SOME MODIFICATIONS OF THE PICRIC ACID METHOD 
FOR SUGARS 1 
By J. J. Willaman and F. R. Davison, Division of Agricultural Biochemistry, 
Minnesota Agricultural Experiment Station 
INTRODUCTION 
The writers recently had occasion to use the picric acid colorimetric method 
(commonly called the Benedict-Lewis method) 2 for determining the sugars in 
cornstalk juices. 3 Although it has been used mostly as a micro method for 
blood and urine analyses, it has been used to some extent as a macro method. 
(3, 4 5, 12.) It proved to be more rapid than, and approximately as accurate as ? 
a copper reduction method after certain modifications had been introduced to 
eliminate certain sources of error. The following is a description of the errors 
met with and the corresponding modifications. 
METHOD 
The basic procedure, the various items of which were made the subject of 
the present investigation, was taken from Rose (16) and is as follows: 
One cubic centimeter of sugar solution of proper concentration is transferred 
to a long narrow test tube (so-called sugar tube, graduated to 10 cc.) containing 
2 cc. of saturated picric acid and 1 cc. of 20 per cent sodium carbonate solution. 
The test tube is then immersed in boiling water for 20 to 30 minutes to effect 
reduction of the picrate to picramate. The solution is then made up to volume. 
The color present represents the reducing sugars and is compared to a standard 
in a colorimeter. 
When sucrose is to be determined 1 cc. of sugar solution is mixed with 2 cc. 
of saturated picric acid in a sugar tube and heated in boiling water 10 minutes to 
effect hydrolysis. One cubic centimeter of 20 per cent sodium carbonate is then 
added and the tube heated in boiling water for 20 to 30 minutes to develop the 
color. This color represents the total sugars. The difference between the two 
sets represents the sucrose present. 
All color measurements were made in a Kober colorimeter. 
The various items for which modifications are suggested are the color standard, 
the effect of picric acid on glucose, the dilution of the unknown color, clarification, 
the calculation of results, and the color factors for various sugars. 
THE COLOR STANDARDS 
Several color standards have been suggested, but all of them have some dis¬ 
advantageous feature. The picramate color does not match ferric acetate ( 8 ), 
potassium bichromate, or o-nitrophenolate (5). Dehn and Hartman (6) used 
the color produced by a standard glucose solution, while Lewis and Benedict 
(10) used pure picramic acid. 
1 Received for publication Feb. 19, 1924. Published, with the approval of the Director, as Paper No. 
428, Journal Series, Minnesota Agricultural Experiment Station. 
2 Thomas, W., and Dutcher, R. A. The application of the Benedict-Osterberg method to the quanti¬ 
tative estimation of carbohydrates in plant tissues. Paper presented at convention of Amer. Chem. 
Soc., Milwaukee, Wis., September, 1923. (Not published.) 
3 Willaman, J. J., Burr, Q. O., and Davison, F. R. Cornstalk sirup investigations. In press. 
4 Reference is made by number (italic) to “Literature cited,” pp. 487-488. 
Journal of Agricultural Research, 
Washington, D. C. 
( 479 ) 
Vol. XXVIII, No. 5 
May 3,1924 
Key No. Minn.-51 
