Objective:
To determine the acid value of given fat sample
Theory:
Acid value is defined as the number of milligrams of potassium or sodium hydroxide that is required to neutralize the free fatty acids in 1 gram of fats or oil. It is a measure of the number of carboxylic acid group in a chemical compound such as a fatty acid, as in a mixture of compound in an organic solvent (often isopropanol) and titrated with a solution of potassium hydroxide of known concentration using phenolphthalein as a color indicator.
The acid number is used to quantify the acidity of a substance e.g., biodiesel. It is the quantity of base expressed in milligrams of potassium hydroxide that is required to neutralize the acidic constituents of 1g of sample.
The acid value (AV) is a common parameter, in the specification of fats and oils. It requires the reaction to occur by the action of lipase enzyme and it is an indicator of inadequate processing and storage conditions (i.e., high temperature and relative humidity, tissue damage). Besides free fatty acids, hydrolysis of triglycerides produces glycerol.
Free fatty acids are a source of flavors and aromas. Short chains of free fatty acids tend to be water soluble and volatile with characteristic smell. On the other hand, we have long chain saturated and unsaturated fatty acids. The latter are more prone to oxidation in their form (free) and their breakdown products (aldehyde, ketone, alcohol, and organic acids) provide characteristic flavors and aromas. In such cases, these flavors and aromas are considered as a defect in oils, fats and food that contain them. However, there are instances where hydrolysis of triglycerides and oxidation of free fatty acids are key in the development of desirable flavors in food. This is the case of aged cheese and some processed metals.
There is a standard method for detecting the acid number such as ASTM (American Society for Testing and Materials). ASTM D664 is the standard test method for determining the acid number in petroleum products and lubricants, whereas, ASTM D6751 is the specification for biodiesel (B100) to ensure fuel quality. Acid number (mg KOH/ g oil) for diesel should be lower than 0.50 mg KOH/g.
It is highly recommended to maintain the low acid number which always remain crucial to prevent corrosion of engine components, fuel injectors, and storage tanks, ensuring the longevity and efficiency of fuel systems. This is since FFA (Free fatty acids) produced many corroded automobile parts and these limits protect vehicle engines and fuel tanks.
As oil-fats rancidity, triglycerides are converted into fatty acids and glycerol causing an increase in acid number. A similar overreaction is observed in biodiesel ageing through analogous oxidation process and when subjected to prolonged high temperatures (ester thermolysis or through exposure to acids or bases (acid/base ester hydrolysis).
Material required:
Glass wares and equipment’s
Beaker, Conical flask, pipette, burette, Retort stand (clamp stand)
Chemical and reagents
- Fat solvent (100ml)
- Absolute ethanol (50 ml)
- Diethyl ether (50ml)
- Potassium hydroxide (50ml)
- Fat sample (cooked and uncooked)
- Phenolphthalein indicator
Procedure:
Preparation of reagents
Preparation of fat solvent
100ml of fat solvent was prepared by mixing 50 ml of absolute ethanol and 50 ml of diethyl ether in a beaker.
Preparation of potassium hydroxide solution
200ml of 0.01M KOH solution was prepared. And, the solution was covered to minimize evaporation due to its volatile trail.
Sample preparation and titration
Accordingly, 1.0 ml of fat sample (cooking oil) was dissolved in 10 ml of fat solvent in a beaker. Again, 1.0ml of cooked oil fat sample was dissolved in another 10 ml of fat solvent.
Addition of indicator
Two drops of phenolphthalein indicator were added to each solution and were mixed thoroughly.
Process of Titration
Each prepared sample solution was titrated against 0.01M KOH until a pale pink endpoint was achieved, indication neutralization of free fatty acids (FFA).
Result and calculation:
From the experiment, the following values were obtained from the titration of used cooking oil with KOH and unused cooking oil versus KOH.
| S. No | Oil sample | Fat sample used | Volume of fat solvent | Intial reading (ml) | Final reading (ml) |
| 1 | Cooked oil | 1ml | 10ml | ||
| 2 | Uncooked oil | 1ml | 10ml |