Volumetric Glassware

Transcription

1 There are several types of glassware used to measure the volume of a liquid accurately. These include graduated cylinders, volumetric flasks, burets, and pipets. Graduated cylinders and volumetric flasks are designed to contain an accurately known volume. Burets and pipets are designed to deliver, or dispense, an accurately known volume. Reading the volume of a liquid in a cylinder or other apparatus is complicated by the fact that liquids do not form flat surfaces in glass. Rather, the surface has a marked curvature, called a meniscus. The liquid level is read at the bottom of the meniscus. An illustration of a meniscus is shown below. Graduated Cylinders: A graduated cylinder is just what its name implies. Cylinders are designed to contain an accurately known volume, but are usually used to dispense that volume. Therefore, they are not as accurate as a volumetric flasks, but the fact that they are graduated makes them more versatile than volumetric flasks. They are the workhorse volumetric apparatus for most lab tasks, and are available with capacities from 10 ml to 2 L. Use of a graduated cylinder is straightforward. If one wishes to fill it to a desired volume, the liquid is added to just below the target volume. The last few drops are added with a dropper or Pasteur pipet to bring the bottom of the meniscus to the desired graduation. If one wishes to read the volume contained in a cylinder, one locates the bottom of the meniscus. The volume is read between the lines or graduations, and the last digit contains the uncertainty of the measurement. 1

2 Volumetric Flasks: Volumetric flasks are designed to contain a very accurately known volume of liquid. They are used for the preparation of solutions in which concentrations must be known to four significant figures. Volumetric flasks have a wide base, and a narrow neck with a single scribe mark which marks the fill level. To use a volumetric flask one adds the solute (a carefully weighed solid, or a liquid transferred by a volumetric pipet) and then fills the wide base with solvent. The solvent is swirled to dissolve and mix the solute. Finally, the neck of the flask is filled until the bottom of the meniscus is at the scribe mark. One then caps the flask and inverts it several times to completely mix the contents. The solution is then transferred to a bottle or other container for use. Volumetric flasks are not generally used for storage of solutions. Burets: A buret is long, narrow tube with a stopcock at its base. It is used for dispensing accurately known volumes of liquids or solutions. It is graduated in 0.1 ml increments, with the 0.00 ml mark at the top. Just before use, a buret should be "conditioned" to ensure that any water adhering to the inside walls is removed. Add ~ 5 ml of the liquid that is to be used into the buret. Tip the buret sidewise and rotate it so the liquid rinses the walls of the buret, then drain the liquid through the stopcock. The buret can now be filled with solution. Do this carefully and avoid trapping air bubbles in the tube. The liquid level can be above the 0.00 ml mark. Clamp the filled buret in place if this was not done prior to filling; it's some-times easier to hold the buret while filling it. Open the stopcock and drain enough liquid to fill the buret's tip. Have a beaker for waste solution handy for this and similar operations. There should be no bubbles in the tube or tip of the buret. These will lead to volume errors. If there are bubbles in the tube, carefully tap the 2

3 buret to free them. Use the stopcock to force bubbles out of the tip. It may be necessary to empty and refill the buret. When the buret is clean and bubble free, drain the liquid until the meniscus (the bottom of the curved surface of the liquid) is at or slightly below the 0.00 ml mark. It is not necessary to align the meniscus exactly at the 0.00 mark since the difference between the initial and final volumes is the desired measurement. If there is a drop of liquid clinging to the buret tip, remove it by gently touching the tip to a glass surface, such as the edge of the waste beaker. The volume of a drop is about 0.1 ml, the same volume as the buret's graduations. Find the bottom of the meniscus, and read the liquid level in the buret to the nearest 0.10 ml at that point. This will take a little practice. Remember, you're reading from the top down. Record this value as the initial volume. Although it's tricky to "read between the lines," remember that the last digit of a measurement is expected to have some uncertainty! Onefifth (1/5) of a division (0.02 ml) can be reproducibly estimated if the meniscus is between calibration marks, after a little practice. Now dispense the liquid you need. If you are using the buret to measure a set amount of liquid, determine what the final reading should be to obtain that amount. Dispense the liquid slowly into the receiving vessel. After closing the stopcock, catch any hanging droplet in the receiving vessel. It's part of the measurement at this point, so don't catch it in the waste container. Wait a few seconds for the meniscus to stabilize, then read and record the final volume to the nearest 0.01 ml. The difference between the initial and final readings is the volume you dispensed. When using a buret, it's easier to work with the exact volume dispensed than to try to dispense an exact volume. Plan your work with this in mind. Although burets are sometimes used as dispensers, they are far more frequently used in procedures called titrations. In a titration, one attempts to determine an equivalence point as exactly as possible. This usually involves the first persistent color change of an indicator. With a little practice, one can dispense fractions of drops (less than 0.1 ml) into the titration vessel, and reproduce results within 0.10 ml or less. Reading the buret and recording volumes is handled the same way as in dispensing liquids. When one is finished with a buret, drain the remaining liquid and clean it carefully. Finish with several rinses of deionized water, and make sure the stopcock and tip get their fair share. If solute dries on the buret, it can be very difficult to remove. 3

