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Weak Acids, Weak Bases, and Buffers

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  • 0:06 Strong and Weak Acids…
  • 2:07 Conjugate Acids and Bases
  • 2:53 The Acidity Constant and PKA
  • 4:20 Buffers
  • 6:41 Lesson Summary
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Lesson Transcript
Instructor: Meg Desko

Meg has taught college-level science. She holds a Ph.D. in biochemistry.

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
Acidity Constant

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.

Buffers

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.

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