iQii] John L. Kantor and William J. Gies 265 



ination for protein in the biuret test emphasized this apprehension. 



At that time we feit satisfied that Benedict's modification of the 

 Fehling reagent was superior to the Fehling reagent itself for the 

 detection of reducing carbohydrates, and accordingly a bottle, 

 labeled FEHLING-BENEDICT REAGENT, was reserved for 

 this liquid in the series of reagents referred to above.* In seeking 

 a way out of the difficulty attending the use of excessive amounts of 

 alkali in the biuret test (the difficulty to which we have just alluded), 

 we thought of the possibility of using the Fehling-Benedict reagent 

 itself. We f ound, however, that the Fehling-Benedict reagent would 

 not yield satisfactory results as a Substitute for copper sulfate and 

 caustic alkali in the biuret test (the original Fehling Solution being 

 better for the purpose), and that the selected reagent for the detec- 

 tion of reducing carbohydrate would not serve the double purpose 

 we had in mind.^ 



Fehling reagent, as is well known, may be used satisfactorily 

 for the detection of protein when comparatively large proportions 

 of protein are present in the material under examination, but when 

 relatively small quantities of protein are contained, the addition of 

 the Fehling reagent conveys an excess of copper into the medium 

 and the typical color of the test may be wholly obscured or its recog- 

 nition may be rendered uncertain. A reagent that would convey a 

 small Proportion of copper, with a relatively large amount of caustic 

 alkali, was an alternative which naturally came to mind as these 

 predicaments were considered. 



Ultimately, seeking such a reagent, the senior author tried one of 

 the simplest possible combinations. A few drops of 3 per cent. 

 copper sulfate Solution were added to about 100 c.c. of 10 per cent. 

 NaOH Solution. The precipitated cupric hydroxid was dissolved 

 entirely when the mixture was shaken. The resultant liquid was 

 distinctly and uniformly blue. Several such additions of copper 

 sulfate Solution were made to the blue liquid with the same result so 

 far as Solution of the precipitated cupric hydroxid was concerned, 

 but the blue color was intensified. Finally, after thus imparting a 



* Fehling Solution was not included in the series. 



*The alkalinity of the Fehling-Benedict reagent is too weak, to say nothing 

 of the excess of copper in the liquid, from our Standpoint — two reasons for its 

 excellence as a reagent for the detection of reducing carbohydrate. 



