Acid Base Titration III

Summary

Perform an acid-base titration to determine the concentration of an unknown hydrochloric acid solution using a sodium hydroxide titrant. This laboratory emphasizes visual measurements dealing with the meniscus of the burette, and supports two different configurations. 

The first one is for a potentiometric approach: you will have access to a digital pH sensor and you can use it to determine when the unknown solution has been neutralized. 

The second one is for a colorimetric approach: you can rely on the color change due to the presence of a phenolphthalein indicator, without having a digital pH sensor available.

 

Acid-base Titration

Titrations are a volumetric method that is based on measuring the amount of a known-concentration reactive (known as a primary standard) that is consumed by an unknown-concentration sample known as analyte.

The titration is conducted by adding the titrant to the analyte using a burette, so as to obtain a chemically-equivalent substance between the titrant and the analyte. This is known as the "equivalence point" and it is a theoretical value that cannot be experimentally determined.

The experimental estimation of this point is obtained through an approximation known as "final point". This is determined through a physical change. In that case, the change in color of the solution is achieved after adding an indicator substance: a substance that changes color in certain ranges of pH.

For the acid-base titration we use a phenolphthalein indicator that becomes a light pink after a pH of around 8.4, which is a value that is very close to the equivalence point in the most common acid-base titrations.

Alternatively, in the potentiometric configuration, a digital pH sensor can be used to determine the “equivalence point”.

 

Colorimetric vs Potentiometric approach

The colorimetric approach relies on the color change provided by the phenolphthalein indicator. The potentiometric approach relies instead on the pH raise as measured by the digital sensor. In this version of the laboratory there are two different configurations available, one for each approach. In the colorimetric configuration students may not see the digital pH sensor.

 

Differences with the other titration laboratories

In this version of the laboratory (Acid-Base Titration II) you can perform the acid-base titration for an unknown hydrochloric acid solution. In other versions of the laboratory you can perform the acid-base titration for a citric acid solution instead (Acid-Base Titration I), and acetic acid (Acid-Base Titration II).

Both this version of the laboratory (Acid-Base Titration III) and Acid-Base Titration II emphasize visual burette measurements, including properly reading the meniscus in the burette. The other version of the laboratory (see Acid-Base Titration I) does not emphasize this, and focuses on the calculations instead.

Also, in these versions you can choose between two different configurations: one for the potentiometric approach and one for the colorimetric approach. The configuration for the colorimetric approach does not show the pH sensor. In other version of the laboratory (see Acid-Base Titration I) there is a single configuration and the sensor is always shown.