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Understanding pH and the logarithmic scale
pH is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10([H+]). Because it is logarithmic, each step represents a 10-fold change. pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5. This is why small pH changes can represent large changes in actual acidity.
In pure water at 25 degrees C, [H+] = [OH-] = 1 times 10^-7 mol/L, giving pH = 7. When an acid dissolves, it increases [H+] above 10^-7, lowering pH below 7. When a base dissolves, it decreases [H+] (via reaction with OH-), raising pH above 7.
Buffers and biological pH
Human blood is maintained at pH 7.35-7.45 by carbonate and phosphate buffer systems. A deviation of just 0.2 pH units (to 7.15 or 7.65) is life-threatening. The body prioritizes pH control above almost all other physiological parameters.
Enzyme activity is pH-dependent. Pepsin (stomach enzyme) is active at pH 1-3. Trypsin (intestinal enzyme) works at pH 7-8. Salivary amylase works at pH 6-7. This pH specificity is why the body maintains different pH environments in different digestive compartments.
Frequently asked questions
- What is pH?
- pH is a measure of hydrogen ion concentration in a solution: pH = -log10([H+]). The scale runs from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral (pure water at 25 degrees C).
- What is the relationship between pH and pOH?
- pH + pOH = 14 at 25 degrees C. pOH = -log10([OH-]). If pH = 3, then pOH = 11. [H+] times [OH-] = 10^-14 (the water autoionization constant Kw at 25 degrees C).
- What pH is neutral?
- A pH of 7.0 is neutral (neither acidic nor basic) at 25 degrees C. This is the pH of pure water. [H+] = [OH-] = 10^-7 mol/L at neutral pH.
- What are common pH values?
- Stomach acid: pH 1-2. Lemon juice: pH 2. Coffee: pH 5. Pure water: pH 7. Blood: pH 7.4. Baking soda solution: pH 8.3. Bleach: pH 12-13. Drain cleaner: pH 13-14.