Determination of Total Nitrogen in Plants and Plant Solutions: A Comparison of Methods with Modifications

Abstract

The analyst who has had considerable experience in the determination of nitrogen is impressed by the fact that some substances are analyzed with more ease and accuracy than other substances. Many factors are involved, not the least of which is the combination of the various forms of nitrogen in the sample to be analyzed. For example, if we consider the determination of total nitrogen in whole plants (peas, wheat, barley, etc.) including the, residual nutrient solutions in which they grew, amino-, amide-, and some ammonia-nitrogen are present in the plants in addition to nitrateand possibly ammonia-nitrogen in the residual solutions. In such a case the determination of total nitrogen presents problems that challenge the accuracy of the various methods used. Two methods are used generally: (1) some modification of the Devarda method, and (2) the official salicylic-thiosulphate method (Assoc. Off. Agr. Chemists, '21, p. 8; I, 28). The latter method has been criticized severely by several investigators whose data have been interpreted as indicating the limitations and defects of any method based on the reduction of nitrates in acid medium. Further reference will be made to these criticisms after the presentation of data. On the other hand, the Devarda method for total nitrogen in the presence of organic matter is time-consuming, since (1) there is a preliminary alkaline distillation with the alloy to collect the ammonia obtained from the reduction of the nitrates and other substances acted upon in the process (Allen, '15; Davisson, '18), and (2) the organic matter and remaining nitrogen must be subjected to a Kjeldahl digestion followed by a second distillation into the same or a second lot of standard acid. Unfortunately, then, neither of these methods is entirely satisfactory for the determination of total nitrogen in plants and plant solutions when the two are present in the same sample. In a study of nitrogen fixation by higher plants the total nitrogen content of the plants and residual nutrient solutions was