Metrohm ASTM WK48004 test method for acid number in crude oil

A test method for the determination of the acid number in crude oil and petroleum refinery intermediates and products is under active investigation by ASTM (ASTM WK48004).

The method is claimed to be more accurate, more precise, more robust and more repeatable than the ASTM D664 potentiometric method. It is also said to be faster and to use less solvent, meaning reagent and waste disposal costs are much lower.

The thermometric endpoint titration (TET) method utilises the same titrant (0.1 mol/L KOH in 2-propanol) as ASTM D664. Instead of using a potentiometric probe (which is liable to fouling of the membrane and clogging of the reference junction after only a couple of determinations), the endpoint is detected using a sensitive, fast-responding electronic thermometer.

The probe requires essentially no maintenance, aside from a solvent rinse, and requires no hydration or calibration. Always ready for use, it can be used by operators as a ‘walk-up’ method or fully automated with a sample changer in the laboratory. Because the method is fast, it is said to increase corrective response time to varying process conditions, with consequent savings to equipment and process running costs.