There are two kinds of pipettes used for volumetric work. The "volumetric" or "transfer" pipette measure a particular volume to high accuracy. (The three pipettes at the top of Figure 1 are volumetric pipettes designed to deliver 25, 20 and 10 mL, respectively.) Volumetric pipettes are designed such after a fluid is dispensed, a small drop of liquid will remain in the tip. Do not blow this drop out! The correct volume will be dispensed from the pipette if the tip is touched to the inside wall of the flask and then gently given a half-twist.
The other kind of pipette, the Mohr pipette, has graduations that permit a range of volumes to be dispensed very easily, but somewhat less accurately. (The pipette at the bottom of Figure 1 is a 10 mL Mohr pipette.) The operation of the Mohr pipette is very similar to that of the volumetric pipette, but, in addition to setting the initial volume, the final volume must also be set by stopping the fluid flow at the appropriate location. The volume dispensed is established by calculating the difference, much like the case when using a burette.
Cleaning of Pipettes
Not only can a dirty pipette contaminate your sample, it can also dispense the wrong volume! Liquid sticks to the walls of a dirty pipette, so one test for a clean pipette is to check that the solution does not bead on the walls as it is dispensed. Unless you are being provided a new pipette by the technician, it is a good idea to wash your pipette as outlined in the following steps:
- Place a small volume of the solution to be pipetted into a beaker.
- Draw the solution into the pipette using a propiette or pipette pump, and then tilt and turn the pipette until all of the inner surfaces have been wetted by the solution.
- Discard this washing solution as waste.
- Repeat the previous steps again until beading is not observed on the inner walls of the pipette.
Both the volumetric and Mohr pipettes can be filled using either a Propipette or a Pipette Pump, as described below. WARNING: Never use your mouth to draw fluid into a pipette. There could be serious health consequences!
Note: Press directly on the letter A, S, or E when opening valves. Applying pressure away from the center of these valves will damage them.
Use your thumb and forefinger to press on valve "A"and squeeze the bulb with other fingers to produce a vacuum for aspiration. Release valve "A" once the bulb is completely deflated.
- Hold the pipette close to its upper end and insert into the bottom of the propipette. (See photograph above.) WARNING: Extreme care should be taken not to place stress on the pipette or it may shatter.
- Insert the pipette into the liquid to be transferred. Press on valve "S". Suction will draw liquid up into the pipette. Continue pressing valve "S" until the liquid reaches the desired level. Carefully adjust the fluid level so that the bottom of the miniscus coincides with the calibration line on the pipette.
- Press on valve "E" to expel liquid.
- Carefully remove the propipette (see warning in step 2.) and touch the tip of the pipette to the inside of the receiving flask to drain the last bit of solution.
- Hold the pipette close to its upper end and insert into the chuck of the pump with a slight pressure and twist. This assures a secure fit. (See photograph above.) WARNING-Extreme care should be taken when inserting disposable glass pipettes because of the possibility of shattering.
- Submerge the tip of the pipette into the solution and turn the wheel with the thumb. Fluid will be drawn up into the pipette.
- Press on the wheel gently to hold the liquid in the pipette until you are ready to expel it.
- To expel fluid, turn the wheel in the opposite direction.
- To loosen the pipette, hold it near the chuck, twist slightly and pull. Touch the tip of the pipette to the inside of the receiving flask to drain the last bit of solution.