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. And so recall from our previous lesson videos, we mentioned that many biological processes are actually strongly affected by the concentration of hydrogen ions dissolved in the solution. It's really to the scientist's interest to measure the concentration of hydrogen ions, because once again the concentration of dissolved hydrogen ions matters because 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 really 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.

Now the pH can also indirectly measure the hydroxide ion concentration or the OH^- ion concentration in aqueous solutions, and we'll be able to see this down below 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 that we have down below, what you'll realize is that acidic solutions are going to be solutions that have a pH that is less than 7. If the pH is less than 7, then we have ourselves an acidic solution. A basic solution, on the other hand, is actually a solution whose pH is greater than 7.

If the pH is exactly equal to 7, then we have ourselves a neutral solution. Neutral solutions have a pH that is exactly equal to 7. When it comes to the hydrogen ion concentration and the hydroxide ion concentration, the pH values are actually inversely related to the concentration of hydrogen ion. The lower the pH is, the greater the hydrogen ion concentration. Acidic solutions actually have really, really high hydrogen ion concentrations that are greater than the hydroxide ion concentration.

H⁺ concentration will be greater than OH^- concentration in acidic solutions. In basic solutions, on the other hand, the higher the pH is, the lower the concentration of hydrogen ion. That means that in basic solutions, we can say 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. It's really important to notice how the acidic solution pH sign will flip when it's related to hydrogen ion concentration.

The smaller the pH, the greater the hydrogen ion concentration. The greater the pH of the solution, the smaller the hydrogen ion concentration. So if we take a look at the pH scale image that we have down below, we can go ahead and apply this. Notice that the acidic solutions, which we have colored with a reddish background here, are further to the left of our scale, whereas the basic solutions at the bottom here are further to the right of our scale. In the middle, 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 these different components that show you where they fall on the pH scale. You can see battery acid has a pH that's associated with 0, so it's incredibly acidic.

Then you can see lemon juice here, tomato juice where it falls, black coffee. Milk is slightly under a pH of 7, which means that it is slightly acidic, and then we have pure water right here in the middle as a neutral solution. You can also see where ocean water falls, baking soda, ammonia, bleaches, and drain cleaner as well. Once again, the lower the value of the pH, the more hydrogen ions there are. So if we were to go ahead and fill in our hydrogen ions, we would see that there would be a lot of hydrogen ions over here towards the acidic end.

Whereas towards the basic end over here, there would be a small amount of hydrogen ion, so we could just put one here just for simplicity. Notice that the hydroxide ion concentration is going to be flipped. The more acidic the solution is, the lower the hydroxide ion concentration will be. So we'll just put one here just for simplicity. The higher the pH is, the more hydroxide ions there will be.

You can see that the pH will actually tell us the hydrogen ion concentration, which will basically be very, very high over here, and then as it makes its way over, it gets smaller and smaller and smaller and smaller. And then indirectly, it also tells us the hydroxide ion concentration, which is the inverse. It starts really high over here, and then as it makes its way over, it gets smaller and smaller. Right here in the middle where the pH is neutral, this means that the concentration of hydrogen ion is going to be exactly equal to the concentration of hydroxide ion. So for here in the middle, what we can say is when the pH is equal to 7, the number of hydrogen ions, which we'll just go ahead and put as 2 here, is going to be exactly equal to the number of hydroxide ions, which we'll put here as 2 as well.

This is what happens when the pH is exactly equal to 7, the neutral pH. This here concludes our lesson, our introduction to pH and the pH scale, and the best way to get this down is to get some practice with it. We'll be able to 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. 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 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. In order to ensure that the pH stays around 7, living organisms use what are known as buffers.

Buffers are defined as substances 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. However, 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. Therefore, 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.

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 regarding the pH values. Observe our pH scale, which we know goes from 0 up to 14. We know that values of 0 are acidic and values of 14 are basic. 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 too much to either side, whether too much acid or too much base was added, it could be really harmful. Thus, it's in the cell's best interest to make sure that the pH is maintained and able to resist changes, so that it stays about the same.

