Aim:
To determine the surface tension of a given liquid by Drop Count Method (Stalagmometric Method) using a stalagmometer.

Apparatus / Requirements:

• Stalagmometer (drop pipette)
• Specific gravity bottle (pycnometer / density bottle)
• Rubber tubing with screw pinch cock
• Beaker
• Stand with clamp
• Distilled water
• Experimental liquid (e.g., liquid paraffin, glycerin solution, alcohol, etc.)
• Thermometer
• Analytical balance / digital balance
• Chromic acid mixture (or suitable cleaning agent), alcohol for rinsing

Principle / Theory:
Surface tension is the force acting per unit length along the surface of a liquid that tends to minimize the surface area.

In the drop count (drop number) method, equal volumes of two liquids (reference liquid – usually water, and the experimental liquid) are allowed to fall drop by drop from a stalagmometer. The number of drops formed is counted.

The surface tension is related to the density and number of drops by the formula:

or

Where:

• = Surface tension of experimental liquid
• = Surface tension of water (known, ≈ 72.8 dyne/cm or 72.8 mN/m at 20°C; use exact value corresponding to experimental temperature)
• = Density of experimental liquid
• = Density of water
• = Number of drops of experimental liquid
• = Number of drops of water

(Note: Higher surface tension → fewer drops for the same volume.)

Procedure:

1. Clean the stalagmometer thoroughly with chromic acid mixture, rinse with distilled water, and dry it completely.
2. Attach a short piece of rubber tubing fitted with a screw pinch cock to the upper end of the stalagmometer.
3. For Distilled Water:
   • Immerse the lower tip of the stalagmometer in distilled water and suck the liquid 1–2 cm above the upper mark (A).
   • Adjust the pinch cock so that drops fall at a slow rate of 10–15 drops per minute.
   • Clamp the stalagmometer vertically.
   • Allow the liquid to flow. Start counting the number of drops exactly when the meniscus passes the upper mark (A) and stop counting when it reaches the lower mark (B).
   • Repeat the process 3–4 times and note the readings. Take the average number of drops ().
4. Rinse the stalagmometer with alcohol and dry it thoroughly.
5. For Experimental Liquid:
   • Fill the stalagmometer with the experimental liquid in the same manner.
   • Count the number of drops between marks A and B exactly as done for water ().
   • Repeat 3–4 times and take the average.
6. Determination of Densities (using specific gravity bottle):
   • Weigh the clean, dry empty specific gravity bottle ().
   • Fill it completely with distilled water, wipe the outside dry, and weigh ().
   • Empty, dry, and fill with the experimental liquid; weigh again ().
   • Note the temperature of the liquids.

Density of water () = (or take standard density at the experimental temperature, e.g., 0.997–0.998 g/ml near room temperature).

Density of experimental liquid () =
(or relative density × density of water).

7. Note the room temperature / temperature of the liquids using a thermometer.

Observations:

Table 1: Number of Drops

S. No.	Number of drops of Water ()	Number of drops of Liquid ()
1
2
3
Mean

Table 2: Density Determination

Parameter	Weight (g)
Weight of empty specific gravity bottle ()
Weight of bottle + water ()
Weight of bottle + liquid ()
Weight of water ()
Weight of liquid ()

Temperature = ______ °C

Density of water () = ______ g/ml
Density of liquid () = ______ g/ml

Calculations:

(Substitute the mean values of , , , and the appropriate at the experimental temperature.)

Result:
The surface tension of the given liquid at ______ °C is ______ dyne/cm (or mN/m).

Precautions:

1. The stalagmometer must be perfectly clean and dry; any grease or impurity affects drop size.
2. Maintain a slow and uniform rate of drop formation (10–15 drops/min).
3. Keep the stalagmometer strictly vertical.
4. Count drops carefully between the exact marks A and B.
5. Perform all measurements at constant temperature.
6. Avoid air bubbles inside the apparatus.
7. Wipe the outside of the specific gravity bottle dry before weighing.