| 2C₆H₁₂O₆ 360 parts | = | 5Cu₂O 715 parts | = | 10FeSO₄ 1520 parts | = | K₂Mn₂O₈ 316.2 parts |
It is thus seen that 316.2 parts by weight of potassium permanganate are equivalent to 360 parts by weight of dextrose; or one part of permanganate corresponds to 1.1385 parts by weight of dextrose. If, therefore, the amount of permanganate required in the above reaction to restore the iron to the ferric condition, be multiplied by the factor mentioned above, the quotient will represent in weight the amount of dextrose which enters into the reaction. The standard solution of potassium permanganate should contain 4.392 grams of the salt in a liter. One cubic centimeter of this solution is equivalent to five milligrams of dextrose.
121. Manipulation.—The saccharine solution whose strength is to be determined should contain approximately about one per cent of sugar. Of this solution ten cubic centimeters are placed in a porcelain dish together with a considerable excess of fehling solution. When no sucrose is present, the mixture may be heated to the temperature of boiling water and kept at that temperature for a few minutes until all the reducing sugar is oxidized. There should be enough of the copper solution used to maintain a strong blue coloration at the end of the reaction. A greater uniformity of results will be secured by using in all cases a considerable excess of the copper solution. When sucrose or other non-reducing sugars are present, the temperature of the reaction should not be allowed to exceed 80° and the heating may be continued somewhat longer. At this temperature the copper solution is absolutely without action on sucrose. The precipitated suboxid is allowed to settle, the supernatant liquid poured off through a filter and the suboxid washed thoroughly a number of times by decantation with hot water, the washings being poured through the filter. This process of washing is greatly facilitated by decanting the supernatant liquid from the porcelain dish first into a beaker and from this into a third beaker and so on until no suboxid is carried off. Finally the wash water is poured through a filter-paper bringing as little as possible of the suboxid onto the paper. The suboxid on the filter-paper and in the beakers is next dissolved in a solution of ferric sulfate made strongly acid with sulfuric; or in a sulfuric acid solution of ammonia ferric sulfate which is more easily obtained free from impurities than the ferric sulfate. When all is dissolved from the beakers the solution is poured upon the suboxid which still remains in the porcelain dish. When the solution is complete it is washed into a half liter flask and all the vessels which contain the suboxid are also thoroughly washed and the wash waters added to the same flask. The whole is rendered strongly acid with sulfuric and made up to a volume of half a liter.
The process carried out as directed, when tested against pure sugar, gives good results, not varying from the actual content of the sugar by more than one-tenth per cent below or three-tenths above the true content. The distinct pink coloration imparted to the solution by the permanganate solution as soon as the iron is all oxidized to the ferric state marks sharply the end of the reaction. In this respect this process is very much to be preferred to the usual volumetric processes depending upon the coloration produced with potassium ferrocyanid by a copper salt for distinguishing the end of the reaction. It is less convenient than the ordinary volumetric process by reason of the somewhat tedious method of washing the precipitated cuprous oxid. When a large number of analyses is to be made, however, the whole can be washed with no more expenditure of time than is required for a single sample. One analyst can, in this way, easily attend to fifty or a hundred determinations at a time.
In the application of the permanganate method to the analysis of the juices of sugar cane and sorghum it is directed to take 100 cubic centimeters of the expressed juice and clarify by the addition of twenty-five cubic centimeters of basic lead acetate, diluted with water, containing enough of the lead acetate, however, to produce a complete clarification. It is not necessary to remove the excess of lead from the filtrate before the determination. Ten cubic centimeters of the filtrate correspond to eight cubic centimeters of the original juice. For percentage calculation the specific gravity of the original juice must be known. Before the addition of the alkaline copper solution, from fifty to seventy-five cubic centimeters of water should be added to the clarified sugar juice and the amount of fehling solution used in each case should be from fifty to seventy-five cubic centimeters. The heating at 75° should be continued for half an hour in order to insure complete reduction and oxidation of the sugar. The sucrose can also be estimated in the same juices by inverting five cubic centimeters of the clarified juice with five cubic centimeters of dilute hydrochloric acid, by heating for an hour at a temperature not above 90°. Before adding the acid for inversion, about 100 cubic centimeters of water should be poured over the five cubic centimeters of sugar solution. The washing of the suboxid and the estimation of the amount reduced are accomplished in the manner above described.
This method has been extensively used in this laboratory and with very satisfactory results. The only practical objection which can be urged to it is in the time required for filtering. This fault is easily remedied by adopting the method of filtering through asbestos felt described in the next paragraph.
For the sake of uniformity, however, the copper solution should be boiled for a few minutes before the addition of the sugar in order to expel all oxygen, the sugar solutions should be made with recently boiled water and the precipitation of the suboxid should be accomplished by heating for just thirty minutes at 75°. At the end of this time an equal volume of cold, recently boiled, water should be added and the filtration at once accomplished.
122. Modified Permanganate Method.—The permanganate method as used by Ewell, in this laboratory, is conducted as follows: After the precipitate is obtained, according to the directions given in the methods described, it is thoroughly washed with hot, recently boiled water, on a gooch. The asbestos, with as much of the precipitate as possible, is transferred to the beaker in which the precipitation was made, beaten up with from twenty-five to thirty cubic centimeters of hot, recently boiled water, and from fifty to seventy-five cubic centimeters of a saturated solution of ferric sulfate in twenty-five per cent sulfuric acid are added to the beaker and then poured through the crucible to dissolve the cuprous oxid remaining therein. If the precipitate be first beaten up with water as directed, so that no large lumps of it remain, there is no difficulty in dissolving the oxid in the ferric salt; while if any lumps of the oxid be allowed to remain there is great difficulty. After the solution is obtained, it is titrated with a solution of potassium permanganate of such a strength that each cubic centimeter is equivalent to 0.01 gram of copper.
In triplicate determinations made by this method the precipitates obtained required after solution in the ferric salt, 28.7, 28.9, 28.6 cubic centimeters of potassium permanganate solution, respectively. For the quantities taken this was equivalent to an average percentage of reducing sugars of 4.19. The percentage obtained by the gravimetric method was 4.26.
The method seems to be sufficiently accurate for all ordinary purposes and is extremely rapid.