Introduction:
Volumetric measurements play
an important role in analytical laboratories. Accurate dispense of the solution
will give the accurate and precise results. The volumetric measurement of
liquids is a routine operation in the laboratory. Therefore, volumetric
instruments, such as volumetric flasks, bulb pipettes, graduated pipettes,
graduated cylinders and burettes are standard equipment.
There are two types of
pipettes commonly used in laboratories are,
1. Volumetric
Pipettes (Bulb Pipettes)
2. Graduated
Pipettes
The use of volumetric pipette
(Bulb pipette) which deliver a fixed volume of liquid and graduated pipets.
Carefully inspect graduated pipets to see if the graduations extend to the tip
of the pipet - if they do not then the volume of the tip is not calibrated and
the pipet should be drained only as far as the lowest calibration line.
Good Pipetting Practices:
In general, when planning
dilutions, try to use the largest volumetric pipet. For example, making a 1 to 10
dilution with a 5 mL pipet and 50 mL flask will be less accurate than making
the same dilution using a 10 mL pipet and a 100 mL flask. For even better
accuracy, we can use a 25 mL pipet and a 250 mL flask. These guidance apply
primarily to your standard solutions, and particularly to any that will be used
for subsequent dilutions.
Always use volumetric
pipette for standard dilutions
Select the pipette and
inspect for the damage in the tips. Clean and dry the pipette before using.
Rinse the pipette: Draw a
small quantity of the liquid to be pipetted into the pipet. Tip the pipet
nearly horizontal, and rotate to thoroughly wet the interior surface, up to
above the mark. Discard the liquid and repeat rinsing at least twice more, for
a total of at least three rinsing.
Carefully fill the pipet little
above the mark using pipette bulb. Quickly place a forefinger over the upper
end of the pipet to hold the liquid. Ensure that there are no bubbles in the
bulk of the liquid or foam at the surface.
Tilt the pipet slightly
from the vertical, and wipe the exterior free of adhering liquid.
Slowly allow the sample
to drain into a waste beaker by partially releasing the forefinger
Halt further flow when
the bottom of the meniscus touches the top of the graduation mark.
Place the tip of the
pipet well into the receiving vessel, and allow the liquid to drain. When free
flow ceases, rest the tip of the pipet against the inner wall or bottom of the
receiving vessel for 10 s.
Finally, withdraw the pipet with a rotating motion to remove any droplet still adhering to the tip. The small volume remaining is not blown out or rinsed into the receiving vessel.
Never pipette anything by mouth
Thoroughly rinse the pipet with distilled water after use. Note that only the upper part of the pipette should be handled. Do not touch or hold the bottom of the pipette to avoid contaminating the liquid being pipetted (and yourself!).
Reading the Meniscus:
Correct meniscus reading
is much important for accurate pipetting. The temperature of the liquid also needs
to be considered during the pipetting. The expansion of the liquid at different
temperature must be considered.
For parallax-free
adjustment of the meniscus, the volumetric instrument is held upright and the
observer’s eye must be at the same height as the meniscus. In this position the
ring mark will appear as a line. The meniscus will appear darker and more
easily readable in front of a light background if a piece of dark paper is held
behind the instrument immediately beneath the ring mark or graduation mark.
In case of Colorless liquid, it is easy to observe the phase boundary, whether it is convex or concave hence in colorless liquids lower meniscus may be considered. However, in case of coloured liquids observing this phase boundary is not possible, hence only upper meniscus is considered.
Tolerances of Class A
transfer (volumetric) pipettes
0 Comments