Potentiometric titrator · Calibration procedure
Titre of volumetric solution
Standardises a volumetric solution, for example 0.1 M sodium hydroxide, against a primary standard such as potassium hydrogen phthalate.
PURPOSE
Why this is checked
A volumetric solution's real concentration differs slightly from its nominal value and changes with time (NaOH absorbs carbon dioxide). The exact molarity, or the factor relative to nominal, is used in every assay calculation.
Frequency
When freshly prepared and then at the interval in your SOP (often monthly or before use for unstable solutions).
Learn how the Potentiometric titrator worksYOU WILL NEED
Standards & tools
- Potassium hydrogen phthalate (KHP), primary standard grade, dried at about 120 °C for 2 hours and cooled in a desiccator (molar mass 204.22 g/mol)
- Carbon dioxide-free purified water
- Calibrated analytical balance
BEFORE YOU START
Prerequisites
- Burette flushed with the NaOH solution
- Electrode check passed
- Titrant bottle protected with a soda-lime guard tube
PROCEDURE
Step by step
- 1Accurately weigh about 0.41 g of dried KHP into a beaker (this consumes about 20 mL of 0.1 M NaOH).
- 2Dissolve in about 50 mL of carbon dioxide-free water.
- 3Titrate potentiometrically with the NaOH solution to the equivalence point.
- 4Carry out a blank titration and subtract the blank volume (if significant).
- 5Repeat to obtain at least three determinations; calculate the molarity, mean and %RSD.
RECORD
Example observation table
| Determination | KHP mass (mg) | Volume NaOH, blank-corrected (mL) | Molarity (M) |
|---|---|---|---|
| 1 | 408.6 | 19.92 | 0.10044 |
| 2 | 411.2 | 20.05 | 0.10042 |
| 3 | 405.9 | 19.79 | 0.10043 |
CALCULATION
How to calculate the result
Molarity (M) = W ÷ (204.22 × V); Factor = Actual molarity ÷ Nominal molarity
- W = 408.6 mg KHP, V = 19.92 mL
- M = 408.6 ÷ (204.22 × 19.92) = 408.6 ÷ 4068.06 = 0.10044 M
- Mean of three = 0.10043 M; %RSD = 0.01%; factor = 0.10043 ÷ 0.1 = 1.0043
ACCEPTANCE CRITERIA
Pass if
- %RSD of replicate determinations typically not more than 0.2%.
- Factor typically within 0.95–1.05 of nominal (or the range in your SOP); otherwise adjust and restandardise.
IF IT FAILS
What to do
- Check the KHP was dried and weighed correctly, and that the burette and electrode passed their checks.
- Check the NaOH has not absorbed carbon dioxide (cloudiness, carbonate); prepare fresh if in doubt.
- Repeat the standardisation; do not use the solution until a valid molarity is assigned.
MORE
Other Potentiometric titrator calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.