Acid Value

                                  Acid Value

The acid value or neutralization number 

It is defiend as the mass of potassium hydroxide KOH in milligrams that is required to neutralize one gram of chemical substance.

It is common parameter in the specification of fats and oils where it is ameasure of the free acids present in the fats or oil.

Determination of acid value in fats or oils is based on titration in ethanol using phenolphthalein as indicator.

Determination of acid value:

1- Reagents:

Phenolphthalein indicator

Weigh 1 g of phenolphthalein and dissolve in 100 mL of ethanol.

Sodium hydroxide titrant

Weigh accurately 4.0 g of sodium hydroxide and place it in a 1000 mL volumetric flask. Make up to the mark with water.

Ethanol-ether solution

Prepare a mixture of ethanol and diethyl ether (1:1, v/v). Neutralize with sodium hydroxide titrant and add 1.0 mL of phenolphthalein indicator until pink colouration is observed. Freshly prepare the solution.

2- Standardization of sodium hydroxide titrant:

Weigh accurately 0.6 g of potassium hydrogen phthalate,previously dried to constant weight at 105℃, and place it in a 250-mL conical flask, then add 50 mL of water. Shake it well. Add 2 drops of phenolphthalein indicator. Titrate the solution with the sodium hydroxide titrant until pink colouration can be observed. Towards the end of titration, potassium hydrogen phthalate should be completely dissolved. Calculate the concentration of the sodium hydroxide titrant according to the following equation:

C(NaOH) = (WC8H5KO4 * P C8H5KO4 * 1000) / (VNaOH * Mw C8H5KO4 )    

where 

CNaOH = Molarity of sodium hydroxide titrant (mol/L)

VNaOH = Volume of sodium hydroxide titrant used (mL)

Mw C8H5KO4 = Molecular Weight of potassium hydrogen phthalate.

WC8H5KO4 = Weight of potassium hydrogen phthalate used (g)

PC8H5KO4 = Purity of potassium hydrogen phthalate (%)

3- Titration of test solution:

weigh accurately a quantity of the fatty oil being examined  and place it in a 250-mL conical flask, then add 50 mL of ethanol-ether solution. Shake it well.If necessary, reflux the mixture gently until the substance is completely dissolved. Titrate the solution with sodium hydroxide titrant until pink colouration can be observed which persists for 30 s. Measure the volume of sodium hydroxide titrant used and calculate the acid value according to the following equation:

Acid value = (V NaOH * 5.61) / W

Where

  VNaOH = Volume of sodium hydroxide titrant used (mL)

   W = Weight of the fatty oil being examined (g)