GB/T 11891-1989 is a water quality testing standard for the determination of Kjeldahl nitrogen. Kjeldahl nitrogen contains organic nitrogen and ammonia nitrogen, which reflects the degree of pollution of water bodies by nitrogen-containing organic matter and is an important indicator for evaluating water quality. The standard details the operation process from water sample digestion, distillation to separate ammonia, to acid titration to determine ammonia nitrogen, and then to calculate Kjeldahl nitrogen content. The method is standardized, reliable, and suitable for all kinds of water samples. It provides key data support for water quality monitoring, water eutrophication prevention and control and related research, and helps maintain a good water environment mass.