Nitric Acid Synthesis

Warning:

Do not attempt to perform any of the following experiment(s) without taking extreme caution.
Always add acid to water, never the reverse.
Chemicals that fall under the categories below must be handled under a fume hood:

• Chemicals with a National Fire Protection Association (NFPA) Health rating of 3 or 4
• Toxic volatile chemicals
• Flammable chemicals
• Carcinogens
• A procedure that creates a toxic aerosol compound
• Reactive (highly exothermic) chemicals that may spatter
• Toxic gases (NH3, CO, F2, Cl2, H2S, NO2, etc.)
• Odorous chemicals

Always wear proper PPE, including:

• Safety goggles
• Disposable (nitrile) gloves
• Closed toed shoes
• Long sleeve shirt and pants

Introduction:

Synthesis of 68% 120mL of nitric acid will be used for future nitrogenation synthesis as part of my organic chemistry work.

Other uses of nitric acid include: rocket propellant, strong oxider, woodwork, and metal etching (refer to wiki link in the reference section for more information).

Reaction:

KNO3 + H2SO4 + heat → HNO3 + KHSO4

MSDS:

Procedure:

1) Prepare a simple distillation apparatus under a fume hood (apply silicone vacuum grease and keck clips at the joints)

 

 

 

2) In a round bottom flask (of at least 500mL) add roughly 50 mL of water and then slowly add approx. 108mL of sulfuric acid (Heat will quickly evolve!)

 

 

 

3) Add approx. 205g of potassium nitrate

 

 

 

4) You may insulate the round bottom flask and the distillation column (optional)

 

 

 

5) Heat to 120°C (water has a boiling point of 100°C and nitric acid distills at 120°C. Therefore, just before 120°C immediately replace the beaker with another container)

 

 

 

6) Distill nitric acid until most of the solution in the round bottom flask has distilled over to the receiver end.

 

 

 

7) The 68% nitric acid product will most likely be a dark brown/reddish color. This coloration will eventually fade if the container is left open as nitrogen dioxide dissipates.

 

 

 

Caution: Nitrogen dioxide gas (red/brown color) will form during synthesis as a result of  nitric acid decomposition to light and heat.  

4HNO3 → 4NO2 + 2H2O + O2

This is a very hazardous substance when inhaled. Therefore, this synthesis must be done under a fume hood.

 

Caution: Have a box of baking soda next you in case of sulfuric acid or nitric acid spills.

 

 

 

8) The other product of this synthesis is potassium bisulfate (white precipitate left over at the round bottom flask). This solid can be discarded by dissolving in  hot water and decanting to the waste container.

Calculation:

Why use 205g of potassium nitrate and 108mL of sulfuric acid? We want to make 125mL of nitric acid, so:

Discussion:

As you might have noticed, synthesis of nitric acid is not really “organic chemistry” because there are no carbons involved. I have put this experiment under organic section because, like I have mentioned before, the nitric acid will be later used as a precursor to nitrogenation synthesis.

Further distillation to make a concentrated nitric acid (above the 68% wt) is difficult to obtain. This is because 68% nitric acid with 32% water is a type of azeotrope solution, whereby they are “a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.” (refer to wiki to learn more about azeotropes and Raoult’s law).

However, there are other means of gaining more concentrated nitric acid, such as changing the vapor pressure between water and nitric acid by adding a third chemical to the mixture. Refer to this link for more info.

The concentration of nitric acid can be verified by calculating its density or more accurately titrating with a known concentration of strong base and indicator dye with an overlapping transition range until the end point (pH 7) is reached. The concentration of nitric acid by titrimetry can be found by stoichiometry equation. 

References:

  1. https://en.wikipedia.org/wiki/Nitric_acid
  2. http://www.sciencemadness.org/talk/viewthread.php?tid=13090