Liquid Meniscus explained
Accurate volume measurements in the laboratory depend on correctly reading the meniscus, also known as the liquid meniscus.
This article explains what a meniscus is, why it forms, and how to measure liquid volumes precisely using graduated laboratory glassware and plasticware.
What is a liquid meniscus?
The term meniscus describes a curvature in the surface of the liquid. The meniscus may be curving upwards or downwards. The curvature develops as a function of the interplay between the adhesion and cohesion forces.
If the diameter of a pipette is narrow enough – as with capillary pipettes – the adhesion force is strong enough to pull up not only the edge but also the entire liquid level (capillary effect).
What is the difference between a Concave and Convex Meniscus?
The difference between a concave and convex meniscus lies in the shape of the liquid surface and the balance between adhesion (attraction between the liquid and the container) and cohesion (attraction between liquid molecules).
How to read a meniscus correctly
1. Get on Eye Level:
For parallax-free adjustment of the meniscus, the volumetric instrument is held upright and the observer’s eye must be at the same height as the meniscus.
In this position the ring mark will appear as a line.
2. Read the meniscus
Tipp: Use dark paper as a reading aid.
The meniscus will appear darker and more easily readable in front of a light background if a piece of dark paper is held behind the instrument immediately beneath the ring mark or graduation mark.
Quick Checklist for reading the meniscus
Before recording your measurement, ask yourself:
✓ Is my eye level with the meniscus?
✓ Am I reading the correct part of the meniscus (bottom for water, top for mercury)?
✓ Is the cylinder standing upright?
✓ Has the liquid stopped moving?
✓ Have I estimated one extra digit?
