precision data obtained from several chromatograms. As would be expected, the calibration factors (mg. per unit area) for the four metal ions are about the same; the slight variations probably arise from differences in diffusion coefficients or from the fact that the true diffusion current is not measured a t the chosen voltage. For any of the metal ions thc factor varies only a few pc'r cent for quantities ranging from 2.0 to 0.2 mg., and for cobalt and copper tlie lower limit evtends to 0.02 nig. The factor for nickel is considerably loner a t the 0.02-nig. level, probably because this ion exhibits the broadrst peak (Figure 2), the area of nhich is greatly affected by any rise in base line. Ilanganrse \vas not deterniiiittl a t this l c ~ e l . Standard deviations of the calibration factors (or areas) obtained for 2mg. quantities range from 0.6 to 3.3%
of the mean. The precision for all but manganese is not much different a t the 0.2-mg. level, the range being 1.8 to 2.4% of the mean. However, manganese values exhibit a standard deviation of 8.i%. This is probably related to the poorly defined polarographic wave, which is closely follon ed by the n-ave from the supporting electrolyte. Thus, 0.2 nig. iq taken as the loner limit for the determination of manganese. At the 0.02-nig. level, tlie standard deviations of nickel, cobalt, and copper areas range from 8.6 to 19% of the mean. These results are expected froni the fact that peak currents are of the Same magnitude as the residual current. Traces of oxygen and other reducible impurities teiid to Cause the base line t o vary considerably. d huge excess of a n y of the ions studicd does not affect the precision of
results for t h r others. For example niangaimr was determined in a bronze (SRS) containing 1.29% manganese. Duplicate determinations obtained with 20-mg. samples gave results of 1.1 and I .370 manganese. ACKNOWLEDGMENT
The Iic~lpful wggcxstions of F. 11. S e l w i arc. gratefully acknon l d g c ~ l . LITERATURE CITED
(1952). ( 3 ) Kraus, I ., l I o o r r > G. E.! J . A - l i ~ i , Chevj. SOC. 75, 1160 (1!%:3). (4) Lingane, J. J., "E:lectroarial?-tical ChPmistry," p. 246, Interscience, S e w l o r k , 195:3. ( 5 ) 1Iann. C. I
