ASTM D3634 Standard Test Method for Trace Chloride Ion in Engine Coolants
3. Summary of Test Method
3.1 The sample is first treated at a pH of 12 to 13 with aqueous hydrogen peroxide to oxidize the mercaptobenzothiazole to soluble, noninterfering sulfonate. The treated sample is dissolved in glacial acetic acid and titrated potentiometrically with dilute standard silver nitrate solution. Since the solubility of silver chloride in this system is sufficient to prevent obtaining a suitable inflection at the very low limit of the test method, some chloride is deliberately added to the glacial acetic acid solvent and then corrected for by a blank titration.
4. Significance and Use
4.1 This test method permits the determination of very low levels of chloride ion in engine coolants containing the common corrosion inhibitor, mercaptobenzothiazole, or related mercaptans, which would normally interfere with the titration by also forming insoluble silver salts with silver nitrate.
