GB/T 5401-2018 "Pulp—Determination of alkali solubility" applies to various types of pulp and specifies the methods for determining alkali solubility using both 1% sodium hydroxide solution and 10% sodium hydroxide solution. The alkali solubility value is calculated based on the dissolution of pulp in sodium hydroxide solutions of specific concentrations. The standard clearly outlines the steps for reagent preparation, sample treatment, constant-temperature dissolution, filtration and washing, drying, and weighing, and standardizes the calculation of results, reflecting the chemical composition and reactivity of the pulp.