pH Scale - Online Tutor, Practice Problems & Exam Prep

The pH scale, ranging from 0 to 14, measures hydrogen ion concentration, indicating whether a solution is acidic (pH < 7), basic (pH > 7), or neutral (pH = 7). Buffers, such as the bicarbonate buffer system, help maintain pH stability around 7, crucial for homeostasis in living organisms. They can either accept or donate hydrogen ions to counteract pH changes, ensuring optimal conditions for biological processes. Understanding pH and buffers is essential for grasping acid-base balance and physiological functions.

pH Scale

In this video, we're going to begin our introduction to pH and the pH scale. Recall from our previous lesson videos, we mentioned that many biological processes are strongly affected by the concentration of hydrogen ions dissolved in the solution. It's to the scientist's interest to measure the concentration of hydrogen ions because the concentration of dissolved hydrogen ions matters; it will strongly affect many biological processes. The measurement of the concentration of hydrogen ions is really where pH comes into play, and that's because pH is just a logarithmic measurement of the hydrogen ion concentration in a solution. The pH can tell us how many hydrogen ions are found within a solution. The pH can also indirectly measure the hydroxide ion concentration in aqueous solutions, and we'll be able to see this when we get to this part of our table.

The pH scale specifically goes from a value of 0 up to a value of 14. Depending on where the pH falls on the pH scale, this will determine if the solution is acidic or basic. If we take a look at this table down below, you'll realize that acidic solutions are going to be solutions that have a pH less than 7. If the pH is less than 7, then we have an acidic solution. A basic solution, on the other hand, is actually a solution whose pH is greater than 7. Of course, if the pH is exactly equal to 7, then we have a neutral solution.

When it comes to the hydrogen ion concentration and the hydroxide ion concentration, the pH values are inversely related to the concentration of hydrogen ion. The lower the pH is, the greater the hydrogen ion concentration. Acidic solutions have really high hydrogen ion concentrations that are greater than the hydroxide ion concentration. In basic solutions, the higher the pH is, the lower the concentration of hydrogen ion, meaning that the hydrogen ion concentration will be smaller than the hydroxide ion concentration, which will be greater. Right in the middle, if we have a neutral solution whose pH is exactly equal to 7, this means that the hydrogen ion concentration is going to be exactly equal to the hydroxide ion concentration.

Notice how acidic solutions, which we have colored with a reddish background, are further to the left of our scale, whereas the basic solutions are further to the right of our scale, and the neutral solutions are right at a pH of 7. The more acidic the solution is, the lower the pH value will be. The higher the pH is, the more basic the solution will be. Throughout this pH scale, we have different components that show where they fall on the pH scale. You can see battery acid has a pH associated with 0, so it's incredibly acidic. Then you can see lemon juice, tomato juice, black coffee, and milk, which is slightly under a pH of 7, indicating it is slightly acidic. Pure water right here in the middle as a neutral solution, and then you can see where ocean water, baking soda, ammonia, bleach, and drain cleaner as well.

Towards the acidic end, there will be a lot of hydrogen ions, whereas towards the basic end, there will be a smaller amount. The hydroxide ion concentration will be flipped, meaning that the more acidic the solution is, the lower the hydroxide ion concentration will be and vise versa for basic solutions. This inverse relationship between hydrogen and hydroxide ions is important to understand the effects of pH on solutions. In the middle, where the pH is neutral, the concentration of hydrogen ion is going to be exactly equal to the concentration of hydroxide ion. This concludes our lesson on pH and the pH scale, and the best way to get this down is to get some practice with it. We'll see you in our next couple of videos to get some more practice.

In a neutral solution, the concentration of __________.

• A. Hydrogen ions is less than the concentration of hydroxide ions.
• B. Water molecules is less than the concentration of hydroxide ions.
• C. Hydrogen ions is greater than the concentration of hydroxide ions.
• D. Hydrogen ions is equal to the concentration of hydroxide ions.

