A study of the specificity of the Berthelot colour reaction

Evaluation of the Berthelot colour reaction has shown that reagent sequence is an important determinant of the chromogenic response. Color is formed from various non-protein nitrogen substances first treated with alkaline hypochlorite followed by phenol reagent. The time lapse before addition of phenol reagent to alkaline hypochlorite-containing mixtures is critical. The high pH promotes the volatilization of ammonia from non-protein nitrogen solutions treated with alkaline hypochlorite. This loss can be prevented by adding phenol reagent before alkaline hypochlorite. The phenol-hypochlorite reagent sequence appears to enhance the specificity of Berthelot colour formation. This reaction provides a basis for measuring urea nitrogen concentration of serum or protein-free filtrates with a degree of reliability satisfactory for most routine or research purposes.