4 Pipets: Pipets are calibrated tubes that are used for transferring and measuring liquids. There are many types of pipets, some for specialized analyses and tests. They can be made from glass or plastic. The following types of glass pipets (plastic varieties are also available) are common: Volumetric pipets Volumetric pipets are used for dispensing known volumes of liquids. They are glass tubes that usually have an expanded midsection and a single scribe mark for the volume they are designed to deliver. Volumetric pipets are made in many sizes, from 0.5 to 200 ml. They are the most accurate pipets, delivering volumes to +/ ml or so, depending on the volume of the pipet. They are designed to drain with gravity, leaving a small amount of liquid in the tip. Do not blow or otherwise force this liquid out of the pipet. Measuring pipets Measuring pipets are graduated, usually to 0.1 ml. Typical sizes are 1 to 25 ml total volume. There are two types, serological and Mohr. Serological pipets are designed to be filled to the desired volume, then emptied completely (usually blown out). They are less accurate than volumetric pipets, because the tubes have larger diameters. However, they are more flexible than volumetric pipets because they can be used for a wide range of volumes. Mohr pipets are very similar to serological pipets except that they are not designed to be completely drained. One should inspect graduated pipets to determine which type they are before use. The photograph shows a serological pipet on top, and a Mohr pipet below it. There are many types of automated, semiautomated, and multiple-tipped pipets for specialized medical and biochemical uses. Most of these have plastic disposable tips. They cannot be used with organic liquids or with strong aqueous acids and bases. Glass pipets remain the workhorses for such applications. Pipets are filled by suction, usually from a rubber bulb or other pipet-filling device. 4

5 Glass pipets should be conditioned by rinsing them with a few milliliters of the solution to be measured before actual measurements are made. The liquid is drawn into the pipet (for a volumetric pipet, stop at the expanded part of the tube), then the pipet is tipped on its side and turned so the liquid coats the inner surface at least to the top graduation. The wash liquid is discarded. To measure liquid with a pipet, use the pipet bulb to pull liquid into the pipet until the liquid level is above the graduation. Use the index finger of your dominant hand to cover the top of the pipet. Liquid will not drain while your finger is in place. Wipe the tip of the pipet to remove any adhering droplets of the liquid in which it was immersed. Carefully allow excess liquid to drain from the pipet until the bottom of the meniscus is at the scribe mark. Now hold the pipet so its tip is above the vessel to be filled and allow its contents to drain. The amount of liquid that should drain from the pipet depends on its type, as described above. Cleaning of Volumetric Glassware: Most volumetric glassware is designed to be accurate to four significant figures. Obtaining that accuracy requires some care. Details on how to use various types of glassware have been given in the discussions above. Cleanliness and careful storage of volumetric glassware is general, and will be discussed here. Chemically clean glassware supports a uniform film of water, with no beads as one would see on a newly waxed car. Maintaining that level of cleanliness is fairly simple, but getting it back once it s gone requires work, and cleaning with fairly harsh chemicals. When you have finished using a piece of glassware, immediately rinse it three or four times with a small volume of distilled water. This will remove the chemical you measured, and leave no residue. If a piece of glassware is not supporting a uniform film of water, it should be set aside for cleaning. Your instructor will do this or show you how. 5