14.7 Acid-Base Titrations
4 min read•june 25, 2024
Acid-base titrations are powerful tools for analyzing and quantifying acids and bases in solution. By carefully adding one reactant to another, we can determine concentrations and explore the unique properties of different acid-base pairs.
The shape of curves reveals important information about acid and base strength. Understanding how to interpret these curves and calculate values at key points allows us to gain deeper insights into acid-base chemistry and equilibria.
Acid-Base Titrations
Analysis of titration curves
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- Rapid pH increase near the due to complete and formation of a neutral salt
- occurs at pH 7 because the molar concentrations of and ions are equal
- titration curve
- Gradual pH increase before the equivalence point as the weak acid is slowly neutralized by the strong base
- region near the maintains a relatively stable pH due to the presence of the weak acid and its conjugate base
- Equivalence point occurs at pH > 7 because the salt formed is slightly basic (sodium acetate)
- titration curve
- Gradual pH decrease before the equivalence point as the is slowly neutralized by the
- region near the half-equivalence point maintains a relatively stable pH due to the presence of the weak base and its conjugate acid
- Equivalence point occurs at pH < 7 because the salt formed is slightly acidic (ammonium chloride)
Calculation of pH in titrations
- Initial pH
- Strong acid: where is the initial concentration of the strong acid ()
- Weak acid: , where is the initial concentration of the weak acid (acetic acid) and is the negative logarithm of the
- Half-equivalence point (weak acid-strong base titration)
- because the concentrations of the weak acid and its conjugate base are equal at this point
- Equivalence point
- Strong acid-strong base: because the molar concentrations of and ions are equal
- Weak acid-strong base: , where is the negative logarithm of the ( at 25 ℃)
- Weak base-strong acid: , where is the negative logarithm of the base dissociation constant
- After equivalence point
- Excess strong base: , then to calculate the pH of the basic solution ()
- Excess strong acid: to calculate the pH of the acidic solution (hydrochloric acid)
Function of acid-base indicators
- are weak acids or bases that change color depending on the pH of the solution, providing a visual indication of the progress and of a titration
- Indicator dissociation reaction:
- is the acid form of the indicator, which exhibits one color ( is yellow in acidic solution)
- is the base form of the indicator, which exhibits another color (bromothymol blue is blue in basic solution)
- Indicator color change occurs over a specific pH range
- Range depends on the indicator's value, which is the pH at which the concentrations of the acid and base forms are equal
- Suitable indicator for a titration
- Color change should occur near the equivalence point to accurately determine the endpoint of the titration
- Selected based on the expected pH at the equivalence point ( for weak acid-strong base titrations)
- Examples of common indicators
- Phenolphthalein: colorless (acid) to pink (base), , suitable for weak acid-strong base titrations
- : red (acid) to yellow (base), , suitable for weak base-strong acid titrations
- Bromothymol blue: yellow (acid) to blue (base), , suitable for strong acid-strong base titrations
Acid-Base Chemistry Fundamentals
- Neutralization: The reaction between an acid and a base, resulting in the formation of water and a salt
- : Pairs of chemical species that differ by a proton, playing a crucial role in buffer systems and acid-base equilibria
- Acid dissociation constant (Ka): A measure of the strength of an acid in solution, influencing the position of acid-base equilibria
- Ion product of water (Kw): The product of hydrogen ion and hydroxide ion concentrations in aqueous solutions, maintaining a constant value of 1.0 × 10^-14 at 25°C
- : The quantitative relationship between reactants and products in chemical reactions, essential for calculating titration endpoints
- : A method of quantitative chemical analysis that involves measuring the volume of a solution of known concentration required to react with a given amount of
Key Terms to Review (49)
Acid anhydrides: Acid anhydrides are chemical compounds derived from acids by the removal of water. They typically react with water to form corresponding acids.
Acid Dissociation Constant: The acid dissociation constant, denoted as $K_a$, is a quantitative measure of the strength of an acid in a solution. It represents the equilibrium constant for the dissociation of an acid into its conjugate base and hydrogen ions, and is a critical factor in understanding pH, buffers, and acid-base titrations.
Acid-base indicators: Acid-base indicators are substances that change color in response to changes in pH, allowing the determination of the acidic or basic nature of a solution. They are commonly used in titrations to signal the endpoint of a reaction.
Analyte: An analyte is a substance or chemical constituent that is being measured and analyzed in a laboratory sample. It is the target of quantitative chemical analysis to determine its concentration within a mixture.
Analyte: The analyte is the specific chemical species or compound of interest that is being measured or detected in a quantitative chemical analysis or an acid-base titration. It is the target substance that the analysis aims to identify and/or quantify.
Arrhenius: Arrhenius is a fundamental concept in chemistry, particularly in the context of acid-base titrations. It refers to the Swedish chemist Svante Arrhenius, who developed a theory to explain the behavior of acids and bases in aqueous solutions.
The Arrhenius definition of an acid is a substance that increases the concentration of hydrogen ions (H+) when dissolved in water, while a base is a substance that increases the concentration of hydroxide ions (OH-) when dissolved in water. This understanding forms the basis for understanding the neutralization reactions that occur during acid-base titrations.