Here we're showing you a specific type of buffer called the bicarbonate buffer system, which is found in our blood and helps to maintain the pH of our blood. 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. Notice that there are not many free hydrogen ions on one side, which means that the pH on that side is going to be higher and it's going to be a basic solution.

Notice here that \( \text{HCO}_3^- \) is part of a buffer system because \( \text{HCO}_3^- \) is capable of accepting hydrogen ions if the hydrogen ion concentration gets too high. In such a scenario when the hydrogen ion concentration gets too high, bicarbonate can act to lower the hydrogen ion concentration by accepting hydrogen ions, and that is going to cause the reaction to go in the direction towards the right, which will increase the pH. So, the pH is increased when \( \text{HCO}_3^- \) accepts hydrogen ions.

However, if the hydrogen ion concentration were to get too low, then this other molecule, which we will refer to from now on as \( \text{HHCO}_3 \) is capable of donating hydrogen ions if it gets too low. You can see that depending on the situation, depending on if the hydrogen ions are too high or too low, buffers, which would include \( \text{HCO}_3^- \) and \( \text{HHCO}_3 \), are capable of either decreasing or increasing the hydrogen ions in the solution. This once again is going to help maintain homeostasis regarding the pH. This year concludes our introduction to buffers and we will 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.

Here’s what students ask on this topic:

What is the pH scale and how does it work?

The pH scale is a logarithmic scale ranging from 0 to 14 that measures the concentration of hydrogen ions (H⁺) in a solution. A pH value below 7 indicates an acidic solution, where the concentration of H⁺ ions is high. A pH value above 7 indicates a basic (alkaline) solution, where the concentration of hydroxide ions (OH⁻) is high. A pH of 7 is considered neutral, meaning the concentrations of H⁺ and OH⁻ ions are equal. The pH scale is crucial for understanding the acidity or basicity of a solution, which can significantly impact biological processes.

Why is pH important in biological systems?

pH is vital in biological systems because many biochemical processes are pH-dependent. Enzyme activity, for instance, is highly sensitive to pH changes; enzymes have an optimal pH range where they function best. Deviations from this range can denature enzymes, reducing their activity and potentially halting metabolic processes. Additionally, pH affects the structure and function of macromolecules like proteins and nucleic acids. Maintaining a stable pH, often around 7, is essential for homeostasis and the proper functioning of cells and tissues.

What are buffers and how do they help maintain pH balance?

Buffers are substances that resist changes in pH when acids or bases are added to a solution. They work by either accepting hydrogen ions (H⁺) when the concentration is too high or donating H⁺ when the concentration is too low. This ability to neutralize excess acids or bases helps maintain a stable pH, which is crucial for homeostasis in living organisms. An example is the bicarbonate buffer system in blood, which helps maintain the blood pH around 7.4, ensuring optimal conditions for physiological processes.

How does the bicarbonate buffer system work in the human body?

The bicarbonate buffer system in the human body involves the equilibrium between bicarbonate ions (HCO₃⁻) and carbonic acid (H₂CO₃). When the blood pH drops (becomes more acidic), bicarbonate ions can accept excess hydrogen ions (H⁺), forming carbonic acid, which then dissociates into water and carbon dioxide (CO₂). Conversely, if the blood pH rises (becomes more basic), carbonic acid can release H⁺ ions, lowering the pH. This dynamic equilibrium helps maintain the blood pH around 7.4, crucial for normal cellular functions.

What happens to biological systems if the pH is not maintained?

If the pH is not maintained within a narrow range, biological systems can suffer severe consequences. Enzymes, which catalyze biochemical reactions, may become denatured and lose their functionality outside their optimal pH range. This can disrupt metabolic pathways and cellular processes. Additionally, changes in pH can affect the ionization state of molecules, altering their structure and function. For example, a significant drop in blood pH (acidosis) or a rise (alkalosis) can impair organ function and lead to life-threatening conditions.