Fehling s solution

Fehling s solution A solution of copper sulphate, sodium potassium tartrate and NaOH used for estimating and detecting reducing sugars. 

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,... 

Add 2 -3 drops of phenylhydrazine to about 2 ml. of Fehling s solution in a test-tube and shake the mixture vigorously nitrogen is evolved and reddish-brown cuprous oxide is precipitated. The reaction proceeds rapidly on gentle warming, more slowly in the cold. 

Fehling s solution. Aqueous solutions of aliphatic aldehydes are almost invariably acidic owing to atmospheric oxidation, and therefore... 

Aliphatic aldehydes reduce Fehling s solution rapidly, benzaldehyde very slowly and indecisively, salicylaldehyde does not reduce it. 

Fehling s solution. Reduces Fehling s solution on warming an excess of the latter should be used. The odour of chloroform is also noticeable, due to the action of the alkali in Fehling s solution. 

It does not reduce Fehling s solution, but turns it a pale green colour. (Note that salicylaldehyde turns ordinary copper sulphate solution a pale green.)... 

Do not reduce ammoniacal AgN03 or Fehling s solution (dis tinction from aldehydes). 

Oxidation, (a) Unsaturation test. Dissolve about o-i g. of cinnamic acid or of a soluble cinnamate in about 5 ml. of 10% NajCOg solution. To the cold solution add 1% aqueous KMn04 drop by drop. Immediate decolorisation denotes unsaturation. (Note. Many easily oxidisable substances, e.g.y formic acid, acetaldehyde, etc.y also rapidly decolorise alkaline permanganate. Cinnamates, however, do not reduce Fehling s solution.)... 

NOTE. Many esters reduce Fehling s solution on warming. This reduction occurs rapidly with the alkyl esters of many aliphatic acids, but scarcely at all with similar esters of aromatic acids (f.g., ethyl oxalate reduces, but ethyl benzoate does not). Note also that this is a property of the ester itself thus both methyl and ethyl oxalate reduce Fehling s solution very rapidly, whereas neither oxalic acid, nor sodium oxalate, nor a mixture of the alcohol and oxalic acid (or sodium oxalate), reduces the solution. 

Reduction of Fehling s solution. Add 5 ml. of the glucose solution to 5 ml. of Fehling s solution and boil. Reduction takes place and a precipitate of cuprous oxide is formed the latter is at first yellow but may become red on standing. 

Does not reduce ammoniacal silver nitrate or Fehling s solution. If, however, the sucrose solution is warmed for some time with the reagent in question, slight hydrolysis to glucose and fructose does take place and reduction then occurs occasionally samples of sucrose will rapidly give a silver mirror, presumably owing to impurities. 

Reduces ammoniacal silver nitrate, and Fehling s solution. 

Does not reduce ammoniacal silver nitrate or Fehling s solution, and does not form an osazone. 

Reduction of Fehling s solution. Dissolve o i g. of uric acid in NajCOs solution, and to the clear solution add Fehling s solution drop by drop. Note the formation of a white precipitate of copper... 

Reduction of Fehling s solution. Boil i ml. of chloroform gently with 3 ml. of Fehling s solution with constant shaking for 3-4 minutes. Reduction occurs and reddish cuprous oxide slowly separates. 

Formation of silver mirror or precipitate of silver indicates reducing agent. (This is often a more sensitive test than I (a) above, and some compounds reduce ammoniacal silver nitrate but are without effect on Fehling s solution.) Given by aldehydes and chloral hydrate formates, lactates and tartrates reducing sugars benzoquinone many amines uric acid. 

Fehling s solution (reduction) + + + + very faint reduction -—... 

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). 

The standardised Fehling s solution can then be used to determine, for example, the percentage amount of glucose in a crude sample. The method... 

When the Fehling s solution is required, transfer equal volumes of solutions A and B (at room temperature) to a dry flask, and mix thoroughly by shaking. Since however Fehling s solution deteriorates slowly on keeping, only sufficient of the solutions A and B should be mixed together to meet immediate requirements. 

Repeat the titrations until consistent values (i.e., values which do not differ by more than 0 1 ml. of glucose solution) are obtained. Then calculate the weight of glucose equivalent to 1 ml. of the Fehling s solution. 

It is usually found that 1 ml. of the above Fehling s solution = 0 0050 g. of glucose. 

The weights of other monosaccharides and reducing disaccharides which will reduce i ml. of this standard Fehling s solution are galactose, 0 00511 g- fructose. 0 00514 g.. mannose, 0 00431 g. lactose, 0 00678 g. maltose, 0 00807 g. 

This is clearly the reverse of the standardisation of the Fehling s solution described above. 

Weigh out accurately about 1 25 g. of the sample, dissolve it in water and make the solution up to 250 ml. Titrate this solution against 25 ml. of the standard Fehling s solution, precisely as before. 

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