Bromothymol Blue: Bromothymol blue is a pH indicator dye that is commonly used in acid-base titrations to determine the endpoint of the reaction. It is a weak acid that changes color in response to changes in pH, making it a useful tool for monitoring the progress of a titration.
Brønsted-Lowry: Brønsted-Lowry is a theory of acid-base behavior that defines acids as proton (H+) donors and bases as proton acceptors. This theory provides a more comprehensive understanding of acid-base reactions compared to the earlier Arrhenius definition, which was limited to aqueous solutions.
Brønsted-Lowry acid: A Brønsted-Lowry acid is a substance that can donate a proton ($H^+$) to another substance. It is one-half of the Brønsted-Lowry acid-base theory, which focuses on proton transfer reactions.
Buffer: A buffer is a solution that resists significant changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.
Buffer: A buffer is a solution that resists changes in pH upon the addition of small amounts of an acid or base. It maintains a relatively stable pH, which is crucial for many chemical and biological processes to function properly.
Burette: A burette is a graduated glass tube with a tap or stopcock at the bottom, used in volumetric analysis to deliver an accurately measured volume of a liquid, typically for titrations in acid-base chemistry.
Color change interval: The color change interval is the pH range over which an acid-base indicator changes color. It is used to signal the endpoint of a titration.
Conjugate Acid-Base Pairs: Conjugate acid-base pairs are related chemical species that differ by the presence or absence of a single proton (H+). When an acid donates a proton, it becomes a conjugate base, and when a base accepts a proton, it becomes a conjugate acid. These pairs are fundamental to understanding the Brønsted-Lowry theory of acids and bases, as well as the concepts of pH, relative acid-base strengths, hydrolysis, polyprotic acids, and acid-base titrations.
Endpoint: The endpoint in a titration is the point at which the reaction between the analyte and the titrant is complete, signaling the end of the titration process. It is the point where a specific property, such as pH or color change, indicates that the reaction has reached its endpoint.
Equivalence point: The equivalence point in a titration is when the amount of titrant added is exactly enough to neutralize the analyte solution. It is the point at which stoichiometrically equivalent quantities of reactants have been mixed.
Equivalence Point: The equivalence point in a titration is the point at which the amount of titrant added is exactly equal to the amount required to completely react with the analyte. It is the point at which the reaction is complete, and the pH changes rapidly.
Erlenmeyer Flask: An Erlenmeyer flask is a type of laboratory glassware widely used in chemistry and other scientific disciplines. It is characterized by its distinctive conical shape with a flat bottom and a narrow neck, which makes it suitable for various experimental and analytical procedures.
H+: H+ is the chemical symbol for the hydrogen ion, which is a proton that has been separated from a hydrogen atom. This hydrogen ion plays a crucial role in various chemical processes, including the topics of hydrolysis of salts, buffers, and acid-base titrations.
H3O+: H3O+ is the hydronium ion, a positively charged species formed when a proton (H+) interacts with a water molecule (H2O). It is a crucial concept in understanding the behavior of acids, bases, and the pH of aqueous solutions.
Half-Equivalence Point: The half-equivalence point in an acid-base titration is the point during the titration process where the volume of the titrant added is equal to half the volume required to reach the equivalence point. At this point, the solution contains equal concentrations of the conjugate acid-base pair.
HIn: HIn, or the conjugate acid of a weak base, is a key concept in the understanding of acid-base titrations. It represents the protonated form of a weak base, which plays a crucial role in the equilibrium and pH calculations involved in these types of titrations.
Hydrochloric Acid: Hydrochloric acid (HCl) is a strong, corrosive acid that is found in the gastric juices of the human stomach. It plays a crucial role in the digestive process and is also widely used in various industrial and laboratory applications.
In-: The prefix 'In-' is used in chemistry to denote the direction or location of a reaction or process. It can indicate an inward movement, insertion, or inclusion of a substance or element within a larger system or structure.
Ion Product of Water: The ion product of water, also known as the equilibrium constant for the self-ionization of water, is a fundamental concept in acid-base chemistry that describes the equilibrium between water molecules, hydrogen ions, and hydroxide ions in an aqueous solution. It is a measure of the tendency of water to undergo self-ionization, forming equal concentrations of hydrogen and hydroxide ions.
Methyl Orange: Methyl orange is a pH indicator dye commonly used in acid-base titrations to determine the endpoint of the reaction. It is an azo dye that changes color based on the acidity or basicity of the solution it is added to.
Molarity: Molarity is a measure of the concentration of a solution, specifically the number of moles of a solute dissolved per liter of solution. It is a fundamental concept in chemistry that is used to quantify the amount of a substance present in a given volume of a solution and is essential for understanding various chemical processes and analyses.
Molarity (M): Molarity (M) is a measure of the concentration of a solute in a solution, expressed as moles of solute per liter of solution. It is commonly used to quantify the concentration of chemical solutions in laboratory settings.
