse’s Solution.=—The solution of mercuric salts proposed by Sachsse, is made by dissolving eighteen grams of mercuric iodid in twenty-five cubic centimeters of an aqueous solution of potassium iodid. To this solution are added 200 cubic centimeters of potash lye, containing eighty grams of caustic potash. After mixing the solution, the volume is completed to one liter. The sugar solutions used to reduce this mixture, should be more dilute than those employed with the mercuric cyanid, and should not be over one-half per cent in strength. The end of the reduction is determined as already described. After a preliminary trial, nearly all the sugar necessary to complete reduction, should be added at once, and the end of the reduction then determined by the addition of successive small quantities. One hundred cubic centimeters of the mercuric iodid solution prepared as directed above, require the following quantities of sugar to effect a complete reduction:

325 milligrams of dextrose, 269 ” ” invert sugar, 213 ” ” levulose, 491 ” ” maltose, 387 ” ” lactose.

By reason of the great difference between the reducing power of dextrose and levulose in this solution, it has been used in combination with the copper reduction method, to be described, to determine the relative proportion of dextrose and levulose in a mixture.

It is now known that copper solutions require slightly different quantities of dextrose, levulose, or invert sugar to effect complete reduction, but the variations are not great and in the calculation above mentioned, it may be assumed that these differences do not exist.

Instead of using stannous chlorid as an indicator, the end of the reaction may be determined as follows: A disk of filtering paper is placed over a small beaker containing some ammonium sulfid. A drop of the clear hot solution is placed on this disk, and if salts of mercury be still present a dark stain will be produced; or a drop of the ammonium sulfid may be brought near the moist spot formed by the drop of mercury salt. An alkaline solution of zinc oxid may also be used.

The methods depending on the use of mercuric salts have, of late, been supplanted by better processes, and space will not be given here to their further discussion.

=112. The Volumetric Copper Methods.=—The general principle on which these methods depend, is found in the fact that certain sugars, notably, dextrose, (glucose), levulose, (fructose), maltose and lactose, have the property of reducing an alkaline solution of copper to a lower state of combination, in which the copper is separated as cuprous oxid. The end of the reaction is either determined by the disappearance of the blue color of the solution, or by the reaction produced by a drop of the hot filtered solution, when placed in contact with a drop of potassium ferrocyanid acidified with acetic.

The copper salt which is found to give the most delicate and reliable reaction, is the tartrate. The number of volumetric processes proposed and which are in use, is very great, and an attempt even to enumerate all of these can not be made in this volume. A few of the most reliable and best attested methods will be given, representing if possible, the best practice in this and other countries. The rate of reduction of the copper salt to suboxid, is influenced by the rate of mixing with the sugar solutions, the temperature, the composition of the copper solution and the strength of the sugar solution.

The degree of reduction is also modified by the rate at which the sugar solution is added, and by the degree and duration of heating, and all these variables together, make the volumetric methods somewhat difficult and their data, to a certain extent, discordant. By reason, however, of the ease with which they are applied and the speed of their execution, they are invaluable for approximately correct work and for use in technical control.

=113. Historical.=—It is not the purpose in this paragraph to trace the development of the copper reduction method for the determination of reducing sugars, but only to refer to the beginning of the exact analytical application of it.

Peligot, as early as 1844, made a report to the Society for the Encouragement of National Industry on methods proposed by Barreswil and Fromherz for the quantitive estimation of sugar by means of copper solution. These methods were based on the property of certain sugars to reduce alkaline copper solution to a state of cuprous oxid first announced by Trommer. This was followed by a paper by Falck on the quantitive determination of sugar in urine.

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.

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. 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. 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.

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.

=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.

Following is the formula for the preparation of the fehling solution:

Pure crystallized copper sulfate CuSO₄.5H₂O, 34.64 grams: Potassium tartrate, 150.00 ” Sodium hydroxid, 90.00 ”

The copper sulfate is dissolved in water and the potassium tartrate in the aqueous solution of the sodium hydroxid which should have a volume of about 700 cubic centimeters. The two solutions are mixed and the volume completed to a liter. Each cubic centimeter of this solution will be reduced by five milligrams of dextrose, equivalent to four and a half milligrams of sucrose.

The reaction which takes place is represented by the following molecular proportions:

C₆H₁₂O₆ = 10CuSO₄.5H₂O Dextrose. Copper sulfate. 180 2494