Fehling’s solution, preparation

Soxhlet s modification of Fehling s solution.—Prepared by mixing immediately before use, equal volumes of (a) and (b). 

Fehling s solution, preparation, 8-1 to 4 FEM, definition, 12-1 to 4 Fermi Energy and Related Properties of Metals, 12-214 to 215 Fermium see also Elements... 

Tartaric acid is noteworthy for a) the excellent way in which the majority of its salts Crystallise, and h) the frequent occurrence of salts having mixed cations. Examples of the latter are sodium potassium tartrate (or Rochelle salt), C4H40 NaK, used for the preparation of Fehling s solution (p. 525), sodium ammonium tartrate, C4H OaNaNH4, used by Pasteur for his early optical resolution experiments, and potassium antimonyl tartrate (or Tartar Emetic), C4H404K(Sb0). The latter is prepared by boiling a solution of potassium hydrogen tartrate (or cream of tartar ) with antimony trioxide,... 

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

Place 10 ml. of 1% starch solution (prepared as described above) in a boiling-tube, add 2 ml. of 1% sodium chloride solution and place the tube in a water-bath maintained at 38-40 . Place about 5 ml. of water in a series of test-tubes and to each add a few drops of 1% iodine solution. Now add 4 ml. of the diluted saliva solution to the starch solution, mix well and note the time. At intervals of about 30 seconds transfer 2 drops of the reacting mixture, by means of a dropping tube, to one of the test-tubes, mix and note the colour. As in the previous experiment, the colour, which is blue at first, changes to blue-violet, red-violet, red-brown, pale brown, and finally disappears at this stage the solution will reduce Fehling s solution. If the reaction proceeds too quickly for the colour changes to be observed, the saliva solution should be diluted. 

To prepare Fehling s solution, mix equal volumes of Solutions... 

Preparation of Fehling s solution. Solution No. 1. Dissolve 34-64 g. of A.R. copper sulphate crystals in water containing a few drops of dilute sulphuric acid, and dilute the solution to 500 ml. 

However, in 1939 this difficulty was obviated by Brauns extraction of about 3% lignin from spruce wood by means of the solvent, ethyl alcohol, at room temperature (9). He termed this preparation native lignin. It was found to be soluble in methanol, ethanol, dioxane, dilute sodium hydroxide and pyridine, and insoluble in water, ether, petroleum ether and benzene. Chemically it behaved the same as lignin as it exists in woody tissues. It also reduced Fehling s solution and gave a strong... 

Experiments.—Being a primary hydrazide (of carbamic acid), semicarbazide reduces ammoniacal silver solutions and Fehling s solution. It reacts readily with aldehydes and ketones with the elimination of water and formation of semicarbazones, which, since they are more easily hydrolysed than are phenylhydrazones and oximes, are to be preferred to the latter for purposes of separation and purification of carbonyl compounds. Shake an aqueous solution of the hydrochloride (prepared as described above) with a few drops of benzaldehyde, isolate the semicarbazone and purify it by recrystallisation from alcohol. Melting point 214° decomp. Benzaldehyde semicarbazone is decomposed into its constituents by gentle warming with concentrated hydrochloric acid. 

N-Methylhydroxylamine.—An aqueous solution of nitromethane is mixed with about its own weight of ammonium chloride, the mixture is then cooled (temperature about 10°) and three parts of zinc dust are added in small portions with constant shaking. Zinc dust is now removed by filtration, and it is found that the filtrate reduces ammoniacal silver and Fehling s solutions. The preparation of this easily accessible alkylhydroxylamine as hydrochloride is described by Beckmann, Annalen, 1909, 365, 204. 

When various successive tests are to be made, it is obvious that greater accuracy in the measurement of the volumes may be attained by preparing a quantity of the diluted Fehling s solution (e.g., 50 c.c. erf each of the two liquids + 400 c.c. of water) and taking 50 c.c. of the mixture for each test. The two liquids should be well mixed before dilution with the water and the diluted solution should not be kept from one day to the next,... 

The determination is made on the uninverted liquid prepared as described above and diluted, if necessary, to give a solution containing not more than 1% of reducing sugars (0-5-1%), so that 5-10 c.c. will be required to reduce completely the above quantity of Fehling s solution. 

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