The Analytical Balance

Transcription

1 Chemistry 119: Experiment 1 The Analytical Balance Operation of the Single-Pan Analytical Balance Receive instruction from your teaching assistant concerning the proper operation of the Sartorius BP 210S analytical balances that you will be using in the C119 laboratory. Learn what is meant by direct weighing and weighing by difference. With few exceptions, weighing of samples of chemicals must be performed by placing those materials in a vessel and obtaining the combined weight of the vessel and sample. The weight of the sample is obtained by subtracting the weight of the clean vessel. In the Sartorius balances, a taring mechanism exists by which the balance is adjusted to read zero mass when the clean vessel is on the balance pan. Then the mass of sample added to the pan can be read directly. (The analytical balance determines the mass of the object, i.e., the result is independent of the local gravitational field. The weight (gravitational force on the object) does depend on local conditions. For example, your weight on earth s moon would be much less than on the earth itself. Nevertheless, scientists tend to be fuzzy about this distinction, i.e., we say weight when we really mean mass ). For accurate weighing of corrosive materials, approximate amounts should be weighed first on a triple-beam or other auxiliary balance to protect the analytical balance from spillage and corrosive vapors. The procedure for weighing silver nitrate or iodine, for instance, is as follows. Weigh an empty vessel on an analytical balance (using a cover if the material is volatile). Record the weight. Take the vessel to a rough balance and weigh into it the approximate amount of material to be taken. Accurately reweigh the vessel plus material on the analytical balance. The vessel must not be handled directly between analytical weightings; use tongs, finger cots, or other protection. Note that accurate weighing of materials, the density of which differs greatly from that of the stainless steel weights (which have a density of 7.8 g/ml) that are used to calibrate the balance, will require a correction for air buoyancy. This correction can be as large as 10 mg, as is demonstrated in the pipette calibration which follows in Experiment 2. 1

2 Rules for Using an Analytical Balance 1. On the front of the balance are seven keys or push switches. Of these, only the key labeled / Ó and the two TARE keys at the extreme left and right are needed for operation of the balance in this course. The / Ó key is a Standby control. If the balance display shows a small O in the lower left corner, the balance has been set to standby and needs to be activated by pressing the / Ó key. Any deviations from the above should be reported immediately to the laboratory instructor. 2. With the balance activated and nothing on the balance pan, the reading should be close to g. To restore the readout to exactly g, press one of the two TARE keys. The readability of the balance is 0.1 mg. You should always record your weights to the nearest 0.1 mg. 3. You are now ready to weigh an object. Open the balance door and gently place the object on the center of the balance pan. Close the balance door while weighing an object to prevent air currents from disturbing the reading. Read the weight from the balance display. You may need to wait 5-10 seconds for the reading to stabilize. When finished, the operator should close the balance door to prevent dust and dirt from entering the balance. The balance may be left in the activated condition for use by the next student. 4. Only glass, ceramic, metal (nonreactive), or plastic objects and containers should be placed in direct contact with the balance pan to prevent corrosion. 5. Handle dried objects with tongs (or finger pads) not with bare hands to prevent grease and moisture from being added. 6. To be weighed accurately, all objects must be at room temperature. A warm object sets up convection currents inside the balance case, which will make an object appear lighter than it really is. Also, warm air inside a container is less dense than the air it displaces and this also leads to a negative 2

3 determinate error. The error can exceed 20 mg for a 50 g container! 7. Objects that possess a static charge may give erroneous weight readings. Waiting a few minutes may be sufficient for the charge to dissipate, or the object may be wiped with a faintly damp chamois cloth. 8. Volatile materials require special precautions, such as weighing in sealed glass containers, such as an ampoule. Even when weighed in conventional containers, the containers should be tightly covered to prevent loss and corrosion to the balance. 9. To prevent reading errors, record the weight in your notebook and then consciously read the number again and place a check next the recorded number to signify that you have verified the reading. An inversion of numbers, for example, will lead to an error in the analysis just as surely as any other mistake in procedure. 10. When weighing chemicals (dry solids) that you place in a container (vial or beaker), you should use the TARE feature to zero out the mass of the container. Then the weight you read is the actual mass of the chemical that has been added to the container. Under no circumstances should the chemical be added to the container while it is on the balance pan. Remove the container from the balance case, add the chemical, then return the container to the balance case and determine its weight. The reason for this stricture is to avoid spilling chemicals inside the balance case. 11. The balance and case should be kept scrupulously clean. A camel's hair brush is useful for the removal of dust or spilled material. 12. Under no circumstances should a balance be adjusted without prior consent from the instructor. Any balance that is not level (bubble indicator) or that cannot be zeroed should be reported immediately. 3

4 Laboratory Exercise: Direct Weighing of Pennies In this experiment, we will practice the use of the analytical balance by weighing an inert material, such as a penny. We will try to determine the average weight of a penny from several weighings. Some experience with simple statistics will also be included. Apparatus: 210-g capacity Sartorius BP 210S analytical balance 2, 100-mL beakers 10 new pennies drying oven (110 o C) tongs or forceps Chemicals: 95% ethanol, 10 ml per student Procedure: 1. Obtain a clean, dry 100-mL beaker. Weigh this to ±0.1 mg after zeroing the balance carefully. 2. Add 10 clean pennies to the beaker, and add just enough 95% ethanol to cover the pennies with liquid. Swirl and pour off the liquid into another clean beaker. 3. Place the beaker in a 100 o C oven for 5 minutes (no more!), then let it cool to room temperature. Weigh each of the 10 pennies (in the 100-mL beaker), taking care not to touch them with your fingers. 4. To obtain some idea of the reproducibility of weighing a given object, determine the mass of one of the pennies five times. 5. Weigh the beaker and all 10 pennies together. Calculations: 1. Calculate the mean and standard deviation for weighing the single penny five times. Compare the reproducibility and readability of the analytical balance. 4

5 2. Calculate the weights of each penny and the weight of all ten pennies. Then calculate the mean, median, standard deviation and confidence interval at 90 and 95% confidence for the ten penny weights. Calculate the relative standard deviation by the following formula: RSD(%) = s 100 x 3. Calculate the fraction of pennies having weights within 1 and 2 standard deviations from the mean. 4. Add the 10 penny weights, and compare with the measured total weight. Do these agree? If not, explain why there is a discrepancy on your report sheet. Report: 1. Answer all questions given above. 2. Report the estimate of the mean weight of a your group of pennies, with standard deviation, relative standard deviation and 90 and 95% confidence intervals. 3. Report the sum of weights of the ten pennies and the weight of all ten pennies taken together. 4. Comment on any discrepancy between the measured total weight and sum of the individual weights. 5. Assume that your mean penny weight and standard deviation are the same as the mean weight and standard deviation (σ) of the population of all existing new pennies and that this population of weights is normally distributed (by the way, there is no basis for the truth of such an assumption). What are the chances that you will find a rich penny, one that exceeds the mean weight by 10%? 6. Pennies are constructed principally of zinc (with a thin copper coating). Assuming that the average density of a penny is identical to that of pure zinc, calculate the mean volume of a penny. Compare this to the volume of the same mass of pure copper. The periodic table of the elements should give you an idea of why these two volumes are very similar. Explain. This experiment has been adapted from a laboratory manual authored by Professor S. D. Brown. Last revision: 8/22/97. 5