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