Preparation of Anhydrous Nitric Acid - Lu Le Laboratory

Nitric acid (HNO3), also known as aqua fortis and spirit of niter, is a highly corrosive strong mineral acid. The pure compound is colorless, but older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. Most commercially available nitric acid has a concentration of 68%. When the solution contains more than 86% HNO3, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as white fuming nitric acid or red fuming nitric acid, at concentrations above 95%. Nitric acid is also commonly used as a strong oxidizing agent.

White fuming nitric acid, also called 100% nitric acid or WFNA, is very close to anhydrous nitric acid. One specification for white fuming nitric acid is that it has a maximum of 2% water and a maximum of 0.5% dissolved NO2. Anhydrous nitric acid has a density of 1.513 g/mL and has the approximate concentration of 24 molar. Anhydrous nitric acid is a colorless mobile liquid with a density of 1.512 g/cm3, which solidifies at −42 °C to form white crystals. It boils at 83 °C. The anhydrous acid reqiures storage below 0 °C to minimize decomposition. (Wikipedia)

White fuming nitric acid could not be prepared from the distillation of 68% nitric acid, because water and nitric acid form azeotrope at 121℃ . So if we want to get pure nitric acid, we could prepare it from the distillation of the mixture of potassium nitrate and concentrated sulfuric acid (98%).

When potassium nitrates react with sulfuric acid, it produces nitric acid and potassium bisulfate that is because the K_a1 value (dissociation constant) of sulfuric acid is about 10⁵, the K_a value of nitric acid is 10^1.3 but the K_a2 value of sulfuric acid is only 1.03x10^-2. So if we want to use the method to produce more anhydrous nitric acid, we should add more sulfuric acid but not just dependent on the moles of protons.

KNO₃ + H₂SO₄ → KHSO₄ + HNO₃

In another view point, when we heat the reacting mixture (KNO₃ + H₂SO₄) , the boiling point of pure nitric acid is 83℃ and sulfuric acid is 337℃ . So the reaction may tend to form nitric acid and potassium sulfate at the boiling point of nitric acid (83℃).

KNO₃ + KHSO₄ → K₂SO₄ + HNO₃

To ensure our yield of anhydrous nitric acid is higher, adding some excess sulfuric acid is very helpful. It makes the final product less water and produces more nitric acid. So we add ~2 eq. sulfuric acid into 1 eq. potassium nitrate to produce anhydrous nitric acid.

Chemical

1.      Potassium nitrate: 30 g

2.      Sulfuric acid: 98% 35mL

Procedure

1.      Place 30 g potassium nitrate into a round-bottomed flask.

2.      Pour 35 mL 98% sulfuric acid and place a stir bar into the flask.

3.      Set up a simple distillation apparatus and start heating the round-bottomed flask with a hot plate with stirring. Soak the collecting vessel in a cold-water bath.

A part of nitric acid decomposes into 

nitrogen dioxide which appears dark red

4.      Collect the 75~85℃ distillate and prevent it from light and heat.

Experimental Record

Weight of potassium nitrate	30.00g
Volume of 98% sulfuric acid	35.0 mL
Anhydrous nitric acid obtained	19.38 g
Theoretical weight of anhydrous nitric acid	20.11 g
Yield	96.4 %