Optimization of a wet chemical continuous flow analysis method exemplified by the determinations of kjeldahl nitrogen and total phosphorus

The optimization of a rather complex wet chemical analysis method, such as the measurement of Kjeldahl nitrogen or total phosphorus with the Technicon AutoAnalyzer, is extremely tedious when purely empirical approaches are used. A mathematical model of the different stages of the measuring method (digestion, neutralization and color reaction) is described. The system can then be optimized for maximum measuring sensitivity. Optimization is done by solving numerically the non-linear optimization problem with constraints. The starting values for the optimization algorithm were found by varying these values systematically within the tolerated range, with checks that none of the constraints were violated. The theoretical results predict an increase in sensitivity by a factor of 15 compared to the method used previously. In practice, the sensitivity was increased by a factor of 10 for the total phosphorus method. For the simultaneous low-level determinations of Kjeldahl nitrogen and total phosphorus some problems of stability remain.