뒤로pH and Acid-Base Chemistry: Study Notes for General Biology
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pH and Acid-Base Chemistry
Introduction to pH
The concept of pH is fundamental in biology, as it describes the acidity or basicity of a solution. pH affects biochemical reactions, enzyme activity, and the structure of biological molecules.
pH is a measure of the hydrogen ion concentration in a solution.
The pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.
pH is calculated using the formula:
Where is the molar concentration of hydrogen ions.
Effects of Acids and Bases on pH
Adding acids or bases to a solution changes its pH, which can have significant biological consequences.
Acid Addition: Adding an acid increases the hydrogen ion concentration, lowering the pH (making the solution more acidic).
Base Addition: Adding a base decreases the hydrogen ion concentration, raising the pH (making the solution more basic).
Neutral Solution: A neutral solution has a pH of 7.
Relative Acidity: Each unit change in pH represents a tenfold change in hydrogen ion concentration. For example, a solution with pH 5 is 100 times less acidic than a solution with pH 3.
Acid-Base Reactions in Aqueous Solutions
Acid-base reactions in water can alter the pH of the solution. The direction and effect of these reactions depend on the reactants and products involved.
General Rule: Reactions that produce ions decrease pH (increase acidity), while those that consume ions increase pH (decrease acidity).
Examples of Acid-Base Reactions
Reaction | Effect on pH | Explanation |
|---|---|---|
HCl → H+ + Cl- | Decrease | Strong acid dissociates, increasing [H+] |
CaF2 → Ca2+ + 2F- | No effect | No H+ or OH- produced or consumed |
NaOH → Na+ + OH- | Increase | Strong base dissociates, increasing [OH-], which decreases [H+] |
NH4+ + H2O → NH3 + H3O+ | Decrease | Ammonium ion acts as a weak acid, releasing H+ |
H2SO4 → HSO4- + H+ | Decrease | Sulfuric acid dissociates, increasing [H+] |
NaCl → Na+ + Cl- | No effect | Neither ion affects [H+] or [OH-] |
Calculating pH from Hydrogen Ion Concentration
To determine the pH of a solution, use the formula .
Example: If M, then:
Lower means higher pH (more basic); higher means lower pH (more acidic).
Sample Calculations
For M:
For M:
For M:
Buffer Systems in Biology: The Carbonic Acid-Bicarbonate Buffer
Buffers are solutions that resist changes in pH when acids or bases are added. The carbonic acid-bicarbonate buffer system is crucial in maintaining blood pH.
The main reaction is:
Carbonic acid (H2CO3): Formed from CO2 and water; acts as a weak acid.
Bicarbonate (HCO3-): The conjugate base of carbonic acid.
This system helps maintain blood pH around 7.4.
Buffer Response to pH Changes
If blood becomes too acidic (pH drops), the reaction shifts to the left, converting H+ and HCO3- into H2CO3 and then into CO2 and H2O, thus removing H+ from the solution.
If blood becomes too basic (pH rises), the reaction shifts to the right, producing more H+ to lower the pH.
Example: During exercise, increased CO2 production can lower blood pH, but the buffer system helps restore normal pH.
Additional info: The Henderson-Hasselbalch equation is often used to describe buffer systems:
Where is the concentration of the conjugate base and is the concentration of the acid.
