A spectrophotometric study of the thorium-morin mixed-color system

Thorium reacts with morin in solutions at a pH of 2.0 to yield a single complex that has a thorium:morin ratio of 1:2. The yellow
complex has a maximum absorbance at 410 m and is stable for at least
7 hours. The sensitivity of the reaction is such that as little as
about 0.2 microgram of ThO₂ in 50 ml can be determined using a light
path of 5 cm. An average apparent equilibrium constant of approximately l x 10⁶ was found for the reaction.

The effects of acidity, alcohol content, morin content, time, age
of the morin reagent and temperature of the reaction have been studied
as have the effect of some other ions. Zirconium and aluminum interfere seriously and must be absent. The interference from ferric iron
is also serious but the effect may be appreciably decreased if hydroxylamine
is added to the solution. Relatively large amounts of calcium and
lanthanum may be present without causing interference, and these ions
can be used as carriers in separations of thorium from other ions.

Morin is a suitable reagent for the determination of thorium in
pure solutions. If appropriate separations are found for the separation
of thorium from other ions, the reaction will be useful for the determination
of trace amounts of thorium in complex materials. The information
presented in this paper is fundamental to any specific adaptation of
the reaction to the analysis of complex materials.