PURPOSE: The titled alkali salt useful as a physiological activator and an optical catalyst, readily being decomposed into single nucleus ions, having improved stability of H3O+ and improved solubility, obtained by degasing a strong acid solution containing a specific ion, irradiating the solution with ultraviolet rays to form an electron reducing seed in the solution, cooling the solution and precipitating the seed.

CONSTITUTION: An strong acid solution containing a Keggin type heteropolyacid ion shown by formula I (X is element to become cation of element of group III of the periodic table; M is W or Mo; p is positive interger) is purged with N2 or degased in vacuum to remove a dissolved oxygen gas, the solution is irradiated with ultraviolet rays from ultrahigh-pressure mercury vapor lamp at 500W for about 70hr in a nitrogen atmosphere, an optical reaction is caused and an electron reducing seed is formed in the solution. Then, the reaction solution is cooled to 3W4°C and allowed to stand for about 2 weeks to give precipitate, which is separated, fully washed with diethyl ether and dried to give the titled alkali salt shown by formula II (x is 1, 2, 6, 12 or 18) having 6W25 ionic size and 103W104mol.wt.