It is recommended that the reduction be accomplished in an atmosphere of hydrogen, but it is apparent that the use of kerosene is permissible in this case, and on account of its greater simplicity it is to be recommended as the best means of excluding the oxygen. The reduction is accomplished at a temperature of about 80°.
It is also proposed to reoxidize the copper by substituting a current of air for the hydrogen at the end of the reaction, and thus use the same copper a number of times. The danger of loss of ammonia, and the difficulty of determining when the oxidation is complete, render this regeneration of the reagent undesirable.
132. Method of Gerrard.—The method of Gerrard does not depend upon the use of ammonia, but the principle involved is the same, viz., the holding of the separated cuprous oxid in solution and the determination of the end of the reaction by the disappearance of the blue color. As first proposed by Gerrard, the copper sulfate solution is made of double the strength usually employed and to each 100 cubic centimeters thereof, before use, three and three-tenths grams of potassium cyanid are added. This is sufficient to hold the precipitated cuprous oxid in solution.[99]
The original method of Gerrard is found difficult of execution and the author, in conjunction with Allen, has lately modified it and reduced it to a practical working basis.[100]
In the new method the ordinary fehling solution is employed and it is prepared for use in the following way: Ten cubic centimeters of the fehling solution, or half that quantity of each of the component parts kept in separate bottles, are placed in a porcelain dish with forty cubic centimeters of water and brought to the boiling-point. To the boiling liquid is added, from a pipette, a five per cent solution of potassium cyanid until the blue color just disappears, or only a very faint tint of blue remains, avoiding any excess of the cyanid. A second portion of the fehling solution equal to that first employed is added, and to the boiling mixture the solution of sugar is added, from a burette, until the blue color disappears. The contents of the dish should be kept boiling during the addition of the sugar solution. The volume used will contain fifty milligrams of dextrose. The sugar solution should be of such a strength as to contain no more than half a per cent of reducing sugar.
The principle of the preparation of the solution may be stated as follows: If to a solution of copper sulfate, potassium be added until the blue color disappears, a double cyanid of copper and potassium cyanid is formed according to the following reaction:
CuSO₄ + 4KCN = Cu(CN)₂.2KCN + K₂SO₄.
This double cyanid is a salt of considerable stability. It is not decomposed by alkalies, hydrogen or ammonium, sulfid. With mineral acids it gives a whitish, curdy precipitate. With fehling solution the same double cyanid is formed as that described above. If, however, fehling solution be present in excess of the amount necessary to form the double cyanid of copper, this excess can be used in the oxidation of reducing sugar and the colorless condition of the solution will be restored as soon as the excess of the fehling is destroyed. The double cyanid holds in solution the cuprous oxid formed and thus complete decoloration is secured.
133. Sidersky’s Modification of Soldaini’s Process.—In all cases where the sugar solutions are not too highly colored, Sidersky finds that the method of reduction in a large test tube, as practiced by Violette, is applicable with the copper carbonate solution.[101] For more exact work it is preferred to determine the quantity of copper reduced by an indirect volumetric method. The sugar solution, properly clarified and the lead removed if subacetates have been used, is made of such a volume as to contain less than one per cent of reducing sugars. In a flask or large test tube are placed 100 cubic centimeters of the copper solution, which is boiled for a short time and the sugar solution added, little by little, from a pipette, at such a rate as not to stop the ebullition. The boiling is continued for five minutes after the last addition of the sugar. The vessel is taken from the flame and 100 cubic centimeters of cold water added, the whole brought on an asbestos felt and the cuprous oxid washed with hot water until the alkaline reaction has disappeared. The residual cuprous oxid is dissolved in a measured quantity of set sulfuric acid, semi- or fifth-normal, a few particles of potassium chlorate added, and the mixture boiled to convert any cuprous into cupric sulfate. The reaction is represented by the following formula:
3Cu₂O + 6H₂SO₄ + KClO₃ = 6CuSO₄ + KCl + 6H₂O.