Nitric acid is a highly corrosive, inorganic mineral acid with the chemical formula HNO3. It’s typically sold at a concentration of 68% by volume, although stronger varieties are available. While nitric acid has a range of commercial and industrial applications, it’s primarily used in the manufacture of fertilisers as a precursor to ammonium nitrate.
In this post, you’ll learn more about the chemistry, properties, and characteristics of nitric acid. We also consider its important industrial and environmental applications.
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Introduction to Nitric Acid
The discovery of mineral inorganic acids such as nitric acid can be traced as far back as the 13th century. During this time, alchemists found ways to synthesise nitric acid from the flowers of the salt-tolerant Nitre bush, Cytrus vitriol (copper sulphate), and Yemen alum. Today, the substance is industrially produced through the Ostwald and Haber process (more on this later).
Nitric acid has a range of uses and industrial applications. However, before we look at these in more detail, let’s remind ourselves of exactly what nitric acid is.
Definition and Basic Properties of Nitric Acid
Nitric acid (HNO3) is classified as a mineral acid, which means it’s derived from inorganic compounds. The nitric acid that’s commercially available is an azeotrope with water. An azeotrope is a liquid mixture with a constant boiling point, despite the different components.
Commercially available nitric acid solutions, which are typically sold at a concentration of 68% by volume, have a boiling point of 120.5 °C. They are a colourless liquid at room temperature, although they can sometimes turn yellow over time.
The Chemistry of Nitric Acid
To understand what nitric acid is and how it is commercially used, it’s important to first understand the properties of an acid and what happens when it’s involved in a chemical reaction. We’ll then look at the chemical composition of nitric acid and its common by-products.
Characteristics of Acids
What makes an acid an acid? Well, an acid is a chemical substance that readily accepts electrons or donates protons (typically hydrogen ions) in a chemical reaction. An acid has a pH below 7 (as opposed to a base substance, which has pH of above 7.)
The pH scale is a logarithmic scale from 0 to 14. The scale indicates whether a solution of a substance either tends to donate hydrogen (proton) ions or accepts hydrogen ions.
In order to calculate the pH of an aqueous (water-based) solution, you need to know the concentration of the hydronium ion in terms of mole per litre. The computation is based on this formula:
pH = – log [H3O+]
Acidity and pH Scale
Nitric acid is one of the seven strong acids. All other acids are classified as weak acids, regardless of their concentration.
Strong acids are defined by their capacity to completely dissociate into ions when dissolved in water. The pH of nitric acid at 1 mM concentration is 3.01.
Reactivity Factors of Acids
Just like other chemicals, many factors influence how acids react. They include variables such as the pH level and concentration, the strength of the acid, its electronegativity, the chemical composition of both the acid and the reactant, and the polarity of the reactants.
All of these factors determine the rate at which acids react, as well as how much energy is released and the by-products of the reactions.
- Acid pH level – Each step in the scale from neutral pH 7, either lower or higher, is ten times the previous one. For example, a solution of pH6 is ten times more acidic than a solution of pH7. Conversely, a solution with a pH of 8 is ten times more basic (alkaline) than a solution with a neutral pH of 7. Therefore, acids with lower pH level are more reactive or corrosive.
- Chemical composition – Aside from the pH level, the reactivity of an acid also depends on its chemical composition. Likewise, the composition of the reactant also matters. For example, nitric acid reacts with most metals but it does not react with precious metals like pure gold.
- Electronegativity – Acid reactivity also has something to do with the electronegativity of a reactant substance, or how strongly atoms attract bonding electrons. Most metals have low electronegativity, which means that they easily give up electrons.
- Polarity – Most mineral and inorganic acids do not react and won’t dissolve in oil because oil is a nonpolar substance. This is the reason why some very corrosive acids like hydrofluoric acid are stored in plastic containers. Plastics are oil-based.
Understanding Nitric Acid
Nitric acid forms naturally during thunderstorms when the elemental nitrogen and oxygen in the air react with each other. The heat of the electron bolts facilitates the reaction between nitrogen and oxygen, forming nitrogen oxide and nitrogen dioxide. These gases then form nitric acid when they react with the water vapour in the air.
The industrial production of nitric acid, however, requires much more precision and control. It’s mainly produced through the Ostwald and Haber process. Firstly, anhydrous ammonia is burned into nitrogen oxide, which is then oxidised by atmospheric oxygen into nitrogen dioxide. It is then passed through water to form nitric acid and the nitric oxide feedstock.
Chemical Structure of Nitric Acid
Nitric acid is a relatively simple compound containing only three elements: hydrogen, nitrogen, and oxygen. Chemically, its formula is written as HNO3.
You can see that nitrogen is at the centre and one oxygen is double-bonded with the nitrogen, while the other two oxygen atoms are single-bonded. They are in what is called a trigonal planar form (a molecular shape in which there are three bonds and no lone pairs around the central atom) while the hydrogen is attached to one of the oxygen atoms.
