Concentration and Mass Flow Measurement of Concentrated Nitric Acid The industrial production of nitric acid (HNO 3 ) is based on the Ostwald process. Ammonia is oxidised via a platinum catalyst and the resulting nitrogen dioxide (NO 2 ) 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 NO 2 is dimerised to dinitrogen tetroxide (N 2 O 4 .). After purification, the N 2 O 4 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 analyzes are carried out to ensure the required concentration is maintained. If the concentration is below 98.5%

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