Beer and Water - Part 4: Basics of Acids and Bases
Continuing from last time
Last time, I talked about the basic properties of water. This time, I wanted to discuss pH, which is deeply related to beer brewing, but... to understand pH, we first need a basic understanding of acids and bases, so let's confirm their definitions.
Three Definitions of Acids and Bases
Arrhenius Definition
This is the definition of acids and bases proposed by Svante Arrhenius of Sweden in 1884. Roughly speaking, it states that "an acid is a substance that releases hydrogen ions (H+) when dissolved in water, and a base is a substance that releases hydroxide ions (OH-) when dissolved in water". It's simple and good. For beer, you might think the Arrhenius definition is sufficient since we basically just need to know the state when substances are dissolved in water (wort or beer). However, the Arrhenius definition doesn't explain alkalinity (buffering capacity), so ultimately, the Brønsted-Lowry definition is necessary.
The image of the Arrhenius definition is like the figure above. I will discuss hydronium ions and coordinate bonds later.
Brønsted-Lowry Definition
This definition was proposed independently in 1923 by Johannes Brønsted of Denmark and Martin Lowry of the United Kingdom. It is often abbreviated as "Brønsted acid" or "Brønsted definition," which makes me feel a bit sorry for Mr. Lowry. Please remember Mr. Lowry as well. Now, roughly speaking, the definition is that "an acid is a substance that donates a hydrogen ion (H+, proton) when substances react, and a base is a substance that accepts a hydrogen ion". The Arrhenius definition assumes dissolution in water, so it doesn't clarify whether water itself is an acid or a base (in reality, water can be both). Also, it cannot grasp the properties of substances like bicarbonate (HCO3-), which appears in alkalinity and can act as either an acid or a base depending on the reaction partner. The Brønsted-Lowry definition clarifies these points.
Considering the reaction between lactic acid and water as shown in the figure above, the hydrogen ion of the lactic acid moves to the water molecule. Lactic acid is an acid because it donates a hydrogen ion, and water is a base because it accepts a hydrogen ion.
Lewis Definition
The Lewis definition was proposed by Gilbert Lewis of the United States in 1923, the same year as the Brønsted-Lowry definition. Roughly speaking, it states that "an acid is a substance that accepts an electron pair to form a covalent bond, and a base is a substance that donates an electron pair to form a covalent (coordinate) bond". Since acids and bases can be classified even if hydrogen ions (H+) are not involved in the reaction, it covers a wider range than the Brønsted-Lowry definition.
Although it is difficult to understand, the figure above is an example of the Lewis definition. Lactic acid is an acid because it accepts an electron pair, increasing its lone pairs by one and becoming a lactate ion; water is a base because it provides its own lone pair to form a coordinate bond. It is an extension of the Brønsted acid-base definition, and in the case of reactions involving hydrogen ions (protons), the result is the same because it is just looking at the same phenomenon from the perspective of the electron pair. While it has the advantage of covering a wide range, it can be said to be intuitively difficult to understand.
For a Basic Understanding of Acids and Bases
Definitions of Acids and Bases Related to Beer
We have looked at three definitions of acids and bases. For a brewer, an understanding of alkalinity (buffering capacity) and pH is essential. Therefore, the Arrhenius definition, which does not consider the movement of hydrogen ions (protons) or electrons, is insufficient. Since beer is an aqueous solution and reactions related to beer involve water and protons, there are not many situations where one would use the Lewis definition to think about acids and bases. Therefore, in practice, it seems best for a brewer to think about acids and bases using the intuitively easier Brønsted-Lowry definition. That said, understanding the movement of electrons reveals the essence of chemical bonds and reactions beyond just water, so if you have the capacity, it is worth grasping the Lewis definition as well.
Bases and Alkalis
We have looked at the definitions of acids and bases, so let's organize the difference between "base" and "alkali." Roughly speaking, alkali is a narrower concept than base. Among bases, those that are dissolved in water are called alkalis. Bases also include those that are not dissolved in water. For example, since the Arrhenius definition deals with reactions in aqueous solutions, one can think of alkali = base, but since the Brønsted-Lowry and Lewis definitions also deal with reactions where water is not involved, it is necessary to use the term base. In English, base = Base, basic = Basic, alkali = Alkali, and alkaline = Alkaline. Originally, alkali meant wood ash in Arabic, and it is said that it was named "base" because the stronger alkaline substance that remains when ash is further heated was considered the "foundation" of the ash. By the way, acids are acids whether in an aqueous solution or otherwise. In English, acid = Acid, and acidic = Acidic.
What is a Hydronium Ion?
We will also explain hydronium ions. The H+ released by an acid coordinates with water molecules in an aqueous solution and exists as a hydronium ion.
A coordinate bond occurs when a cation bonds to a lone pair of electrons in a molecule where electron pairs are already stably arranged in orbitals. Once a coordinate bond is formed, the electron pair takes on covalent characteristics and becomes indistinguishable from other shared electron pairs.
The general term for an oxygen cation (positive ion) with three bonds is an oxonium ion, and among these, the specific ion H3O+ is called a hydronium ion. This is why H3O+ is sometimes referred to as an oxonium ion.
In pH explanations and formulas, it is treated as a hydrogen ion, H+. This means we are thinking in terms of the difference from a water molecule.
To be continued
This time, we covered the definitions and basic understanding of acids and bases. Next time, we will finally discuss pH.
Thank you for reading. If you found this article interesting, or if you suddenly felt thirsty and wanted a beer, please feel free to visit us here.
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