Stalagmometer: Parts, Diagram, Uses & Surface Tension Formula

Stalagmometer: Parts, Diagram, Uses & Surface Tension Formula

What is a stalagmometer?

A stalagmometer is a simple, hand-operated glass device used to measure the surface tension of liquids. It is a capillary tube with a widened bulb in the middle and a narrowed tip at the bottom. To use it, you draw a liquid up past a fixed upper mark, then let it drain by gravity, counting the drops as the liquid level passes down to a fixed lower mark.

The name comes from the Greek stalagma, meaning “drop.” The technique itself is called the stalagmometric method (also known as the drop-number or drop-weight method), and it remains one of the simplest and most widely taught ways to compare the surface tension of two liquids without specialised electronic equipment.

You’ll typically meet a stalagmometer in an undergraduate physical chemistry lab, alongside instruments like the Ostwald viscometer, and in pharmacy or quality-control settings where surface tension is used as an indicator of liquid purity or surfactant concentration.

Parts of a stalagmometer (labelled diagram)

A stalagmometer has five main parts, from top to bottom: the suction bulb, the reservoir bulb, the two fixed graduation marks, and the capillary tip. The diagram below labels each one.

• Suction bulb: A small rubber bulb (or a pipette filler) attached at the top, used to draw the liquid up past the upper mark before letting it drain freely.
• Bulb (reservoir): The widened glass section that holds the liquid sample between fillings.
• Upper mark: A fixed etched line on the narrow stem — drop counting starts the moment the liquid meniscus passes this mark.
• Lower mark: A second fixed etched line further down — drop counting stops here. The fixed volume between the two marks is the same every time, for every liquid.
• Capillary tip: The fine, narrow outlet at the bottom where the liquid gathers into a drop and detaches under gravity.

How a stalagmometer works

A drop forms at the capillary tip and grows until its weight can no longer be held up by the liquid’s surface tension — at that point it detaches and falls. This is described approximately by Tate’s law:

For a fixed volume of liquid V (the volume between the upper and lower marks), if the liquid forms n drops, then the weight of one drop is Vρg/n, where ρ is the liquid’s density. Combining this with Tate’s law and comparing two liquids measured with the same instrument (so r and V cancel out) gives the surface tension formula below.

Surface tension formula

In practice, you rarely calculate absolute surface tension from Tate’s law directly — you compare a test liquid against a reference liquid of known surface tension (almost always water), measured with the same stalagmometer:

Water is used as the reference because its surface tension is precisely known and tabulated at any given temperature (approximately 72.8 dyn/cm at 20°C). Since both liquids are measured with the same instrument, the capillary radius and the fixed volume between the marks cancel out of the calculation entirely — you only need the drop counts and the two densities.

How to determine surface tension: step-by-step

1. Clean the stalagmometer. Rinse thoroughly with distilled water, then with a small amount of the liquid you’re about to test, to avoid contamination affecting the reading.
2. Fill with the reference liquid (water). Use the suction bulb to draw water up past the upper mark.
3. Let it drain and count the drops. Allow the water to fall freely, drop by drop, and count the number of drops (n₁) as the meniscus passes from the upper mark to the lower mark. Repeat 2-3 times and average the count.
4. Empty and rinse, then fill with the test liquid. Draw the test liquid up past the same upper mark.
5. Count the drops again. Record the number of drops (n₂) as the test liquid drains between the same two marks. Repeat and average, as before.
6. Note the densities. Look up or measure (using a specific-gravity bottle or pycnometer) the density of the test liquid (ρ₂); the density of water (ρ₁) is taken as 1.00 g/mL at room temperature (or the exact tabulated value at your working temperature).
7. Apply the formula. Substitute n₁, n₂, ρ₁, ρ₂ and the known γ₁ of water into γ₂ = γ₁ × (n₁/n₂) × (ρ₂/ρ₁) to find the surface tension of the test liquid.

Observation table

Common uses of a stalagmometer

• Undergraduate physical chemistry practicals: The standard experiment to determine the surface tension of a liquid relative to water using the drop-number method.
• Pharmacy practicals: Studying how surfactants and formulation additives lower surface tension.
• Quality control & purity checks: Surface tension is sensitive to trace contamination, so a shift in drop count can flag an impure or contaminated sample.
• Comparing liquids: Any situation where you need a quick, low-cost comparison of surface tension between two or more liquids without electronic tensiometry equipment.

Stalagmometer vs Ostwald viscometer vs burette

These three are all calibrated glass instruments that work by comparing a test liquid against a reference under identical conditions — but each measures something different:

If you’re looking for the volumetric-glassware reading method instead, see our burette vs pipette guide.

Buying guide: what to look for

For school and college physical chemistry labs, look for:

• Borosilicate glass construction for chemical resistance and thermal stability
• Clearly etched, permanent upper and lower graduation marks
• A capillary tip fine enough to form clean, well-separated drops
• Bulk/institutional availability for setting up multiple lab benches at once

Common errors & accuracy tips

• Not cleaning between liquids: Residue from the previous liquid changes the effective surface tension of the next reading. Always rinse with the new liquid before the real measurement.
• Draining too fast: Tilting the instrument or draining too quickly causes drops to merge or break early, giving an inconsistent count. Let it drain freely under gravity alone.
• Counting only once: A single trial can be off by a drop or two. Repeat 2-3 times per liquid and average.
• Using an outdated density or reference surface-tension value: Both water’s density and surface tension change with temperature — use values for your actual working temperature, not just a generic textbook figure.
• Vibration or draughts: Air currents and bench vibration disturb drop formation. Work on a stable, draught-free surface.

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