Acid Base titration is a fundamental method in the quantitative chemical analysis used to determine the concentration of acids or bases in a solution. This process involves the controlled addition of a known concentration of an acid or base (titrant) to a solution of the substance being analyzed until the reaction reaches a neutral point. The moles of acid and base are equal at this equivalence point, resulting in a neutral solution. A pH indicator is often used to monitor the progress of the reaction, and a titration curve can be constructed to visualize the pH changes throughout the titration process.
In this article, we look into what is acid-base titration, titration curve, choice indicator, etc.
Table of Content
What is Acid Base Titration?
An acid-base titration is a quantitative analysis technique used to determine the concentration of acids or bases in a solution. In this process, a known concentration of an acid or base (titrant) is added to an unknown concentration of the opposite reactant (analyte). The reaction continues until the equivalence point is reached, where stoichiometric amounts of acid and base have reacted. This point is identified using indicators that change colour at the endpoint of the titration. By knowing the volume and concentration of the titrant at the equivalence point, one can calculate the concentration of the unknown solution.
Titration Curve, Endpoint and Equivalence Point
To understand acid-base titrations we need to learn about titration curves, endpoints, and equivalence points. These terms are explained below in detail.
Titration Curve
A titration curve is a graphical representation of a titration analysis, typically showing pH against the volume of titrant added. It helps identify the endpoint and equivalence point more precisely than using indicators. Different properties like pH, absorptivity, temperature, conductivity, or electrical current can reveal the endpoint of a reaction.
Endpoint
The endpoint of a titration is the stage where a color change or intensity shift indicates the completion of the reaction. It signifies the point at which the reactants have reacted completely. Indicators are often used to signal the endpoint of a titration visually.
Equivalence Point
The equivalence point is when chemically equivalent quantities of analyte and titrant exist in the system. At this point, moles of both species are equal, indicating that enough titrant has been combined with the analyte to neutralize it. It differs from the endpoint as it represents the exact stoichiometric point in the reaction.
Choice of Indicators
In acid-base titrations, the choice of indicator plays a crucial role in accurately determining the endpoint of the titration. Here are some of the points to consider:
- Indicator Function: An acid-base indicator is a weak organic acid or base that changes color depending on the pH of the solution. This color change helps signal the endpoint of a titration when the reaction is complete.
- pH Range: The effectiveness of an indicator is determined by its pH range. An indicator’s pH range is the span of pH values where it changes color from its acidic to basic form. It extends from the highest pH, where only the acid form is visible, to the lowest pH, where only the base form is visible.
- Indicator Selection: When choosing an indicator for titration, selecting one whose color changes occur as close to the pH at the equivalence point is essential. For example, Phenolphthalein with a pKa value around nine is suitable for titrations where the equivalence point falls within a pH range of eight to ten.
- Accuracy: Using an indicator with a pH range that aligns closely with the expected pH at the equivalence point ensures accurate endpoint detection. For strong acid-strong base titrations, where there is a significant pH change around the equivalence point, indicator choice is less critical than titrations involving weak acids or bases.
- Indicator Equilibrium: Indicators exist in equilibrium between their protonated and deprotonated forms, with distinct colors associated with each state. The equilibrium shifts based on changes in H+ or OH- concentrations in the solution, leading to color changes that indicate different stages of the titration process.
What Are Acid-Base Indicators?
Acid-base indicators are essential chemicals used to determine the acidity, neutrality, or alkalinity of a solution. These indicators undergo color changes based on the pH of their solution. Acid-base indicators play a vital role in analytical chemistry by providing visual signs to determine when a reaction has reached completion during titrations.
Use of Acid Base Titration
Here are some of the uses of acid-base titrations in industrial settings:
- pH Control: Acid-base titrations are crucial for maintaining optimal pH levels in various industrial processes, ensuring the desired chemical reactions occur efficiently.
- Evaluation of Acidity Constants: In industries like pharmaceuticals and food chemistry, acid-base titrations are used to evaluate the acidity constants of polyprotic systems, aiding in understanding chemical properties and reactions.
- Analysis of Polyprotic Systems: Acid-base titrations are employed to determine the speciation of polyprotic acids, providing insights into their composition and behavior in different solutions.
Types of Acid Base Titrations
These types of acid-base titrations play a crucial role in analytical chemistry, allowing for the determination of unknown concentrations of acids and bases through precise neutralization reactions using indicators to monitor the progress and endpoint of the reaction. Here are some of the acid-base titrations:
Titration of a Strong Acid with a Strong Base
This type of titration involves a strong acid and a strong base, leading to a neutralization reaction resulting in water and salt. The purpose is to determine the concentration of the acidic solution by titrating it with a strong base until neutralization occurs.
Equivalence Point:
- At the equivalence point, equal amounts of H+ and OH– ions combine to form water, resulting in a pH of 7.0 (neutral).
- The pH at the equivalence point for this titration will always be 7.0, specific to titrations of a strong acid with a strong base.
Procedure:
- A burette is used to dispense the strong base into a container of the strong acid, or vice versa, to determine the equivalence point.
- An indicator or pH meter is employed to locate the equivalence point during titration accurately.
Chemical Reactions: Strong acids completely ionize in aqueous solutions, dissociating into their ions when placed in water.
The general equation for the dissociation of a strong acid is HA → H+ + A–, while for a strong base, it is BOH → B+ + OH–.
When paired, the reaction between a strong acid and a strong base results in water and a neutral salt.
Example: This titration is the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) to form water, sodium ions, and chloride ions.
Titration of a Weak Acid with a Strong Base
This titration involves the reaction between a weak acid and a strong base, forming water and salt. The process aims to determine the concentration of the weak acid solution by adding a strong base until neutralization occurs.
