Question

You are titrating an acidic solution with a basic one, and just realized you forgot to add the indicator that tells you when the equivalence point is reached. In this titration, the indicator turns blue at the equivalence point from an initially colorless solution. You quickly grab a bottle of indicator and throw some into your titration beaker, and the whole solution turns dark blue. What do you do now? [Section 4.6]

Short answer

Since the indicator was added late in the titration and the solution turned dark blue, the exact equivalence point cannot be determined from the current titration, which may lead to inaccurate results. The best course of action is to discard the current titration, prepare a fresh sample of the acidic solution, and add the indicator at the beginning of the titration process. Perform the titration again, and monitor the color change to determine the equivalence point accurately. Record the volume of titrant required and use this information to calculate the concentration of the acidic solution.

Step-by-step solution

Understanding the situation

When performing a titration, an indicator is used to determine the equivalence point, which is the point at which the amount of titrant added is stoichiometrically equivalent to the amount of analyte present in the solution. In this case, the equivalence point is reached when the solution turns dark blue. It is essential to add the indicator at the beginning of the titration to precisely identify the equivalence region. However, in this exercise, the indicator was added after some titrant had already been introduced, and the solution turned dark blue.

Assessing the accuracy of the current titration

Since the indicator was added late and the solution is already dark blue, it is impossible to know the exact point when the equivalence was reached. This means that the current titration results might be inaccurate, as we cannot determine the amount of titrant that was required to reach the equivalence point.

Deciding on an action plan

To have accurate titration results, it is necessary to pinpoint the exact amount of titrant needed to reach the equivalence point. In this case, it would be wise to discard the current titration and start over.

Performing the new titration

To ensure accurate results, follow these steps: 1. Prepare a fresh sample of the acidic solution that needs to be titrated. 2. Add the appropriate indicator to the acidic solution at the beginning of the titration process. 3. Slowly add the titrant (basic solution) to the acidic solution while continuously stirring. 4. Monitor the color change in the solution, and stop adding the titrant when the solution turns dark blue, indicating the equivalence point.

Reporting accurate results

Once the titration is performed correctly and the equivalence point is reached, record the volume of the titrant required to reach the equivalence point. This information can be used to calculate the concentration of the acidic solution, providing accurate and reliable results.

Key concepts

Equivalence Point

In titration, the equivalence point is crucial. It signifies the moment when the moles of the titrant added equal the moles of the substance present in the solution being titrated. For acid-base titrations, this means that the acid has completely reacted with the base. The implication of reaching this point is that any further addition of titrant will start leading to an excess, potentially altering the pH of the solution drastically. Finding the exact equivalence point allows chemists to determine the precise concentration of the unknown solution. It involves using stoichiometry to know exactly when neutralization occurs. Without identifying this point accurately, the results of the titration could be meaningless. Every titration needs keen observation around this point, ensuring precision in scientific results.

Indicator

An indicator is a substance that changes color to show the completion of a chemical reaction, like reaching the equivalence point. It’s a vital part of a titration. Indicators are chosen based on the pH range they can signal, ideally shifting their color at the equivalence point of the reaction. In our case, the indicator turns the solution blue, denoting the completion of acid-base titration.

Choosing the right indicator is essential as using an incorrect one could lead to misleading conclusions. For example, if the indicator's color change happens before or after the equivalence point, it won’t provide accurate results.

• Indicators such as phenolphthalein change around pH 8.2 to 10.
• Other indicators like bromothymol blue change from yellow to blue as pH goes from 6.0 to 7.6.

Ensuring the proper conditions for the indicator to function is also important. This means adding it at an appropriate stage of the titration and in the correct quantities.

Acid-Base Titration

Acid-base titration is a method used to find the concentration of an unknown acid or base by neutralization. It involves slowly adding a solution of known concentration (titrant) to a measured volume of a solution of unknown concentration (analyte) until the reaction reaches the equivalence point. This is usually indicated by a color change from an indicator.

In acid-base titrations, a few steps are always followed:

• Preparing the titrant and analyte solutions.
• Selecting and adding the appropriate indicator.
• Conducting the titration by adding titrant gradually to the analyte until the indicator changes color.

The main goal is accurate measurement and calculation of the unknown solution's concentration. The use of a burette allows precise delivery of the titrant, and all results are recorded to calculate molarity using the formula:
\[ \text{Molarity of Analyte} = \frac{\text{Molarity of Titrant} \times \text{Volume of Titrant}}{\text{Volume of Analyte}} \]