Titration

A titration measures how much of a known solution (the titrant, in a burette) it takes to react exactly with the sample. From that volume and the balanced equation you get the amount in the sample. It's cheap and, done carefully, accurate to a few parts per thousand.

The equivalence point is where the titrant has exactly reacted with the sample. You see it as the endpoint: an indicator changes color, or a pH meter shows a jump. Phenolphthalein turns pink above about pH 8.2 and suits a strong acid with a strong base; methyl orange changes around pH 3 to 4.4.

The sum: moles of titrant = M × V (in litres). Use the ratio from the balanced equation to get moles of the analyte, then divide by the sample's volume. 25.00 mL of HCl takes 18.40 mL of 0.1000 M NaOH: 0.1000 × 0.01840 = 0.001840 mol NaOH = mol HCl (1:1), and 0.001840 ÷ 0.02500 = 0.07360 M.

Watch the ratio. Sulfuric acid, H₂SO₄, gives two protons, so H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O: moles of acid are half the moles of base.

A titration curve plots pH against volume added. For a strong acid and strong base it's flat, then jumps steeply around pH 7 at the equivalence point. A weak acid starts higher and its equivalence point is above 7. Titrations aren't only acid-base: redox titrations (with permanganate, say) and EDTA titrations for water hardness use the same idea.