Titration: Videos & Practice Problems

Titrations are essential for determining the pKa values of weak acids and their concentrations. The titrant, a solution of known concentration, is added to the analyte until neutralization occurs, indicated by a color change. The equivalence point, where moles of titrant equal moles of analyte, varies in pH depending on the acid's strength. For polyprotic acids, multiple inflection and equivalence points correspond to each acidic hydrogen's pKa. The Henderson-Hasselbalch equation is useful for analyzing these relationships in titration curves.

Titrations

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.

Which of the following titration curves expresses the titration of a weak acid with a strong base?

• A.
• B.
• C.
• D.

Answer: (D)

Titration of Monoprotic Weak Acids

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.

You have an analyte solution of 50 mL of 0.2 M acetic acid (pK_a = 4.8). What volume of 0.05 M NaOH titrant needs to be added to get the final pH = pK_a?

• A. 20 mL
• B. 50 mL
• C. 100 mL
• D. 150 mL

Answer: (C)

Titration of Polyprotic Weak Acids

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.

Use the titration curve below.

What is the predominate species in the solution of phosphoric acid at pH = 5?

• A. H₃PO₄
• B. H₂PO₄^-
• C. HPO₄^-2
• D. PO₄^-3

Answer: (B)

Titration confirms an acetic acid solution to be 0.1 M. Calculate the pH. (acetic acid K _a = 1.76 x 10^-5 M).

• A. 2.1
• B. 3.6
• C. 2.9
• D. 8.3

Answer: (C)

Here’s what students ask on this topic:

What is the purpose of a titration in biochemistry?

The purpose of a titration in biochemistry is to determine the pKa values of weak acids and their concentrations. Titrations involve gradually adding a titrant, a solution of known concentration, to an analyte, a solution of unknown concentration, until neutralization occurs. This process helps identify the point where the moles of titrant equal the moles of analyte, known as the equivalence point. Titrations are crucial for understanding the behavior of acids and bases in biological systems, where most acids are weak acids.

How do you determine the pKa value from a titration curve?

To determine the pKa value from a titration curve, identify the inflection point, also known as the midpoint, on the curve. This point occurs when half of the acid has been neutralized, and the pH of the solution equals the pKa value of the weak acid. At this point, the concentration of the conjugate base is equal to the concentration of the conjugate acid. The inflection point is typically marked by a noticeable change in the slope of the titration curve.

What is the equivalence point in a titration?

The equivalence point in a titration is the exact point where the moles of titrant added are equal to the moles of analyte present in the solution. It is indicated by a significant change in the pH of the solution, often marked by a color change if an indicator is used. For strong acid-strong base titrations, the equivalence point typically occurs at a pH of 7. However, for weak acid-strong base titrations, the pH at the equivalence point can be higher than 7.

What are polyprotic acids and how do their titration curves differ?

Polyprotic acids are acids that have multiple acidic hydrogens, each with its own pKa value. Their titration curves differ from monoprotic acids by having multiple inflection points and equivalence points, corresponding to each acidic hydrogen. For example, phosphoric acid (H₃PO₄) has three acidic hydrogens, resulting in three distinct pKa values and three sets of inflection and equivalence points on its titration curve. Each inflection point indicates the pKa of an acidic hydrogen, and each equivalence point marks the complete neutralization of one acidic hydrogen.

How does the Henderson-Hasselbalch equation apply to titrations?

The Henderson-Hasselbalch equation is useful for analyzing titration data, especially for weak acids. The equation is given by:

${\mathrm{pH}}_{}={\mathrm{pKa}}_{}+\frac{\log }{\frac{\mathrm{[A^-]}}{\mathrm{[HA]}}}$

where [A^-] is the concentration of the conjugate base and [HA] is the concentration of the conjugate acid. At the inflection point, where pH = pKa, the concentrations of the conjugate base and acid are equal. This relationship helps in determining the pKa value from the titration curve.