This is a rapid procedure for estimating mercaptan sulfur in naphtha or other low-boiling petroleum distillates. Copper number is defined as the number of milligrams of copper required to titrate 100 mL of sample. Mercaptan sulfur is calculated from the copper number. Hydrogen sulfide, if present, will interfere and be included with the mercaptan. To determine the mercaptan content more precisely and distinguish mercaptans from hydrogen sulfide, use UOP Method 163. Mercaptan sulfur can be determined over a range of 15 mass-ppm to several percent.