CORRECTION Determination of Thorium a n d Uranium in Ores and Mill Tailings by Alpha Spectrometry In the paper by C. W. Sill, Anal. Chem., 49,618 (1977),part of a sentence was omitted. On page 619, the sentence beginning on the tenth line from the bottom in the right-hand column should read as follows: “When dry, place the plate first on a hot plate covered with a piece of asbestos cloth for 1 min, then on an uncovered hot plate for about 5 min and cool.” Also, it now seems necessary to call attention to the need to correct the thorium-234 tracer recovered for that contributed by the sample itself when relatively large samples of uranium ores, uranium concentrates, or fresh mill tailings are being analyzed for thorium-230. With ores, or with concentrates older than a few months since the last chemical separations were made, the correction can be determined from the uranium-238 activity, if known. However, with concentrates in which the ingrowth of thorium-234 is unknown, with tailings less than a few months old from which the thorium-234 will not have decayed sufficiently, or if uranium-238 is not being determined, the correction must be determined by other means. The simplest way is to increase the thorium-234 added to about lo5 dpm instead of the “at least l o 4 dpm” recommended previously. Because 0.27% uranium ore contains 2 X lo3 dpm/g of thorium-234, the error will be less than 2% for samples up to 1 g, even if no corrections are applied. Similar arguments can be made for uranium concentrates and tailings. With larger samples or higher-grade ore, corrections will have to be made. It should also be emphasized that this method for the determination of thorium-230 in uranium ore is not applicable to ores containing relatively high concentrations of natural thorium and/or the light lanthanides, e.g., uraniferous monazites. Unless the total quantity of these elements in the aliquot taken for analysis is kept less than about 1mg, they will so saturate the barium sulfate lattice positions that precipitation of thorium-230 will be grossly incomplete. See Anal. Chem., 41, 1624 (1969) for effects of these elements on precipitations with barium sulfate. Also, the thorium, cerium, lanthanum, etc., carried in the barium sulfate form complexes with the potassium sulfate also present that are very insoluble in even hot 72% perchloric acid, making it appear that the barium sulfate fails to dissolve in the perchloric acid. Because of their solubility in water, however, the insoluble precipitates dissolve readily on addition of the strong aluminum nitrate solution.
1648
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
