in methanol addn. of acid in anhyd. methanol to slight excess of Tl2CO3, refluxing and stirring (several h); filtn. excess of Tl2CO3, concn. and pptn. by addn. of ethyl ether. Recrystn. from abs. ethanol, vac.-dried. Purity 99.7 mol-percent (d.s.c.);