Páginas: 12 (2849 palabras) Publicado: 26 de diciembre de 2011
Precision and Accuracy of Glassware
Bring your ThinkPad to lab today! Background: One of the most common measurements in the chemical laboratory is a liquid volume measurement. The most common graduated or marked glassware (i.e. glassware with volume markings on them) used to hold liquids include: beakers, Erlenmeyer flasks, volumetric flasks, graduated cylinders, pipettes, and burettes. Eachone has its advantages and disadvantages; and the objective of today’s lab is to identify some of them, especially as they pertain to accuracy of the graduations and markings. There are two subtly different considerations in measuring liquid volumes with graduated or marked glassware. In one case one measures a prescribed amount of a liquid which is to be added to another vessel. In the other caseone wants to know the volume contained in the piece of glassware, as when making a solution. Glassware used to dispense particular volumes is said to be calibrated “to deliver” or TD. There will necessarily be a little liquid left in the vessel after the liquid is poured out. The amount of this remnant must be fairly consistent because the graduations must account for it. For example a graduatedcylinder calibrated TD and filled to the 10mL mark will actually contain a little more than 10mL, but the amount that comes out when it is emptied should be very close to 10mL. Calibration “to deliver” is common for graduated cylinders and volumetric pipettes. One has different concerns when making solutions. In those cases one is interested in the amount of liquid in the vessel holding thesolution, since that will determine the volume associated with a particular amount of solute. The most common type of vessel for making solutions is a volumetric flask, and it is said to be calibrated “to contain” or TC. For example, a 50mL volumetric flask may be used to make a 1.000M solution by adding 0.05000 moles of solute to the flask and, after dissolving, filling the flask to the 50.00mL mark. Theamount that can be poured out will be slightly less than 50mL since some small amount will remain with the flask, but the concentration of what was poured out will be exactly 1.000M since there were exactly 0.0500 moles of solute and 50.00mL of solution in the flask when the solution was made. Attempting to make such a solution in a 50mL graduated cylinder calibrated TD would yield a solutionwhich was slightly less than 1.000M since the actual volume in the filled cylinder would be a small undetermined amount more than 50mL; but the amount of solution that could be poured out would be close to 50mL. Consequently one shouldn’t make solutions in graduated cylinders because one has limited control over the actual volume in making the solution – not to mention the fact that stirring anddissolving are quite problematic in long skinny vessels like that. Clearly the amount of liquid that remains in TD glassware when liquid is poured out is an important quantity. It will depend largely on the design of the piece, i.e. the shape of the spout or the diameter and length of any holes; but it will also depend on factors that affect

how liquid sticks to glass. Specifically, liquids willtypically stick more to dirty glassware, and the dirtier the glassware, the more liquid will stick. The calibration of TD glassware presumes the glassware is clean. So, in addition to the general virtues of cleanliness, one wants to keep glassware as clean as possible in order to get the most accurate and reproducible volume measurements. Cleaning glassware can be done at several different levels.The most common and simplest technique involves simply scrubbing with a brush and soapy water, followed by copious rinsing. One can also soak glassware in solutions that are likely to attack whatever may be stuck to the glass; such solutions commonly include Cr(VI) or Na2S2O8 in H2SO4 which are very strong oxidizing agents, or KOH dissolved in ethanol which is good on most residues except metals...
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