(a) The potassium ferricyanide titration method described in appendix A to part 425 shall be used whenever practicable for the determination of sulfide in wastewaters discharged by plants operating in all subcategories except the hair save or pulp, non-chrome tan, retan-wet finish subcategory (subpart C, see § 425.30). In all other cases, the modified Monier-Williams method as described in appendix B to part 425 shall be used as an alternative to the potassium ferricyanide titration method for the determination of sulfide in wastewaters discharged by plants operating in all subcategories except subpart C.

(b) The modified Monier-Williams method as described in appendix B to part 425 shall be used for the determination of sulfide in wastewaters discharged by plants operating in the hair save or pulp, non-chrome tan, retan-wet finish subcategory (subpart C, see § 425.30).