GB/T 5400-1998 "Determination of copper number of pulp" specifies the method for determining the copper number of pulp, applicable to various types of pulp. The method involves the reaction of reducing substances in the pulp with a copper sulfate solution, followed by the separation and weighing of the elemental copper reduced in the process to calculate the copper number. The copper number reflects the content of reducing substances in the pulp, providing an important basis for evaluating pulp cooking quality, bleaching degree, and stability. The standard outlines the preparation of reagents, reaction conditions, and quantitative analysis procedures.