Vin] FOODS AND FEEDING STUFFS 87 



similar period with 10% sodium hydroxid solution, and then treat for a few hours 

 with hot alkaline tartrate solution (old alkaline tartrate solutions that have stood 

 for some time may be used for this purpose) of the strength employed in sugar de- 

 terminations. Then wash the asbestos free from alkali, digest for several hours 

 with dilute nitric acid (1 to 3) and, after washing free from acid, shake with water 

 into a fine pulp. In preparing the Gooch crucible, make a film of asbestos i inch 

 thick and wash thoroughly with water to remove fine particles of asbestos. If the 

 precipitated cuprous oxid is to be weighed as such, wash the crucible with 10 cc. of 

 alcohol, then with 10 cc. of ether, dry for 30 minutes at 100°C., cool in a desiccator 

 and weigh. 



(b) The solution used is described under 19. 



25 PRECIPITATION OF CUPROUS OXID. 



Transfer 25 cc. each of the copper sulphate and alkaline tartrate solutions to a 

 400 cc. beaker of alkali-resisting glass and add 50 cc. of reducing sugar solution, or, 

 if a smaller volume of sugar solution is used, add water to make the final volume 

 100 cc. Heat the beaker upon an asbestos gauze over a Bunsen burner, regulate the 

 flame so that boiling begins in 4 minutes, and continue the boiling for exactly 2 

 minutes. (It is important that these directions be strictly observed and, in order 

 to regulate the burner for this purpose, it is advisable to make preliminary tests, 

 using 50 cc. of the reagent and 50 cc. of water before proceeding with the actual 

 determination.) Keep the beaker covered with a watch glass during the heating. 

 Filter the cuprous oxid at once on an asbestos mat in a porcelain Gooch crucible, 

 using suction. Wash the cuprous oxid thoroughly with water at a temperature of 

 about 60°C., and either weigh directly as cuprous oxid as in 26, or, determine the 

 amount of reduced copper by one of the methods under 29-34, respectively. Con- 

 duct a blank determination, using 50 cc. of the reagent and 50 cc. of water, and, if 

 the weight of cuprous oxid obtained exceeds 0.5 mg., correct the result of the reducing 

 sugar determination accordingly. The alkaline tartrate solution deteriorates on 

 standing and the amount of cuprous oxid obtained in the blank increases. 



Determination of Reduced Copper. 



26 7. Direct Weighing of Cuprous Oxid. — Tentative. 



Prepare a Gooch as directed under 24 (a). 



Collect the precipitated cuprous oxid on the mat, as directed under 25, wash 

 thoroughly with hot water, then with 10 cc. of alcohol, and finally with 10 cc. of 

 ether. Dry the precipitate for 30 minutes in a water oven at the temperature of 

 boiling water; cool and weigh. Calculate the weight of metallic copper. Obtain 

 from 27 the weight of invert sugar equivalent to the weight of copper found. 



This method should be used only for determinations in pure sugar solutions. In 

 all other products the copper of the cuprous oxid should be determined by one of the 

 following methods, since the cuprous oxid is very apt to be contaminated with 

 foreign matter. 



The number of milligrams of copper reduced by a given amount of reducing sugar 

 differs when sucrose is present and when it is absent. In the tables the absence of 

 sucrose is assumed except in the two columns under invert sugar, where one for 

 mixtures of invert sugar and sucrose containing 0.4 gram of total sugar in 50 cc. of 

 solution, and one for invert sugar and sucrose when the 50 cc. of solution contains 2 

 grams of total sugar are given, in addition to the column for invert sugar alone. 



