pH and pOH

Do you know anyone that suffers from lactose intolerance? It is estimated that 65% of the world's population suffers from lactose intolerance, and these people have a hard time digesting milk products. The disaccharide lactose is found in milk and is broken brown by the enzyme lactase to make glucose and galactose.

However, some people lack this enzyme, so their bodies cannot break down lactose. The lactose remains intact and undergoes a process known as bacterial fermentation, producing gases and causing nausea, abdominal cramps, and diarrhea. Lactase functions better at pH 6, so changing pH has a big effect on lactase activity.

What exactly is pH and how does it relate to pOH? Keep reading to find out!

• First, we will look at the difference between pH and pOH
• After, we will learn about the pOH scale and how it differs from the pH scale.
• Then, we will talk about the pH of strong acids and bases
• Lastly, we will dive into the calculations involving pH and pOH.

Difference between pH and pOH

Before diving into the world of pH and pOH, we need to recall the definition of acids and bases. Acids and bases have different definitions, depending on who you ask!

Svante Arrhenius was the first chemist to classify acids and bases. Arrhenius acids are acids that dissociate in water and form H⁺ ions. Arrhenius bases, on the other hand, dissociate in water and form OH^- ions.

Other chemists who came up with acid and base definitions were Johannes Brønsted and Thomas Lowry. Together, they defined acids and bases by their ability to donate and accept protons (hydrogen ions). Brønsted-Lowry acids are proton (H⁺) donors, whereas Brønsted-Lowry bases are proton (H⁺) acceptors.

Now that we know a bit more about acids and bases, let's define what pH and pOH are!

pH is measured using a pH scale that ranges from 0 to 14. The smaller the pH, the more acidic a solution is, and the higher the concentration of H⁺ ions in the solution. So, acidic solutions have a very small concentration of OH^- ions compared to the high amount of H⁺ ions in the solution.

• Acidic solutions have a pH of smaller than 7
• Basic solutions have a pH of greater than 7
• Solutions containing pH 7 are considered neutral.

Fig.1-pH scale

pOH scale

The sum of the pH and pOH of an aqueous solution equals 14. The relationship between pH and pOH is as follows:

$\mathrm{pH}+\mathrm{pOH}=14$

So, if you know either the pH or pOH, you can easily calculate the missing value! For example, if you have a pOH of 3, then the pH would be equal to 11, because 11 + 3 = 14. Remember:

• The lower the pH value, the more acidic a solution is, the higher the H⁺ concentration and the lower the OH^- concentration.
• The lower the pOH value, the more basic a solution is, the higher the OH^- concentration and the lower the H⁺ concentration.

Fig.2-Scale showing pH and pOH value

pH and pOH Formula

The formula to calculate pH and pOH is:

$$\begin{gathered} \mathrm{pH}=-{\log }_{10}\left[{\mathrm{H}}^{+}\right] \\ \mathrm{pOH}=-{\log }_{10}\left[{\mathrm{OH}}^{-}\right] \end{gathered}$$

Those formulas might look a bit confusing, but don't worry, we will put them to practice!

Now, let's look at an example involving pOH.

Using pH and pOH to calculate Ion Concentration

We can rearrange the pH and pOH formulas to find the concentrations of hydrogen and hydroxide ions!

$$\begin{gathered} \left[{\mathrm{H}}^{+}\right]={10}^{-\mathrm{pH}} \\ \left[{\mathrm{OH}}^{-}\right]={10}^{-\mathrm{pOH}} \end{gathered}$$

Let's solve another example!

pOH to pH

During your AP chemistry exam, you will most likely be asked to convert pOH to pH. This can be accomplished by using the different formulas that we learned so far. The simplest way is by using the formula pH + pOH = 14.

However, sometimes you will have to use more than one formula to find pH. Let's look at an example.

pH of Strong Bases

_References:

_{Malone, L. J., Dolter, T. O., & Gentemann, S. (2013). Basic concepts of Chemistry (8th ed.). Hoboken, NJ: John Wiley & Sons.}

_{Swanson, J. W. (2020). Everything you need to Ace Chemistry in one big fat notebook. Workman Pub.}

_{Saunders, N. (2020). Supersimple Chemistry: The Ultimate Bitesize Study Guide. London: Dorling Kindersley.}