What Is the Acid Number?

The acid number is a chemical measurement used to represent the acidity of an oil, such as petroleum or cooking oil. It is calculated from the mass of base, measured in milligrams of potassium hydroxide, needed to neutralize the acids in one gram of the oil. Acid number, also called acid value, is often an important guideline in determining the degradation of oil products.

The test for acid number uses the principles of acid-base chemistry to determine the acidity of the oil being tested. Acids and bases are opposing substances that can cancel each other out when mixed. When a base, such as the potassium hydroxide, is added to an acidic substance, a neutralization reaction occurs. By measuring how much potassium hydroxide is required to complete the neutralization reaction, it is possible to deduce the amount of acid in the test sample.

Acids and bases are chemical compounds that dissociate in water into their constituent ions — atoms with a positive or negative overall charge. Bases give off hydroxide ions, while acids give off hydrogen ions. For example, the base potassium hydroxide, represented by the chemical formula KOH, dissociates in water into positively-charged potassium ions (K+) and negatively charged hydroxide ions (OH-). Hydrochloric acid (HCl), on the other hand, dissociates into positively charged hydrogen ions (H+) and negatively charged chlorine (Cl-) ions.

The acidity or basicity of a substance is determined by the amount of hydrogen or hydroxide ions in that substance. When combined, acidic and basic solutions produce a neutral substance because the hydrogen from the acid reacts with the hydroxide from the base. For example, although KOH and HCl are both toxic substances in their separate forms, when they react they become a neutral mixture of potassium chloride (KCl) and water, neither of which is toxic. In the acid number test, the goal is to add just enough base to the test sample for the hydroxide ions to react with the hydrogen ions and produce a neutral solution.

The process of adding a known concentration and amount of base to an acid, or vice versa, to achieve an endpoint is known as titration. In the acid number test, a base is titrated, or dripped gradually, from a calibrated glass tube known as a buret into the oil sample until the endpoint of the reaction — the complete neutralization of the acid — is reached. The total amount of base used is then determined based on the calibrated marks on the buret. This quantity can be used to determine how many milligrams of base were needed to neutralize one gram of the oil sample, providing the acid number.

Chemical components of cooking oil and petroleum products frequently degrade over time, becoming more acidic as their molecules dissociate and release hydrogen ions. This can affect the performance of the oil in the case of petroleum used in vehicles and cause corrosion and other ill effects. Cooking oil degradation is a concern for the food and restaurant industry because increased acidity adversely affects taste and may even cause health problems. The acid number test is commonly used to monitor the quality of the oil and prevent problems caused by degradation.