Volatile acids are formed as intermediates during the first phase of the biphasic anaerobic degradation of organic substances.
In anaerobic digestion, 72% of the total COD is due to the acetic acid while the rest accounts for propionic acid. Direct distillation method is commonly used. All low molecular weight fatty acids have pH in the range of 6.5 to7.5.
Under these conditions organic acids existing in the ionic form get converted to unionic from as shown below.
CH3COO + H+ CH3COOH
Procedure
1. Take 100 ml sample and 100ml distilled water. Add 5ml H2SO4 (1:1) so as to bring the pH to 3.5. Distill the contents in such a fashion so as to collect 150 ml in 25 minutes.
2. Discard first 10-15 ml of the distillate, and then collect exactly 150 ml of distillate and titrate with standard NaOH (0.1N) using phenolphthalein as an indicator.
Calculation
Mg/L volatile acids as CH3COOH = mL NaOH x N of NaOH x 60000 /mL sample x R.F.
Where R.F. is a recovery factor, and is generally 0.7. However, it changes with the geometry of the apparatus and existing experimental conditions.
Note: Recovery factor can be calculated as follows:
A) Standard acid solution is prepared by diluting 1.9 ml glacial acetic acid (sp. Gravity 1.043) to 1 liter (1ml = 1mg) 50 ml of stock solution is diluted with 150 ml of distilled water and than distilled. 150 ml of distillate is collected and titrated with standard NaOH solution (0.1N) using phenolphthalein indicator.
Recovery factor =a/b
Where,
a = Volatile Acid conc. recovered in distillate, mg/L
b = Volatile Acid conc. In std. Solution used, mg/L