Acid Base Titration Theory

In chemistry, acid-base titration is used to determine the concentration of a solution. The process of titrating utilizes a solution of unknown concentration, which is either an acid or a base. A solution of known concentration is then added to the solution being testing. The added solution is the opposite of what the tested solution is: If one is a base, the other is an acid. The type of chemical reaction occurring between the two solutions is called a "neutralization reaction." The amount of one required to exactly neutralize the other tells you when the amount of acid and base are equal. The known concentration can therefore be used to calculate the unknown concentration. Acid-base titrations are the most common type of titration.

Basic Process

The solution of unknown concentration is called the "titer." The added solution is called the "titrant." In acid-base titration, enough titrant is added to the titer to neutralize it. So if the titer is a base, an acid is added as the titer.

A color indicator is added to the titer before starting to indicate to the lab technician when the neutralization point is approaching. This is important because if he adds the titrant too fast, the technician can go right by the neutralization point and not know exactly how much titrant was needed to reach it.

Indicators

In acid-base titration, the neutralization point occurs at a pH of 7.0. Litmus is a good indicator for an acid-base titration, because it changes color at a pH of around 6.5---close enough, as will be explained below. Since indicators react with the solution being measured, they should be used in moderation---only a few drops if possible.

Equivalence Point

The point at which the titrant fully neutralizes all the titer, leaving neutral water, is called the "equivalence point." This is when the titrant has "used up" all of the titer. The acid and base have fully canceled each other out. An example of this sort of mutual cancellation is illustrated in this chemical formula:

HCl+NaOH-->NaCL+H2O

At equilibrium, the solution's pH is 7.0.

Titration Curve

If a pH meter is used, pH can be recorded on a regular basis as titrant is added. A plot of the pH (as the vertical axis) against the volume of titrant would produce a sloping curve that is particularly steep around the equivalence point. PH is a measure of the H3O+ concentration in a solution. Adding one or two drops to a neutral solution greatly changes the H3O+ concentration, by a factor of 10 or more. Doubling the amount added doesn't change the concentration nearly as much. This is what makes the titration curve so steep in that one region, and therefore makes the equivalence point so easy to identify in the graph. The amount of titrant needed to neutralize the titer is therefore easy to accurately quantify.

Potentiometric Titration

A titration curve can also graph conductivity (as the vertical axis) against titrant. Acids and bases conduct electricity. Therefore, conductivity can be measured by inserting electrodes into the titer. The electrodes would be attached to a battery and ammeter (or voltmeter). The titration curve will change precipitously at the equivalence point. In this case, conductivity will have a noticeable minimum at the equivalence point. This method has the benefit of not needing an indicator, which could interfere or participate in the neutralization reaction, affecting its results.

References

• Virginia Tech: Titration
• Potentiometric Titration
• Acid-Base Indicators