After completing the surface tension measurement of the solutions (at any time of the day that surface tension is to be measured), the stalagmometer must be properly rinsed with distilled water until no colour remains. Note: This may require several rinses.
After rinsing, place the stalagmometer horizontally in a tray of fresh distilled water and allow the stalagmometer to soak in the distilled water until next use.
9.2 If, after rinsing, beading of liquid occurs inside the stalagmometer, this is an indication that the stalagmometer is dirty. Beading can occur due to insufficient or improper rinsing of the stalagmometer; if the solution has been allowed to dry within the stalagmometer; or if oil or another material has coated the inside. If this situation occurs, more aggressive measures using nitric or sulphuric acid must be used to clean the stalagmometer. Use the following procedure to clean the stalagmometer:
9.2.1 Place a piece of 0.5 cm (ΒΌ") (inside diameter) tubing on the top of the stalagmometer. Place a tubing clamp on the tubing between one and two centimetres from the stalagmometer.
9.2.2 Use the pipette bulb to draw up 50% nitric or 25% sulphuric acid (prepared as per subsection 3.2) to above the upper line marking on the stalagmometer. CAUTION: These acids are corrosive and produce a lot of heat when mixing with water. Wear personal protective equipment at all times: goggles, acid-resistant gloves, chemical apron and lab coat.
9.2.3 Close the clamp and ensure that no liquid is released. Allow the acid to remain in the stalagmometer for one to two hours. More time may be required for very dirty equipment.
9.2.4 Drain the acid from the stalagmometer and rinse the stalagmometer at least three times with distilled water.
9.2.5 If oil is suspected to cause the beading, then, after soaking the stalagmometer in acid, rinse once with distilled water, then rinse once with full strength acetone or methanol, and finally rinse at least three times with distilled water.
