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The industrial production of nitric acid (HNO3) is based on the Ostwald process. Ammonia is oxidised via a platinum catalyst and the resulting nitrogen dioxide (NO2) is then absorbed in water. The acqueous solution can only be distilled to a maximum concentration of 68% (azeotropic mixture of nitric acid and water). In order to achieve a higher concentration, the NO2 is dimerised to dinitrogen tetroxide (N2O4.). After purification, the N2O4 is mixed with diluted nitric acid in the presence of oxygen and under pressure to form the concentrated nitric acid by direct reaction.
Regular sampling and lab analyses are carried out to ensure the required concentration is maintained. If the concentration is below 98.5%, the nitric acid is fed back into the process until it reaches the required concentration.

In order to optimise the process and thereby increase productivity, the plant's process engineers were looking for a method that would allow them to continuously monitor the concentration. Non-invasive measurement with FLEXIM's PIOX S non-invasive process analyser, which is simply mounted onto the outside of the pipe wall, proved to be the most suitable solution for this extremely corrosive medium.
PIOX S not only supervises the nitric acid concentration with an accuracy of 0.1%, but also measure the mass flow rate of the medium.


  • Reliable and accurate non-invasive flow and concentration measurement from the pipe wall outside
  • Process control in real time
  • No wear and tear – by using a durable acoustic coupling pad, maintenance is reduced to a minimum
  • No interruption to production, No risk of leakages, No risk of corrosion by the aggressive medium