Kjeldahl digestion

The  Kjeldahl digestion FOSS  systems enable fully automated digestion for convenient, safe and flexible Kjerdahl analysis. Two-way PC communication supports traceability and GLP. Capacity for eight or twenty tubes.

The basic digestion systems DT 208 and DT 220 for Kjeldahl analysis consist of a digestion block and a tube rack. The digester blocks are insulated to minimize heat transfer to the environment and allow rapid and uniform heating for good working conditions and minimal energy consumption.

The Labtec Line digestion systems are based on a digestion unit and tube rack that allows digestion for safe and flexible Kjeldahl analysis. These versatile digestion systems are capable of carrying eight or twenty tubes in 100ml, 250ml or 400ml volumes. depending on the selected configuration

Efficient and cost-effective operation

Smooth operation saves chemicals, energy, labor, space and time. Efficient smoke containment is provided via an optional exhaust and fumes can be removed via an optional scrubber.

Safe and simple routine analysis

Labtec Line digesters have an internal controller and display, user definable, time and temperature. Various accessories are available to make routine analysis safe and simple. They include a wide range of test tubes, tube racks, holding plates for washing, smoke removal systems, welding rods and catalyst tablets.

Program support

These systems come with extensive application support and are compatible with FOSS Kjeldahl distillation units as well as other distillation units.

Types of samples

Raw materials and finished products in food, feed, agriculture and related water and wastewater matrices and a wide range of industrial compounds


Kjeldahl digestion, chemical oxygen demand and other reflux chemicals, trace metal analysis by AAS or ICP instruments


Lifting and racking system, exhaust manifolds and scrubber

Exhaust manifolds, smoke removal and containment systems
Many digestive applications, e.g. Kegeldals produce fumes that are unpleasant and caustic. Exhaust manifolds designed for each digestion unit facilitate smoke removal and containment and are highly recommended for use in all digestion stages. The cost of replacing a smoke cabinet that has been damaged by ineffective smoke removal is much higher than the relatively low cost of an approved exhaust.

Exhaust manifolds must be connected to the supplied water aspirator or to a suitable scrubber unit.

SR 210 scrubber
The compact desktop Labtec Line SR 210 Scrubber Unit is independent and therefore not affected by water supply issues. During wet digestion, acidic fumes from the connected exhaust are drawn through the scrubber unit. Acid vapors are first condensed and diluted in a large acid trap.

Any residual vapor is collected, washed and neutralized before passing through a second small acid trap that protects the vacuum pump if the cleaning agent runs out. The cleaned air is then discharged through a pipe outlet. In the interest of GLP and H&S, this ventilation pipe should be routed into the fume cabinet.

Reflux headaches
When a digestion unit is used for reflux chemicals, such as chemical oxygen demand (COD), a reflux header connected to a suitable cold water supply should be used instead of an exhaust manifold. Reflux heads are easily installed in racks that match the tube rack in the digestion unit. Pellet condensers are designed for use with tubular digesters.

Digestive tubes
Digester tubes are available in three sizes (400 mL, 250 mL, and 100 mL) and three shapes to accommodate various applications and manual/automatic handling systems. Straight tubes are recommended for most digestion applications. Volumetric tubes with shrink necks are recommended for applications where the digestate needs to be diluted to a constant volume prior to analysis such as FIA, SFA and AA etc.

Ball joint tubes, available in 250 mL size only, are required for reflux chemistry using a reflux condenser head. Straight 250 mL Kjeldahl tubes are recommended because they can accommodate samples of very different sizes. Samples of heterogeneous materials often need to be relatively large to ensure they are representative. There is no lower limit for sample size in 250 ml tubes. They simply provide more flexibility for most types of samples. Straight 100 ml tubes can be used where the material is homogeneous and small samples are fully representative or where specified by the approved method. A special 400ml tube that fits into 250ml systems is designed to handle large liquid volumes, typically 50 or 100ml in water and beer.

For Kjeldahl digestion, salt is used to increase the boiling point and catalyst is used to increase the reaction rate. As a convenient way to get the correct dose, FOSS offers Kjeltabs – tablets containing potassium sulfate and a catalyst (copper, selenium, or copper/titanium). The digestion time may depend on the catalyst used. Throughout history mercury has been used as the most efficient catalyst.

Additional information


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