Acids, Bases, Buffers, and pH

Acids and bases. Acidity is about hydrogen ions (H⁺) in solution:

• An acid is a substance that releases H⁺ (increases H⁺ concentration) — e.g., hydrochloric acid (HCl) in the stomach.
• A base (alkali) is a substance that accepts H⁺ or releases hydroxide (OH⁻), lowering H⁺ concentration — e.g., bicarbonate (HCO₃⁻).

The pH scale. pH measures H⁺ concentration on a scale from 0 to 14:

• pH 7 is neutral (pure water).
• pH below 7 is acidic (more H⁺); pH above 7 is basic/alkaline (less H⁺).
• The scale is logarithmic: each 1-unit change means a 10-fold change in H⁺ concentration. So pH 5 is 10× more acidic than pH 6 and 100× more acidic than pH 7. This is why "small" pH changes are actually large chemically.

Human blood pH is normally kept in a narrow range around 7.35–7.45 (slightly basic). Values outside this range (acidosis below, alkalosis above) impair body function.

Buffers. A buffer is a system that resists changes in pH by absorbing or releasing H⁺ as needed. Buffers typically consist of a weak acid and its conjugate base working as a pair: when H⁺ rises, the base part soaks it up; when H⁺ falls, the acid part releases some. The body's most important buffer system is the bicarbonate buffer system (carbonic acid / bicarbonate, H₂CO₃ ⇌ HCO₃⁻ + H⁺), which — together with the lungs (which adjust CO₂) and kidneys (which adjust bicarbonate and H⁺) — keeps blood pH stable. Buffers act fast; lungs adjust within minutes; kidneys provide slower, powerful long-term control.

Why pH stability matters. Enzymes and proteins are sensitive to pH — they work best in a narrow range, and abnormal pH can denature proteins and disrupt reactions. This is why the body invests so much in tight pH control.

How it works

Acid–base basics:

Acid = releases H+ (↑ acidity) | Base = accepts H+ / releases OH− (↓ acidity)
pH 0–14: 7 neutral; <7 acidic; >7 basic; LOGARITHMIC (each unit = 10× change in H+)
Blood pH ~7.35–7.45 (narrow); <7.35 acidosis, >7.45 alkalosis
Buffer (weak acid + conjugate base) resists pH change → bicarbonate buffer system
   backed by lungs (CO2, minutes) + kidneys (HCO3−/H+, slower, powerful)
Why: enzymes/proteins need stable pH (abnormal pH denatures/disrupts)

Comparisons

Common confusions

• Acid releases H⁺ (pH < 7); base accepts H⁺ (pH > 7) — it's about hydrogen ions.
• The pH scale is logarithmic — each unit is a 10× change, so small numbers matter a lot.
• Buffers resist pH change (weak acid + conjugate base); they don't prevent it entirely.
• Lungs (CO₂, fast) and kidneys (bicarbonate/H⁺, slow) both regulate pH — not just one.

Memory aids

• "Acid Adds H⁺; Base Bites (accepts) H⁺."
• "Lower pH = more H⁺ = more acidic."
• "Blood pH 7.35–7.45 — narrow and non-negotiable."
• "Buffer = the body's shock absorber for pH."

Quick review

• Acids release H⁺ (pH < 7); bases accept H⁺ (pH > 7); pH 7 is neutral, and the scale is logarithmic (each unit = 10× change in H⁺).
• Blood pH is tightly held at ~7.35–7.45; outside this is acidosis or alkalosis.
• Buffers (weak acid + conjugate base, esp. the bicarbonate buffer system) resist pH change, backed by the lungs (CO₂, fast) and kidneys (bicarbonate/H⁺, slow but powerful).
• Stable pH is essential because enzymes and proteins work only in a narrow pH range.