Part 19

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. 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:

34.65 grams copper sulfate; 173 grams potassium-sodium tartrate; 480 cubic centimeters sodium hydroxid solution, 1.14 specific gravity; 67.3 grams sodium hydroxid; fill to one liter; 0.180 gram grape sugar reduces according to Bödeker, 36.1 cubic centimeters of the copper solution = 0.397 gram copper oxid. The same quantity of milk sugar reduces, however, only twenty-seven cubic centimeters copper solution = 0.298 copper oxid.

4. Boussingault:

40 grams copper sulfate; 160 grams potassium tartrate; 130 grams sodium hydroxid.

5. Dietzsch:

34.65 grams copper sulfate; 150 grams potassium-sodium tartrate; 250 grams sodium hydroxid solution, 1.20 specific gravity; 150 grams glycerol.

6. Fleischer:

69.278 grams copper sulfate dissolved in about half a liter of water, add to this 200 grams tartaric acid; fill to one liter with concentrated sodium hydroxid solution; twenty cubic centimeters copper solution = forty cubic centimeters sugar solution, that contain in every cubic centimeter five milligrams grape sugar.

7. Fehling:

40 grams copper sulfate; 160 grams di-potassium tartrate = 600-700 cubic centimeters sodium hydroxid solution, 1.12 specific gravity, or from 54.6 to 63.7 grams sodium hydroxid, fill to 1154.4 cubic centimeters.

8. Gorup-Besanez:

34.65 grams copper sulfate; 173 grams potassium-sodium tartrate; 480 cubic centimeters sodium hydroxid solution, 1.14 specific gravity; equal 67.3 sodium hydroxid. Fill to one liter.

9. Grimaux:

40 grams copper sulfate; 160 grams potassium-sodium tartrate; 600-700 cubic centimeters sodium hydroxid solution, 1.20 specific gravity, equal to 92.5-107.9 grams sodium hydroxid. Fill to 1154.4 cubic centimeters. Ten cubic centimeters of this solution are completely decolorized by 0.050 gram glucose.

10. Holdefleis:

34.632 grams copper sulfate in one liter of water; 125 grams potassium hydroxid, equivalent to 89.2 grams sodium hydroxid; 173 grams potassium-sodium tartrate. Fill to one liter.

11. Hoppe-Seyler:

34.65 grams copper sulfate; 173 grams potassium-sodium tartrate; 600-700 cubic centimeters sodium hydroxid solution, 1.12 specific gravity; equal to 63.0-73.5 grams potassium hydroxid. Fill to one liter. One cubic centimeter is reduced by exactly 0.005 gram grape sugar.

12. Krocker:

6.28 grams copper sulfate; 34.6 grams potassium-sodium tartrate; 100 cubic centimeters sodium hydroxid solution, 1.14 specific gravity. Fill to 200 cubic centimeters. In 100 cubic centimeters of this solution is contained 0.314 gram copper sulfate, which is reduced by 0.050 grape sugar.

13. Liebermann:

4 grams copper sulfate; 20 grams potassium-sodium tartrate; 70 grams sodium hydroxid solution, 1.12 specific gravity. Fill to 115.5 cubic centimeters.