A Kinetic Experiment for Determination of Traces of Cu2+ M. Lampard 3 Nockolds Avenue. Punchbowl, NSW 2196, Australla
This experiment is an adaptation of a spot technique used
to identify trace amounts of cupric ion14 (in the absence of tungsten and selenium), by its catalytic effect on the ferric ion/tbiosulfate reaction. Ferric salts react with thiosulfates via the following equations, reaction l giving a violet complex ion
Reaction 1is fairlv ranid: redox reaction 2 is slow but is enormously accelerated 6y traces of copper salts. If the reaction is conducted with thiocvanate mesent, which serves as an indicator for ferric ion &d also ;etardsreaction 2, then
'
Vogel, A. I. A Textbook of Macro and Semi-Micro Qualitative inopnic Analysis, 4th ed.; Longmans: London, 1954: p 228. 2 Goto, H.; Sudo, E. Sci. Repis Research Insts. Tohoku Univ. 1949, 7., 30-dn -- .-. Goto. H.; Suzuk. S. Sci. Repis. Research insts. Tohoku Univ. 1952,4.35-40.
Yatsimirskil, K. 6. Zhur. Anal. Khim 1955. 10, 339, 344.
the reaction velocity (proportional to the time taken for complete decolorization) may be used for detecting minute amounts of cupic ion. Experlmenlal The "ferric thiocyanate" reagent is prepared by dissolving 1.5 g of ferric chloride and 2.0 g of potassium thiocyanate in 100 mL of distilled water. A set of standards, each in distilled water, is prepared to contain 1wglml, 0.5 pgImL, and 0.45 pglmL. In adjacent cavities of a spot plate a drop of distilled water and a drop of the mwt concentrated copper standard are placed. One drop of the "ferric thiocyanate" reagent is added and then 3 drops of 0.05 M sodium thiosulfate solution; a seconds stopwatch is started on addition. The decolorization of the copper-free solution occurs in 1'12-2 min; decolorization is instantaneous if the test solution con-
- -" -.-----. .
The deedoriration times for each of the standards and the blank are recorded and a graph plotted of copper content against time. "Unknown" copper contents can then be interpolated from the - graph. The limit of detection (0.02 up Cu2+)could be investigated;so also could the effectof appropriate tungsten and selenium ~ a l w(in place ofcopper).asat kart some oxidation scares of these elements have a similar catalytic accelerating srtion,seienrum toa lesrereatent than tungsten.
Volume 6 7
Number 7
July 1990
601
