You probably know the pH scale from previous years. It looks something like this:

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Where would you place the following substances on the scale if you had to guess?

Balsamic vinegar. Beer. Seawater. Hand soap.

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The pH scale ranges from 0 to 14, with values below 0 or above 14 being uncommon. pH measures how acidic a substance is. However, we'll delve deeper into this concept to enhance your understanding. Understanding pH is crucial for many reasons, including maintaining healthy skin, hair, and teeth.

What is pH?

The pH scale measures the concentration of hydrogen ions in a solution. In simpler terms, it tells us how acidic or basic a substance is. Stronger acids have a lower pH value, while stronger bases have a higher pH value. For example, balsamic vinegar is a stronger acid than soap because it has a lower pH value.

The pH scale was invented by a Danish chemist named Søren Peder Lauritz Sørensen in 1909. He wanted to control the acidity of his beer to ensure healthy yeast growth while preventing the growth of harmful bacteria. The letter "H" in pH stands for hydrogen, but the origin of the "p" is unclear. It could refer to the test solution or come from the French and German words for "power" and "potential."

We can use an equation to calculate pH based on the concentration of hydrogen ions in a solution. A pH of less than 7 is considered acidic, while a pH greater than 7 is considered basic. A pH of 7 is not necessarily neutral; instead, a neutral solution has equal concentrations of hydrogen and hydroxide ions. Knowing pH is important for many reasons, such as maintaining healthy skin, hair, and teeth. Calculating pH can be made easier by using the "key" on your calculator.

What are strong acids and bases?

In our previous article, we learned that acids donate protons in a solution, which leads to ionisation. Ionisation is a reversible reaction that separates negative ions and positive hydrogen ions. However, some acids are incredibly efficient at giving up their hydrogen ions, making the reaction one-way. These are known as strong acids.

Strong acids dissociate completely in a solution, and the equation to represent this is:

Similarly, we can obtain strong bases. A strong base is a base that also dissociates completely in a solution. When we add a strong base to water, we get the following equation: 

The pH value of strong acids and bases

Strong acids have a low pH because they have a high concentration of hydrogen ions in the solution. Examples are hydrochloric acid (), nitric acid () and sulfuric acid ().

Strong bases have a low concentration of hydrogen ions in the solution. As a result, they have a high pH. Examples of this are all group 1 and group 2 hydroxides, such as sodium hydroxide ().

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How do we find the pH of strong acids?

Remember that a strong acid dissociates completely in an aqueous solution. Let us take hydrochloric acid as an example.

Find the pH of  moles of hydrochloric acid dissolved in  of water. If we put  moles of hydrochloric acid in  of water, the acid will completely dissociate into  mole of hydrogen ions, , and 0.1 moles of chloride ions, . To find the concentration, we divide the number of moles by the volume of the solution. So, to find the hydrogen ions concentration in this particular solution, we do .

Now what? How can we find the pH value?

Well, let's go back to our original equation for pH:

We now know the concentration of hydrogen ions in the solution. Therefore, we can substitute this into our equation as shown: Note that pH is always given to two decimal places. It also has no units.

Finding [H+] from pH

If you know the pH of a strong acid and want to find its hydrogen ion concentration, you can rearrange the pH equation. Start by switching the minus sign around, and then take antilogs of both sides. This will give you the hydrogen ion concentration, which you can then use to find the number of moles of the acid in the solution.

Let's do an example to help understand this process better. Suppose the pH of a solution of hydrochloric acid is 0.75, and we want to find the number of moles of HCl in the solution. We can start by substituting the pH value into the equation: This gives us the hydrogen ion concentration of the solution. Since hydrochloric acid is a strong acid and dissociates completely in solution, we know that the concentration of HCl is also . To find the number of moles of HCl, we can multiply the concentration by the volume of the solution: Remember to check the units of your numbers before doing any calculations. Hydrogen ion concentration is given in moles per liter, so make sure to convert your volume to liters as well. In this way, we can use the pH equation to calculate the concentration of hydrogen ions and the number of moles of a strong acid in a solution.

