To a four neck flask of 200 ml are added 3.80 g (20 mmol) of lanthanum hydroxide, 19.94 g (60 mmol) of di(2-ethylhexyl) phosphate [Starting Material A] and 150 g of cyclohexane at 20° C., and then 675 mg (2.6 mmol) of 35% hydrobromic acid is added and agitated, whereby reaction is immediately started and a temperature of a reaction solution is raised up to 35° C. As the reaction solution becomes translucent while the reaction is continued at this temperature, the temperature of the reaction solution is raised to 70° C. by gradually heating. Further, the reaction is continued at this temperature, and the reaction is completed at a time point that turbidity disappears and the reaction solution becomes transparent. The reaction time is 120 minutes. Then, the same procedure as in [Example 1] is conducted to obtain 110.2 g of lanthanum di(2-ethylhexyl) phosphate having a concentration of about 20%. The analytical results are shown in Table 1.
