🍎 Acidity of fats (acid value)
Determination of acid value and acidity in edible fats and oils by alkalimetric titration. Indicator of fat quality and freshness.
Overview
Acid value (AV) defines the amount of mg KOH needed to neutralize free fatty acids in 1 g of fat. Acidity is also expressed as % oleic acid. This parameter is an indicator of fat quality and freshness — increasing AV indicates triacylglycerol hydrolysis (hydrolytic rancidity).
The method involves dissolving the fat sample in a solvent mixture (ethanol/diethyl ether) and titrating free fatty acids with KOH solution using phenolphthalein indicator or potentiometrically.
Method principle
Weighed fat sample (2–20 g) is dissolved in solvent mixture (ethanol/diethyl ether 1:1, neutralized with KOH). Free fatty acids are titrated with standardized KOH solution (0.1 or 0.5 mol/L in ethanol) to phenolphthalein color change (colorless → pink) or to potentiometric endpoint (pH 8.5). AV [mg KOH/g] = (V × c × 56.1) / m.
Applications
- Quality control of vegetable oils (rapeseed, sunflower, olive)
- Analysis of animal fats (lard, butter)
- Monitoring fat rancidity during storage
- Quality control of frying oils (thermal degradation)
- Assessment of fat raw materials in oleochemical industry
Key parameters
| Parameter | Value |
|---|---|
| Result | Acid value [mg KOH/g] or % oleic acid |
| Range | 0.05–200 mg KOH/g |
| Limit (extra virgin olive oil) | ≤0.8% oleic acid |
| Precision (CV) | 1–3% |
| Sample mass | 2–20 g (depending on AV) |
Standard
- Standard number
- PN-EN ISO 660:2020
- Title (PL)
- Tłuszcze i oleje roślinne oraz zwierzęce — Oznaczanie liczby kwasowej i kwasowości
- Title (EN)
- Animal and vegetable fats and oils — Determination of acid value and acidity
Step-by-step procedure
1. Solvent preparation
Mix 50 mL ethanol + 50 mL ether. Add 2–3 drops phenolphthalein. Neutralize with KOH to light pink.
2. Sample weighing
Weigh fat sample (mass depends on expected AV: 20 g for AV<1, 2 g for AV>20) into conical flask.
3. Dissolution
Add 100 mL neutralized solvent. Mix until fat dissolves (heat to 40°C if needed).
4. Titration
Titrate with 0.1 mol/L KOH to persistent pink color (30 s). Record consumption with 0.05 mL accuracy.
5. Calculations
AV = (V × c × 56.1) / m [mg KOH/g]. Acidity [%] = (V × c × 28.2) / m (as oleic acid).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Burette 25 mL (class A) | Brand, DURAN, accuracy 0.05 mL | 200–500 PLN |
| Analytical balance | Mettler Toledo ME204, accuracy 0.1 mg | 5 000–15 000 PLN |
| Conical flasks 250 mL | DURAN Erlenmeyer, wide neck | 15–30 PLN/pc |
| Automatic titrator (optional) | Metrohm 888 Titrando, Mettler Toledo T5 | 15 000–35 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium hydroxide KOH 0.1 mol/L in ethanol | 1310-58-3 | Standardized, for titration |
| Ethanol 96% | 64-17-5 | Solvent, neutralized with KOH |
| Diethyl ether | 60-29-7 | Solvent (1:1 with ethanol), without peroxides! |
| Phenolphthalein 1% | 77-09-8 | Indicator, solution in ethanol |
Health and safety (OHS)
- Diethyl ether — extremely flammable, may form explosive peroxides! Keep away from fire, check expiry date.
- KOH — corrosive, causes burns. Gloves and safety glasses.
- Ethanol — flammable. Fume hood recommended.