20.3 Biological Acids and the Henderson–Hasselbalch Equation
3 min read•may 7, 2024
The ###-Hasselbalch_Equation_0### is a powerful tool for understanding acid-base behavior in biological systems. It helps calculate the ratio of to forms of weak acids, crucial for predicting their behavior at different levels.
Most exist as at due to their low values. This equation allows us to quantify the percentages of and species, providing insights into acid-base equilibria and buffer systems in living organisms.
Biological Acids and the Henderson-Hasselbalch Equation
Ratio calculation with Henderson-Hasselbalch equation
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- Henderson- equation calculates ratio of dissociated (deprotonated) to undissociated (protonated) forms of a weak acid (carboxylic acids)
- Equation:
- : solution pH
- : negative logarithm of constant
- : concentration of dissociated (deprotonated) acid form
- : concentration of undissociated (protonated) acid form
- Equation:
- Calculate ratio by solving equation for , subtracting from both sides
- Find ratio by taking (10 to the power) of both sides
- Example: at pH 5.5
- , meaning more dissociated than undissociated form
- The Henderson-Hasselbalch equation uses a to relate pH and pKa
Carboxylic acids as anions at physiological pH
- Physiological pH around 7.4, higher than of most carboxylic acids (4-5)
- pH higher than shifts equilibrium towards dissociated (deprotonated) acid form
- Higher pH indicates lower ion concentration in solution
- At physiological pH, ratio much greater than 1, majority of carboxylic acid molecules in deprotonated (anionic) form
- Henderson-Hasselbalch equation demonstrates this:
- If , then , and
- Therefore, , deprotonated form dominant
- Example: at physiological pH (7.4)
- , vast majority in deprotonated form
- The deprotonated form of the acid acts as a in solution
Percentages of acid species at specific pH
- Calculate ratio using Henderson-Hasselbalch equation
- Total acid concentration equals sum of protonated and deprotonated form concentrations
- Divide ratio equation by :
- Calculate protonated form percentage:
- Calculate deprotonated form percentage:
- Example: Benzoic acid at pH 4.0
Buffer Solutions and Equilibrium
- resist changes in pH when small amounts of acid or base are added
- They consist of a weak acid and its
- explains how buffers maintain pH balance
- The () determines the strength of the acid-base pair in the buffer
- can be used to visualize the buffering capacity of a solution
Key Terms to Review (30)
Acetic Acid: Acetic acid is a weak organic acid with the chemical formula CH3COOH. It is a colorless liquid with a distinctive sour odor and is the main component of vinegar. Acetic acid is a versatile compound that plays important roles in various organic chemistry topics, including functional groups, oxidation of alkenes, reduction of carbonyl compounds, naming of carboxylic acids, and the chemistry of esters.
Acid Dissociation: Acid dissociation is the process by which an acid compound, when dissolved in a solvent, separates into its constituent ions. This fundamental chemical process is central to understanding the behavior and properties of acids in biological and chemical systems.
Acid Species: Acid species refer to the various forms that an acid can take in a solution, including the protonated acid, the conjugate base, and the hydronium ion. These different acid species are crucial in understanding the behavior and properties of acids in biological and chemical systems.
Anions: Anions are negatively charged ions that form when an atom gains one or more electrons, resulting in an excess of electrons and a net negative charge. These charged particles play a crucial role in chemical bonding and biological processes.
Antilog: The antilog, also known as the inverse logarithm, is a mathematical operation that reverses the process of taking the logarithm of a number. It is used to find the original number from its logarithmic value, and is particularly relevant in the context of biological acids and the Henderson-Hasselbalch equation.
Biological Acids: Biological acids are organic compounds found in living organisms that possess one or more carboxyl groups (-COOH) and can donate protons (H+) in aqueous solutions. These acids play crucial roles in various metabolic processes and are central to the Henderson-Hasselbalch equation, which describes the pH of biological systems.
Branched-chain alkane: A branched-chain alkane is an alkane that has one or more alkyl groups attached to its continuous chain of carbon atoms, creating a non-linear structure. These compounds are a type of hydrocarbon where the carbon atoms are connected by single bonds in a branching pattern, differing from straight-chain alkanes.
Buffer Solutions: Buffer solutions are aqueous solutions that resist changes in pH upon the addition of small amounts of an acid or base. They maintain a relatively stable pH and are essential in various chemical and biological applications, including organic chemistry and biochemistry.
Carboxylic Acids: Carboxylic acids are a class of organic compounds containing a carboxyl functional group (-COOH) attached to an alkyl or aryl group. They are characterized by their acidic properties and play a crucial role in various chemical reactions and biological processes.
Carboxylic acids, RCO2H: Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH), where "R" represents an alkyl or aryl group attached to the carbon atom of the carboxyl group. They are known for being acidic due to the ability of the hydroxyl (OH) part of the carboxyl group to release a proton (H+).
