Studies of sample cleaning for the determination of ultratrace amounts of carbon in high-purity iron by infrared absorption after combustion

For a precise and highly sensitive determination of ultratrace amounts of carbon in high-purity iron, cleaning methods of the sample surface were investigated. The cleaning methods tested were 3 kinds of chemical etching reactions (etching in HF-H₂O₂, electrolytic etching in acetic acid-HCIO₄ and etching in HCl); the effects of these cleaning on the determination of carbon were investigated. Contaminated carbon amounts, not only by adsorption on sample surface, but also by invasion inside sample, the could be removed in all cleaning methods. In etching with HF-H₂O₂, the lowest analytical value of carbon was obtained by using an electric tube furnace for sample combustion, while the highest analytical value was obtained for oxygen. However, the analytical values of carbon using an induction furnace were in good agreement in all methods. When samples were allowed to stand in a CO₂-H₂O atmosphere, the amounts of carbon adsorbed on the sample surface was found for all cleaning methods. Moreover, when etching using HCl, contaminated the carbon amounts were found not only on the surface, but also inside the sample. Only adsorbed of carbon amounts on the sample surface could be removed by heating at 623 K for 10 minutes in an ambient atmosphere. Both contaminated carbon amounts could be removed by etching in HCl.