Phosphoric acid-esterified fine cellulose fiber and method for producing the same

A phosphoric acid-esterified fine cellulose fiber, which contains 0.6 mmol/g or more of phosphoric acid groups, which is prepared by a method comprising (a) a step of allowing a compound having a phosphoric acid group or/and a salt thereof to act on a fiber raw material containing cellulose in the presence of urea or/and a derivative thereof to introduce phosphoric acid groups into the fiber raw material, and (b) a step of refining the fiber raw material of step (a) wherein phosphoric acid groups have been introduced by step (a), and wherein strongly acidic groups and weakly acidic groups of the phosphoric acid groups are introduced into the cellulose, and wherein a difference between an amount of strongly acidic groups in the cellulose and an amount of weakly acidic groups in the cellulose is made to be 0.5 mmol/g or smaller;

• and
• wherein the phosphoric acid-esterified fine cellulose fiber has a polymerization degree of 400 or higher as determined by the copper ethylenediamine method.

A phosphoric acid-esterified fine cellulose fiber, which contains 0.6 mmol/g or more of phosphoric acid groups, and has a polymerization degree of 400 or higher as determined by the copper ethylenediamine method.