Determination of Cl^- in Chromic Acid Anodizing Solution for Aluminium Alloy by Automatic Potentiometric Titration

The chromic acid anodizing solution (5.00 mL) was transferred into a beaker of 150 mL, then 10 mL of HNO₃ (1+1) solution, 10.0 mL of 100 g·L^-1 ascorbic acid solution were added. The solution was mixed well to make Cr⁶⁺ completely reduced to Cr³⁺ to eliminate the interference of chromic acid. Water was added to make the volume of the solution up to 60 mL. Cl^- in the solution was determined by automatic potentiometric titration, using 0.010 mol·L^-1 AgNO₃ standard solution as titrant at a stirring rate of 40-50 r·min^-1. The proposed method was applied to analysis of chromic acid anodizing solution containing Cl^- of 0.200, 0.500, 1.00 g·L^-1 and 3 batches of substantial samples from production lines, giving RSDs (n=6) ranged from 0.51% to 3.0%. Recovery tests of 3 batches of substantial samples from production lines were performed by standard addition method, and values of recovery were in the range of 97.0%-102%.