pH and [H⁺]

Understanding the relationship between pH and hydrogen ion concentration is fundamental to understanding acid-base balance

pH and Hydrogen Ion Concentration [\(H^+\)]

• Acids and Bases
  • Acids: Substances that donate hydrogen ions (\(H^+\)) in solution
  • Bases: Substances that accept hydrogen ions (\(H^+\)) in solution
• pH Definition: The pH is a measure of the acidity or alkalinity of a solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration:

\[pH = -log [H^+] = -log [H_3O^+]\]

• Hydrogen Ion Concentration [\(H^+\)]
  • The hydrogen ion concentration [\(H^+\)] refers to the amount of hydrogen ions present in a solution
  • [\(H^+\)] is typically expressed in units of moles per liter (mol/L) or nanomoles per liter (nmol/L)
• Relationship Between pH and [\(H^+\)]
  • Inverse Relationship: pH and [\(H^+\)] are inversely related. As [\(H^+\)] increases, pH decreases, and as [\(H^+\)] decreases, pH increases
  • Acidic Solutions: Have a high [\(H^+\)] and a low pH (pH < 7)
  • Basic Solutions: Have a low [\(H^+\)] and a high pH (pH > 7)
  • Neutral Solutions: Have a [\(H^+\)] equal to the [\(OH^-\)] and a pH of 7
• The pH Scale
  • The pH scale ranges from 0 to 14
  • pH 0-7: Acidic
  • pH 7: Neutral
  • pH 7-14: Basic (alkaline)
• Normal Physiological pH
  • Arterial Blood: 7.35-7.45
  • Venous Blood: 7.32-7.42
  • Cerebrospinal Fluid (CSF): 7.35-7.40
  • Intracellular Fluid: Varies depending on the cell type, but typically slightly lower than extracellular fluid
• Importance of Maintaining Physiological pH
  • Enzyme Activity: Most enzymes function optimally within a narrow pH range
  • Protein Structure: pH affects the ionization state of amino acids, which influences protein folding and stability
  • Oxygen Transport: pH affects the affinity of hemoglobin for oxygen
  • Electrolyte Balance: pH affects the distribution and transport of electrolytes
  • Cellular Metabolism: pH influences various metabolic processes
• Calculations Involving pH and [\(H^+\)]
  • Calculating pH from [\(H^+\)]: \(pH = -log[H^+]\)
  • Calculating [\(H^+\)] from pH: \([H^+] = 10^{(-pH)}\)
• Henderson-Hasselbalch Equation
  • This equation relates pH to the concentrations of bicarbonate (\(HCO_3^-\)) and carbon dioxide (\(CO_2\)) in the blood: \[pH = 6.1 + \log \left( \frac{[HCO_3^-]}{0.03 \times pCO_2} \right)\]
    • \(pH\): Blood pH
    • \(pKa\): Dissociation constant (e.g. carbonic acid \(pKa\) = 6.1)
    • [\(HCO_3^-\)]: Bicarbonate concentration (mmol/L)
    • 0.03 is a solubility coefficient, converting Pa\(CO_2\) to dissolved \(CO_2\)
    • Pa\(CO_2\): Partial pressure of carbon dioxide (mmHg)
• Clinical Significance
  • The Henderson-Hasselbalch equation is used to assess acid-base disturbances
  • Arterial Blood Gas (ABG) analysis measures pH, Pa\(CO_2\), and \(HCO_3^-\), allowing for the diagnosis and classification of acid-base disorders
• Acid-Base Imbalances
  • Acidosis: A condition in which the blood pH is below 7.35
    • Increased [\(H^+\)]
  • Alkalosis: A condition in which the blood pH is above 7.45
    • Decreased [\(H^+\)]

Key Terms

• Acid: A substance that donates hydrogen ions (\(H^+\))
• Base: A substance that accepts hydrogen ions (\(H^+\))
• pH: A measure of the acidity or alkalinity of a solution
• Hydrogen Ion Concentration [\(H^+\)]: The amount of hydrogen ions present in a solution
• Acidosis: A condition in which the blood pH is below 7.35
• Alkalosis: A condition in which the blood pH is above 7.45
• Buffer: A substance that resists changes in pH
• Henderson-Hasselbalch Equation: An equation that relates pH to the concentrations of bicarbonate and carbon dioxide
• pKa: A number that characterizes the strength of an acid