Kjeldhal Distillation Assembly

Quick Answer: A Kjeldahl distillation assembly is the glass set-up used in the second stage of the Kjeldahl method, where a digested sample is made alkaline and its ammonia is distilled into a receiving acid. The amount of ammonia collected gives the nitrogen, and from that the protein, content of foods, feeds, soils and fertilisers.

From Digest to Distillate

After digestion in hot concentrated sulfuric acid, the nitrogen of the sample is held as ammonium sulfate in a strongly acidic solution. The distillation stage releases it. The cooled digest is diluted, concentrated sodium hydroxide solution is added in excess, and the ammonia set free is boiled over through a splash-head trap into a condenser. The condensed distillate runs into a flask of boric acid or standard acid, where the ammonia is held for titration.

A macro assembly of this kind is built around the long-necked Kjeldahl flask, so digestion and distillation can take place in the same vessel. The splash head, often a bulb with an inner tube, is essential: the strongly alkaline mixture foams and spits as it boils, and any droplets reaching the receiver would neutralise acid there and inflate the result. The condenser outlet ends in a delivery tube that dips into the receiving solution.

The listing spells the name Kjeldhal and does not state the parts, flask size or joints supplied. For smaller samples and faster distillations by steam, see the micro Kjeldahl nitrogen distillation apparatus.

Specifications

Applications

• Crude protein determination in cereals, pulses, milk powder and animal feed
• Total nitrogen in soils and organic manures in agricultural laboratories
• Nitrogen content of fertilisers such as urea and ammonium salts
• Food chemistry and biochemistry practicals in university and pharmacy courses

Care & Handling

• Add the sodium hydroxide slowly down the neck so it forms a layer under the digest, and connect the assembly before swirling to mix.
• Check that the receiver tube dips into the acid before heating starts, and lift it out of the liquid before heating stops.
• Add a few granules of zinc or anti-bumping chips to calm the violent boiling of the alkaline mixture.
• Rinse the condenser and delivery tube into the receiver at the end, then flush the assembly with water before the next sample.

Why Choose LabEquip

Food testing, feed and soil laboratories that run the Kjeldahl method on batches of samples need assemblies that can be duplicated along a bench. LabEquip offers the assembly under chemistry lab equipment; give the flask size and number of units through the contact page.

Frequently Asked Questions

Why is so much sodium hydroxide added before distilling?

The digest is strongly acidic from the sulfuric acid. Enough alkali must be added to neutralise all of it and leave the mixture clearly alkaline, otherwise the ammonium ions are not converted to ammonia and part of the nitrogen stays behind.

What is the purpose of the splash head?

The alkaline digest boils vigorously and throws up spray. The splash head traps these droplets and returns them to the flask, so only ammonia and steam reach the condenser and the result is not falsely high.

Is boric acid or standard acid better in the receiver?

Boric acid needs no exact concentration and allows direct titration of the ammonia, which makes it popular. A measured excess of standard sulfuric or hydrochloric acid is used for back titration and is still required by some older methods.

How do I know when distillation is complete?

Methods usually specify a volume of distillate to collect. A simple check is to hold moist red litmus paper at the delivery tube tip: if it no longer turns blue, ammonia has stopped coming over.

Why run a reagent blank?

Sulfuric acid, catalyst and alkali can contain traces of nitrogen, and laboratory air can contain ammonia. A blank carried through digestion and distillation with no sample measures this so it can be subtracted from each result.

What is the difference between macro and micro Kjeldahl distillation?

Macro assemblies distil the whole digest from a large flask by direct heating. Micro apparatus takes a small portion of the digest into a compact steam-distillation unit, using less sample and reagent and finishing each run faster.

Last Updated: September 2026