The tip of the pipet can then be placed in the solution which is to be drawn up and the bulb slowly released. This method requires a little practice but in the final analysis may be considerably more practical and satisfying than the use of the high-tech bulb Dr. Reilly will show you next. You must exercise care not to allow the tip of the pipet to break the surface of the liquid while you are drawing in the solution or the sudden decrease in viscosity at the tip will cause a large amount of liquid to contaminate the inside of your rubber bulb because of the entry of air pushing the liquid up beyond the mouth of the pipet.
Draw up the solution until the meniscus is several centimeters above the calibration line, then quickly put your finger over the open hole of the pipet. Making sure that your line of sight is perpendicular to the length of the pipet, allow a tiny amount of air in so that the meniscus drops to the calibration mark, as Dr.
Reilly is showing at the right. When the bottom of the meniscus coincides with the calibration mark, your pipet contains a precisely measured volume, as in the image at the left. The pipet can then be removed from your reagent solution, transferred to the receiving flask and allowed to drain.
A volumetric pipet should not be "blown out" to eject all liquid at the tip because volumetric pipets are calibrated in a manner that takes into account the solution which remains at the tip due to surface tension. The "high-tech"pipet bulb is an Eppendorf bulb. It can be placed firmly on the mouth of the pipet.
At the side of the Eppendorf is a protruding lever attached to a slide. Pull it down to create a vacuum inside the bulb. Use your thumb to push the two way valve up when you are ready to draw your solution into the pipet, as shown at the left, but make sure the nozzle of your pipet remains below the surface of the solution right so that you don't suck air into your pipet causing lots of solution to pass into the filter in the bulb area which will require disassembly, drying out and replacement of parts.
When a sufficient amount of the solution has been drawn in so that the meniscus is above the calibration mark, use your thumb to slide the two-way valve down, as shown at the left. Do it gently so that the meniscus drops slowly to the calibration mark.
Then you can transfer the solution in the pipet to the receiving flask and push the two-way valve lever down to empty the pipet. The nozzle of the pipet can be kept in the open air for the transfer, as shown at the right. Finally, pipettes and burettes accumulate inert solid material which must be removed from time to time. Here at the left is the nozzle of a burette which has material which will not pass through. You may have to use a wire, available on the lower ledge of the burette case, to clean out this material.
It is best to do it with the petcock valve removed so that when you do a reverse wash after poking it free, the material can be washed out at the point of the valve instead of at the other end of the burette cylinder. Two major types of graduated pipettes are Mohr pipettes and serological pipettes. Both types have bi-directional graduations and are made to deliver volumes with varying accuracy. Serological pipettes such as these also have color-coded stripes for simple identification.
The difference between these two types of pipettes is that the volume of liquid that is ultimately transferred is different from the volumes indicated on the pipette before and after delivery. Calibration markings on Mohr pipettes begin above the tip, where serological pipettes are calibrated to the end of the tip.
Another popular variety of graduated pipettes are small graduated transfer pipettes. These are ideal for one-time-use, simple liquid transfers. The one-piece plastic pipettes made out of polyethylene are unbreakable and sterile; making them the best choice for working with tissue culture application, microbiology, or hematology. Small pipettes such as these also come in different sizes for different needs, and allow for precise drops and reproducible spotting.
Both volumetric pipettes and graduated pipettes have their place in any lab, university, or research facility. The pros and cons related to each system of measurement are unique, and dependent on the build, make, and type of pipette and what their intended use is. Volumetric pipettes are the clear choice when working with chemical analysis, research, or anything requiring high accuracy.
Graduated pipettes offer a range of different sizes, and provide durable and reliable measurement systems when working with different volume sizes. When choosing which types of laboratory tools are right for your lab, it is important to consider the requirements for your specific application.
For over 40 years, Lab Pro Inc. To learn more, visit the biggest Lab Supply showroom in California, or contact us online or at View profile. Menu Categories. Volumetric pipettes transfer a single, predetermined volume of liquid.
They are often called a bulb pipette for their shape, which is a long tube-like shaft at the bottom and top and a bulb in the center where the bulk of solution is held.
The name is also indicative of the bulb that attaches to the top of the pipette, often made of rubber, which must be manually squeezed to create a vacuum. As the rubber bulb begins to fill with air, a solution is drawn into the pipette. While seeming rudimentary, volumetric pipettes are extremely precise up to four significant figures. They are limited to the particular amount calibrated and cannot be used to accurately measure liquid amounts less than their specified capacity.
Measuring pipettes have graduated volumes, like a graduated cylinder, and can dispense different volumes. Measuring pipettes are calibrated with small divisions and are often adjustable, allowing users to accurately draw up the necessary amount of liquid. Micropipettes are accurate and precise and can transfer measured volumes of microliters of volume. Micropipettes are spring-loaded instruments and require calibration every few months either from the manufacturer or by a process of weighing water at a known temperature.
Pasteur pipettes, named for Louis Pasteur, are similar to eye droppers and can transfer a small amount of liquid. They are made of glass or plastic, have a separate bulb and are not graduated for a certain volume. Similar to Pasteur pipettes, they are made of plastic but molded, so the bulb may contain some of the liquid.
Squeezing the bulb will create a vacuum and liquid may be drawn up. Regardless of the type of pipette, care and attention is needed. To prevent damage, especially to glass pipettes, hold them at a distance from the bottom of the container before drawing liquid. When the required volume has been drawn up, gently tap the side of the pipette to remove excess droplets.
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