Estimation of Microgram Amounts of se4+ by Its Catalysis of the Ferric-lon/Thiosulfate Reaction
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have described the estimation of traces of c o o ~ e (Cu2+ r 1 and tunesten (W6+1on a micro scale Twnnrevious by rhmr camiys~softhe nhnrt- rearrmn. Oxidntmn states ofsrlenrurn, pnmded they r : m be rcdurrd by thiosulfntr then rrr,xdwed hy ferrtc >on,durtngthe cvurscofthe mcrhan~sm,also rntslyrc,, but ro a I e s d curnnt, and t h i i 1% thc. +ubjcrt d t h v prrsrmt l r t l d ~('oppcrnnd tungsten mujt benhsent tllercare n ~ ~ u t l ~ e r i n r r r f t r e nSea* ~ r z has hem in\r=tigatcd as the S e O P i o n , the latter purely far convenience. Experimental: As in the other two studies, distilled water and reagent-grade chemicals were used throughout. Solutions were stored in polyethylene bottles as was the ferric thiocyanate. The thiosulfate solution was 0.05 M. A solution of selenious acid in pure water was prepared to contain 20 vg Se4+lmLor pgldrop. In adjacent cavities of a ceramic spot plate were placed drops of the standard to give 1, 2, 3, and 4 vg Se4+,and a vacant cavity was left for the blank. The procedure thence was exactly the same as in the other two studies. Average times for the disappearance of the red thiocyanate color for the above were 192, 166, 144, 132 s, and, for the blank, 263 s. Room temperature was 14 "C . A plot of micrograms of Se4+ versus reciprocal time gave a linear relationship from which unknowns may be interpolated. I t appears t h a t the limit of detection is of the order of 1 pg. 7~
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'Lampard, M . J. Chem. Educ. 1990, 67,601. 'Lampard, M, submitted for publication in J. Chem. Educ.
M. Lampard 3 Nockolds Ave. Punchbowl, NSW, Australia 21 96
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Journal of Chemical Education
