Problems with Titrations

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If you take a chemistry lab in high school or college, you'll quickly become familiar with a technique called titration. In titration experiments, a chemical called a titrant is dispensed from a buret into a flash, where it reacts with and neutralizes a chemical of unknown concentration. This test substance is called the analyte, and you can use the amount of titrant dispensed to calculate its concentration. While titrations are straightforward in principle, a number of complications can crop up in practice.

Reaction

  • A titration can only be used where there is a reliable way to track the progress of the reaction between the titrant and the analyte. With acids and bases, for example, the pH will change over the course of the titration; with oxidizing and reducing agents, the progress of the reaction can be monitored with a chemical called a redox indicator. For some reactions, however, progress is difficult to track precisely. If the product of the reaction is a solid precipitate, for instance, it's very difficult to determine concentration based on titration. Reactions whose progress is difficult to track do not lend themselves to titration.

Indicator

  • Many titrations use an indicator, a chemical that changes color at the endpoint, i.e., when the titration is complete. It's important, however, to choose an indicator whose endpoint is as close as possible to the equivalence point of the titration -- the point at which all the analyte has been neutralized by titrant. If the equivalence point is too far from the endpoint, your calculations will be off by the amount of additional titrant it took to reach the endpoint. If the equivalence point pH of a titration is 7, for example, but the indicator only changes color once you reach pH 11, your results would be flawed because you would have no way to know when you reached pH 7. Many different indicator chemicals are available; it's important to select the right one when preparing your titration.

Rate of Addition

  • Imagine you are performing the most common kind of titration -- an acid-base titration. At the beginning, adding a lot of titrant will have only a small effect on pH. Near the equivalence point of the titration, however, the pH will start to change rapidly. Consequently, it's important to decrease the rate at which you are adding titrant as you approach your endpoint. If you continue to add titrant too rapidly, you may find that you overshoot the endpoint and need to repeat the titration.

Weak Acid Weak Base

  • Titrating a strong acid and weak base or strong base and weak acid such as its pKa value. Titrating a strong acid and a strong base also works very well. Titrating a weak acid and a weak base, however, is inadvisable, because it will be very difficult to find the equivalence point from your data for this kind of experiment. In general, one of the reactants should always be a strong acid/strong base. These chemicals, however, are more hazardous, which can sometimes present a problem in grade-school science classes.

References

  • "Chemistry 7L Lab Manual"; Sandrine Berniolles; 2010
  • "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins, et al.; 2008
  • Photo Credit Jupiterimages/Goodshoot/Getty Images
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