Question
Thu August 09, 2012 By:
 

sir,What is meant by theory of steam distillation?

Expert Reply
Thu August 09, 2012

If the sample that has to be distilled is only slightly soluble in water and may decompose at its boiling point and experience violent bumping with a vacuum distillation, it is better to steam distill. Mixtures of oils and tars do not dissolve well in water; thus, they can be steam distilled.

Heating the compound in the presence of steam makes the compound boil at a lower temperature. This has to do with partial pressures of water and organic oil.

A helpful relationship when considering steam distillation in a theoretical sense is the ideal gas law, PV = nRT, where P = pressure, V = volume, n = moles, R = the gas constant and T = temperature. It is important to remember that all of these parameters refer to gaseous molecules. Since distillation involves the expansion of a liquid into a gas in a fixed volume (the still), the gas law can be useful in predicting the amount of water needed to complete a steam distillation or to figure out the proportion in which the organic and aqueous layers will co-distill. To gain a more practical expression, take the ratio of a gas law written for the gaseous water and one written for the organic gas. If this is done, one obtains the following expression.

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