Acids, bases and pH

An acid gives away a proton (an H⁺ ion, which is just a hydrogen nucleus). In water the proton lands on a water molecule and makes hydronium, H₃O⁺. A base takes a proton, or puts hydroxide, OH⁻, into the water. Hydrochloric acid, HCl, is an acid; sodium hydroxide, NaOH, is a base.

pH measures how much H₃O⁺ there is: pH = −log₁₀[H₃O⁺], where the square brackets mean concentration in mol/L. The log turns powers of ten into plain numbers: [H₃O⁺] = 10⁻³ M gives pH 3. Each step of one pH unit is ten times more or less acid. Pure water at 25 °C is pH 7, neutral; below 7 is acidic, above 7 basic.

Water always holds a little of both, and at 25 °C [H₃O⁺] × [OH⁻] = 10⁻¹⁴. So pH + pOH = 14, where pOH is the same idea for hydroxide. A 0.01 M NaOH solution has [OH⁻] = 10⁻² M, pOH 2, so pH 12.

Strong acids such as HCl, nitric and sulfuric acid give up their protons completely in water. Weak acids, such as acetic acid in vinegar, give up only a few, so the same concentration has a higher pH. 0.1 M HCl is pH 1; 0.1 M acetic acid is about pH 2.9.

Acid and base cancel each other: H₃O⁺ + OH⁻ → 2H₂O, neutralization. A titration uses it to measure an unknown: add a base of known concentration from a burette until an indicator changes color, then moles of base = moles of acid (for a one-proton acid and a one-hydroxide base). 20.0 mL of 0.10 M NaOH is 0.0020 mol, so it neutralizes 10.0 mL of 0.20 M HCl.