The invention provides a preparation method of high-purity isosulfur blue, which comprises the following steps: by taking 2-chlorobenzaldehyde as a
raw material, after sulfonation reaction of fuming
sulfuric acid, adding
calcium carbonate to convert excessive
sulfuric acid into
calcium sulfate precipitate, and filtering to remove the
calcium sulfate precipitate, thereby reducing inorganic salt residues in an intermediate I; carrying out
substitution reaction on the intermediate I under the action of
sodium sulfite and
sodium hydrogen sulfite, adding a calcium
chloride solution after the reaction to enable
sulfite to form
calcium sulfite precipitate, and carrying out
filtration, vacuum concentration and
methanol pulping to obtain a high-purity intermediate II; the intermediate II reacts with N, N-
diethylaniline to generate an intermediate III, then the intermediate III is oxidized by
silver oxide to generate an isosulfur blue silver salt,
sodium carbonate is added to directly convert the silver salt into a
sodium salt form, and a high-purity isosulfur blue finished product is obtained through
ethyl acetate crystallization. According to the method, inorganic salt residues are remarkably reduced, the product purity and yield are improved, meanwhile, the technological process is simplified, the
solvent usage amount is reduced, operation is safe and
environmentally friendly, and industrial production is facilitated.