The pipette are used to transfer a particular volume of solution. As such, it is often used to deliver a certain fraction (aliquot) of a solution.
To ascertain the fraction ,the original volume of solution from which the aliquot is taken must be known , but it need not all be present, so long as it has not evaporated or been diluted.
Types of Pipette:
There are two common pipette types, the volumetric or transfer and the measurement or graduation pipette. Variations of the latter are also called clinical or serological pipets.
Designed of Pipette:
Pipets are designed to deliver a specified volume at a given temperature , and they are marked “TD.” Again the volume can be considered to be constant with small changes in temperature.
Pipette are calibrated to account for the drainage film remaining on the glass walls. This drainage film will vary somewhat with the time taken to deliver. Usually the solution is allowed to drain under the force of gravity and the pipet is removed shortly after the solution is delivered. A uniform drainage time should be adopted.
Function of volumetric Pipette:
The volumetric pipette is used for accurate measurements since it is designed to deliver only one volume and is calibrated at that volume.
Generally speaking, precision to four significant figures is achieved, although if necessary five figures can be obtained with proper calibration. See the table on the back cover for tolerances of class A transfer pipets. Measuring pipets are straight-bore pipette that are marked at different volume intervals.
This are not accurate why?
These are not as accurate because non uniformity of the internal diameter of the device, will have a relatively larger effect on total volume than is the case for pipette with a bulb shape.The drainage film will also vary with the delivered volume.
At best , accuracy to three significant figures can be expected from these pipets , unless you make the effort to calibrate the pipette for a given volume delivered.
Many volumetric pipette is designed to deliver with a small volume left in the tip. This should not be shaken or blown out. In delivering , the pipet is held vertically and the tip is touched on the side of the vessel to allow smooth delivery without splashing and so that the proper volume will be left in the tip. The forces of attraction of the liquid on the wall of the vessel will draw out a part of this.
Some pipetare types of blowouts (including pipet measurements calibrated to the whole volume of the tip). The final volume of solution must be blown out from the tip to deliver the calibrated amount.
Pipette are easy to identify
These pipette are easy to identify , as they will always have one or two ground bands or rings around the top. (These are not to be confused with a colored ring that is used only as a color coding for the volume of the pipet.) The solution is not blown out until it has been completely drained by gravity.
Blowing up the delivery level would change the drainage
Volumetric pipet size:
Volumetric pipet are available in sizes of 100 to 0.5 mL or less. Measuring and serological pipet range from a total capacity of 25 to 0.1 mL.