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Chemistry

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Apr 3, 2024

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Balances and Pipet Calibration During the first laboratory period, you will familiarize yourself with the lab, the equipment, and other small tasks. Here is a checklist of what you should have accomplished by the end of the lab period. • Check locker contents • Obtain and start drying your soda ash and (Chem 2400 only) KHP unknowns • Obtain dried sodium carbonate from oven • Perform the balance assignment and familiarization exercise • Calibrate your 25.00 mL pipet • Practice reading a buret • Verify and change the desiccant in your desiccator if necessary. • Check the seal on your desiccator, lubricate if necessary. • Have your TA initial the results in your lab manual. These can be performed in any order, but your TA will need to give the safety talk and assign lockers first and initial your lab manual last. Operation of the Balances

The major difference between the top loading and analytical balances is that the analytical balance has greater sensitivity. Sensitivity is defined as the smallest increment of mass that can be measured. On an analytical balance it is usually 4 decimals or more. Use the top loading balance to prepare approximate solutions, and the analytical balance to accurately weigh your limiting reagents, primary standards, and unknowns. As a rule of thumb, anything that is dried and stored in your desiccator is weighed on the analytical balance. Warm up As long as the balances are plugged in for at least 60 minutes (and they are), they are warmed up and ready to use as soon as you turn them on. It is good lab practice to turn them off after use. Operation of the top loading balance To turn the balance ON, press the on/off button (upper left). To zero (or tare) the balance, press TARE. To turn the balance OFF, press the on/off button. Operation of the analytical balance The description of the analytical balances given in this manual refers to the METTLER TOLEDO electronic balance model MS-TS. Exact horizontal positioning is required for repeatable results. Verify that the level indictor bubble

located at the back of the balance is centered. You can adjust the leveling feet (at the rear of the balance) until the air bubble in the indictor is centered. • To turn the balance ON, press O/T. • To zero (or tare) the balance, press O/T. • To turn the balance OFF, press and hold O/T until the display indicates OFF. Calibration The balances have internal weights and do not require outside calibration. Although you will not be calibrating the balances, you will verify the calibration of the balance using a calibrated metal block. 46 Weigh this mass at the beginning of the semester and whenever you want to verify the accuracy of the balance. Let your TA know immediately if you balance does not weigh reproducibly within 0.0002 grams. Your TA may initiate an internal calibration routine on your designated balance. Weighing objects The glass enclosure around the weighing pan prevents drafts from affecting the readings. After placing a dry, room-temperature object on the center of the weighing pan, close the glass doors. Wait until the

stability indicator “o” disappears and take the reading. Hot objects within the glass enclosure cause convection currents that produce erratic readings. Moreover, the light hot air displaces heavier cool air causing the balance to read lighter than it should. On the other hand, water from damp or wet objects evaporates causing the readings to drift towards lighter readings. Maintenance If you spill chemicals on the balance that cannot be removed with the small brushes provided, let you TA know immediately. Preventing corrosion of the balance is essential to the longevity of the balance. Analytical Weighing Techniques Significant digits Analytical chemists collect the maximum number of significant digits available. In the case of the analytical balances this is to the nearest 0.0001 gram (0.1 mg). If you have too many significant digits, you can always eliminate some later. Weighing by difference This technique is recommended for weighing hygroscopic samples (samples that absorb water) or any dry samples. Use this technique for all primary standards and unknowns. First weigh the vial containing your previously dried reagent, then tap some reagent from the vial into a receiving flask and weigh your

vial again. The difference in weight is the weight of your reagent. Normally you would keep the cap on for this process to minimize exposure to the atmosphere and to avoid spilling your sample inside the balance. Unfortunately, the caps we use have a paper liner which is difficult to keep at constant weight. Luckily none of our reagents are particularly hygroscopic so we weigh with the lid off. Fingerprints Fingerprints can add mass to an object by more than 1 milligram, particularly if your hands are moist or greasy. Handle the vials or other object to be weighed on an analytical balance with a paper towel or a clean tissue. Questions (Self-test, not for marks) 1. Why must the balance be level? 2. Why must the outside of objects taken into the balance room be free of all water droplets? 3. Why must an object to be weighed at room temperature? 4. Why should the vials be capped when they are on the balance? 5. How do we weigh by difference?

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