Primary standards for titrimetry
Common primary standards: molar mass (computed from the formula), the titrant each standardises, the stoichiometry, the mass that uses about 40 mL of titrant, and how to dry and use it.
| Potassium hydrogen phthalate (KHP) | 204.22 | 1.0 | 0.817 | dry at 110 °C for 1–2 h | Large molar mass (204 g/mol), not hygroscopic, 99.99 % pure; phenolphthalein endpoint. | |||
| Sodium carbonate | 105.99 | 2.0 | 0.212 | heat at 270–300 °C to remove NaHCO₃ and water | Boil near the second equivalence point to drive off CO₂ and sharpen the endpoint (methyl red or bromocresol green). | |||
| Borax | 381.36 | 2.0 | 0.763 | kept over sucrose–saturated NaCl so the decahydrate keeps its water | Very large molar mass (381 g/mol), so the weighing error is relatively smaller; methyl red endpoint. | |||
| Potassium dichromate | 294.18 | 6.0 | 0.196 | dry at 150 °C | Weigh it straight into a standard solution that keeps for years — but Cr(VI) is toxic and carcinogenic. | |||
| Sodium oxalate | 134.00 | 0.4 | 0.268 | dry at 105 °C; titrate in H₂SO₄ at 55–60 °C | The classic standard for KMnO₄: the reaction starts slowly, then the Mn²⁺ it forms catalyses it. | |||
| Potassium iodate | 214.00 | 6.0 | 0.143 | dry at 110 °C | With excess KI and acid it releases an exact amount of I₂ for the thiosulfate: 1 mol IO₃⁻ ≙ 6 mol S₂O₃²⁻. | |||
| Sodium chloride | 58.44 | 1.0 | 0.234 | heat at 500 °C to drive out occluded water | Standardises AgNO₃ (Mohr or Fajans); AgNO₃ itself decomposes in light. | |||
| Calcium carbonate | 100.09 | 1.0 | 0.040 | dry at 110 °C; dissolve in a little HCl, then bring to pH 10 | Standardises EDTA; the disodium dihydrate can serve itself but often carries a little extra water. |
The mass for 40 mL assumes 0.1 M titrant (0.02 M for KMnO₄, 0.01 M for EDTA). ν is the moles of titrant consumed per mole of standard; Na₂CO₃ and borax are taken to their second equivalence point.