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 micro Kjeldahl 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.