This note describes the potentiometric determination of hydrogen sulfide, carbonyl sulfide and mercaptans in liquid products of the oil industry (used absorption solutions, distillate fuels, aviation petrol, gasoline, kerosene, etc.). The samples are titrated with alcoholic silver nitrate solution using the Ag combined electrode coated with Ag2S for the equivalence point indication.
Reagents
Silver nitrate (AgNO3)
Isopropanol (IPA) or ethanol (EtOH)
Sodium acetate trihydrate (CH3COONa · 3 H2O)
Glacial acetic acid
Ammonia, w(NH3) = 25%
Potassium chloride standard, c(KCl) = 0.1000 mol/L
Required Solutions
Titrant: AgNO3 = 0.01 mol/L. suitable weight of AgNO3 is mixed with 80 mL dist. water and made up to 1000 mL with ethanol or IPA.
Alkaline solvent for low molecular mercaptans: 2.7 g NaCH3COO · 3 H2O is dissolved in 25 mL dist. water. Add 975 mL IPA as well as 10 mL w(NH3) = 25% and mix them.
Acidic solvent for higher molecular mercaptans: 2.7 g NaCH3COO · 3 H2O is dissolved in 25 mL dist. water. Add 975 mL IPA as well as 4.6 mL glacial acetic acid and mix them.
Analysis
suitable volume of sample is added to 100 mL solvent*. While passing nitrogen over the solution, the solution is titrated with alcoholic c(AgNO3) = 0.01 mol/L until after the first or second equivalence point, depending on the sample. If the sample contains only H2S or mercaptans only one equivalence point is found, if the sample contains both two equivalence points are found. If the sample does not dissolve completely in the solvent, some toluene can be added.
1) For aviation petrol, kerosene and distillate fuels, which normally contain higher molecular mercaptans the acidic
solvent is used.
2) For low boiling hydrocarbon fractions, which also contain low molecular mercaptans, the alkaline solvent is used.
Instrument
Autotitrator (AT-710B/ AT-710S)
Combined Silver Electrode with Ag2S coating
Sealed Cell Flask Assembly
Reference: ASTM D 3227, IP342, UOP 163, ISO3012:1999, UOP209-00
