Titration: Overview, Curves & Calculations

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Titrations are useful to determine the molarity of a sample or a compound in a sample. We will learn how to perform a titration and how to calculate the molarity of a sample based on the titration.

Titration Overview

When looking at the juice carton, you realize that it says it contains 65 mg of vitamin C. Have you ever wondered how they determined how much vitamin C is in that juice? Is that number just an estimate, or do they have a way to determine exactly how much vitamin C is in that juice? There is an official method for measuring vitamin C in juice, and it uses a method called titration.

A titration is the method of slowly adding a titrant, which can be an acid or base, to the sample to watch when a specific reaction occurs. By indicating how much titrant, the compound reacting with the sample, was needed for a reaction to occur, we can calculate the amount or property of the sample in question.

Titrations can be used to determine the isoelectric point of compounds, to find the molarity of a sample (the number of moles of solute per liter of solution), to measure compounds such as vitamin C in a mixture, and to test the quality of products.

The Titration Experiment

Let's walk through how a vitamin C (which is typically found in the form of ascorbic acid) titration will occur. The juice is accurately weighed and put into a beaker. A buret is filled with indophenol, which is a dye. When this indophenol reacts with the vitamin C in the sample, it becomes reduced so it has no color. But once all of the vitamin C has been used up (to react with the indophenol before) then the indophenol is no longer reduced, and it appears as a pink color.

Color is not the only way to observe a titration. The pH can also be monitored in order to observe the progression of the reaction. This is particularly helpful for determining the isoelectric point. So a pH probe is inserted into the acidic sample. As the base (in this case, the titrant indophenol is considered a base) is slowly added, the pH doesn't change much to start out with. But it will suddenly increase a lot. The volume at the half-way point of this sudden pH increase is the isoelectric point.

Titration Curve

When indicating the amounts of titrant used and the pH (or color change) a titration curve can be drawn.

The titration starts out, with the sample in a beaker, and the titrant is in the buret as shown:

The titrant is in the buret, and the sample is in the beaker.
Titration begins

As the first few mL of titrant are added to the beaker, the pH doesn't change very much:

At the start of the titration, the pH does not change much.
Titration begins

But suddenly, the pH will increase a lot, after just a small amount of titrant is added:

Suddenly, after a small difference in titrant is added, the pH increases by a lot.
Titration peak

When we look at the resulting curve, we can draw a line along the 'straight up' portion, and the half-way point is the isoelectric point, or equivalence point, of the compound:

The halfway point is called the isoelectric or equivalence point.
Equivalence point

Titration Calculations Equation

So we know how much titrant was used, but what does this tell us about the molarity or how much vitamin C was in the sample?

The equation is actually a very simple equation, which is what makes titrations so nice.

The titration equation

The basic equation is simple molarity of sample times the volume of the sample is equal to the molarity of the titrant times the volume of the titrant. This equation only works if the ratio of analyte, the resulting compound from the reaction, to the titrant is 1:1. If the ratio is not 1:1, then the equation needs to be slightly changed.

If the ratio of analyte to titrant is 1:2 then the equation would look like this:

When the ratio is not one to one, then we multiply the titrant or sample by the change in ratio.
Titration equation 2

So we simply multiply the titrant side by 2, in order to account for there being twice as much titrant as analyte.

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