1. Nitric acid properties
2. Azeotropic acid

Nitric acid properties

Nitric acid (HNO₃) is a strong acid and a powerful oxidizing agent. The concentration of commercial nitric acid destined for fertilizer production depends on the method of production and is usually in the range of 42% to 65%. Such acid is known as weak acid, unlike concentrated acid (96.0%-98.5%) that is used for other industrial purposes. Weak nitric acid is usually directed for captive use. Further properties of nitric acid are shown in Table 1.

Anhydrous HNO₃ does not normally exist in liquid form. When strong solutions are distilled under special conditions, one obtains almost pure nitric acid having a concentration of 99.7%. When concentrated nitric acid is heated at atmospheric pressure, it boils at 78.2°C but begins to decompose and eventually yields a 68% HNO₃ solution with a maximum boiling point of 120.5°C. When dilute solutions of nitric acid are concentrated under atmospheric pressure, a maximum boiling-point solution again corresponding to 68% HNO₃ results (azeotropic concentration).

Further properties of nitric acid can be found in the literature. Typical impurities in HNO₃ originate from ammonia or the process. The standard limits are the content of Cl₂ – less than 5 ppmw, and HNO₂ – less than 5 ppmw in most cases.

For the effect of other contaminants, including the recommended thresholds, the reader is referred to the Kirk-Othmer Encyclopedia and related references.

Azeotropic nitric acid

Nitric acid forms an azeotrope with water at about 68 wt% of nitric acid and a boiling point at atmospheric pressure of 122°C. There are various implications from the occurrence of such an azeotropic mixture and the reader is referred to the dedicated literature. Most industrial nitric acid plants produce a weak acid with a concentration below 68%, usually 55-65%. Few plants are designed to produce a 68% nitric acid concentration.

References

Nitric acid, Kirk-Othmer Encyclopedia of Chemical Technology, M. C. E. Groves, John Wiley and Sons, June 2020. https://doi.org/10.1002/0471238961.1409201803120118.a01.pub3.

Links to related IFS Proceedings and recordings

540, (2004), Nitric Acid Production and Abatement Technology Including Azeotropic Acid, L M Marzo

Links to External Resources

Fertilizer Manual, edited by the United Nations Industrial Development Organization (UNIDO) and the International Fertilizer Development Center (IFDC), Kluwer Academic Publishers, 1998, pages 209-220.

The nitrogen industry – properties, G. D. Honti, Akademia Kiado, Budapest, Hungary, 1976, pages 409-413.

Various processes for the production of commercial-grade nitric acid, G. D. Honti, 1985, Fertilizer science and technology series, USA, pages 61-98.

Various processes for the production of concentrated nitric acid, G. D. Honti, Nitric acid and fertilizer nitrates, Marcel Dekker, New York – Basel, 1985, pages 99-130.