Duty: To measure the concentration of dissolved H2S in a waste water stream and to calculate total dissolved sulphides.

Process water flows through a sensor equipped with a propriety membrane. Due to the partial pressure effect, the membrane is only pervious to gases, not to ions, liquids or solids.

The sensor contains a buffer solution with a redox catalyst and 3 electrodes. A polarisation voltage is applied to the electrodes to give a ratio of the oxidised/reduced redox catalyst. The H2S reacts, first chemically in presence of the redox catalyst to form a reaction product and then the electrochemical oxidation of the reaction product takes place at the measuring electrode. This causes a flow of current proportional to the concentration of dissolved H2S present in the sample. The measurement is compensated for pH, pressure and temperature changes on a real time basis, and these can be reported as separate 4-20mA outputs (optional) if required.

The measurement of dissolved H2S in water is important as it can be highly toxic and foul smelling. Also, H2S can biologically degrade to sulphuric acid which can cause corrosion in concrete and metal pipes and sewage systems.

Applications can be found in:

• Ethylene quench towers
• Waste water treatment
• Sewage systems
• Coke oven quench towers
• Steel plant scrubbers
• Sour water plants
• Well-head injection

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