How do we find the pH of strong bases?

The pH of a 0.05 M solution of Sodium Hydroxide is 13.3. This can be calculated using the equation [pH = -log([H+])], where [H+] is the concentration of hydrogen ions in the solution. To calculate the concentration of hydrogen ions, we can use the Ionic Product of Water, which is equal to 1.0 x 10-14 at 25°C. We can use this value to calculate the concentration of hydrogen ions in the solution, by dividing both sides of the equation by [Kw][^3]:

[H+] = [Kw]/[OH-]

[H+] = 1.0 x 10-14/0.05

[H+] = 2 x 10-13

We can then put this value into the equation for pH:

pH = -log(2 x 10-13)

pH = 13.3 

How do we find the pH of mixtures?

To calculate the pH of a mixture of strong acids and bases, you need to determine which reagent is in excess, calculate the concentration of the remaining hydrogen ions or hydroxide ions, and then calculate the pH as before. Here's a summary of the process:

1. Convert the volumes of the acids and bases into moles.
2. Determine which reagent is in excess by comparing the number of moles of hydrogen ions and hydroxide ions.
3. Calculate the number of moles of hydrogen ions or hydroxide ions remaining after the neutralisation reaction.
4. Calculate the concentration of hydrogen ions or hydroxide ions remaining using the total volume of the solution.
5. Calculate the pH using the concentration of hydrogen ions or hydroxide ions remaining.

Let's do an example to illustrate this process. Suppose we have a mixture containing 25 mL of 0.1 M hydrochloric acid (HCl) and 35 mL of 0.05 M sodium hydroxide (NaOH). To calculate the pH of this mixture, we can follow these steps:

1. Convert the volumes of the acids and bases into moles:

moles HCl = 0.1 M x 0.025 L = 2.5 x 10^-3 mol
moles NaOH = 0.05 M x 0.035 L = 1.75 x 10^-3 mol

2. Determine which reagent is in excess. Since HCl and NaOH react in a 1:1 ratio, we can see that HCl is in excess because it has a larger number of moles.
3. Calculate the number of moles of hydroxide ions remaining after the neutralisation reaction:

moles H+ remaining = moles HCl - moles NaOH = 0.5 x 10^-3 mol

4. Calculate the concentration of hydrogen ions remaining using the total volume of the solution:

[H+] = (moles H+ remaining) / (total volume of solution)
= (0.5 x 10^-3 mol) / (0.06 L)
= 8.33 x 10^-3 M

5. Calculate the pH using the concentration of hydrogen ions remaining:

pH = -log[H+]
= -log(8.33 x 10^-3)
= 2.08

Therefore, the pH of this mixture is 2.08.

In summary, calculating the pH of a mixture of strong acids and bases involves determining which reagent is in excess, calculating the concentration of the remaining hydrogen ions or hydroxide ions, and then calculating the pH as before.

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In summary, finding the pH of a solution can be done using a universal indicator for a rough measurement or a pH meter for a more accurate measurement. The pH scale measures the concentration of hydrogen ions in a solution, and a pH below 7 is acidic, while a pH above 7 is alkaline. A neutral solution has equal concentrations of hydrogen and hydroxide ions. Strong acids and bases dissociate completely in solution, making it easier to calculate their pH. To find the pH of a strong acid, base, or mixture of the two, you need to calculate the concentration of hydrogen ions in the solution, followed by using the formula pH = -log[H+].

pH

What is pH?

pH is a measure of hydrogen ion concentration in solution. It tells you how acidic or alkaline a substance is.

How do you calculate pH?

To calculate pH: Calculate the moles of hydrogen ions in the solution. Calculate the concentration of hydrogen ions in the solution. Take a negative log of the hydrogen ion concentration. For a further explanation and worked example, check out the rest of this article.

How do you measure pH?

You measure pH using a universal indicator or a pH probe.

What is pH balance?

The term pH balance describes the way your body maintains a constant pH. When parts of your body become too acidic or too alkaline, their systems stop functioning properly. Your body has therefore developed several mechanisms to prevent this.

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