SOP: Titratable Acidity Measurement
While pH tells you the intensity of acidity, titratable acidity tells you how much acid is actually there. Both matter: a yogurt and a citrus drink can share a pH but taste entirely different. This titration is a daily workhorse in beverage, dairy, and fruit processing labs.
Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.
Purpose
To determine titratable acidity by neutralization titration for flavor control, fermentation monitoring, and specification compliance.
Scope
Applies to fruit juices, beverages, dairy products, wines, vinegars (diluted), and fermented foods. Results are expressed as the predominant acid of the product.
Equipment and Materials
- Burette, 25 or 50 mL, Class A
- Standardized sodium hydroxide, 0.1 N
- Phenolphthalein indicator, 1 percent in ethanol
- Conical flasks, 250 mL
- Volumetric pipettes
- Deionized water, freshly boiled and cooled (carbon dioxide free)
- Magnetic stirrer (optional)
Procedure
- Standardize the sodium hydroxide against potassium hydrogen phthalate if not done within the last month. Record the exact normality.
- Pipette 10 mL of juice or beverage (or 10 g of homogenized sample) into a 250 mL conical flask.
- Add about 50 mL of carbon dioxide free deionized water and 3 drops of phenolphthalein.
- Titrate with 0.1 N sodium hydroxide, swirling continuously, to the first permanent faint pink that persists 30 seconds.
- Record the burette reading to 0.05 mL.
- For strongly colored samples where the pink endpoint is hard to see, titrate to pH 8.2 using a pH meter instead of the indicator.
- Run duplicates; titration volumes must agree within 0.2 mL.
Recording Results
Calculate acidity as: percent acid equals titration volume times normality times milliequivalent weight of the reference acid times 100, divided by sample volume or weight. Common factors: citric acid 0.064, malic acid 0.067, lactic acid 0.090, acetic acid 0.060, tartaric acid 0.075. Report to two decimal places with the reference acid named.
Quality Control
Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.
Troubleshooting
- Overshooting the endpoint: titrate slowly near the end, adding dropwise with thorough swirling.
- Fading endpoint: carbon dioxide in the water or sample. Use freshly boiled deionized water.
- Endpoint invisible in dark juices: dilute the sample further or switch to the pH 8.2 potentiometric endpoint.
- Drifting normality: sodium hydroxide absorbs carbon dioxide. Restandardize monthly and store protected from air.
Safety Notes
- Sodium hydroxide is corrosive to eyes and skin. Wear safety glasses and gloves.
- Rinse burettes after use to prevent crystallization in the stopcock.
- Wipe spills of sodium hydroxide promptly; it damages benchtops.