In an organic laboratory, accurately measuring and transferring liquids is a crucial aspect of conducting successful experiments. There are several liquid volume measuring devices that are commonly used in the laboratory, including graduated cylinders, pipets, burets, dispensing pumps, syringes, and beakers and flasks with volume markings on them. The choice of equipment used for measuring a specific volume of liquid depends on the accuracy with which the volume needs to be known.
When measuring liquids in the laboratory, it is essential to ensure that the measurement is as accurate as possible. For example, when working with a liquid reagent that is the limiting factor in a mini-scale reaction, it is important to measure the volume of the liquid with a graduated pipet and then weigh it to determine the exact amount. This level of precision is necessary to ensure that the reaction proceeds as planned and to avoid any unnecessary waste of reagents.
If the liquid is a solvent or is present in excess of the limiting reagent, volume measurement can be done with a graduated pipet for micro-scale work and with either a graduated pipet or a graduated cylinder for mini-scale work. Graduated cylinders are not as accurate as pipets but can be used for measuring quantities of liquids other than limiting reagents. It is important to note that the volume markings on beakers and flasks can only be used to estimate approximate volumes and should never be used for measuring a reagent that will go into a reaction.
Dispensing pumps are another tool used for measuring liquids in the organic laboratory. Dispensing pumps fitted to glass bottles come in a variety of sizes designed to deliver a preset volume of liquid with an accuracy of +/- 0.1 mL. Dispensing pumps should never be used for measuring the volumes of limiting reagents as they are not precise enough for this application. However, pumps in the 1-, 2-, and 5-mL range may be used in mini-scale reactions for other than limiting reagents and may sometimes be used in micro-scale work for dispensing solvents.
Before using a dispensing pump, it is important to check that the spout of the pump is filled with liquid and contains no air bubbles that could cause a volume less than the preset one to be delivered. If air bubbles are present in the spout, pull up the plunger and discharge one or two samples into another container until the spout is completely filled with liquid. The discarded samples should be placed in the appropriate waste container. To use the pump, hold the receiving container or reaction vessel under the spout and slowly pull up the plunger until it reaches the preset volume stop. Then, gently push the plunger down as far as it will go to discharge the preset volume. Be sure that the last drop of liquid on the spout is transferred by touching the tip to the inside of the receiving vessel.
The exact amount of liquid delivered by a dispensing pump depends on the liquid’s viscosity, density, surface tension, and vapor pressure. Normally, an instructor will adjust the pump to deliver the proper amount before the laboratory period. In the unlikely event that the pump has not been preset and checked, it may be calibrated by weighing a sample transferred with the pump. Dividing the mass of the liquid sample by the liquid’s density provides the actual volume transferred, which can be compared to the nominal preset volume. If the actual and nominal volumes do not match, the preset volume position can be changed to accommodate the error, and the pump recalibrated to verify the adjustment.
In addition to analog pumps, digital pumps are also available in the laboratory. These pumps are more precise and can be programmed to deliver a specific volume of liquid. Your instructor will demonstrate the specific operating procedure of the bottle-top dispensing pumps used in your laboratory.
When working with small volumes used in micro
-scale experiments, there are some additional considerations to keep in mind. One important factor is the precision of the measuring device being used. When measuring very small volumes, even small errors can have a significant impact on the accuracy of the final result. Therefore, it is important to use measuring devices that are specifically designed for micro-scale measurements, such as micropipettes or microsyringes. These devices are capable of accurately measuring volumes as small as a few microliters.
Another important factor to consider when working with small volumes is the viscosity of the liquid being transferred. Viscosity is a measure of how resistant a fluid is to flow, and it can have a significant impact on the accuracy of a liquid transfer. High viscosity liquids, such as oils, require more force to move through a pipette or syringe, and this can result in inaccuracies in the final volume measurement. It is therefore important to choose a measuring device that is appropriate for the viscosity of the liquid being transferred.
When transferring small volumes of liquid, it is also important to take into account the surface tension of the liquid. Surface tension is the tendency of a liquid to form a cohesive surface that resists external forces. This can cause small droplets of liquid to form on the outside of a pipette or syringe, which can result in a lower volume being transferred than intended. To minimize the effects of surface tension, it is important to properly wet the tip of the pipette or syringe before transferring the liquid. This can be done by gently touching the tip to the surface of the liquid, or by pre-wetting the tip with a small amount of the liquid being transferred.
In addition to these considerations, there are some general tips that can help ensure accurate and precise liquid transfers in the lab. First, it is important to always use clean and dry measuring equipment to prevent contamination or dilution of the liquid being transferred. It is also important to properly calibrate and maintain measuring devices to ensure accurate measurements. Finally, it is important to take your time and work carefully when transferring liquids, especially when working with small volumes. Rushing or being careless can lead to inaccurate measurements or spills, which can compromise the results of an experiment.
In conclusion, measuring and transferring liquids in organic lab is an essential skill that requires careful attention to detail and precision. By using appropriate measuring devices, considering factors such as viscosity and surface tension, and following general best practices for lab work, it is possible to accurately and reliably measure and transfer liquids in the lab. With practice and patience, these skills can be mastered, and will serve you well in any organic chemistry lab setting.