Pipets (or pipettes) are used to transfer reagents from a holding container to a reaction vessel like a test tube or beaker. They may be precisely graduated for one or more volumes or may be completely ungraduated. Due to the narrow width of pipets, graduations are very precise and allow the recording of four significant digits.
Graduated pipets have a full graduated scale to their maximum volume and may be used to measure many different volumes of liquid. As with graduated cylinders, the smallest graduated pipet that will hold the desired measured volume should always be used. This maximizes the precision of the measurement. Measured volumes are recorded to one decimal place beyond that which is indicated by the graduations.
Graduated pipets come in three types:
Type 1 - Reverse Graduated - Partial Delivery: These pipets feature the zero mark at the top of the pipet and include a maximum volume (nominal volume) somewhere near the bottom of the pipet. The pipet is first filled to the zero mark, then allowed to drain to the required volume. The pipet is never drained completely during measurement.
Type 2 - Standard Graduated - Full Delivery: These pipets feature the zero mark at the bottom of the pipet with the nominal volume at the top of the pipet. The pipet is filled until the required volume (up to the nominal volume) is achieved, then is allowed to drain completely into the reaction vessel.
Type 3 - Reverse Graduated - Full Delivery: These pipets are similar to Type 1 Pipets except the nominal value can only be achieved by draining the pipet completely.
Pasteur pipets (or dropper pipets) are essentially eye droppers used to transfer liquids in a dropwise fashion. They may be plastic or glass, will have a detachable siphon bulb, and are ungraduated. Often volume is not a concern when using dropper pipets, but when they are used quantitatively, the accepted volume equivalence used is 20 drops = 1mL.
- The tip of the dropper should always be held high enough above the reaction vessel so that the tip cannot be splashed when the drops fall.
- Pipets should never be placed on the lab bench due to the potential for sample contamination and spreading of chemicals.
- When using pipets from a dropper bottle, the bottle should always remain in your hand while the pipet is out to prevent pipets from being inserted into the wrong bottle.
Transfer pipets (or Beral pipets) are one-piece plastic pipets used for transferring reagents. They may or may not be graduated but are not considered to be precise or accurate. They may be used, however, to precisely fill graduated cylinders or volumetric flasks to the fill line. Transfer pipets are typically disposable.
Volumetric pipets are designed to measure only one very precise volume. For example, a 5mL volumetric pipet is designed to only measure a volume of 5mL - it has no other graduations. The measurement, if made correctly, will have a level of precision to four significant digits (i.e., the volume should be recorded as 25.00mL). Volumetric pipets often have a bulb in the middle, but the graduation mark will always appear on the narrow portion of tube above the bulb.
- Get the appropriate amount of the solution you wish to pipet in a small, clean, dry beaker. Never pipet directly out of the stock bottles of solution. This creates a contamination risk.
- Insert the tip of the pipet into the beaker of solution so that it is about 1/4” from the bottom. Be sure not to press the tip against the bottom of the container.
- If you are right handed, hold the pipet in your right hand, leaving your index finger free to place over the top of the pipet. With your left hand, squeeze the pipet bulb. Press it firmly over the top of the pipet, but DO NOT INSERT THE PIPET INTO THE BULB!! Release the pressure on the bulb and allow the solution to flow into the pipet until it is above the volume mark. Do not allow the solution to reach the bulb.
- Quickly remove the bulb and place your index finger firmly over the top of the pipet. Slowly roll you finger to one side and allow the liquid to drain until the bottom of the meniscus is aligned with the volume mark. With practice you will be able to lower the liquid very, very slowly.
- When the bottom of the meniscus is even with the volume mark, press your index finger firmly on the top of the pipet so no liquid leaks out. Pull the pipet out of the solution and touch the tip once to the side of the container.
- To transfer the solution, place the tip of the pipet against the wall of the receiving container at an angle of 10-20 degrees. Slowly allow the liquid to drain from the pipet. Keep the flow slow so that no droplets cling to the inside of the pipet.
- When the solution stops flowing, touch the pipet once to the side of the receiving container to remove any hanging drops. DO NOT blow out the remaining solution. The pipet has been calibrated to deliver the appropriate amount of solution with some remaining in the tip.