Measuring the Volume

 

ISO standard requires the volume to be read at the lowest point of the liquid level. This lowest point has to touch the upper limit of the graduation mark. Meniscus is the curvature in the surface of the liquid. The curvature is caused by the surface tension of the iquid. On the volumetric glassware the shape of the meniscus is concave (curved downwards). For accurate and most precise results the eye of the observer must be at the same level to the meniscus. To get a better, faster and recise reading and measurement a black strip should be help at the back, just below the graduation mark and meniscus line.

Measuring the Volume

Delivery and Waiting Times

The delivery time is the time needed for the meniscus to pass from the upper mark to standstill wich is mostly the tip of the pipette or burette.

The waiting time starts after the delivery process finishes. During the waiting time remaining liquid continues to flow down from the glass wall. For the most accurate measure of volume with burettes and pipettes someone has to pay attention to the waiting time. Specially with pipettes you have to wipe off the pipette tip on the wall of the glassware that is used.

How to use burettes

Burettes are used for titration and are calibrated « to deliver ». First rinse the burette very well with solution you will use for titration. Then fill the burette a bit higher than the zero mark. To activate the stopcock drain the solution but never below the capacity of zero mark. Pay attention that there will be no air bubbles left inside the burette. Fill more titration solution up to max 10mm above the zero mark and pay attention not to wet the burette wall above the liquid level. By reading the meniscus at your eye level drain the solution to the exact level of the zero. At this step you have to wipe the drops at the tip of the burette tip. Then you have to open the stopcock very slowly and start the titration and also pay attention not to touch the tip to the wall of the container you use. When the colour changes stop the titration process and wait for 30 seconds for the waiting time. You have to read the discharged volume at your eye level. Do not forget to wipe the remaining drop of liquid at the burette tip since this is also a part of the titrated volume.

How to use pipettes

Pipettes are calibrated « to deliver ». The first stage is to fill the pipette for filling process you always have to use a pipette filler or a pipette pump. Fill the liquid max 10mm above the volume mark. Then you must wipe and make dry the outer surface of the pipette. Hold the pipette to your eye level and place the tip of the pipette to the wall of a vessel with a slight angle. Then you have to discharge the access volume until meniscus reaches the calibration mark of the pipette. Second step is to empty yhe pipette. You must empty the pipette by placing the tip of the pipette to the wall of the vessel with a slight angle. Pay attention to empty the pipette completely (or until the 2nd mark of bottom graduation line).

How To Use Cylinders

Cylinders are mainly used for exact measurement of liquids and they are calibrated « to contain ». Fill in the liquid so that meniscus will be exactly at the same level of the desired mark. Pay attention to read the meniscus at the eye level.

How To Use Volumetric Flasks

Volumetric flasks are mainly used for the preparation of highly accurate standard solutions. They are calibrated according to « to contain ». First put the exact and precise weight of the substance into the flask. Then fill the volumetric flask with water (distilled water) about half of the volume and mix the content fully. Then fill in with more liquid just a bit below of the calibration ring mark. Then add some more water so that the meniscus will be exactly at the same level of the ring mark. Pay attention to read the meniscus at the eye level and pay attention not to wet the neck wall above the ring mark level.