Potassium Hydrogen Phthalate
Properties
| State | Solid (white crystalline powder) |
| Color | White |
| Solubility | Moderately soluble in water (80 g/L at 20°C) |
| Melting Point | 295°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Potassium Hydrogen Phthalate
Potassium hydrogen phthalate (KHP), KHC8H4O4 at 204.222 g/mol, is the white crystalline acid that almost every analytical chemist has weighed at least once. It is the textbook primary standard for titrating sodium hydroxide because it ticks every box: it crystallizes to 99.95%+ purity straight from water, doesn't pick up moisture from the air the way oxalic acid dihydrate does, has a high enough formula weight that ±0.1 mg on the analytical balance translates into ±0.05% precision in a 0.4 g sample, stays stable indefinitely on a closed reagent shelf, and reacts cleanly 1:1 with hydroxide at an endpoint near pH 8.7 — right in phenolphthalein's color-change window. KHP is monoprotic in the titration: only the free -COOH on the phthalate dissociates, while the carboxylate that's already neutralized as the potassium salt stays put. The compound also serves as a NIST-traceable pH 4.005 buffer at 25°C, which is the low end of the standard 4-7-10 calibration trio for any pH meter you'd find in a lab. It also doubles as a TOC analyzer reference standard because its carbon content is exactly stoichiometric.
Where you'll encounter it
If you've ever standardized a fresh bottle of 0.1 M NaOH or calibrated a pH meter at the start of a buffer-prep day, you've used KHP. The standard undergraduate procedure is: dry KHP in a 110°C oven for at least an hour, weigh roughly 0.4 g into an Erlenmeyer to four decimal places, dissolve in 50 mL of CO2-free water, add two drops of phenolphthalein, and titrate the new NaOH solution to the first persistent pink. Three replicates should agree to within 0.2%, and the actual molarity calculated this way is what gets recorded on the bottle label and cited in every downstream titration the lab runs that month. Outside teaching labs, KHP appears in QC labs that need a primary acid for routine USP buret calibrations.
Common Uses
- Primary standard for sodium hydroxide titration in analytical chemistry teaching labs
- NIST-traceable pH 4.005 buffer standard for pH meter low-point calibration
- Reference material for total organic carbon (TOC) instrument calibration
- Quality control reagent for buret and pipette volumetric verification
- Calibration standard for UV-Vis spectrophotometer linearity checks at 270 nm
- Reference acid for Karl Fischer titration coulometric system suitability
- Internal standard for HPLC method development with UV detection
Safety Information
GHS: H315 skin irritation Cat 2, H319 eye irritation Cat 2. Oral LD50 in rats around 3200 mg/kg, so acute toxicity is low. The hazard during routine handling is mostly mechanical — fine KHP dust is mildly irritating to mucous membranes if you're scooping a kilogram bottle without a fume hood draft. Standard nitrile gloves and safety glasses are sufficient for analytical-scale work. Solutions are dilute weak acid (pKa2 = 5.41) and pose no special disposal concern; spent titration mixtures can be neutralized and drain-disposed in jurisdictions that allow it. Keep the bottle dry between uses to preserve the 99.95% purity that makes it useful as a primary standard in the first place.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.