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Calculating Isoelectric Point: Definition & Formula

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  • 0:05 Introduction to pH and pKa
  • 2:14 What Is the Isoelectric Point?
  • 4:09 Calculating the…
  • 5:37 Lesson Summary
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Lesson Transcript
Instructor: Sarah Erhart

Sarah has taught college physical, organic, and general chemistry and high school biology. She has a master's degree in chemistry.

In this lesson, you will learn about isoelectric points. We will review pH and pKa, explore the definition of isoelectric points with use of examples, and then describe how to calculate the value knowing pKa values.

Introduction to pH and pKa

Every solution has a pH value, which simply describes how many hydrogen ions are in the solution. This can be measured with the use of a pH meter. This value also tells us if the solution is acidic or basic. Values less than 7 are acidic, values at 7 neutral, and values greater than 7 are basic. The pH of a solution also informs us about what will occur to compounds when they are put into the solution. Some compounds have functional groups that can give up a proton or can accept a proton. Their ability depends on the pH of the solution and the acid disassociation constant, or pKa as we'll be calling it from now on, of the group.

For now, and as an example, let's consider this figure on screen where there are two compounds represented: compound A, phenol, and compound B, 4-chlorophenol.

Figure 1
Comparison of pKa values of phenol and 4-chlorophenol

Although very similar compounds, 4-chlorophenol has a lower pKa value than phenol. The solution does not need to be as basic for 4-chlorophenol to lose its OH proton. In both cases, the hydrogen of the OH group, known as an alcohol, can leave as a proton. However, the pKa is different. The pKa tells us the pH that is required to remove 50% of the hydrogens from the molecule. Therefore, for compound A, phenol, the pH would need to be quite basic, specifically at a pH of 9.95 before 50% of the phenol molecules would be deprotonated.

Compound B, 4-chlorophenol, would need to be less basic, at a pH of 9.41 before 50% of the molecule would be deprotonated. The image shows that the pKa is lowered due to the difference in substitution. Understanding why pKa is different is not required for this topic. Instead simply understanding that the pKa can vary based on the type of functional group and molecule needed. If the pH is a full unit higher than the pKa the molecule is considered to be fully deprotonated. If the pH is a full unit lower than the pKa, then the molecule is considered to be fully protonated.

What Is the Isoelectric Point?

Compounds can have multiple pKa values, because they can contain multiple functional groups capable of being protonated or deprotonated. Amino acids, the building blocks of proteins, are common examples of this. This new figure on screen illustrates what occurs to the amino acid, glycine, in different pH solutions.

Figure 2 The form of glycine changes at different pH values. At the pH that matches the pI, the molecule is neutral.
Gylcine at different pH values

At low pH values both functional groups, the ammonia, or NH3+, and carboxylic acid, or COOH group, are both protonated. As the pH increases the COOH group becomes deprotonated first, as its pKa value is low. Then after the pH value is increased to a basic pH the NH3+ group is deprotonated.

Notice that the charge of the overall glycine compound changes in different pH values. At low pH, or in acidic solution, the overall compound has a positive charge, due to the NH3+ group. At moderate pH, or close to neutral, the overall compound is neutral, due to the loss of proton and subsequent minus charge on the COO- group and positive charge on the NH3+ group. At high pH, or in basic solution, the overall compound is negative, as both groups have lost their protons, leaving a neutral charge on the NH2 group and negative charge on the COO- group.

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