Estimation of Nitrate Nitrogen in Plant Juice: a Study of the Expression and Clarification of the Juice.

Since the publication by GILBERT (3) of the colorimetric method for the determination of nitrate nitrogen, phosphate, and potash in plant juices, several workers have presented criticisms and modifications of this method as applied to nitrates. These include particularly HOLTZ and LARSON (7) and EMMERT (2). Considerable work has been done in this laboratory on this particular determination and the purpose of this paper is to present the modifications which have evolved here during the past four years. The GILBERT method is essentially as follows: The fresh plant tissue is ground and the juice expressed through fine mesh cloth. This juice is decolorized with carbon black and finally clarified by the addition of solutions of AgSO4, CuSO4, and a mixture of solid Ca(OH)2 and MgCO3 as recommended by HARPER (5). It was originally the plan to determine nitrate nitrogen, phosphorus, and potash on aliquots of the original juice. As the phosphate determination does not permit the use of alkaline reagents for clarification, the treatment with carbon black was extremely important. Later it was believed that heating would coagulate much of the colloidal material, allowing its separation with the carbon black on the filter. This was adopted shortly after publication of the original method, but has never been described in the literature. HOLTZ and LARSON (7) have published a criticism of the recovery of nitrate nitrogen by the GILBERT method, stating that, using the procedure as published, they were only able to recover 40-45 per cent. of the nitrate added to extracts of wheat plants. This extremely low recovery theyattribute to the use of MgCO3 in the final clarification, which prolonged the final evaporation. These workers have suggested a modified procedure