TESTS FOR SUGAR. 297 



149. Tests for Sugar. 



1. Trommer's Test depends upon the fact that in alkaline solutions 

 sugar acts as a reducing agent ; in this case a metallic oxide is changed 

 into a suboxide. To the fluid to be investigated, add J of its volume of 

 a solution of caustic potash (soda) specific gravity 1*25, and a few 

 drops of a weak solution of cupric sulphate, which causes at first a 

 bluish precipitate consisting of hydrated cupric oxide, but it is redis- 

 solved giving a clear blue fluid, if sugar be present. Heat the upper 

 stratum of the fluid, and a yellow or red ring of cuprous oxide is 

 obtained, which indicates the presence of sugar; 2 CuO = Cu 2 0. 



The solution of hydrated cupric oxide is caused by other organic substances; but 

 the final stage, or the production of cuprous oxide is obtained only with certain 

 sugars grape, fruit and milk (but not cane) sugar. Fluids which are turbid must be 

 previously filtered, and if they are highly coloured they must be treated with basic 

 lead acetate; the lead acetate is afterwards removed by the addition of sodium 

 phosphate and subsequent filtration. If very small quantities of sugar are present 

 along with compounds of ammonia, a yellow colour instead of a yellow precipitate 

 may be obtained. In doing the test, care must be taken not to add too much 

 cupric sulphate. 



[2. Fehling's Solution is an alkaline solution of potassio-tartarate of 

 copper. Boil a small quantity of the deep-blue coloured Fehling's 

 solution in a test tube, and add to the boiling test a few drops of the 

 fluid supposed to contain the sugar. If sugar be present the copper 

 solution is reduced, giving a yellow or reddish precipitate. The 

 reason for boiling the test itself is, that the solution is apt to decom- 

 pose when kept for some time, when it is precipitated by heat alone. 

 This is one of the best and most reliable tests for the presence of 

 sugar. In Pavy's modification of this test, ammonia is used instead of 

 a caustic alkali.] 



3. Bottger's Test. Alkaline bismuth oxide solution (5 grammes each of basic 

 bismuth nitrate, and tartaric acid, 30 cubic centimetres water, and caustic soda 

 sufficient for neutralisation) is reduced to bismuth suboxide by sugar, with the 

 formation of a dark olive green and ultimately black precipitate. 



4. Moore and Heller's Test. Caustic potash or soda is added until the 

 mixture is strongly alkaline; it is afterwards boiled. If sugar be present, a yellow, 

 brown or brownish-black colouration is obtained. If nitric acid be added, the 

 odour of burned sugar (caramel) and formic acid is obtained. 



5. Mulder and Neubauer's Test. A solution of indigo-carmine rendered 

 alkaline with sodic carbonate, is added to the sugar-solution until a slight bluish 

 colour is obtained. When the mixture is heated the colour passes into purple, red, 

 and yellow. When shaken with atmospheric air, the fluid again becomes blue. 



In all cases where albumin is present it must be removed in urine by acidula- 

 ting with acetic acid and boiling; in blood, by adding four times its volume of 

 alcohol and afterwards filtering, while the alcohol is expelled by heat. 



