Automated potentiometric titrations of sulphides and thiols in petroleum systems

Abstract

An automated titration system, based on a Sinclair ZX Spectrum microcomputer, for titrating thiols and sulphide using an Orion 94-16A solid-state ion-selective electrode as an indicator electrode is described. Ethane-, propane-, pentane- and heptanethiols were used in assessments for developing the titration-calibration system. The titrants employed were silver nitrate and mercury(II) chloride, and a statistical analysis showed that there was no significant difference between the two titrants. However, silver nitrate required the addition of 0.1 cm³ of 2 M ammonia solution to the otherwise optimum aqueous sodium hydroxide (9 cm³ of 1 M solution) plus ethanol (1 cm³) titration medium, and also shielding from light.

The automated titration system was applied to the monitoring of sulphur compounds in refinery process streams with emphasis on the spent caustic soda solution in the sweetening process and on the washed LPG stream, and was found to be suitable for titrating the sulphide and thiol components at the relevant <1% and ca. 1 p.p.m. ranges, respectively.