Determination of Nitrate and Fluoride in Spent Pickling Baths for Stainless Steel by Spectrophotometry and Potentiometry

Abstract

Hydrofluoric and nitric acid baths are widespread in pickling of stainless steel due to their rapid performance and excellent quality of treated surfaces. The concentrations of nitrate and fluoride play an important role in the process and therefore require regular monitoring. Simple methods for spectrophotometric determination for nitrate and potentiometric determination for fluoride were verified based upon current procedures for water analysis. After removing metals by cation exchange, the recoveries were 98.25–98.83% for nitrate and 98.15–100.6% for fluoride. These values were obtained by comparison between these methods and a reference ion chromatography procedure for real baths and the material balance measurements of artificial mixtures. Due to high metal contents in the bath, the proposed methods fail without sample pretreatment; the spectrophotometry of nitrates with sulphosalicylic acid is impossible, and fluoride potentiometry provides the results 20% lower compared to the references samples. The conditions for ion exchange pretreatment (i.e., exchanger capacity, specimen dilution, and flow rate) were optimized. The combination of simple fluoride and nitrate determination with sample pretreatment by cation exchange is suitable for monitoring the bath quality in small pickling plants, where an automatic analyzer would represent a high investment.