Analytical instruments - VELP Scientifica
VELP Scientifica - Via Stazione, 16 - 20865 Usmate (MB) ITALY - Tel. +39 039 62 88 11 - Fax. +39 039 62 88 120
Analytical instruments - VELP Scientifica
Send us your request
Contact Us
08:30 – 17:30 (GTM +1)
VELP Inc. U.S.
8.30 AM – 5.30 PM (EST)

Kjeldahl apparatus

Modern Kjeldahl apparatus combine digestion and distillation/titration in an automated method.

VELP Scientifica has developed the widest range of Kjeldahl apparatus consisting on digestion units (traditional or automatic models) and four distillation units with multiple innovations and features that distinguish them from conventional instruments, to respond to the different needs in R&D, QC and QA laboratories today.
VELP Scientifica Kjeldahl apparatus has been designed to maximize automation, liberating laboratory staff and providing the best conditions for reproducible results. In addition to the objectives for improving the Kjeldahl analysis, substantial effort has been invested in creating eco-friendly equipment – eco in terms of ecology, and also in terms of economy.

Very early Kjeldahl digestions were performed using stone fume hoods and gas mantles as a heat source. Then, around 1930’s, they were replaced by classical Kjeldahl digestion and distillation apparatus.

The traditional Kjeldahl apparatus for digestion consists in a 250 ml flask capacity. Macro Kjeldahl flasks started to appear, for volume from 400 to 800 ml, suggested for those samples with a very low amount of nitrogen, and handle relatively big sample size. A smaller version that appeared on the market was the microKjeldahl apparatus, consisting of smaller capacity flasks of 30 to 100 ml volume, commonly used with low sample amount.
Aluminum heating blocks are designed to accept a number of straight digestion tubes at once. They usually accommodate from 6 up to 20, simultaneously. In all cases, since the Kjeldahl analysis involves significant corrosive fumes, appropriate attention must be given to fume removal.

Typically block digesters have one controller that adjusts the temperature of the entire block and the operating time, that automates and allows timed ramping during the course of a digestion.
Many governmental and regulating organizations have developed methods that specify the classical Kjeldahl apparatus. The Kjeldahl apparatus is available freestanding, if appropriately combined with fume removal system as specified above, or can be positioned under a fume hood.

Kjeldahl apparatus for digestion followed by steam distillation is sometimes referred to as “rapid Kjeldahl analysis” partly because a number of block digestion methodologies are shorter than the classical method.

Kjeldahl apparatus

Original apparatus as designed by the Danish chemist Johan Kjeldahl (1849-1900) for the determination of Kjeldahl nitrogen in organic samples. Johan Kjeldahl developed the process to assist his studies into the protein content of various grains used for brewing, as the existing techniques were unreliable or inaccurate


The typical Kjeldahl apparatus for distillation is designed to accept straight digestion tubes, directly from block digesters. In this way the sample does not need to be transferred.
Steam distillation is much more rapid than classical Kjeldahl distillation, typically taking from 3 to 5 minutes.
Kjeldahl apparatus for distillation are available with different levels of automation, ranging from relatively manual models to highly automatic Kjeldahl distillation units. Entry-level models dispense sodium hydroxide under control of a push button. Many models include a timer to control the length of distillation. Some units automate the entire Kjeldahl nitrogen process once the digestion flask is in place, and others automatically titrate to end point after Kjeldahl distillation, and calculate and display the results in a printed report.

Receiving solutions may be individually hand titrated using an indicator solution and burette, but several models of benchtop automatic titrator instruments are also available. Some units will titrate one receiving solution at a time to a set end point; others will automatically titrate a number of receiving flasks sequentially.

VELP Scientifica started to manufacture Kjeldahl apparatus since the 1980s and offers now a comprehensive know-how and experience on this application, especially under the technical and analytical point of view.

First VELP Kjeldahl apparatus

First VELP Scientifica Kjeldahl apparatus.
On the left, the first Series of DK 20 and DK 6, Keldahl digestion units; on the right, the first UDK 126, Kjeldahl distillation unit, semi-automated

Today, various scientific associations approve the Kjeldahl method, including the AOAC International (Association of Official Analytical Chemists), AACC (Association of American Cereal Chemists), AOCS (American Oil Chemists Society), EPA (Environmental Protection Agency), ISO (International Standards Organization), and many others. All VELP Scientifica equipment for Kjeldahl nitrogen determination work in accordance with the above-mentioned associations.

Modern Kjeldahl apparatus for digestion

Modern Kjeldahl apparatus for distillation and titration