109. Classification.—Among the volumetric methods will be given those which are in common use or such as have been approved by the practice of analysts. Since the use of mercuric salts is now practiced to a limited extent, only a brief study of that process will be attempted. With the copper methods a somewhat extended description will be given of those depending on the use of copper sulfate, and a briefer account of the copper carbonate process.
In the copper sulfate method two distinct divisions must be noted, viz., first an indirect process depending first upon the reduction of the copper to a suboxid, the subsequent action of this body on iron salts, measured finally by titration with potassium permanganate; and second, a direct process determined either by the disappearance of the blue color from the copper solution, or by the absence of copper from a drop of the solution withdrawn and tested with potassium ferrocyanid. This last mentioned reaction is one which is found in common use. The volumetric methods are not, as a rule, as accurate as the gravimetric, depending on weighing the resultant metal, but they are far more rapid and well suited to technical control determinations.
110. Reduction of Mercuric Salts.—The method of determining sugar by its action on mercuric salts, is due to Knapp.[70] The method is based on the observation that dextrose and other allied sugars, will reduce an alkaline solution of mercuric cyanid, and that the mercury will appear in a metallic state.
The mercuric liquor is prepared by adding to a solution of ten grams of mercuric cyanid, 100 cubic centimeters of a solution of caustic soda of 1.145 specific gravity, and making the volume to one liter with water. The solution of sugar to be titrated, should be as nearly as possible of one per cent strength.
To 100 cubic centimeters of the boiling solution, the sugar solution is added in small portions from a burette and in such a way as to keep the whole mass in gentle ebullition.
To determine when all the mercuric salt has been decomposed, a drop of the clear boiling liquid is removed and brought into contact with a drop of stannous chlorid solution on a white surface. A brownish black coloration or precipitate will indicate that the mercury is not all precipitated. Fresh portions of the sugar must then be added, until no further indication of the presence of mercury is noted. The approximate quantity of sugar solution required to precipitate the mercury having thus been determined, the process is repeated by adding rapidly, nearly the quantity of sugar solution required, and then only a few drops at a time, until the reduction is complete.
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: