How does a dispensing pump work in the lab

Dispensing pumps are a convenient and efficient tool for delivering a preset volume of liquid from a glass bottle. They are designed to dispense a precise amount of liquid with an accuracy of approximately 0.1 mL. However, it is important to note that dispensing pumps should never be used to measure the volumes of limiting reagents in chemical reactions, as even small errors in measurement can have significant consequences.

Dispensing pumps are available in a range of sizes, from 1 mL to 5 mL, and may be used in miniscale reactions for non-limiting reagents. They may also be used in microscale work for dispensing solvents, provided that the exact volume is not critical.

To ensure accurate measurements, it is essential to check that the spout of the dispensing pump is filled with liquid and does not contain any air bubbles that could cause the delivered volume to be less than the preset one. If air bubbles are present, they should be removed by discharging one or two samples into another container until the spout is completely filled with liquid.

When using a dispensing pump, it is important to slowly pull up the plunger until it reaches the preset volume stop. The receiving container or reaction vessel should be held under the spout, and the plunger should then be gently pushed down as far as it will go to discharge the preset volume. It is important to ensure that the last drop of liquid on the spout is transferred by touching the tip to the inside of the receiving vessel.

The exact amount of liquid delivered by a dispensing pump depends on various factors such as the liquid's viscosity, density, surface tension, and vapor pressure. Normally, instructors will adjust the pump to deliver the proper amount before the laboratory session. However, if a dispensing pump has not been preset and checked, it can be calibrated by weighing a sample transferred with the pump.

In addition to analog pumps, digital pumps are also available. Your instructor will demonstrate the specific operating procedure of the bottle-top dispensing pumps used in your laboratory. By following the correct procedures and regularly checking for accuracy, dispensing pumps can be a useful tool for delivering precise volumes of liquid.

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