Trisodium naphthalene-1,3,6-trisulphonate

The free acid is obtained by passing the salt through an ion-exchange column and converting it to the lanthanum salt by treatment with La2O3. This salt is crystallised twice from hot water. [The much lower solubility of La2(SO4)3 and its retrograde temperature dependence allows a good separation from sulfate impurity]. The lanthanum salt is then passed through an appropriate ion-exchange column to obtain the free acid, the sodium or potassium salt. (The sodium salt is hygroscopic.) [Atkinson et al. J Am Chem Soc 83 1570 1961.] It can also be recrystallised from aqueous acetone [Okahata et al. J Am Chem Soc 108 2863 1986]. 574 Purification of Metal-Organic Compounds