Comparison of Conductimetric and Colorimetric Methods with Distillation–Titration Method of Analyzing Ammonium Nitrogen in Total Kjeldahl Digests

Abstract

The distillation–titration method (DTM) is a standard procedure used by most laboratories to measure ammonium-nitrogen (NH4-N) in the total Kjeldahl N (TKN) digests of various kinds of agricultural and environmental samples. These samples may have TKN contents ranging from less than 100 ppb to as high as percentage levels. However, the DTM procedure generally leads to a very low throughput because it is labor intensive and time-consuming. At the current practical quantitation limit (PQL) of 300 ppb established at the Feed and Environmental (FEW) Laboratory, University of Georgia, the DTM procedure is less applicable to low TKN surface water samples. In this study, we therefore compared the performance of diffusion conductivity method (DCM) and colorimetric method (CM) with DTM in measuring NH4-N in the TKN digests of 29 different samples representing surface waters, lagoons, manures, poultry litters, and environmental wastes. Acceptable accuracy and precision were achieved for various QC samples by all three methods. For widely different sample matrices and TKN contents, the NH4-N in the TKN digests measured by DCM and CM both agreed well with that measured by DTM. However, the linear working range of CM is limited within 0.2 to 5.0 ppm, whereas DCM is linear at a wider range of 0.01 to 2000 ppm. With DCM, the PQL of TKN is at 13 ppb, much less than the 300 ppb in DTM and 520 ppb in CM. Both DCM and CM require increasing the pH of the working TKN digest to a highly alkaline range. To meet such pH requirement, the minimum dilution need for DCM is twofold, where as that CM is fourfold. Because of greater mandatory dilution requirement coupled with a greater PQL, CM may often fail to measure NH4-N in the working TKN digest of some low TKN surface water samples. On the other hand, with some environmental waste samples containing TKN at percentage level, CM would require multistep dilution of the digests prior to measurement, thus allowing dilution-related error as well as requiring additional labor. In contrast, DCM can measure both low TKN surface waters and high TKN environmental wastes without any major limitations. Moreover, DCM may work well without any adjustment of sample background in the calibration standards. Thus DCM appears to be an attractive alternative to the labor-intensive and time-consuming DTM for measuring NH4-N in the TKN digests of various kinds of agricultural and environmental samples in the analytical services laboratories.