Potentiometric determination of hydrogen sulfide, carbonyl sulfide and mercaptans in petroleum gas products

This note describes the potentiometric determination of hydrogen sulfide, carbonyl sulfide and mercaptans in gaseous products of the oil industry (unnatural gas and liquefied petroleum gas). 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 (NaCH3COO · 3 H2O)
Glacial acetic acid
Ammonia, w(NH3) = 25%
Sodium hydroxide (NaOH) or potassium hydroxide (KOH)
Disodium ethylendiaminetetraacetic acid (Na2EDTA)
Monoethanolamine (MEA)
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.
Absorption solution for H2S and mercaptans: w(KOH) = 30% or w(NaOH) = 30%; addition of 5 g/L Na2EDTA
Absorption solution for carbonyl sulfide w(MEA) = 5% in ethanol
Sample preparation
The sulfur compounds to be determined are absorbed in alkaline solutions. The first two absorption vessels are filled with w(KOH) = 30% or w(NaOH) = 30%, the third absorption vessel with w(MEA) = 5% in ethanol.
Analysis
H2S and mercaptans
The contents of the absorption vessel are rinsed into the titration vessel with oxygen-free dist. water. The solution is titrated with alcoholic c(AgNO3) = 0.01 mol/L while passing nitrogen over.
Absorption solutions from refineries usually contain little H2S besides large quantities of mercaptans. In this case, the titration is stopped after the first equivalence point (which corresponds to H2S) and a second titration is carried out with a smaller sample size in order to determine the mercaptans.
Carbonyl sulfide
The absorption solution is rinsed into the titration vessel with oxygen-free ethanol. 1 mL w(NH3) = 25% is added and titrated with alcoholic c(AgNO3) = 0.01 mol/L while passing nitrogen over the solution.
Autotitrator (AT-710B/ AT-710S)
Combined Silver Electrode with Ag2S coating
Sealed Cell Flask Assembly
Reference: Reference: ISO 6326-3:1989, IP 272, UOP212-05

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