7
$\begingroup$

A volumetric flask does not look like an Erlenmeyer flask, but has more of a pear shape. Is it just to distinguish more easily between them while they are on the shelf?

$\endgroup$

2 Answers 2

14
$\begingroup$

Why do volumetric flasks have narrow necks?

A volumetric flask is designed to store a relatively large volume of liquid with great accuracy. If you want to reach a determinate volume of liquid in a large beaker, when you add liquid to it so that the liquid height increases a little bit, then the volume increases a lot. What you measure with your eyes when determining the volume is the height of the liquid, so the error in the volume measured is directly linked to the error in measuring the liquid height. The relation between the increase of the height ($\Delta h$) and the volume of a cylinder is very simple:

$$ V= \pi r^{2} \Delta h $$

So suppose that you make a systematic error $\Delta h$ of 0.5 mm. That is a quite good estimate of average max human accuracy in measuring the liquid height in a cylinder with the naked eye. If you make this error in glassware with a different radius you will change the accuracy of your measurements, according to the previous equation:

enter image description here

As you can see it is an exponential function so, for instance, if you use a beaker with a radius of 20 mm and you make an error of 0.5 mm in the reading of the height you will have an absolute error of 2.5 ml but if you use a radius of 40 mm you will have an absolute error of 10 ml. By absolute error, I mean that difference (positive or negative) from the expected volume. Of course, if you are a beginner and make a 1 mm error in the reading of the height your error will double! So a small cylinder radius is a good choice for getting consistent measurements and minimizing operator errors!

Why the body has not the conical shape of the Erlenmeyer?

This actually is not so important. Generally, the spherical surface more evenly distributes stress across its surface and is less prone to fractures. This is why we use round-bottom flasks. It doesn't seem there are a lot of differences in the production of both Erlenmeyer and volumetric flask, so it's probably mainly due to a convention. In fact, you can find also volumetric flasks with Erlenmeyer style. Like in the image below.

enter image description here

$\endgroup$
2
  • 2
    $\begingroup$ Speaking of roundbottom flasks, those things are just absolute tanks. I've dropped them in the past after washing them, straight to the floor, and had them just bounce instead of shattering, not even any visible crack. $\endgroup$ Jul 10, 2019 at 14:38
  • $\begingroup$ @AmphotericLewisAcid the magic of spherical shape or you might consider investigating the properties of the compounds you were producing maybe you discover a glass reinforcement compound $\endgroup$
    – G M
    Jul 10, 2019 at 17:50
10
$\begingroup$

When the glassware was made by hand, the bulb at the bottom varied in size, so each flask had to be filled with the correct volume before the line could be etched on the glass. A tube was used for the neck since the volume filling the tube would change at a consistent rate. So you have a bulb holding most of the liquid, and a tubular neck to indicate when you have the correct volume. The flask looks like a pear shaped container.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.