Abstract: Disclosed are a sodium salt represented by the average formula (I) below, and a method for producing such a sodium salt. (In the formula, m1, n1 m2 and n2 independently represent 0 or a positive number not greater than 2, while satisfying m1 + n1 = 2, m2 + n2 = 2, 0 < m1 + m2 < 4 and 0 < n1 + n2 < 4) Also disclosed is a decomposition suppression method which makes a sodium salt represented by average formula (I) coexist with a sodium salt represented by the general formula (IV) below. This method enables improvement of long-term stability of a sodium salt represented by general formula (IV).
