Disclosed are a sodium salt represented by the average formula (I) below, and a method for producing such a sodium salt. (In the formula, m.sub.1, n.sub.1, m.sub.2 and n.sub.2 independently represent 0 or a positive number not greater than 2, while satisfying m.sub.1+n.sub.1=2, m.sub.2+n.sub.2=2, 0