The ASTM D2674 standard specifies three different test procedures:
The sulfuric acid concentration of a sulfochromate solution is measured using Method A. The hexavalent and trivalent chromium concentration of a sulfochromate solution is measured using Method B. Method C is a substitute for determining the hexavalent and trivalent chromium concentration of a sulfochromate solution.
The following three test methods are employed in ASTM D2674 testing:
o Method A:
A 5-mL sample of sulfochromate etch solution is pipetted into a beaker and water is added. Then, the sample is titrated with the NaOH solution to a pH of 3.5, using a pH meter.
o Method B:
➢ For determination of trivalent chromium as sodium dichromate:
1 mL of the sample is pipetted into a beaker, followed by water and boiling H2SO4. The AgNO3 and ammonium persulfate solutions are added while the mixture is still hot, then boiled and cooled to room temperature. A drop of HCl and one drop of the phenanthroline ferrous complex indicator are added, and the solution is titrated with the standard ferrous ammonium sulfate solution until it becomes pink. The total volume added is recorded after adding an excess of 5 mL of titrating solution. Back titration is done with the potassium permanganate solution until the color changes to apple green. The amount of potassium permanganate that was used to get to this point is kept track of.
➢ For determination of hexavalent chromium as sodium dichromate:
1 mL of the sample is pipetted into a beaker, then 5 mL of H2SO4 is added, and the indicator is titrated with the standard ferrous ammonium sulfate solution until it turns pink, and then the standard ferrous ammonium sulfate solution is added in excess. The total volume of the ferrous standard solution added is kept track of. The potassium permanganate solution is then back titrated until the color of the solution becomes very faintly green.
o Method C:
Method C is intended as an alternative method in the ASTM D2674 standard for measuring the hexavalent and trivalent chromium content of a sulfochromate solution.
For ASTM D2674, the sulfochromate etch solution is itself used as a test specimen.
According to the ASTM D2674, the amount of sulfuric acid present/added, the amount of hexavalent and trivalent chromium present, and the amount of sodium dichromate added are all reported.
The ASTM D2674 test is a standard test method for the analysis of sulfochromate etch solutions used in the surface preparation of aluminum.