An acid is a chemical, which when added to water, increases the amount or concentration of hydrogen ions, or H⁺, making it acidic.

A hydrogen ion (H⁺) is just a hydrogen atom without its electron. And, as a hydrogen atom is just one electron and one proton, the term hydrogen ion (H⁺) can be used interchangeably with proton.

How does a chemical increase the concentration of H⁺ when added to water?

Historically, three models were proposed to describe how a chemical increases H⁺. The first model was proposed by Svante Arrhenius. He defined an acid as a chemical that “releases” H⁺ into water.

The second model was developed independently by chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry. They defined an acid as a chemical that “donates” H⁺ to water, like the Arrhenius model.

The third model was proposed by Gilbert Lewis. He defined an acid as a chemical that can “accept” a pair of electrons. According to this model, H⁺ is considered an acid because it can accept a pair of electrons from a hydroxide base (OH^–). This definition is the most comprehensive as it covers all chemical acids. But without a background in chemistry, it’s more difficult to grasp.

General Properties of Acids

• They are liquids
• They have a sour taste like vinegar
• They have a sticky texture
• They have a sharp smell that can burn your nose
• They turn litmus paper red

Classes of Acids

Acids can be classified into different subgroups depending on their properties and atomic structure. There are strong acids, weak acids, concentrated and dilute acids, oxyacids and hydracids, and organic and inorganic acids.

Strong Acids

Strong acids are chemicals that donate all their H+ when dissolved in water. The most common example is hydrochloric acid or HCl. When dissolved, it completely splits into H⁺ and Cl^–.

Strong acids can be corrosive because they donate all the H⁺ to water. A high concentration of H⁺ in water can react with biological tissue. Protective clothing must be worn when handling strong acids. If handled properly, they can be useful. Here is a list of some common strong acids and uses.

• Hydrochloric acid (HCl) is best known as the acid in our stomachs. It helps with digestion and protection from infectious microbes. But it’s also used as a cleaning agent, from toilet bowl cleaners to steel production.
• Sulphuric (H2_SO₄) acid is another example of a strong acid. It completely splits into 2H⁺ and SO₄^2- (sulphate). It has a variety of industrial processes such as making medicines, fertilizers, dyes and pigments, explosives, and it can also be used in the home as a drain cleaner.
• Nitric acid (HNO₃) is a common strong acid. It’s mostly used to produce fertilizer, but it’s also used as a cleaning agent, to make nylon, and even artificially aging wood to make it look antique.
• One of the most powerful strong acids is perchloric acid (HClO₄). It’s dangerously corrosive and readily forms explosive mixtures. It’s so powerful, it’s used to produce rocket fuel and to etch and polish metal.

Weak Acids

Weak acids, on the other hand, donate only a tiny fraction of H⁺ when dissolved in water.

• The most well-known weak acid is acetic acid (CH₃COOH), the acid in vinegar. In the home, we use acetic acid as a food condiment, to pickle vegetables, or to descale coffee machines. Industrially, acetic acid is used in photography, adhesives, paints, dyes, and perfumes.
• Another well-known acid is lactic acid (C₃H₆O₃). It’s found in sour milk products like yogurt of kefir, as well as sourdough bread and even sour beers. And when you exercise, the burning sensation you feel is caused by a buildup of lactic acid, which is an important metabolite.
• Phosphoric acid (H₃PO₄) is also a weak acid. It’s mainly used to make fertilizers, however, its uses extend to foods, such as soft drinks and jam. Other industrial uses include making soaps, detergents, toothpaste, as well as water treatment.
• Formic acid (HCOOH) is an acid primarily used as a preservative and antibacterial agent in livestock feed. It’s also used to descale household appliances, to tan leather, and to process latex.
• Hydrofluoric acid (HF) is used as a starting material for cleaning, silicon chip manufacturing, industrial chemistry, mining, and glass finishing.

Concentrated and Dilute Acids

Concentrated and dilute acids shouldn’t be confused with strong and weak acids. Whether an acid is strong or weak depends on properties of the acid itself, as we already discussed. When you add an acid to water, we dilute it. If we dilute it a little, it’s concentrated. The more water we add, the more diluted it becomes.

Concentrated acids can be dangerous even if they’re strong or weak acids. Concentrated weak acids like acetic acid or phosphoric acid can cause burns. But diluted acids can be useful, like acetic acid in vinegar, which is only 5% acetic acid and 95% water.

When we use acids, we typically dilute them with water to a useful strength. Pure acids can be either solid, like a powder, or liquids. We can calculate how much water to add to achieve the strength we want. For example, an 80% HCl solution can be made by adding 4 parts 100% HCl to 1-part water. Likewise, a 5% acetic acid solution (vinegar) can be made by adding 1-part acetic acid to 19-parts water.

Oxyacids and Hydracids

Acids can have alternative classifications based on their chemical structure. An oxyacid is an acid that has at least one oxygen atom, such as sulphuric acid (H₂SO₄) or acetic acid (CH₃COOH). A hydracid is an acid that doesn’t have any oxygen atoms, such as hydrochloric acid (HCl).

Organic and Inorganic Acids

By definition, any chemical with carbon atoms is an organic chemical. So, organic acids have carbon, such as acetic acid (CH₃COOH) or lactic acid (C₃H₆O₃), while inorganic acids don’t contain carbon, like hydrochloric acid (HCl) or sulphuric acid (H₂SO₄).

