A comparative study of hydroxy-benzopyrone derivatives has been made with reference to their capacity to form chelate combinations with metals. Based on the determination of stability constants it could be concluded that 1-hydroxy xanthone forms the most stable complex. Addition of a methoxyl group in the 3-position has a favourable influence. Among the flavones a hydroxyl group in the 5-position forms definitely a stronger complex as compared with the 3-hydroxyl group. Here also addition of a methoxyl group in the 7-position is an advantage. Similar results are obtained with the chromones though their complexes are comparatively weaker. Isoflavone derivatives unexpectedly form the weakest complexes and in this case steric factors seem to be involved. The presence of this factor could be noticed in the case of 2: 3-diphenyl chromones also which also have abnormally weak complexes.

1-Hydroxy-3-methoxy xanthone forms a complex ThL₄ in aqueous alcoholic solution at pH 2·6–4·0 and it can be precipitated with water, dried and ignited to yield thorium dioxide. This provides a satisfactory method of estimation of the metal in the presence of a large excess of cerite earths and ceric salts since these are not precipitated by this reagent and also in the presence of uranium which gets precipitated only at higher pH. The method can be used for separation also.