Flask, Kjeldahl

Quick Answer: A Kjeldahl flask is a round-bodied glass flask with a long, wide neck, used to digest organic samples in hot concentrated sulfuric acid during the Kjeldahl method for nitrogen and protein. The long neck is set at an angle during heating so that acid vapour condenses and runs back into the bulb.

The Kjeldahl Method in Three Steps

Johan Kjeldahl published his method for determining nitrogen in 1883, and it is still used to estimate protein in foods, feeds, fertilisers and soils. The sample is first digested in concentrated sulfuric acid with a catalyst and a salt such as potassium sulfate, which raises the boiling point, so that organic nitrogen is converted to ammonium sulfate. The digest is then made alkaline and the released ammonia is distilled into a measured quantity of acid, which is finally titrated to find how much nitrogen the sample contained.

The Kjeldahl flask is the vessel for the first, digestion stage. Its pear or round bulb holds the sample and acid, and its long neck, tilted at roughly 45 degrees in a digestion rack or over a burner, acts as an air condenser: acid vapour and water condense on the cooler upper neck and drain back, which limits acid loss during long heating. In many laboratories the same flask is then connected to a distillation unit for the ammonia step.

Crude protein is estimated by multiplying the nitrogen found by a conversion factor, conventionally 6.25 for general foods and feeds, with other factors used for some products such as milk and cereals. The listing does not state capacity, neck length or glass type, and because digestion involves sustained strong heating, heat resistance should be confirmed.

Applications

• Estimating crude protein in food, feed and grain samples in college and food-testing laboratories
• Measuring total nitrogen in soils and fertilisers in agricultural science practicals
• Teaching digestion, distillation and titration as linked analytical steps
• Wet oxidation of organic matter with acid before some elemental analyses

Care & Handling

• Digest in a fume cupboard or under a fume extraction manifold; hot sulfuric acid gives off dense, corrosive white fumes.
• Let the digest cool fully before adding water, then add the water slowly with swirling, because concentrated sulfuric acid releases a great deal of heat on dilution.
• Use anti-bumping granules during digestion and distillation to prevent violent boiling.
• Inspect the neck for cracks after each run, since repeated strong heating and cooling stresses the glass.

Specifications

Why Choose LabEquip

Food, agriculture and dairy science departments, along with quality-control laboratories that test protein, are the main buyers of Kjeldahl flasks. For digestion racks of a given size, share the capacity and number of places on the contact page; the flask is supplied from LabEquip’s chemistry lab equipment section.

Frequently Asked Questions

Why does a Kjeldahl flask have such a long neck?

The neck works as an air condenser. When the flask is tilted and heated, vapour rises into the cooler neck, condenses and runs back into the bulb, so less acid is lost during digestions that can last an hour or more.

Why is the flask tilted during digestion?

Tilting at around 45 degrees stops boiling liquid spurting straight up and out of the neck, spreads the heat over the side of the bulb, and gives condensing vapour a longer path back into the digest.

What does potassium sulfate do in the digestion?

It raises the boiling point of the sulfuric acid, so digestion runs hotter and organic matter is broken down faster and more completely. A catalyst, commonly a copper compound, speeds the reaction further.

How is protein calculated from the nitrogen result?

Multiply the percentage of nitrogen by a conversion factor. The factor 6.25 assumes protein contains about 16 per cent nitrogen and is used for many foods and feeds; different factors apply to some products, such as milk and wheat.

Can a Kjeldahl flask be heated over a Bunsen burner?

It can, on a digestion stand or clamped at an angle over the flame, but heating blocks and dedicated digestion racks give more even heat and are safer for several samples at once. Always work in a fume cupboard.

What safety precautions does the digestion need?

Wear goggles, a face shield and acid-resistant gloves, work in a fume cupboard, and keep sodium bicarbonate or a spill kit nearby. Let the flask cool before handling, and dilute the cooled digest slowly because heat is released.

Last Updated: September 2026