Enhanced ammonia recovery and a simplified method are described for a rapid Kjeldahl digestion using sulfuric acid and hydrogen peroxide as the sole digestion reagents. This micro procedure uses a Vigreux fractionating head fitted to a 100 mL volumetric flask and a hot plate with a solid-state controller. Continuous-flow peroxide addition is controlled by a capillary funnel, and fumes are evacuated through a side-arm vent leading to a water aspirator. Complete recovery of nitrogen from the refractory compound, nicotinic acid, is obtained with less than 10 min digestion. The described method reduces digestion time by 25-50% over the open-manifold peroxy method. A digestibility index (DI), scaled 0-10, establishes the difficulty of digestion for each sample and assigns values to compounds. A useful tool for determining the minimal amount of reagent and digestion time required, the DI assigns zero for compounds not needing digestion and 10 for nicotinic acid. Digested samples obtained from the described method are suitable for direct colorimetric analysis of many elements in addition to Kjeldahl nitrogen. Distillation of the digested sample is not required

A method for the determination of Kjeldahl nitrogen uses peroxymonosulfuric acid (Caro's acid) as a strong oxidant for rapid sample decomposition without added salts or metal catalysts. Peroxymonosulfuric acid is formed in a hydrogen peroxide-sulfuric acid mixture and flows at 2 mL/min into a sample carbonized in concentrated sulfuric acid. The peroxy method obtains full recovery of nitrogen from nicotinic acid in 17.5 min. Decomposition occurs 5-10 times faster in the peroxy method than with conventional Kjeldahl methods. The digestion is followed by rapid colorimetric determination by an improved Nesslerization. A Digestion Index rates the difficulty of digestion of compounds and enables the digestion time and reagent to be minimized. Results with this method are accurate, fast, and comparable to standard Kjeldahl methods. The digest is not contaminated by salts or metal catalysts and can be further analyzed for other elements.1948). More recent work used dropwise addition of hydrogen peroxide for the initial clearing, followed by a 30min digestion to obtain good recoveries (Florence and Milner, 1979).

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