Sometimes volumetric flasks are designed with a bulge in the neck right below the stopper opening. What is the design intent of that bulge?

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  • $\begingroup$ I'd assume just an artifact of joining the ground glass joint to the neck of the flask. $\endgroup$ – MaxW Sep 29 '17 at 21:39
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    $\begingroup$ It's also there to help you grip better. This way there's a stop if it starts to slide in your hand. $\endgroup$ – Zhe Sep 29 '17 at 23:49

From ASTM manual [1, p. 204], emphasis mine:

Volumetric flasks should be diluted to just below the line, mixed thoroughly, then allowed to stand for 4 to 24 h to allow any heat from an acid-water reaction to dissipate. [...] Thorough mixing consists of at least 20 complete inversions of the volumetric flask, allowing enough time on each for the air bubble to completely traverse the length of the vessel. Some borosilicate volumetric flasks contain a bulge in the neck above the line as an aid to proper mixing.

Volumetric pipetts also have a large bulge in the middle, but it serves a different purpose – to keep the $\rm length : volume$ ratio minimal and still maintain quick liquid collection with high precision.


  1. Dulski, T. R. A Manual for the Chemical Analysis of Metals; ASTM manual series; ASTM: West Conshohocken, PA, 1996. ISBN 978-0-8031-2066-2.
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