Fuming HNO3 is a highly Corrosive and Toxic acid with additional oxidising properties. FNA is formed when the solution contains more than 86% of the Acid. It can be either Red or White depending on the amount of Nitrogen Dioxide present in the solution.
Decomposition starts to occur at room temperatures above 95% when the solution becomes a yellow colour. Pure Nitric Acid with no dissolved Nitrogen Dioxide is classed as White Fuming Acid and should be stored at temperatures below freezing to avoid decomposition of the substance. The name 'Fuming Nitric Acid' generally refers to the Red Fuming Acid which is found at room temperature.
The category of White FNA is determined as follows; it is very close to the anhydrous substance and must contain no more than 2% water and 0.5% Nitrogen Dioxide. Various grades of Fuming HNO3 are available commercially and classified upon their concentration. One such grade which is used widely to manufacture explosives is approximately 90% Nitric Acid; this contains more water than the anhydrous substance so does not produce fumes at the same rate. The specification for Red Fuming Nitric Acid is a minimum of 10% Nitrogen Dioxide.
One of the properties of Fuming Nitric Acid is its ability to corrode metals. To overcome this problem, an inhibitor can be added in small amounts e.g. Hydrogen Fluoride at approximately 0.6%, which forms a metal fluoride layer on the surface.
The various ACID forms can be given the following abbreviations:-
- White Fuming Nitric Acid - WFNA
- Red Fuming Nitric Acid - RFNA
- Inhibited Form - either INFNA or IRFNA
Because of the Hazardous nature of the substance, FNA is classified as a dangerous goods for transport.
The UN number either refers to the Red Fuming composition and is UN 2032, Nitric Acid, Red Fuming or UN2031, Nitric Acid, other than Red Fuming with more than 70% HNO3. Label requirements for its carriage are Corrosive, Oxidising and Toxic for Red Fuming and Corrosive, Oxidising for other forms. The Packing Group is I with no Limited Quantities.
White Fuming HNO3 is a safer alternative to the red form with regard to toxicity because it contains less or no Nitrogen Tetroxide, a highly volatile and toxic chemical. Both forms can be used as rocket propellants.
Fuming Nitric Acid can be prepared by distillation of concentrated acid solutions to remove the water, or by mixing the concentrated acid with Sulphuric Acid prior to distillation at higher temperatures. An alternative method is to react dry Nitrogen Dioxide with Hydrochloric Acid gas in a reaction chamber to produce the Nitric Acid and Nitrosyl Chloride gas. This would be less energy intensive, less wasteful and performed at atmospheric temperature and pressure