THE CARBOHYDRATES 69 



the solution shows bi-rotation. Its final specific rotatory power at 

 20 C. is 52-74. 



TESTS FOR GLUCOSE. Trommer's test depends on the power possessed in 

 common with the other sugars of reducing cupric hydrate to cuprous oxide. 

 The sugar solution is made alkaline with caustic potash or soda, and a few drops 

 of copper sulphate solution added. On heating the blue solution thus obtained 

 to boiling, it turns yellow, and a yellowish-red precipitate of cuprous hydrate 

 is produced. This test is generally performed with Fehling's solution, which 

 consists of an alkaline solution of cupric hydrate in Rochelle salt. The propor- 

 tions in making the solutions are so arranged that 10 c.c. of Fehling's solution 

 are completely reduced by '05 gramme glucose. This reaction is made use of 

 for the quantitative determination of glucose in solution. The determination 

 may be carried out either volumetrically, as in Fehling's or Pavy's method, or 

 gravimetrically, as in Allihn's method. 



Moore's Test. A solution of glucose treated with a little strong caustic potash 

 or soda and warmed, becomes first yellow and then gradually dark brown, and 

 gives off a smell of caramel. 



With ordinary yeast, glucose solutions ferment readily, giving off CO 2 , and 

 form alcohol with small traces of amyl alcohol, glycerin, and succinic acid. 



With phenyl hydrazine glucose gives well-marked needles of glucosazone. 

 These are precipitated when the liquid is still hot, the precipitate being increased 

 as the liquid cools. The crystals form bundles of fine yellow needles which are 

 almost insoluble in water, but are soluble in boiling alcohol. When purified by 

 recrystallisation they melt at 204-205 C. 



On treating a watery solution of glucose with benzoyl chloride and caustic 

 soda and shaking till the smell of benzoyl chloride has disappeared, an insoluble 

 precipitate is produced of the benzoic ester of glucose. This method has been 

 often used for isolating glucose from fluids in which it occurs in minute quantities. 



Molisch's Test. On treating 0'5 c.c. of dilute glucose solution with one drop 

 of a 10 per cent, alcoholic solution of n-naphthol, and then pouring 1 c.c. of 

 concentrated sulphuric acid gradually down the side of the tube, a purple ring 

 is produced at the junction of the two fluids, which on shaking spreads over the 

 whole fluid. This reaction depends on the formation of furfurol from the glucose. 



In order to identify glucose in a normal fluid, the following tests may be 

 applied, after removing any protein which may be present : 



(1) Reduction of cupric hydrate or Fehling's solution. 



(2) Estimation of reducing power of solution. 



(3) Estimation of rotatory power of solution on polarised light. 



(4) Formation of osazone crystals with phenyl hydrazine. These crystals 

 must come down while the fluid is still hot. They must be purified and their 

 melting-point taken. A determination by combustion of their nitrogen content 

 will give direct information whether the sugar is a monosaccharide or disaccharide. 



(5) The solution is made acid and boiled for some time. It is then made 

 up to its former volume and its reducing power and effect on polarised light once 

 more taken. In the case of a disaccharide, which would be converted into mono- 

 saccharide by boiling in acid solution, these two readings would be altered, whereas 

 neither the rotatory power nor the reducing power of glucose would undergo 

 any change. 



(6) Fermentation with ordinary yeast. 



A positive result would exclude glycuronic acid. 



D-FRUCTOSE, or LEVULOSE, occurs ^mixed with dextrose in 

 honey and in fruit sugar. It is also, with glucose, formed by the 



