To ensure proper measurement of large volumes in the laboratory, volumetric apparatus should be calibrated regularly. Two of the most common apparatus calibrated are volumetric flasks and serological pipettes.
This is a simple, manual procedure on how to calibrate volumetric apparatus:
Volumetric flask:
Weigh the volumetric flask.
Fill the volumetric flask with distilled water, up to the mark
Weigh the volume of distilled water contained in the flask.
Calculate the true volume of glassware by the formula:
Weight of dist. H2O
Experimental volume of glassware = –
(EV) Density of H2O AST
(Constant value =1)
Calculate the percent error.
% Error = Theoretical volume (TV) – EV
– X 100
TV
* Record data.
* Do the same procedure with the serological pipette.
* Record again your data.
* Allowable percent error is +- 0.03 %
*If the error is more than this then the apparatus should be discarded or changed into a volumetric apparatus that could measure more accurately.
This could be done with all volumetric apparatus. The important thing to remember is that accuracy of measurement is always a vital requirement in a laboratory.
Volumetric apparatuses include the following:
Serological pipettes Volumetric flasks Erlenmeyer flasks Graduated cylinders Beakers Mohr pipettes Volumetric pipettes Between –two-mark pipettes Micropipettes Burettes
For pipettes, the following should be noted:
They should not have chipped ends. This would decrease the volume measured. Operator should observe proper pipetting to ensure accurate measurements. You should wipe any excess fluid clinging at the sides of the pipette. This will increase the volume inaccurately. Blow pipettes that have two blasted rings at the upper end. Drain pipettes that have no blasted rings at the upper end. Viscous fluids should be read at the upper meniscus. Clear liquids should be read at the lower meniscus. Avoid formation of bubbles. This will lead to inaccurate volumes. Never pipet by mouth. Solutions maybe corrosive or harmful. Rubber bulbs are available for sucking and dispensing.
If volumetric apparatus is broken, it should be replaced and never be used. Proper care should also be observed when calibrating them, because any error in the procedure will invalidate calibration. If unsure of the method that you used. Consult the supplier of the apparatus for further information.
