1989 Volume 75 Issue 1 Pages 181-186
A rapid potentiometric titration mothod of free sulfuric acid in the colouring solution for stainless steels is described.
The solution was composed of chromic acid (CrO3, 250 g/l), sulfuric acid (H2SO4, 500 g/l) and metallic sulfate ions as Fe2 (SO4)3, Cr2( SO4)3 etc.
For the control of the tints and qualities of coloured films, the determination of free sulfuric acid was needed. But the conventional procedures could not be employed because of co-precipitation of both sulfuric acid and metallic sulfate as BaSO4.
It was found the follwing procedure was effective to take away the interference of the metallic sulfate ions.
(1) At first, total [H+] at 1st titration end point of H2SO4 and H2Cr2O7 in the sample solution was determined with potentiometric titration method after addition of Zn-EDTA for masking metallic ions.
(2) Free sulfuric acid concentration was determined after subtraction of [H+]CrO3 of 1st dissociation of CrO3 from total [H], which was determined by the another chemical analysis of Cr(VI).
The time required for this method was about 30 minutes for one sample and the results were accurate and reproducible.
