pH Scale - Online Tutor, Practice Problems & Exam Prep

The pH scale, ranging from 0 to 14, measures hydrogen ion concentration in solutions, indicating acidity or basicity. A pH less than 7 signifies acidity, while a pH greater than 7 indicates a basic solution; a pH of 7 is neutral. Buffers, such as the bicarbonate buffer system, help maintain pH stability around 7, crucial for living organisms. They can either accept or donate hydrogen ions to resist pH changes, ensuring homeostasis. Understanding pH and buffers is essential for grasping biological processes and maintaining cellular function.

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 as 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, and depending on where the pH falls on the pH scale, this will determine if the solution is acidic or basic. 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. On the other hand, a basic solution is actually a solution whose pH is greater than 7. And of course, 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. This means the lower the pH is, the greater the hydrogen ion concentration. Acidic solutions actually have really high hydrogen ion concentrations that are greater than the hydroxide ion concentration. In basic solutions, on the other hand, the higher the pH is, the lower the concentration of hydrogen ion. This means that in basic solutions, we can say that the hydrogen ion concentration will be smaller than the hydroxide ion concentration. In the middle, if we have a neutral solution whose pH is exactly equal to 7, the hydrogen ion concentration is going to be exactly equal to the hydroxide ion concentration.

It's 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, and the greater the pH of the solution, the smaller the hydrogen ion concentration. If we take a look at the pH scale image down below, we can apply this. 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, and 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, and 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; lemon juice, tomato juice, black coffee are indicated. Milk, which is slightly under a pH of 7, means that it is slightly acidic. Then we have pure water right here in the middle as a neutral solution. You can also see where ocean water falls, along with baking soda, ammonia, bleach, and drain cleaner as well.

One thing to note here is that 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 ions, so we could just put 1 here 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. We'll just put one here for simplicity. And of course, the higher the pH is, the more hydroxide ions there will be. You can see that the pH will tell us the hydrogen ion concentration, which will be very high over here and then as it makes its way over, it gets smaller and smaller. 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 once again that the concentration of hydrogen ion is going to be exactly equal to the concentration of hydroxide ion. When the pH is equal to 7, the number of hydrogen ions, which we'll just 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 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 for 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. 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.

• 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. So, 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 make sure 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. Once again, this is beneficial for living organisms because if the pH changes too much then that can be really harmful. 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 are going to use buffers to help maintain the pH or help maintain homeostasis in regards to the pH values.

Now, if we take a look at our image down below over here 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. And 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 in order for them to survive. If the pH were to tip to either side, if too much acid was added, that could tip the scales towards the acidic side. That 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. 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.

We're showing you in the example 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. On the right-hand side, we're focusing on the bicarbonate buffer system, which includes these two molecules that we see here, HCO3− and HHCO3. Notice that on the left-hand side over here, 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. So this side over here would be the acidic side, 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.

It says that HCO3− is part of a buffer system because HCO3− 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. That is going to cause the pH to increase when HCO3− accepts hydrogen ions. However, if the hydrogen ion concentration were to get too low, then this HHCO3 over here is capable of donating hydrogen ions if it gets too low. Depending on the situation, whether the hydrogen ions are too high or too low, buffers, which would include HCO3− here and HHCO3, 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.

• 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.

• 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 less than 7 indicates an acidic solution, where the concentration of H⁺ ions is high. A pH greater than 7 indicates a basic (alkaline) solution, where the concentration of hydroxide ions (OH^-) is higher. 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 chemical environment of biological processes, as many enzymes and cellular functions are pH-dependent.

Why is pH important in biological systems?

pH is critical in biological systems because it affects the structure and function of molecules, especially proteins and enzymes. Enzymes, which catalyze biochemical reactions, have an optimal pH range in which they function most efficiently. Deviations from this optimal pH can lead to denaturation or reduced activity, disrupting metabolic processes. Additionally, pH influences the solubility and availability of nutrients and ions, impacting cellular homeostasis and overall organism health. Maintaining a stable pH is essential for the proper functioning of cells and tissues.

What are buffers and how do they work?

Buffers are substances that help maintain a stable pH in a solution by neutralizing added acids or bases. They work by either accepting hydrogen ions (H⁺) when the pH decreases (becomes more acidic) or donating H⁺ ions when the pH increases (becomes more basic). A common biological buffer is the bicarbonate buffer system, which involves bicarbonate ions (HCO₃^-) and carbonic acid (H₂CO₃). This system helps maintain the pH of blood and other bodily fluids around 7.4, which is crucial for physiological processes and homeostasis.

How does the bicarbonate buffer system maintain blood pH?

The bicarbonate buffer system maintains blood pH by using bicarbonate ions (HCO₃^-) and carbonic acid (H₂CO₃). When blood pH drops (becomes more acidic), bicarbonate ions react with excess hydrogen ions (H⁺) to form carbonic acid, which then dissociates into water (H₂O) and carbon dioxide (CO₂). Conversely, when blood pH rises (becomes more basic), carbonic acid dissociates to release H⁺ ions, lowering the pH. This dynamic equilibrium helps keep blood pH around 7.4, essential for normal cellular functions and metabolic processes.

What happens to biological systems if pH is not maintained?

If pH is not maintained within a narrow range, biological systems can suffer severe consequences. Enzymes and proteins may denature, losing their functional shape and activity. Metabolic processes can become inefficient or halt altogether, leading to cellular damage or death. For example, in humans, blood pH must be maintained around 7.4; deviations can result in conditions like acidosis (low pH) or alkalosis (high pH), both of which can be life-threatening. Therefore, maintaining pH homeostasis is vital for the survival and proper functioning of living organisms.