A reliable method for the colorimetric determination of Total Kjeldahl Nitrogen (TKN) in water and waste water using the Optrode sensor, according to ISO 5663, DIN EN 25 663 and 40 CFR part 136.3 is introduced. In this study, the Limit of Detection and Limit of Quantification, which are a benchmark for the method’s sensitivity, were determined for water samples. To test the system’s performance, the nitrogen recovery of different concentrations of a urea reference sample was tested. The samples were digested mercury and selenium free using the KjelDigester K-449. The distillation and boric acid titration were performed with the KjelMaster K-375 with KjelSampler K-376. Equivalent to the international norms, the detection principle of the boric acid titration is colorimetric. The KjelMaster K-375 is equipped with the colorimetric set composing of the Optrode sensor and corresponding BUCHI accessories for increased stirring and air bubble repression. The fast heating and cooling of the KjelDigester K-449, the synchronized process steps of the KjelMaster system K-375 / K-376 combined with the titration mode “Online” increases the sample throughput per work day.