scope:

This method is a convenient, though not rigorous, test for the plant control of refinery caustic solutions used to remove hydrogen sulfide, mercaptans and certain other weak acids from petroleum products. The method is also applicable to Merox circulating caustic. The results are significant only when correlated with plant experience.

While the Double Indicator Procedure may be a convenient procedure to use, a potentiometric procedure is more accurate and can distinguish between strong bases; i.e., sodium phenolate and sodium hydroxide. See UOP Method 209.