Estimation of Nitrogen -- Kjeldahl's Method

Kjeldahl's method is the more commonly used of the two nitrogen-estimation methods precisely because it is easier to carry out than Dumas's, and it sees heavy practical use in the routine analysis of foods and fertilisers. It rests on a different chemical fact: heating a nitrogenous organic compound with concentrated sulphuric acid quantitatively converts every nitrogen atom present into ammonium sulphate, (NH4)2SO4, in solution. Treating that solution with excess alkali then liberates the nitrogen back out as ammonia gas, which is absorbed into a precisely measured EXCESS of standard acid; finally, back-titrating that leftover (unreacted) acid against a standard alkali tells you exactly how much of the acid the ammonia actually consumed -- and from that, exactly how much nitrogen was present.

Practically: a weighed 0.3-0.5 g portion of the substance, together with about 25 mL of concentrated H2SO4 and a small amount of potassium sulphate and copper sulphate (added as a catalyst), is heated gently in a special long-necked Kjeldahl flask, loosely stoppered and held at an incline, until the initially dark-brown digestion mixture clears completely -- at that point, every nitrogen atom in the original sample has been converted into ammonium sulphate. The cooled, diluted flask contents are then transferred into a separate 1-litre round-bottom flask, excess sodium hydroxide solution is poured in down the side (to liberate the ammonia), and the flask is fitted with a Kjeldahl trap (which simply catches any alkali splashed up by vigorous boiling, stopping it from being carried over) and a water-jacketed condenser, whose outlet dips directly into a separately measured, excess volume of standard (commonly N/20) sulphuric acid. Heating the round-bottom flask then distils the liberated ammonia gas straight into that standard acid, where it is absorbed and neutralised. Once no further ammonia is detected coming over (tested by holding red litmus paper at the outlet), the receiving flask is removed, and the ACID THAT WAS NOT CONSUMED by the ammonia is titrated against a standard alkali solution, using phenolphthalein as the indicator, to find out exactly how much of the original standard acid is still left unreacted. …