Equivalence Point and pH:
The equivalence point for this titration is above 7 (basic) due to the presence of the anion from the weak acid reacting with water to produce hydroxide ions, leading to a basic pH. The pH at the equivalence point is higher than 7 due to the formation of hydroxide ions from the reaction between the anion and water.
Chemical Reactions:
The general reaction for this titration involves the weak acid reacting with hydroxide ions from the strong base to form water and the conjugate base of the weak acid.
CH3COOH(aq) + OH–(aq) → CH3COO–(aq) + H2O(l).
Titration Curve:
The titration curve for a weak acid with a strong base starts at a higher pH value compared to strong acid-strong base titrations. It maintains higher pH values throughout until the equivalence point. After the equivalence point, both curves become identical as pH is primarily determined by the excess hydroxide ions added in both cases.
Example Calculation:
Calculating the pH at different points during titration involves understanding equilibrium concentrations, using ICE tables, and applying relevant equations like Henderson-Hasselbalch for buffer solutions.
Titration of a Strong Acid with a Weak Base
This titration involves the reaction between a strong acid and a weak base, forming water and salt. The primary objective is to determine the concentration of the strong acid solution by titrating it with a weak base until neutralization occurs.
Equivalence Point and pH: The equivalence point for this titration is below 7 (acidic) due to the presence of excess hydronium ions from the strong acid reacting with the anion from the weak base, leading to an acidic pH. The pH at the equivalence point is lower than 7 due to the excess hydronium ions in the solution after neutralization.
Chemical Reactions: The general reaction for this titration involves the strong acid reacting with the weak base to form water and the conjugate base of the weak acid.
HCl(aq) + NH3(aq) → NH4+(aq) + Cl–(aq)
The titration curve for a strong acid with a weak base starts at a lower pH value than strong acid-strong base titrations and maintains lower pH values until the equivalence point.
Titration of a Weak Acid with a Weak Base
This titration involves the reaction between a weak acid and a weak base, forming water and salts specific to the reactants involved. The primary goal is to determine the concentration of the weak acid solution by titrating it with a weak base until neutralization occurs, leading to unique equilibrium conditions.
Equivalence Point and pH: The equivalence point for this titration can vary depending on the specific weak acid and weak base involved, resulting in pH values that are not necessarily 7.0. The pH at the equivalence point is determined by the particular equilibrium conditions of the weak acid and weak base reaction, leading to varying pH values around neutrality.
Chemical Reactions: The general reaction for this titration involves the weak acid reacting with the weak base to form water and salts, which is characteristic of the reactants involved, leading to unique equilibrium compositions.
CH3COOH(aq) + NH3(aq) → NH4+(aq) + CH3COO–(aq)
Titration Curve: The titration curve for a weak acid with a weak base exhibits gradual changes in pH around the equivalence point due to the characteristics of both reactants and their equilibrium properties. The shape of the curve is influenced by the identities of the weak acid and weak base, their dissociation constants (Ka and Kb), and their concentrations, resulting in distinctive titration curves.
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Solved Example Acid-Base Titration
Example 1: A 25 mL solution of 0.5 M NaOH is titrated until neutralized into a 50 mL sample of HCl. Determine the concentration of the HCl solution.
Solution:
The concentration of the HCl solution is calculated to be 0.25 M through titration.
Example 2: A 30 mL solution of 0.2 M HCl is titrated with 0.15 M NaOH. Calculate the volume of NaOH required to reach the equivalence point.
Solution:
Using the formula MacidVacid = MbaseVbase
0.2 × 30 = 0.15 × Vbase
(0.2 × 30) ÷ 0.15 = Vbase
Vbase = 40ml
Hence, the volume of NaOH required is 40 ml.
Example 3: Titrate a 40 mL acetic acid (CH3COOH) solution with a 0.1 M NaOH solution. Determine the pH at the half-equivalence point.
Solution:
At the half-equivalence point, half of the acetic acid has reacted with NaOH, forming equal amounts of acetic acid and acetate ions, leading to a buffer solution with a specific pH value.
Practice Questions on Acid Base Titration
1. What will be the pH at the equivalence point of the reaction when 50 mL of 0.257 M HBr is titrated with 0.450 M KOH solution? (Answer: pH = 7)
2. What is the trend of pH change in the titration between a weak acid and a weak base.
3. Why do the salts of weak acid and weak base hydrolyze in the solution?
4. In the titration of strong acid vs strong base, what trend of pH is observed?
5. How is an indicator selected for detecting the pH change in acid-base titrations?
Frequently Asked Questions on Acid Base Titration
Why can’t strong acids or bases act as buffers?
Strong acids and bases cannot act as buffers because they ionize entirely in solution. This leads to no undissociated acid or base reservoir to maintain the buffer’s pH.
Why does finding pKa from an acid-base titration only work with a strong base-weak acid pair?
The process of finding pKa from an acid-base titration works best with a strong base and weak acid pair because, at the halfway point, the concentrations of the weak acid and its conjugate base are equal, simplifying the calculation of pKa.
Why must the amount (volume) of titrant added be considered to determine pH during titration?
The volume of titrant added directly affects the pH during titration as it changes the concentration of reactants and products, impacting the solution’s acidity or basicity at any given point along the titration curve.
What is the significance of hydrolysis in salt analysis during acid-base titrations?
Hydrolysis plays a crucial role in salt analysis by influencing the pH of solutions containing salts, especially those derived from weak acids or bases, impacting their behavior in solution and aiding in identifying unknown substances.
How is an indicator selected for detecting pH changes in acid-base titrations?
Indicators are chosen based on their color changes at specific pH ranges. For example, Methyl orange changes from red (acidic) to yellow (basic), while Phenolphthalein shifts from colorless (acidic) to pink (basic), helping visualize pH transitions during titrations.