A method and system that characterizes hydrogen sulfide in petroleum fluid employs a tool that includes a fluid analyzer for performing fluid analysis (including optical density (OD) for measuring carbon dioxide concentration) of a live oil sample, and a storage chamber for an analytical reagent fluidly coupled to a measurement chamber. An emulsion from fluid of the sample and the reagent is produced into the measurement chamber. The reagent changes color due to pH changes arising from chemical reactions between components of the sample and the reagent in the measurement chamber. The tool includes an optical sensor system that measures OD of a water phase of the emulsion at one or more determined wavelengths. The pH of the water phase is derived from such OD measurements. The pH of the water phase and the carbon dioxide concentration in the sample is used to calculate hydrogen sulfide concentration in the sample.