Each of the three oxygen atoms carries a negative 2/3 charge while the nitrogen atom carries a positive 1 charge. This arrangement is crucial in the reactivity of the compound.
When it disintegrates, nitric acid produces water and nitrogen dioxide. For instance, when nitric acid reacts with copper, the balanced chemical reaction can be written as:
4 HNO3(l) + Cu(s) ==> Cu(NO3)2(s and aq) + 2 NO2(g) + 2 H2O(l)
Nitrogen dioxide is liberated as a noxious brown gas while cupric nitrate is formed as a blue precipitate. During the reaction, the solution will first turn green, then greenish-brown before finally turning blue as it is diluted with water. Just like in other acid-metal reactions, the process is exothermic, releasing excess heat energy.
The Strength of Nitric Acid Compared
When dissolved in water, strong acids dissociate their ions completely. Nitric acid is a strong acid and it is one of the seven strong acids. All other acids are weak ones. The other strong acids are the following:
- HCl – hydrochloric acid
- H2SO4 – sulfuric acid
- HBr – hydrobromic acid
- HI – hydroiodic acid
- HClO4 – perchloric acid
- HClO3 – chloric acid
Nitric acid is present in small amounts of rain. It is commercially available at 68% concentration. It is most effective in this concentration. It’s important not to confuse acid strength with acid concentration.
A concentrated acid is more corrosive compared to diluted acid. But the measure of strength is ionic dissociation in water. Strong acids like nitric acid are more reactive this way.
Applications of Nitric Acid
Nitric acid has important roles in both our ecosystem and in industry. For example, the nitric acid that’s created naturally during thunderstorms reacts with soil minerals to form nitrates. These nitrates are used by plants to synthesise amino acids, which in turn serve as the building blocks of proteins.
When it comes to industrial applications, nitric acid is mainly used as a precursor to organic nitrogen compounds including fertilisers and explosives.
TNT, for instance, is made when nitric acid reacts with toluene to produce trinitrotoluene (preferred IUPAC name 2-methyl-1,3,5-trinitrobenzene), which is highly explosive.
C6H5CH3 + 3 HNO3 → C6H2(NO2)3CH3 + 3 H2O
As an oxidant, nitric acid is also essential for nylon production. The precursor to nylon, adipic acid, is synthesised on a large-scale basis by using nitric acid as an oxidising agent for KA oil. This oil is a mixture of cyclohexanone and cyclohexanol.
Industrial and Environmental Uses of Nitric Acid
Nitric acid naturally forms in the atmosphere when nitrogen dioxide is dissolved in rain droplets. Lightning facilitates the chemical reaction by splitting the strong molecular bonds of elemental nitrogen gas in the atmosphere, allowing it to combine with oxygen.
The nitric acid in rain forms nitrates in the soil, which are essential for plant growth. The nitrates serve as the basis for amino acids, which in turn become the building blocks of proteins. Nitrogen fixing-bacteria, on the other hand, follow a different pathway in converting the nitrogen in the air into usable forms such as ammonia.
Nitric acid is used in many types of industrial applications that require the production of other nitrogen compounds. Nitric acid is used in the following:
- Manufacture of fertilisers
- Production of explosives
- Nylon precursors
- Speciality organic compounds
- Rocket propellant
Nitric acid also has niche uses such as artificially ageing wood. A 10% solution of the acid is used to change the colours of maple and pine into grey-gold.
It is also very useful as a cleaning agent when combined with water and phosphoric acid. The blend is commonly used as a cleaning agent for food and dairy equipment.
Nitric acid is also combined with alcohol such as methanol or ethanol to form a solution known as nital. The solution is used for etching metal.
Is Nitric Acid an Oxidizer?
As a strong acid, nitric acid completely dissociates its ions in water solution into hydronium (H3O+) and nitrate (NO3−) ions. This means that it is a powerful oxidizing agent because it readily accepts electrons during oxidation-reduction reactions.
The positively-charged ion component of the acid removes the electrons from the atoms of a reactant. For instance, when nitric acid reacts with carbon, it oxidises it, forming carbon dioxide. The balanced chemical equation for this reaction is the following:
4HNO3 + C —> 2H2O+ CO2 + 4NO2
Nitric acid reacts well with other non-metallic elements such as sulphur, phosphorous, and iodine. In these reactions, the acid acts as the oxidising agent.
The Oxidizing Nature of Nitric Acid
An oxidising agent is a substance that can strip away the electrons of other substances, thereby getting reduced in the process. Meanwhile, an acid, according to the Brønsted–Lowry definition, is a substance that donates its protons.
Some acids can also act as reducing agents, depending on the oxidation state of the non-metal part of the acid. The nitrogen in nitric acid has an oxidation state of +5, the highest state. This makes nitric acid a very strong oxidising agent.
Nitric acid also releases oxygen when heated, which makes it even more reactive as an oxidising agent.
