POTENTIOMETRIC DETERMINATION OF SULFIDE

2.1 The distillations in Methods 9030 and/or 9031 are performed, except that the scrubber solution is sulfide anti-oxidant buffer (SAOB), with ascorbic and salicylic acids added as oxygen scavengers. The distillates are then analyzed potentiometrically using a sulfide ion-selective electrode (ISE) in conjunction with a double-junction reference electrode and a pH meter with an expanded millivolt scale or an ISE meter capable of being calibrated directly in terms of sulfide concentration. 2.2 This method is an alternative to the iodometric titration procedure, where a solution of thiosulfate is standardized against the thiosulfate solution, and the sulfide standard is standardized against the iodine solution/thiosulfate solution (which requires daily calibration itself). This method allows for standardizing the sulfide calibration standards by a potentiometric titration with standardized silver nitrate using the sulfide ISE as the working electrode. Silver nitrate solutions are stable when stored properly and are easily standardized against sodium chloride. 2.3 The key to acceptable recoveries is the use of the proper apparatus and careful assembly of the distillation apparatus prior to distillation. If these steps are not taken, low recoveries will result.