Quantifying Acids

pH – Measures Acid Strength

pH is a way to measure the acidity or the H⁺ concentration in a solution. pH is calculated using the equation: pH = -log[H⁺], where [H⁺] is the concentration of H⁺ in water. So, if [H⁺] is 0.001 Molar (M) or 10^-3, then the pH is 3. If the [H⁺] is 0.1 M or 10^-1, then the pH is 1. The higher the [H⁺], the lower the pH value.

We can measure the pH of a solution using a pH meter. A pH meter is like a voltmeter that measures voltage. H⁺ has a positive charge. The more H⁺, the higher the positive charge, and the higher potential to generate an electrical current.

To measure the pH of a solution, the pH meter must first be calibrated with a solution of a known pH. Typically, pH meters are calibrated with solutions at pH 4, 7, and 10. Once the pH meter has been calibrated, it’s dipped into the solution and the pH can be read.

pKa – Describes Acid Strength

The pKa of an acid is a numerical way of describing the strength of an acid. As we discussed, strong acids are strong because they release all H⁺ to water, while weak acids release only a small fraction of H⁺. The pKa value tells us how much H⁺ is released.

For example, when we mix acetic acid (CH₃COOH) and water, some of the H⁺ is released (or dissociates). In Figure 1, the released H⁺ binds to water (H₂O) to form H₃O⁺ (hydronium ion) leaving a negatively charged acetate ion (CH₃COO^–), called the conjugate base.

The Ka value, or dissociation constant, is the ratio of released H⁺ and CH₃COO^– to acetic acid and is described by the equation: Ka = ([H₃O⁺][CH₃COO^–]) / [CH₃COOH)]. Acids that release more H⁺ have a higher Ka value. And like pH, we use a log scale for convenience. So, the pKa = -log(Ka).

For example, acetic acid has a pKa = 4.76, meaning a ([H₃O⁺][CH₃COO^–]) / [CH₃COOH)] ratio of 10^-4.76 or 0.000017. Indeed, a tiny fraction. On the other hand, HCl has a pKa of -6.3. Likewise, the ratio of ([H⁺][Cl^–]) / [HCl] is 10^6.3 or almost 2,000,000!

It’s also possible to determine the pKa of an acid by adding small amounts of a strong base such as NaOH (titration). As NaOH is added to an acetic acid solution, it splits into Na⁺ and OH^–. The OH^– removes H⁺ from acetic acid to generate H₂O and an acetate ion, CH₃COO^–, lowering the amount of available H⁺, causing the pH to rise. With each addition of NaOH, the pH rises incrementally. Eventually, all the H⁺ is removed from the acid and the pH will reach a limit. If we plot on a graph the pH values versus the amount of NaOH (titration curve), the pKa value will be the midpoint in the rise (or inflection point).

Preparation of Acids

Acids can be made by mixing other chemicals together in specific ways. Here are some examples.

Strong Acid Preparation

• HCl is made by the chlor-alkali method. This involves electrolyzing a NaCl solution which produces hydrogen gas (H₂), NaOH, and chlorine gas (Cl₂). Under UV light, these gases combine to form HCl. HCl can also be made by taking advantage of acid boiling points. Mixing H₂SO₄ (which has a higher boiling point than HCl) with NaCl (the salt of HCl) generates HCl.
• Sulphuric acid (H₂SO₄) is made by first burning sulphur with oxygen to generate SO₂ (sulphur dioxide). Then SO₂ is converted to SO₃ (sulphur trioxide) in the presence of a vanadium(V) oxide catalyst. Finally, SO₃ is mixed with water to produce H₂SO₄. Other oxides like carbon dioxide (CO₂) reacts with water to form carbonic acid (H₂CO₃).

Weak acid Preparation

• Acetic acid is primarily made by carbonylation. In this process, methanol (CH₃OH) and carbon monoxide (CO) react in the presence of a catalyst to produce acetic acid. A small amount of acetic acid is made by bacterial fermentation, which metabolically converts sugars like glucose to acetic acid.
• Phosphoric acid can be synthesized by a wet process and a thermal process. The wet process involves mixing a phosphate-containing mineral, such as calcium hydroxyapatite with sulphuric acid to generate phosphoric acid and calcium sulphate (CaSO₄ or gypsum). The thermal process takes phosphate ore and burns to make elemental phosphorus, which is distilled out of the furnace with air to make phosphorus pentoxide (P₂O₅). When dissolved in water, P₂O₅ produces phosphoric acid.

Biology

Deoxyribonucleic Acid (DNA)

Acids are critical for life. The most famous biological acid is deoxyribonucleic acid or DNA. DNA contains phosphoric acid. But as physiological pH is around 7, all the H⁺ has been removed leaving DNA with a negative charge.

Amino Acids

Amino acids are another important acid (Figure 4). The acid portion of an amino acid is like acetic acid that reacts with an amino group of another amino acid forming a chain. Chains with more than 40 amino acids are otherwise known as proteins.

Figure 4: The general structure of an amino acid. There are 20 different R groups making up 20 different amino acids.

Energy Production with H⁺

The release of H⁺ is used to generate chemical energy used by the plants and animals. Plants cells use light energy from the sun to pump H⁺ across a membrane to create an H⁺ gradient, with more H⁺ on one side of the membrane. The flow of H⁺ back through the membrane generates adenosine triphosphate (ATP). ATP is the chemical energy source plants use to make glucose from water and carbon dioxide (CO₂). In a similar fashion, animal cells use the energy from the breakdown of glucose to pump H⁺ across a membrane to generate ATP for energy.

Conclusion

Acids are an important part of our world. They’re handy in the kitchen and around the home. By understanding their properties, they’ve played a key role in many scientific and industrial applications, and are part of many critical biochemical processes.