What is the practical significance of the shapes of conical flask and Kjeldahl flask?

  1. Kjeldahl flask
  2. Conical flask

Question: What is the practical significance of the shape of conical flask (in US: Erlenmeyer flask)?

An Erlenmeyer flask (also known as a conical flask or a titration flask) is a type of laboratory flask, which consists of a flat bottom, a conical body, and a cylindrical neck. The flask is created by the German chemist, Richard August Carl Emil Erlenmeyer (1825–1909) in 1860, and hence its name. With its wide flat bottom, it was perfect for heating solutions (large surface area). Hot vapor condenses on the upper section of slanted sides of the Erlenmeyer flask, reducing solvent loss. Also, these slanted sides and small mouth of the flask cause the hot solutions in the flask to cool slowly, keeping the empty air space hot. This feature greatly aids certain techniques in chemistry, especially appropriate for recrystallization. For example, during the hot filtration step, when the hot solution is filtered through a fluted filter paper into the receiving Erlenmeyer flask, hot vapors from the boiling solvent would keep the filter funnel warm, avoiding the premature crystallization on the funnel and its stem.

Equally, the sloping sides prevent risk of spillage of solutions in it, therefore ideal to hold content, which needs to be mixed by swirling. This nature also makes them suitable for use in titrations. Again, the flat bottom of the flask has the advantage of giving its content a uniform color across the entire flask, which makes it easier to determine sharp end points.

Question: What is the practical significance of the shape of Kjeldahl flask?

In 1883, Danish chemist Johan Kjeldahl (1849–1900) of the Carlsberg Laboratory published a method to determine nitrogen content in organic matter. This is the first accurate, simple, and speedy method to determine nitrogen content in organic matter including protein content in food, which bares discoverer's name, the Kjeldahl method. The first step of the method is the digestion of sample in concentrated sulfuric acid at $\pu{360–410 ^\circ C}$, which decomposes the sample by oxidation to liberate the reduced nitrogen as ammonium sulfate. This heating would cause splash-back, which is a treat because of conc. $\ce{H2SO4}$. Thus, along with his famous method, Kjeldahl had also developed a piece of laboratory equipment he developed in 1888, which bares his name as well, Kjeldahl's flask. The long-necked, round-bottomed (actually pear shaped) flask was ideal for avoiding splash-back when heating solutions. The long-neck also function as air condenser.

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  • $\begingroup$ Wow you explained it so well thanks! $\endgroup$ – PJ The MADAO Apr 5 at 6:39
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    $\begingroup$ An additional point is that in many South Asian countries, people pronounce it as "Jel-dahl" flask or method. Just like in Knorr noodles, K is not silent, same here. This "j" sound is somewhat close to "y". "K'ell-dahl" may be close. Listen to the actual Danish pronunciation: forvo.com/word/kjeldahl $\endgroup$ – M. Farooq Apr 5 at 12:55
  • $\begingroup$ @M. Farooq thanks for the info!! $\endgroup$ – PJ The MADAO Apr 6 at 10:59

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