Estimation of volatile acids
Volatile fatty acids are water-soluble and they can be distilled at atmospheric pressure. These fatty acids are intermediates in the process of methane production and many other allied reactions. These acids can be removed by steam distillation, in spite of high boiling points, because of their high vapor tension. This group includes water-soluble fatty acids having up to six carbon atoms.
Straight distillation or steam distillation after acidifying the sample with 1 + 1 H2SO4 results in the evaporation of volatile acids which on condensation yield liquefied volatile acids. These acids could be titrated against 0.1N alkali with phenolphthalein indicator to find out the amount of volatile fatty acids.
The steam distillation method is more tedious, requires at least 4h to complete, but results in the recovery of 92% to 98% volatile acids from sewage sludge. The steam distillation is conducted on a MgSo4 saturated, acidified, sludge-free liquor produced by chemical treatment with ferric chloride and filter-aid followed by vacuum filtration. The effects of varying concentrations of dissolved solids and mineral acids during distillation are avoided.
» Dilute Sulphuric Acid 1 + 1: Mix one volume of conc. H2SO4with one volume of water.
» Ferric Chloride Solution: Dissolve 82.5g FeCl3 6H2O in one liter distilled water.
» Diatomaceous-Silica Filter Aid
» Magnesium Sulphate (MgSO4,7H2O)
» Standard Sodium Hydroxide, titrant 0.1N: Dissolve 4g sodium hydroxide in 1L of distilled water and standardize against any standard acid.
» Phenolphthalein Indicator: 0.1% in 95% alcohol.
1. Rand, M C, Greenberg, A E and Taras, M J (eds) (1976) In: Standard Methods for the Examination of Water and Waste Water (14th Ed) American Public Health Association Washington p 529.
2. Olmslead, W H, Whitaker, W M and Duden, C W (1930) J Biol Chem 85 109.
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