Understanding pH
pH measures acidity or basicity on a scale from 0 to 14. It's defined as pH = -log??[H?], where [H?] is hydrogen ion concentration in mol/L. pH 7 is neutral, below 7 is acidic, above 7 is basic. Each pH unit represents a 10-fold change in [H?]. pH 3 is 10 times more acidic than pH 4 and 100 times more than pH 5. The relationship pOH = 14 - pH connects acidity and basicity through the water equilibrium constant.
The pH Scale and Common Substances
The pH scale ranges from 0 (strongly acidic) to 14 (strongly basic). Battery acid is pH 0, lemon juice pH 2, coffee pH 5, pure water pH 7, baking soda pH 9, and drain cleaner pH 14. Body fluids have specific pH ranges: blood is 7.35-7.45, stomach acid is 1.5-3.5. Enzyme activity, protein structure, and biochemical reactions depend critically on pH. Even small pH changes can have significant biological effects.
Applications and Importance
pH control is vital in chemistry, biology, medicine, and industry. Chemists buffer solutions to maintain constant pH. Biologists optimize pH for cell culture and enzyme reactions. Farmers test soil pH for optimal crop growth. Aquarium owners monitor water pH for fish health. Medical labs measure blood pH to assess respiratory and metabolic function. Water treatment plants adjust pH for safety and to prevent pipe corrosion. Understanding pH is essential for agriculture, food processing, pharmaceuticals, and environmental monitoring.
Quick Tips
- Always verify units are consistent
- Use scientific notation for very large/small numbers
- Results are approximations — real conditions may vary
Frequently Asked Questions
At 25 degreesC, pure water has [H?] = [OH?] = 10?? M, giving pH = -log(10??) = 7. This equality of H? and OH? defines neutrality. Note: neutral pH changes slightly with temperature.
Yes, though rare. Concentrated strong acids can have negative pH (e.g., 12 M HCl has pH ~= -1.1), and concentrated strong bases can exceed pH 14. The 0-14 range covers most practical situations.
pH + pOH = 14 at 25 degreesC. This comes from the water equilibrium constant: Kw = [H?][OH?] = 10???. Taking negative logarithms of both sides gives pH + pOH = 14.
pH can be measured with pH meters (electrode-based, accurate to 0.01 pH units), pH paper (quick, visual, less accurate), or indicators (chemicals that change color at specific pH ranges). pH meters are most accurate.
H? concentrations span many orders of magnitude (10??? to 10? M). The logarithmic scale compresses this huge range into manageable numbers (0-14) and reflects that biological responses often scale logarithmically.