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The complexes of the rare earth metals with 2-phenyltrifluoroacetone in the presence of TX-100 are reported. The characteristic absorbances of neodymium, holmium and erbium complexes can be increased by factors of 8.5, 31 and 15 respectively, compared to those of the cholrides. The third-derivative spectra have been used to eliminate the interference of cerium, and the sensitivities are increased again by factors of 7.4, 5.5 and 6.5. A method for the direct determination of neodymium, erbium and holmium in rare earth mixtures is proposed.

The derivative absorption spectra of the neodymium complex with Semi-Xylenol Orange and cetylpyridinium chloride has been investigated. The characteristic absorption of the complex is approximately 350 times that of neodymium chloride. The fourth-order derivative spectrum has been used to eliminate the interference of the other lanthanides, and to increase the sensitivity by a further factor of 6. Beer's law is obeyed for 0-7.5 microg of Nd in 25 ml of solution. The relative standard deviation for 7 determinations of 1.8 microg/25 ml neodymium was 1.3%. The detection limits were 5.8 ng/ml in the absence of lanthanum and 11.2 ng/ml in the presence of 44 ng/ml lanthanum (or 36 ng/ml yttrium). The method has been used for the determination of neodymium in mixed rare earths, with satisfactory results.

Neodymium and erbium can form stable ternary complexes with 8-hydroxyquinoline-5-sulphonic acid and cetylpyridinium chloride, but a photometric method based on this does not give high enough sensitivity and is subject to interference by cerium. Use of the third derivative spectra, however, eliminates the interference by cerium and increases the sensitivity.