Otto Folin and Vintila Ciocalteu present a critical study of the Folin-Looney methods for determining tyrosine and tryptophane in proteins. They discuss the limitations and improvements of these methods, focusing on the phenol reagent's reactivity and colorimetric properties. The authors introduce a new colorimetric method for estimating tyrosine in protein hydrolysates, which involves Millon's reaction with mercuric sulfate. They also detail the hydrolysis of proteins with alkalis and provide methods for purifying mercuric sulfate to ensure accurate results. The paper includes detailed protocols for preparing and analyzing various protein samples, such as casein, egg albumin, edestin, gliadin, and zein, to validate the new methods. The authors conclude that their improved methods yield more reliable and accurate results compared to the Folin-Looney methods, particularly for tryptophane determinations.Otto Folin and Vintila Ciocalteu present a critical study of the Folin-Looney methods for determining tyrosine and tryptophane in proteins. They discuss the limitations and improvements of these methods, focusing on the phenol reagent's reactivity and colorimetric properties. The authors introduce a new colorimetric method for estimating tyrosine in protein hydrolysates, which involves Millon's reaction with mercuric sulfate. They also detail the hydrolysis of proteins with alkalis and provide methods for purifying mercuric sulfate to ensure accurate results. The paper includes detailed protocols for preparing and analyzing various protein samples, such as casein, egg albumin, edestin, gliadin, and zein, to validate the new methods. The authors conclude that their improved methods yield more reliable and accurate results compared to the Folin-Looney methods, particularly for tryptophane determinations.