A base _______:

• A. Has a value of 7 on the pH scale.
• B. Is a chemical that donates hydrogen ions to a solution.
• C. Is a chemical that accepts hydrogen ions from a solution.
• D. Has a value below 7 on the pH scale.
• E. None of the above are correct.

Buffers

In this video, we're going to begin our introduction to buffers. And so first, we need to note that the pH of most living organisms is actually maintained right around a value of 7 or having a neutral pH. Changing the pH even slightly can actually be really, really harmful to living organisms. It's in the best interest of living organisms to make sure that the pH stays around 7 in its neutral range. So, in order to ensure that the pH stays around 7, living organisms use what are known as buffers. Buffers are defined as substances that are capable of resisting changes in the pH even when acids and bases are added to the solution. Typically, acids and bases would change the pH. But if buffers are present in the solution, then even when acids and bases are added to the solution, the substance is capable of resisting changes in pH, which means that the pH will not change. This is beneficial for living organisms because if the pH changes too much, then that can be really harmful. So, these buffers are good things that cells use to make sure that the pH is going to resist changes and stay in that neutral range.

Now, depending on the situation, buffers are capable of either decreasing or increasing the concentration of hydrogen ions in the solution. Living organisms use buffers to help maintain the pH or help maintain homeostasis in regards to the pH values. If we look at the image below on the left-hand side, notice that we're showing you a pH scale, which we know goes from 0 up to 14. We know that values of 0 are going to be acidic, and values of 14 are going to be basic. Then, of course, right in the middle with the value of 7, that is going to be neutral. Most living organisms require a pH of about 7 to survive. If the pH were to tip to either side, too much acid added could tip the scales towards the acidic side, which could be really harmful for a cell. But also if too much base was added, that could tip the scale towards the base side, and that could also be really harmful. So, it's in the cell's best interest to make sure that the pH is maintained and that the pH is able to resist changes, so that it stays about the same.

Here, we're showing you in the example a specific type of buffer called the bicarbonate buffer system, which is found in our blood and it helps to maintain the pH of our blood. We're focusing on the bicarbonate buffer system, which includes these two molecules that we see here, HCO3− and H₂CO₃. Notice that on the left-hand side, we're showing you hydrogen ions, and remember that the pH is going to reflect how many hydrogen ions there are. If there are a lot of hydrogen ions, then we would have an acidic solution. But of course, if there are not many free hydrogen ions like over here, notice there's no H⁺ anywhere, that means that the pH is going to be high and it's going to be a basic solution.

HCO3− is part of a buffer system because it is capable of accepting hydrogen ions if the hydrogen ion concentration gets too high. In a scenario when the hydrogen ion concentration gets too high, bicarbonate can act to lower the hydrogen ion concentration by accepting hydrogen ions. This will cause the reaction to go in this direction towards the right, which will increase the pH. The pH is increased when HCO3− accepts hydrogen ions. However, if the hydrogen ion concentration were to get too low, then H₂CO₃ is capable of donating hydrogen ions if it gets too low. Depending on the situation, buffers, which would include HCO3− here and H₂CO₃, are capable of either decreasing or increasing the hydrogen ions in the solution. This is once again going to help maintain homeostasis in regards to the pH.

This concludes our introduction to buffers, and we'll be able to get some practice applying these concepts moving forward in our course. So, I'll see you all in our next video.

Which of the following statements about buffers is true?

• A. They maintain a consistent pH only when acids are added to them, but not bases.
• B. They maintain a consistent pH of 7.
• C. They fluctuate in pH when acids are added to them.
• D. They maintain a consistent pH when acids or bases are added to them.
• E. They fluctuate in pH when acids or bases are added to them.

Buffers are substances that help resist shifts in pH by:

• A. Donating H⁺ in acidic solutions.
• B. Donating H⁺ to a solution when they have been depleted.
• C. Releasing OH− in basic solutions.
• D. Accepting H⁺ when they are in excess.
• E. Both b and d are correct.