Standard Reference Materials: Determination of Oxygen in Ferrous Materials Srm 1090, 1091, and 1092

A description is presented of methods used for the determination of homogeneity and the establishment of the oxygen values certified by NBS for three ferrous standard reference materials. These standards are represented by two low alloy materials, ingot and vacuum melted iron containing 484 and 28 ppm of oxygen respectively, and a high alloy steel containing 131 ppm of oxygen. The analyses of these materials are based on two vacuum fusion procedures. The poor results for the high alloy steel obtained by the first of these methods are explained. The first method is based on manometric measurements and depends on the conversion of CO to CO^ by copper oxide and on a differential freezing of gases. In the second method a system is used which provides a highly efficient degassing of sample and a direct measurement of CO by infrared absorbancy. Additional data from an analysis by inert gas fusion method also indicate that the homogeneity of a single rod of ingot iron, over a wide range of sample size, has a relative standard deviation of less than two per cent. From control data and homogeneity studies it was ascertained that the uncertainty limits stated in the NBS certificate include the error due to the relative inhomogeneity between rods. Finally, data from eighteen laboratories cooperating in this program are presented.