Impediments in the Characterization of Copper-Nickel-Tin Spinodal Alloys by Atomic Absorption Spectrometry

The metallurgical properties of copper-nickel-tin (Cu-Ni-Sn) spinodal alloys are critically dependent on composition. One of the alloys of primary interest, Cu-15Ni-8Sn-0.25Nb, must be manufactured by suppliers to meet stringent Bell System specifications for nickel, 15 ± 0.5 percent, and tin, 8 ± 0.5 percent. Precision of analysis better than ±1 percent and absolute errors less than ±1 percent have been achieved by atomic absorption spectrometry (AAS). Problems such as limited sensitivity, matrix interferences, and instrument limitation have been resolved in order to obtain these results. A fuel-rich air-hydrogen flame was found to give the most stable and sensitive absorption measurement for tin at 224.6 nm. Significant interfering effects on tin absorption from acids and other metals in the low temperature flame were identified and corrected by proper addition of salts and acids to all solutions. Nickel was determined with an air-acetylene flame at the wavelength, 232.0 nm.

Measurements of absorbance by a rapid direct method were compared with a high precision method. Repeated determinations of both nickel and tin performed on a single sample solution showed that the high precision method was capable of 0.14 percent relative standard deviation (RSD) while the direct method gave 0.5 percent RSD.

A standard addition method was necessary to control the strong suppression and enhancement interferences by acids and cations on niobium absorptions. The presence of 3 percent iron was found to give a threefold enhancement of niobium absorption, allowing quantitative determinations down to 0.05 percent niobium.