Basics Review and Calibration of Volumetric Glassware
There are three types of containers used in lab to contain or deliver liquids: volumetric, ordinary, and disposable glassware. Volumetric glassware are containers that have been calibrated at a specific temperature to deliver or contain VERY PRECISE amounts of liquid. Examples of volumetric glassware that we will use include burets, pipets, and volumetric flasks. Ordinary glassware has less precise volume calibrations and are used whenever the volumes do not have to be measured as accurately. Examples include beakers, Erlenmeyer flasks, and graduated cylinders. Disposable glassware (or plastic ware) is used to transfer or hold liquids temporarily and may not contain any volume markings. Examples include medicine droppers and disposable pipets. In this experiment you will be exposed to a variety of important concepts related to quantitative experimentation, including the proper use of glassware, analytical balances, and statistics. You will calibrate a volumetric pipet or flask, that is, you will experimentally determine what volume a pipet or flask really delivers. You will also calibrate a beaker or Erlenmeyer flask and a 50 mL buret. You will use these calibrated values throughout the remainder of the quarter. Construct a table at the front of your laboratory notebook to record calibration values or tape in the calibration tables from Chapter 2 of Harris’ 7th edition.
How to read the level of liquid in glassware:
The “meniscus” of a liquid refers to the curvature of the liquid surface in a narrow container. If the surface of the liquid is concave (as in water), the liquid level is measured using the bottom of the meniscus. If the surface is convex (upward curving) as in mercury, the liquid level is read from the top of the meniscus. It is important to position the eye at the same level as the meniscus to avoid a parallax error. Sometimes it is easier to use an index card marked with a dark line or piece of black electrician’s tape behind the glassware to contrast the line between the glassware markings and the liquid level. If the liquid level falls between the markings on graduated glassware, the student must estimate the last, uncertain digit in the recorded volume measurement. Example: some of our graduated cylinders have individual milliliter markings so a reading must be to the tenths of a milliliter (10.4 mL).
Care of glassware:
Glassware should be washed with a mild soap solution, rinsed with tap water, and then rinsed with distilled water. If beads of water stick to the inside walls, it be should be cleaned again. If glassware is to be dried, allow it to drain or use paper towels. NEVER SLING GLASSWARE TO REMOVE WATER. NEVER TAP GLASSWARE AGAINST THE SIDE OF THE SINK. A buret or pipet should be rinsed with a small amount of the solution to be used to remove any water droplets from the glass walls. Burets: Burets must be read to the hundredths of a milliliter. Note that the liquid level markings begin at the top or open end. This is because the buret is designed to deliver liquids. All volumes are measured as the DIFFERENCE between an initial and final reading. Chem321 Exp. 1/Calib. of Glassware. Page 1 of 8
Pipet: The volumetric pipet is designed to deliver a single, fixed volume of liquid at a specific temperature. A graduated pipet is used in a similar fashion to the buret to deliver specific amounts of liquid. Remember to touch off the drop of liquid hanging outside the tip into the transferred liquid (it is part of the delivered volume) but DO NOT BLOW OUT ANY LIQUID THAT REMAINS INSIDE THE TIP OF THE PIPET. The pipet has been calibrated to contain this last drop of liquid.
The Analytical Balance
The user of an analytical balance should know how to determine whether it is operating properly and how to make the simple adjustments that are occasionally necessary. A brief overview of analytical balances can be found in...