This lesson covers both strong and weak acids and bases, using human blood as an example for the discussion. Other concepts discussed included conjugate acids and bases, the acidity constant, and buffer systems within the blood.
Strong and Weak Acids and Bases
At least 60% of the average human is made of water. A lot of that water can be found in our blood, and our blood is 83% water. It's really important for our blood to have a pH of about 7.4, which is slightly basic because otherwise we get really sick.
You might recall that an acid is a hydrogen ion donor and raises the hydrogen ion concentration in a solution while decreasing the pH of that solution, and a strong acid is a hydrogen donor, or acid, that completely dissociates in water. For example, hydrogen chloride dissociates from HCl, bonded together, into a hydrogen ion and a chloride ion. On the other hand, a base is a hydrogen ion acceptor - it lowers the hydrogen ion concentration in a solution while increasing the pH of that solution. A strong base is a hydrogen ion acceptor that can completely dissociate in water.
Now, we know that water has a pH of 7, which means that its concentration of hydrogen ions in solution is 1*(10^-7) moles per liter. So this tells us that some dissociation of water is going on. The pH of blood is about 7.4, and it needs to remain there so we don't become ill. Given that most of our blood is water, something must be going on to maintain the pH of our blood around 7.4.
It turns out that the reason our blood can maintain this pH and keep us from getting sick is weak acids and weak bases. These are really important because if all acids and bases were strong acids and bases, they would all completely dissociate when they got into water, and they would all just keep reacting with each other. Plus, humans would just be one giant walking exothermic acid/base reaction, and that wouldn't be good for anyone.
Conjugate Acids and Bases
An important acid in blood is carbonic acid, H2CO3. Carbonic acid can dissociate in water to give hydrogen ions and hydrogen carbonate, HCO3-. In this case, carbonic acid is serving as an acid to give away hydrogen ions and leaving a conjugate base, which is made by removing a hydrogen ion from a particular acid. Similarly, hydrogen carbonate, acting as a base, can react with a hydrogen ion to form carbonic acid, which is the conjugate acid, which is formed by adding a hydrogen ion to a base.
The Acidity Constant and pKa
The acidity constant measures exactly how weak or how strong an acid is
Now, not all weak acids are exactly the same. Some weak acids are stronger than others, and there is a way that we can measure this, which is called the acidity constant. For a particular reaction where we have an acid - in this case I'm going to call it HA, so H is the hydrogen ion that it's going to give away - it can dissociate, partially, to give some A- and some hydrogen ions as its products. To figure out how far this reaction has gone, we can take the concentration of hydrogen ions, multiply it by the concentration of A-, and divide that by the concentration of HA that's still left in the solution. This gives us the Ka.
Now, if we look at the dissociation of carbonic acid, we find that its Ka is equal to the concentration of hydrogen ions, times the concentration of hydrogen carbonate in solution, divided by the amount of carbonic acid there still is in the solution. At room temperature, we find that this is 7.9*(10^-7). Now, these numbers are not very pleasant to look at. So, similarly to pH, we modify this, and we have something called the pKa, which is the negative log of the Ka. So, in this case, the pKa of carbonic acid is 6.1, which is on the stronger end of things. Water has a pKa of 14, so carbonic acid is considerably more acidic than water is, which is no surprise, given that water has a neutral pH.
Now, carbonic acid is an important component of blood, and helps to regulate its pH. And you might wonder why it would be there, since it has a pKa of 6.1 and seems to want to give away a lot of hydrogen ions. It turns out that carbonic acid is part of what we call a buffer system. And this is actually something that's really cool about weak acids and bases, is that we can put a weak acid and its conjugate base in a solution to create a solution whose pH will not change when small amounts of acid or bases are added.
Carbonic acid, dihydrogen phosphate, and proteins are buffer systems that keep blood pH in check
Now, blood, which needs to have a pH of around 7.4, with an acceptable range of 7.35 to 7.45, has three buffer systems to help keep it in check. The first one is this carbonate buffer system that we already talked about. In blood, carbonic acid is primarily responsible for removing acid from the environment, so the conjugate base, hydrogen carbonate, is incredibly important. It reacts with hydrogen ions that are formed in our bodies, for example, when we exercise. It removes these hydrogen ions, reacting with them to form carbonic acid.
This carbonic acid can then travel around in our blood to the lungs, where it can dissociate into water and carbon dioxide, and then the carbon dioxide can be released through our lungs into the atmosphere. This is an important part of respiration and helps to maintain the pH of our blood.
In addition to carbonate as a buffer system in blood, proteins can act as buffers because they can accept and donate hydrogen ions. The other important buffer system in our blood is the phosphate buffer system. The phosphate buffer is really good at dissociating when a base is added to the system, to help increase the hydrogen ion concentration to help maintain the pH of blood at about 7.4. The pKa of dihydrogen phosphate is about 7.1.
Having these three buffer systems in our blood is incredibly important because they can work together to keep the pH of our blood stable. Buffers are really important in biology, in our bodies, and also in the lab. In the lab, scientists use buffers in their biological experiments to help create for cells and proteins the environments that they find were they naturally would come from.
To recap, we've learned that acids give away hydrogen ions and that they can be strong acids that completely dissociate in water, or weak acids that partially dissociate. We've learned that bases are substances that can accept hydrogen ions. These bases can be strong bases, which completely dissociate, or weak bases, which only partially dissociate. We've learned that the conjugate acid of a base is the molecule that is formed when that base has accepted a hydrogen ion.
We've also learned that the conjugate base of an acid is the molecule that is formed when that acid has given away a hydrogen ion. We've learned that blood contains three important buffering systems, and we've learned that the acidity constant and pKa can tell us about how strong or weak an acid is. A lower pKa is a stronger acid.
After watching this lesson, you should be able to:
- Compare strong and weak acids and bases in relation to human blood
- Define conjugate acids and bases
- Interpret how the acidity constant and pKa play a role in how strong or weak an acid is