If you've ever studied or are studying chemistry, you're most likely familiar with how pH is used to indicate attributes of chemical substances, such as whether they are acids or bases. Why is it important to represent the acidity of these various solutions, and how is it calculated? Read to find out!
Introduction to the pH Scale
In your science class, you've most likely learned that acidic solutions taste sour, basic substances have slippery textures, and water is neutral. To be more precise, you'd identify where the substances are on the pH scale, by means such as using pH strips.
The pH scale ranges from 0 to 14. Acids are those who have a pH less than 7, and the lower the pH, the more acidic and stronger the acid is. Bases have a pH greater than 7, meaning the stronger bases have a pH closer to 14. Lastly, solutions that are neutral have a pH of 7.
However, do you know where exactly the numbers on the pH scale are derived from? And how are these numbers correct indicators of the acidity of solutions?
Calculations
The answer is in the concentration of hydrogen (H+) and hydroxide (OH-) ions in each solution! Acids are solutions that are capable of donating protons (hydrogen), meaning that they have a high concentration of hydrogen. On the other hand, bases are solutions that accept protons, and have a lower concentration of hydrogen. Additionally, bases have a high concentration of another ion called hydroxide, and its negative charge is what allows it to accept the positive hydrogen ions.
pH is calculated from this equation:
pH = -log[H+] [H+] = concentration of H+ ions (M)
**Note: log is a function in your scientific calculator, and you would input [H+] in the parenthesis that follow it!
***2nd note: M is the symbol for "molar" which is a unit for measuring the concentration of a solution. It is derived from moles/liter!
Here are some examples of pH being calculated from different solutions' H+ ion concentrations:
Lemon juice has a concentration of 0.01 M of H+ ions. Therefore, you would calculate its pH by doing:
-log(0.01) = 2. Therefore, lemon juice has a pH of 2, making it a strong acid.
Bleach has a concentration of 1.0 x 10^(-12) M of H+ ions. Therefore, you would calculate its pH by doing: -log(10^-12) = 12. Therefore, bleach has a pH of 12, making it a strong base.
Of course, you could also find [H+] from knowing the pH of the solution! For instance, water is a neutral solution, and has a pH=7. To find the concentration of hydrogen in water, you would do the reverse log function which is:
[H+] = 10^(-pH)
Therefore, you would find the concentration of hydrogen ions in water by doing: 10^(-7), which would give you [H+] = 1.0 x 10^(-7) M.
Analyzing the Results
After making these calculations, how do we use them to analyze the characteristics of the solution? As you can see, the pH of the solution is dependent on the concentration of hydrogen ions present within it, and the higher [H+] is, the more acidic the solution is.
For instance, using some of our calculations from earlier, lemon juice is a strong acid since it has 0.01 M of H+ ions (pH = 2), while a weak acid such as milk only has 1.0 x 10^(-6) M or 0.000001 M of H+ ions (pH = 6). Milk has a significantly smaller concentration of hydrogen compared to lemon juice, and lemon juice is also lower on the pH scale, which indicate that it is a stronger acid.
Similarly, bleach is a strong base since it has a concentration of 1.0 x 10^(-12) M or 0.000000000001 M H+ ions (pH = 12). As you can see, this is a really small amount of H+ ions! This means that this solution is a very strong base. On the other hand, a weaker base is hand soap, which has a concentration of 1.0 x 10^(-10) M or 0.0000000001 M H+ ions (pH = 10).
Therefore, knowing the pH or concentration of H+ ions is helpful for identifying whether a solution is an acid or base, along with the strength of its acidity. The creation of pH and the pH scale allowed the process of identifying acids and bases to be easier. Comparing numbers with lots of decimal places could get difficult and would be inconvenient compared to just comparing the numbers calculated from the logarithmic function. However, these two mean the same thing, and pH is just another way of saying "the concentration of [H+] ions in this solution is 10^(-pH) M.
If you want to read more about pH paper specifically, and experiments you can do with pH, feel free to read my previous article: "pH Paper: Explained and How to Make Your Own". Thank you for reading!
Written by: Janice Le
References:
Alley. “Scientists Say: PH.” Science News for Students, 7 Dec. 2019, www.sciencenewsforstudents.org/article/scientists-say-ph.
“PH Scale: Acids, Bases, PH and Buffers (Article).” Khan Academy, khanacademy.org/science/biology/ water-acids-and-bases/acids-bases-and-ph/a/acids-bases-ph-and-bufffers.
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