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 benefits of this technology can be summarised as follows:

• As the measurement is specific to H₂S, there will be no interference from other sulphur components
• Fast response (seconds) compared to ion-selective electrodes (hours)
• The instrument has a wide range capability, measuring between 0-2ppm to 0-1% H₂S
• Configuration of the system is via a touch screen which eliminates the need to open the enclosure.
• Real time compensation for pH, pressure and temperature
• Optional analog outputs for pH, pressure and temperature
• Suitable for gas group IIC environments.

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.