A Kjeldahl flask is a specialized laboratory glassware used in the Kjeldahl method for determining the nitrogen content in a sample. The flask has a round bottom and a long neck, with a sidearm for the addition of a condenser. The Kjeldahl method involves the digestion of a sample with a strong base and sulfuric acid, which releases nitrogen gas that is then collected in the flask. The nitrogen content is then determined by measuring the volume of gas collected.

The Kjeldahl flask is made of heat-resistant glass and is commonly used in analytical chemistry, food analysis, and environmental analysis. The size and capacity of the flask can vary, depending on the size of the sample being analyzed. The flask is typically used in conjunction with other apparatus, such as a condenser, to ensure a complete and accurate analysis.