Kjeldahl Flasks

The Kjeldahl flask is purpose-made for the Kjeldahl method, the classic way to measure the nitrogen content of a sample — and from it, protein. The sample is boiled in concentrated sulphuric acid with a catalyst until it is fully digested, a fierce, fuming, high-temperature step that the flask must survive.

Its defining feature is the long neck: it acts as an air condenser, letting acid vapour condense and run back into the bulb so the digestion does not boil dry or lose sample, while keeping corrosive fumes away from the operator and the heater.

Nitrogen determination by the Kjeldahl method and the protein analysis derived from it, in food and feed testing, agricultural and soil analysis, fertiliser analysis, environmental Total Kjeldahl Nitrogen (TKN) testing, and brewing, dairy and QC labs.

A round-bottomed borosilicate 3.3 bulb with a long tapering neck (plain or with a joint for digestion/distillation units). The round bulb heats evenly and resists the thermal stress of acid digestion; the long neck condenses fumes. Capacities are chosen for the sample size and digestion apparatus.

Digestion uses hot concentrated sulphuric acid, so the work is done in a fume hood with full protection, on a proper digestion rack tilted to manage the fumes. Heat gradually to avoid bumping and frothing over the long neck; never let the flask boil dry. Allow the flask to cool before dilution, and inspect it for cracks before each high-temperature digestion.

Blown from borosilicate 3.3 into the round bulb with the long drawn neck, then annealed for repeated high-temperature acid digestion. Wall evenness (for thermal endurance) and neck length/geometry (for fume condensation) are the key checks.