Of all these products only oxyphenic and lactic acids and their ethers and oxalic acid remain unchanged and they can be isolated. All the others are transformed in an acid state and they can only be detected by operating in the presence of metallic oxids capable of precipitating them at the time of their formation.[79] =115. Fehling’s Solution.=—The copper solution which has been most used in the determination of reducing sugars is the one proposed by Fehling as a working modification of the original reagent used by Trommer.[80] Following is the formula for the preparation of the fehling solution: Pure crystallized copper sulfate CuSO₄.5H₂O, 34.64 grams: Potassium tartrate, 150.00 ” Sodium hydroxid, 90.00 ” The copper sulfate is dissolved in water and the potassium tartrate in the aqueous solution of the sodium hydroxid which should have a volume of about 700 cubic centimeters. The two solutions are mixed and the volume completed to a liter. Each cubic centimeter of this solution will be reduced by five milligrams of dextrose, equivalent to four and a half milligrams of sucrose. The reaction which takes place is represented by the following molecular proportions: C₆H₁₂O₆ = 10CuSO₄.5H₂O Dextrose. Copper sulfate. 180 2494 Fehling’s solution is delicate in its reactions but does not keep well, depositing cuprous oxid on standing especially in a warm place exposed to light. The fehling liquor was soon modified in its constitution by substituting 173 grams of the double sodium and potassium tartrate for the neutral potassium tartrate first used, and, in fact, the original fehling reagent contained forty grams of copper sulfate instead of the quantity mentioned above. Other proportions of the ingredients are also given by many authors as fehling solution. =116. Comparison of Copper Solutions for Oxidizing Sugars.=—For the convenience of analysts there is given below a tabular comparison of the different forms of fehling liquor which have been proposed for oxidizing sugars. The table is based on a similar one prepared by Tollens and Rodewald, amended and completed by Horton.[81] The solutions are arranged alphabetically according to authors’ names: 1. _Allihn_: 34.6 grams copper sulfate, solution made up to half a liter; 173 grams potassium-sodium tartrate; 125 grams potassium hydroxid (equivalent to 89.2 grams sodium hydroxid) solution made up to half a liter. 2. _A. H. Allen_: 34.64 grams copper sulfate, solution made up to 500 cubic centimeters; 180 grams potassium-sodium tartrate; 70 grams sodium hydroxid (not less than 97° NaOH), solution made up to half a liter. 3. _Bödeker_: