General Rules for Volumetric Glassware
The assumption of volume (either delivered or contained) for volumetric glassware is dependent on the cleanliness of the surface. Although this is more important for TD (to deliver) glassware, we must also be aware of the considerations of 'gunk' on the surface of TC (to contain) glassware as well.
The best insurance for accurate volumes is simply to insure that your glassware is clean. For burets and pipets, cleanliness of the glassware is indicated by the absence of any 'water beads' on the inside surface of the glassware. When the item is clean, the solution will exist in a thin, unbroken film on the inside of the glassware.
Cleaning Volumetric Glassware
Typically, a brief soaking in a warm detergent solution will be enough to clean pipets and volumetric glassware. You should avoid soaking the glassware for too long, as if the glassware is left for too long with the detergent solution a rough area is likely to develop at the glass/air interface, which may destroy the usefulness of the equipment. After a brief (2-3 min.) soaking, the glassware should be rinsed thoroughly with tap water, and finally 3-4 rinsings with distilled or deionized water should be done.
A buret can be easily cleaned by using a detergent solution and a 'buret brush', a long handled brush that is designed to be able to reach the bottom of the buret. Rinse the buret thoroughly with tap water, and follow with 3-4 portions of distilled or deionized water.
Another check that you must do for the buret is a leak check. Fill the buret with water, and check for leaks around the stopcock (the valve at the bottom of the buret that you use to release the reagent. Allow the buret to stand for several minutes, and make sure that the indicated volume remains constant. If you find a leak check with the instructor for further instructions.
The proper procedure for delivering the indicated volume for a pipet is relatively straightforward. First, using your pipet aid, draw enough liquid into the pipet such that the level is well past the 'mark' on the neck of the pipet. BE SURE TO AVOID DRAWING REAGENT INTO YOUR PIPET AID! Remove the pipet aid and quickly cover the end of the pipet using your forefinger. At this point make sure there are no bubbles in the bulk liquid, or foam at the reagent/air interface. Wipe any reagent off of the exterior of the pipet at this time. This is easier if you tilt the pipet slightly while wiping. Find a 'waste' beaker, and slowly drain the pipet to the calibration mark while the tip of the buret is in contact with the wall of the waste beaker. Then, carefully move the pipet tip to the receiving vessel, and allow the liquid to drain. When the pipet appears to be drained, touch the tip of the pipet against the wall of the receiving container for at least ten seconds. If this is done properly, there should be a small residual volume in the pipet tip. THIS IS EXPECTED, and the pipet has been calibrated for this!
To calibrate your 25 mL pipet, use a clean pipet and water for which the temperature is known and constant (
temperature-equilibrated water). Using an analytical balance, weigh a clean 125 mL Erlenmeyer flask
with a rubber stopper in the neck and record the mass. Deliver a known volume into the flask, stopper, and reweigh. Using the
following table, calculate the volume of water delivered from the pipet based on the mass of water delivered and the volume corrected to 20OC. Perform this operation at
least 10 times, and calculate the mean and standard deviation for the volume delivered. [NOTE: It is not necessary to scrupulously dry
the flask each time, but it IS necessary to obtain a new initial weight, as some of the water from the previous calibration may adhere to
the inside of the Erlenmeyer flask.]
Table 1. Volume occupied by 1.000 g of water weighed in air against stainless steel weights.
|Temp. OC||Volume @ T||Vol. corrected to 20OC|
Using temperature-equilibrated water, fill the buret past the 0 mL mark (be sure to record the temperature), insuring that no air bubbles are trapped in the tip. Give the water about a minute to drain, and then allow the water to drain to the 0.00 mL mark in your waste beaker. Touch the side of the tip to the beaker to remove any water sticking to the tip. Replace the waste beaker with the weighed 125 mL Erlenmeyer flask, (analytical balance) and slowly transfer 10 mL of water to the Erlenmeyer flask (this should take about 1 minute.) Reweigh the flask (again using the analytical balance) and calculate the volume delivered as you did for the pipet. Repeat this procedure at least 2 more times, or until your delivered volumes are ±0.02 mL. Once you get an appropriate agreement between the 10 mL values, perform the same procedure for 20, 30, 40, and 50 mL. Remember to allow the buret to drain at about 10 mL/min for all of these determinations!
ALTHOUGH WE ARE NOT PERFORMING THIS CALIBRATION FOR THIS EXPERIMENT, if you
were to calibrate a volumetric flask, it must be completely dry. Fill the flask to the calibration mark with temperature-equilibrated water, and
determine the mass delivered by comparing the empty mass to the full mass, calculating the volume as in the previous procedures.
[NOTE: Since the volumetric flask will be 'wet' after this procedure, and since we do NOT dry volumetric glassware in the oven, you
will only be able to perform this calibration once per session.]
For your report, fill out the report form that is provided.