Neutralization: Neutralization is a chemical process in which an acid and a base react to form a salt and water. This process is characterized by the mutual cancellation or counteraction of the acidic and basic properties, resulting in a neutral solution.
Neutralization reaction: A neutralization reaction is a chemical reaction in which an acid and a base react to form water and a salt. This type of reaction typically involves the combination of hydrogen ions ($H^+$) from the acid and hydroxide ions ($OH^-$) from the base.
OH-: OH- is the hydroxide ion, a negatively charged particle composed of one oxygen atom and one hydrogen atom. This ion is a key component in understanding acid-base chemistry, as it plays a central role in the concepts of pH, pOH, relative strengths of acids and bases, hydrolysis of salts, buffers, acid-base titrations, Lewis acids and bases, and coupled equilibria.
PH: pH, or potential of hydrogen, is a measure of the acidity or basicity of a solution. It is a scale that ranges from 0 to 14, with 7 being neutral, values less than 7 being acidic, and values greater than 7 being basic or alkaline. The pH of a solution is directly related to the concentration of hydrogen ions (H+) present, and it is a critical factor in many chemical and biological processes.
Phenolphthalein: Phenolphthalein is a chemical compound commonly used as a pH indicator in acid-base titrations. It is a weak acid that changes color depending on the pH of the solution, making it a valuable tool for quantitative chemical analysis and acid-base titrations.
PKa: pKa is a measure of the strength of an acid, representing the pH at which a weak acid is 50% dissociated. It is a critical parameter that helps determine the relative strengths of acids and bases, the behavior of polyprotic acids, the effectiveness of buffers, and the progress of acid-base titrations.
PKb: pKb is the negative logarithm of the equilibrium constant (Kb) for a base in an aqueous solution. It is a measure of the strength of a base, providing information about its ability to accept protons and form hydroxide ions in water.
PKw: pKw, also known as the ion product constant of water, is a fundamental parameter in acid-base chemistry that represents the equilibrium constant for the autoionization of water. It is a measure of the strength of water as an acid and base, and is essential in understanding the pH scale and acid-base titrations.
POH: pOH, or the negative logarithm of the hydroxide ion concentration, is a measure of the acidity or basicity of a solution. It is a crucial concept in understanding the relationship between pH and the relative strengths of acids and bases, as well as its application in acid-base titrations.
Sodium Hydroxide: Sodium hydroxide, also known as caustic soda or lye, is a highly alkaline chemical compound with the formula NaOH. It is a white, crystalline solid that is widely used in various industrial and household applications due to its strong base properties.
Stoichiometry: Stoichiometry is the calculation of reactants and products in chemical reactions using balanced chemical equations. It involves the use of molar ratios derived from these equations to predict quantities of substances consumed and produced.
Strong Acid: A strong acid is a type of acid that completely dissociates into its constituent ions when dissolved in water, resulting in a high concentration of hydrogen ions (H+) in the solution. This high concentration of H+ ions gives strong acids a very low pH and a high degree of acidity.
Strong Acid-Strong Base: A strong acid-strong base reaction is a type of acid-base reaction where a strong acid and a strong base are combined, resulting in the formation of a salt and water. This type of reaction is characterized by complete proton transfer from the acid to the base, producing a neutral solution with a pH of 7.
Titration: Titration is a quantitative analytical technique used to determine the unknown concentration of a solution by gradually adding a measured volume of a standard solution with a known concentration until a specific endpoint is reached, indicating the completion of a chemical reaction. This process allows for the precise measurement and calculation of the concentration of the unknown solution.
Titration analysis: Titration analysis is a quantitative chemical technique used to determine the concentration of an unknown solution by reacting it with a solution of known concentration.
Titration curve: A titration curve is a graph of the pH of a solution as a function of the volume of titrant added during an acid-base titration. It helps illustrate the point at which neutralization occurs.
Volumetric Analysis: Volumetric analysis is a quantitative analytical technique used to determine the concentration or amount of a substance in a solution. It involves the precise measurement of the volume of a reagent solution required to react completely with the substance being analyzed, allowing for the calculation of the unknown concentration or amount.
Weak Acid-Strong Base: A weak acid-strong base reaction is a type of acid-base reaction where a weak acid, such as acetic acid (CH3COOH), reacts with a strong base, such as sodium hydroxide (NaOH). This reaction results in the formation of a salt and water, and is an important concept in the context of acid-base titrations.
Weak Base: A weak base is a type of base that only partially dissociates in an aqueous solution, producing a relatively low concentration of hydroxide ions. This is in contrast to strong bases, which completely dissociate in water, producing a high concentration of hydroxide ions.
Weak Base-Strong Acid: A weak base-strong acid is a type of chemical system where a weak base (a substance that partially accepts protons) reacts with a strong acid (a substance that completely donates protons). This interaction is particularly important in the context of acid-base titrations, where the reactions and pH changes are closely studied.
Weak bases: Weak bases are substances that partially ionize in water, producing a relatively small number of hydroxide ions ($OH^-$). They have a higher $pH$ compared to strong acids but do not completely dissociate in solution.