Titration: Videos & Practice Problems

Titrations are a fundamental laboratory technique used to measure pH changes in acid-based solutions, primarily focusing on weak acids, which are prevalent in biological systems. The process involves a titrant, a solution of known concentration, being gradually added to an analyte solution, which has an unknown concentration. The goal is to reach the point of neutralization, where the moles of titrant equal the moles of analyte, indicated by a visible color change.

In a typical titration setup, the analyte solution is contained in an Erlenmeyer flask, while the titrant is held in a burette. A pH electrode continuously measures the pH of the analyte as titrant is added. The titration curve, which plots the pH of the analyte against the amount of titrant added, is crucial for visualizing the process. The equivalence point, or endpoint, occurs when the moles of titrant added are equal to the moles of analyte, typically resulting in a pH of 7 for strong acid-strong base titrations.

For example, when titrating a strong acid like hydrochloric acid (HCl) with a strong base such as sodium hydroxide (NaOH), the titration curve will show a low initial pH, indicating the presence of a strong acid. As titrant is added, the pH will rise sharply at the equivalence point, confirming the neutralization of the acid. The equivalence point is marked at 1 molar equivalent of titrant, meaning 100% of the analyte has been neutralized.

Understanding titration curves is essential for predicting the type of titration being performed. The characteristics of the curve, including the initial pH and the pH at the equivalence point, provide insights into the nature of the acids and bases involved. This knowledge is particularly valuable in biochemistry, where weak acids play a significant role in various biological processes.

Titration is a crucial technique in chemistry, particularly for understanding the behavior of acids and bases. In biological systems, most acids are weak acids, and their titration reveals important characteristics, such as the pKa values. The pKa value is significant because it indicates the strength of an acid in solution. During a titration of a weak acid with a strong base, the inflection point, also known as the midpoint, occurs when half of the acid has been neutralized. At this point, the pH of the solution equals the pKa of the weak acid, which means the concentrations of the conjugate base and conjugate acid are equal.

In a titration curve, the equivalence point is where all the weak acid has been neutralized by the strong base. This point is marked on the curve and is crucial for determining the amount of titrant needed. For weak acids, the equivalence point does not necessarily correspond to a pH of 7, which is only true for strong acid-strong base titrations. Instead, the pH at the equivalence point for a weak acid titrated with a strong base is typically higher, often around pH 8.5. This is due to the presence of the conjugate base formed during the reaction, which can affect the pH of the solution.

Understanding these concepts is essential for accurately interpreting titration results and applying them in practical scenarios. As you practice these principles, keep in mind the relationship between pH, pKa, and the equivalence point, as they are fundamental to mastering acid-base chemistry.

Polyprotic acids are characterized by having multiple acidic hydrogens, each associated with its own pKa value. The titration curves of these acids display multiple inflection points and equivalence points, corresponding to the number of acidic hydrogens present. For instance, if an acid has two acidic hydrogens, it will exhibit two inflection points and two equivalence points. This relationship is crucial for understanding the behavior of polyprotic acids during titration.

Phosphoric acid (H₃PO₄) serves as a prime example of a polyprotic weak acid, possessing three acidic hydrogens with pKa values of 2.2, 7.2, and 12.7. The titration curve for phosphoric acid against a strong base illustrates these points clearly. The y-axis represents the pH of the solution, while the x-axis indicates the amount of titrant added. Each inflection point on the curve corresponds to a pKa value, where the concentration of the conjugate acid equals that of the conjugate base.

At the first inflection point, occurring at a pH of 2.2, the concentration of H₃PO₄ (the conjugate acid) equals that of H₂PO₄^- (the conjugate base). This point marks the first equivalence point, where one molar equivalent of titrant has been added, completely neutralizing H₃PO₄. At the second inflection point, with a pH of 7.2, the concentration of H₂PO₄^- equals that of HPO₄^2-, indicating the second equivalence point where two molar equivalents of titrant have been added, fully neutralizing H₂PO₄^-.

The third inflection point occurs at a pH of 12.7, where the concentration of HPO₄^2- equals that of PO₄^3-. This point signifies the third equivalence point, achieved after adding three molar equivalents of titrant, resulting in the complete neutralization of HPO₄^2-.

Understanding these concepts is essential for analyzing titration curves and the behavior of polyprotic acids in various chemical reactions. The Henderson-Hasselbalch equation can also be applied to further explore the relationships between pH, pKa, and the concentrations of acids and bases in solution.