The accurate determination of selenium, which is added to stainless steel in order to improve the corrosion resistance, is important for the steel industry. In a preceding paper', we reported on a sensitive and reproducible method for the determination of selenium in iron and steel without preliminary separation of iron matrix. This consisted of the electrodeposition on a rotating silver disk electrode as silver selenide in an acidic sample solution (pH 2.5) followed by the stripping in an alkaline medium. Voltammetric interferences from iron matrix and high nickel concentrations (more than about 40-fold amounts of selenium) were eliminated by adding sodium oxalate and by pre-electrolyzing at potentials more negative than -0.4 V vs. SCE, respectively. Unfortunately a time-consuming and laborious standard additions method was required because of serious interferences from the coexisting copper.In the present paper, accordingly, a further investigation has been carried out in order to establish an analytical procedure for the direct quantitation of the selenium content in stainless steel from a calibration graph. Compared with other methods for chemical analysis of stainless steel2, the advantages of this method without using harmful chemicals are simplicity, less experimental skill and rapidity as well as high sensitivity and excellent precision.
ExperimentalThe apparatus, reagents and the procedure were the same as those described previously"3, except that sodium bromide was used as the masking agent and that an amount of selenium was determined by reference to a calibration (peak height vs. selenium concentration) graph, which was prepared with 4 cm3 of a 1 % acid mixture (l-to-1-to-198 mixture of hydrochloric acid, nitric acid and water) of pH 2.5 containing 0 to 200 ng of selenium(IV). The sensitivity of the recorder was 31 µA cm'.
Results and DiscussionSodium bromide was found to be suitable for masking iron(III) and copper(II) completely. As shown in Figs. 1 and 2, 2 mol dm 3 sodium bromide was selected, allowing the interferences from up to 0.75 mg cm 3 of iron(III) and 60 ng cm 3 of copper(II) to be eliminated. The surface of the silver electrode changed to grayish color after pre-electrolysis in a 2 mol dm 3 sodium bromide solution containing 1 mg cm3 of iron(III). This seems to be due to the deposit of elementary iron. Pre-electrolysis was performed at -0.5 V vs. SCE in order to suppress the interference from high nickel concentrations.The deposition time and the scan rate were the same as those previously recommended.',3 More than 50% of selenium in a 4-cm3 aliquot of the electrolyte containing 3 mg of iron was electrodeposited on the rotating silver disk electrode (2.0X103 rpm) during 30 min, which yielded adequate sensitivity for selenium determination. The deposited silver selenide was completely stripped during a single Cathodic scan at a rate of 50 mV s-'. The technique of medium exchange, i.e., the use of a 2 mol dm-3 sodium hydroxide solution as the stripping medium, was employed in order to a...