Measuring Volumes

Volumetric vessels and other tools in the chemical laboratory
Various laboratory vessels and devices are used for preparation of solutions and manipulation with liquids. They differ in the intended use and in accuracy. Precise volume must always be measured at constant temperature because, due to thermal expansion, changes in temeprature are followed by changes in volume. For example water at 10 °C has much smaller volume than at 80 °C.

Volumetric vessels are usually calibrated for 20 °C (25 °C in the USA). Calibration temperature is written at every volumetric vessel. Substantial error may result e.g. from change in temperature during dilution of an acid. A lot of heat is released during dilution and temperature increases. When the volume of acid is measured immediately, the real volume after cooling-down will be smaller. The requirement for constant temperature must therefore be observed everytime.

* not suitable for measuring volumes, only approximate volume indicated

Beakers


Beakers are used for approximate determination of volume of liquids. They are used especially for dissolving compounds, diluting liquids, heating and other operations. Because of low accuracy, they are frequently not classified as volumetric devices.

Volumetric flasks and cylinders
Volumetric cylinders and volumetric flasks are used to measure volume of liquids contained in them. They are calibrated for volume included in them - this is indicated by the marking "IN". The liquid has accurate volume when it reaches the corresponding marking on the scale. Volume is usually indicated in mL. During measurement of the volume the vessel must be placed on an even, horizontal support. The required volume is measured when the lower edge of meniscus of the liquid just touches the marking on the vessel. Cylinders are less accurate, volumetric flasks are used for preparation of solutions of exact concentration. Cylinders are used solely for measuring volume, they are not employed for dissolution, dilution or mixing.

Burettes, pipettes, dispensers and syringes
Burettes, pipettes, dispensers and syringes measure volume of a liquid delivered to another vessel.

Pipettes and burettes are usually calibrated for outpouring. It is indicated by letters "EX" (from exclude). The exact volume is measured when the liquid is kept to flow out from a certain marking on the scale. Some liquid may remain in a pipette after outpouring; it should never be blown out, its volume was taken ito account during calibration.


 * Burettes.

Burettes are used for titrations or whenever the same liquid is repetitively measured. Burette is a glass or plastic tube with calibrated scale closed with a stopcock. Burette is attached to a stand in a vertical position. With a closed vent, it is filled with the measured liquid. Some liquid is then outpoured so that the meniscus touches marking of the scale. Now, burette is ready for titration. The liquid is outpoured with the stopcock and the volume is read on the scale. The most important moment is reading the volume; with burette, the volume is always read twice: first to read the starting position of the meniscus and the second time to read the end position. As the difference in volume is calculated, it is not important how exactly the volume is read, it must however be read in the same way everytime.


 * Automatic burettes are employed in routine laboratories.


 * Glass pipettes.


 * Today, glass pipettes are rarely used in contemporary routine laboratories; glass pipettes were replaced by semiautomatic dispensers. The volume of glass pipettes may vary from 1 to 100 mL. Glass pipettes can be non-scaled for measuring of a certain volume or scaled with grades by 1 mL and tenth of milliliter. Numbering of the scale can go from the tip up or in the opposite direction.
 * For security reasons, liquid should never be aspired by mouth. Various adapters and pistons are used instead.


 * When aspiring the solution, the pipette should never touch the bottom of the vessel. Before measuring a sample the pipette should be filled with the solution first and the liquid is then thrown away. Then the exact volume is measured and delivered for further processing. The measured solution must never get into the adapter.




 * Pipettors (automatic pipettes, micropipettes, microdispensers).
 * Another possibility of measuring small volumes is use of micropipettors. They are always calibrated for outpouring.


 * Microsyringes.
 * Microsyringes serve for dispensing small volumes (0.1–1000 &mu;L) of liquids. They consist of a needle attached to a scaled glass cylinder with a piston. Different types vary in the diameter of needles and pistons.




 * Piston dispensers.


 * Piston dispensers consist of a piston with a scale attached to a flask. They are intended for repetitive dispensing of the same volume from the stock container. Dispensers designated for measuring aggressive liquids (e.g. strong acids) are made of borosilicate glass; plastic parts that are in contact with measured liquid are made of PTFE, the other parts of PE or PP. Electronic control modules may automate dispensing in contemporary dispensers.