Conjugate base: A conjugate base is the species that remains after an acid has donated a proton (H+ ion) during a chemical reaction. It is capable of gaining a proton in the reverse reaction, forming the original acid.
Conjugate Base: A conjugate base is the species formed when an acid loses a proton (H+) in an acid-base reaction. It is the base that is left behind when an acid donates a proton to another substance, becoming the conjugate acid-base pair. This term is central to understanding acid-base chemistry, as well as its applications in organic reactions and biological systems.
Deprotonated: Deprotonated refers to the state of a molecule or ion where a proton (H+) has been removed, resulting in the loss of a positive charge. This process is central to understanding the behavior of biological acids and the application of the Henderson-Hasselbalch equation.
Dissociated: Dissociated refers to the process by which a molecule or ion separates into two or more smaller, electrically charged particles called ions. This is a key concept in understanding the behavior of biological acids and the Henderson-Hasselbalch equation, which describes the relationship between the pH of a solution and the concentrations of the dissociated and undissociated forms of an acid.
Equilibrium Constant: The equilibrium constant is a quantitative measure of the extent to which a reversible chemical reaction proceeds to completion. It represents the ratio of the concentrations of the products to the reactants at equilibrium, and provides insight into the position and direction of a reaction at equilibrium.
Hasselbalch: Hasselbalch is a key concept in understanding the relationship between pH, the concentration of hydrogen ions, and the equilibrium of biological acids and amino acids. It is central to the Henderson-Hasselbalch equation, which is used to describe the behavior of these systems.
Henderson: Henderson is a term that is closely associated with the Henderson-Hasselbalch equation, which is a fundamental concept in understanding the behavior of biological acids and the isoelectric points of amino acids. The Henderson-Hasselbalch equation provides a way to calculate the pH of a solution based on the acid dissociation constant (Ka) and the concentrations of the acid and its conjugate base.
Henderson-Hasselbalch Equation: The Henderson-Hasselbalch equation is a mathematical expression that relates the pH of a solution to the equilibrium concentrations of the conjugate acid-base pair. It is a fundamental tool used to predict and understand acid-base reactions in organic chemistry, biological systems, and various other applications.
Ka: Ka, or the acid dissociation constant, 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 a hydrogen ion. The value of Ka is used to determine the pH of an acid solution and to predict the extent of acid-base reactions.
Lactic Acid: Lactic acid is a chemical compound produced in the body during anaerobic respiration, where glucose is broken down without the use of oxygen. It is a key player in various biochemical processes, including the reason for handedness in molecules, spin-spin splitting in 1H NMR spectra, cyanohydrin formation, and the regulation of biological acids through the Henderson-Hasselbalch equation.
Le Chatelier's Principle: Le Chatelier's principle states that when a system at equilibrium is subjected to a change in one of the factors (concentration, temperature, or pressure) determining the equilibrium, the system will shift to counteract the change and establish a new equilibrium. This principle helps predict the direction of a system's response to disturbances.
Logarithmic Scale: A logarithmic scale is a type of scale used to measure quantities that vary over a very wide range of values. It is commonly used in scientific and engineering applications to represent data that spans multiple orders of magnitude, providing a more compact and visually intuitive representation compared to a linear scale.
PH: pH, or the 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.
Photon: A photon is a quantum of electromagnetic energy, essentially a particle of light that carries energy but has no mass. In the context of spectroscopy, photons interact with molecules to cause transitions between energy levels, which is fundamental to understanding molecular structure through techniques like infrared spectroscopy.
Physiological pH: Physiological pH refers to the normal range of pH values found in the body's fluids and tissues that are essential for maintaining optimal biological functions. It is a crucial factor in understanding the behavior and interactions of biological acids and bases as well as the role of biological amines in the body.
PKa: pKa, or the acid dissociation constant, is a measure of the strength of an acid in a solution. It represents the pH at which a particular acid is 50% dissociated into its conjugate base. This value is crucial in understanding the behavior and properties of acids, bases, and their reactions in organic chemistry.
Proton Transfer: Proton transfer is a fundamental chemical process in which a proton (H+) is donated from one species to another. This process is central to understanding acid-base reactions, reaction mechanisms, and the behavior of biological systems involving acids and bases.
Protonated: Protonated refers to a species or molecule that has gained a positively charged hydrogen ion (H+), also known as a proton. This process is crucial in understanding the behavior of biological acids and the Henderson-Hasselbalch equation, which describes the relationship between pH, pKa, and the concentrations of the protonated and deprotonated forms of an acid.
Titration Curves: Titration curves are graphical representations of the changes in pH that occur during a titration process. They provide a visual depiction of the relationship between the volume of a titrant added and the resulting pH of the solution, allowing for the determination of important properties such as the equivalence point and the buffer regions.
Undissociated: Undissociated refers to a molecule or species that has not undergone dissociation, meaning it has not separated into smaller ionic or molecular components. This term is particularly relevant in the context of biological acids and the Henderson-Hasselbalch equation, which describe the equilibrium between the dissociated and undissociated forms of an acid.