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Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 1 Calibration of Glassware Calibration and determination of error in volumetric glassware Objective Students will calibrate a beaker, buret , and volumetric pipette and also determine the error associated with measuring volumes using a micropipette. Logistics The duration is two week s , and students will work individually. The spreadsheet with calculations for error analysis must be submitted to Canvas within one week after you complete the lab . Before lab, prepare your notebook according to the Lab Policies course document o n Canvas . Introduction When we calibrate a piece of volumetric glassware, we are looking to verify the agreement between the claimed volume and the actual volume. If agreement is lacking, a correction is determined. A calibration consists of determining the mass of a liquid of known density (water in this case) that is contained in (or delivered by) a piece of volumetric glassware. From the relationship for density (d = mass/volume), we can determine the exact volume occupied by the liquid. Because the density of a substance varies with temperature, we must measure the temperature of the liquid to correct th e value of d us ed. Table 1 contains densities at various temperatures. Temperature (C) Density (g/mL) 16 0.9989460 17 0.9987779 18 0.9985986 19 0.9984082 20 0.9982071 21 0.9979955 22 0.9977735 23 0.9975415 24 0.9972995 25 0.9970479 Table 1 The density of water at various temperatures. From â€œQuantitative Chemical Analysisâ€ by Daniel C. Harris, 2010
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Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 2 In this experiment, we will calibrate three pieces of glassware: A) a 50 mL beaker; B) a 10 mL Class A volumetric pipet; and C ) a 50 mL buret. We will also learn to use and check the calibration on an adjustable 100 1000 L micropipe tte. This is an applied laboratory skill that is worth spending time becoming proficient at. We will check the calibration of the micropipette by calculating the inaccuracy and imprecision . Inaccuracy (A) compares the average volume measured ( ) to the desired volume being dispensed (). Inaccuracy is a measure of systematic error. It can be calculated as the absolute value, A, and as a relative value, A%, as shown below. Imprecision is a measure of random error and helps us understand the repeatability of pipettings. Imprecision can be calculated as standard deviation (S) and as the re lative value called the coefficient of variation (CV%) as shown below. Procedure Read before beginning any part of this lab: Obtain some deionized water in a large clean beaker to be used for all parts . Record the temperature of the water to the correct number of significant figures . Use the data in Table 1 to create a linear trendline, and use the equation of the line to find the density at your exact temperature. Note: you should add significant figures to the equation of the line before using it to calculate the density. You will need 6 decimal places. Obtain a 100 mL beaker, a 50 mL buret, and a 10 mL Class A volumetric pipet. Check these items for cleanliness; water should drain uniformly while maintaining a "wetted" appearance after draining. Any breaks or nonwetting signifies that residues are present which will decrease the precision and worsen the performance of any calibration. If this is the case, you should thoroughly wash the glassware with soap and rinse several times. A. 100 mL Beaker calibration You will det ermine the accuracy at which a 100 mL beaker is able to measure 10 mL of water . For a total of three trials, record the initial and final mass of the beaker bef ore and after the addition of 10 mL of water. Measure the temperature of your water to the correct number of significant figures. Use the density from your trendline to determine the corrected volume of water at each of the three masses that you determined. Then calculate the average, standard deviation, % RSD, and 95 % confidence interval. Be sure these calculations are formatted with the appropriate equation in Excel.
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Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 3 B. Buret calibr ation You will calibrate the buret at the following volumes: 10, 20, 30, 40, and 50 mL. Additions will be performed in increments of approximately 10 mL and should be reported with the correct number of significant figures . Liquid dispensed from the buret will be collected in a 100 mL beaker . For each of the volumes, you should record the initial and final volumes read from the buret as well as the initial and final masses. Do not empty the flask between additions! Repeat so that you have data for two tr ials . Use the masses measured for each addi tion and the density to determine the volume of water (V). Then calculate the correction from the difference of the measured volume (Vtrial) and the volume calculated from the density (V). Consider the sam ple data in Table 2 for the first four additions. Trial V i V f V trial M i M f M H2O V H2O Correction 1 0.00 10.00 10.00 58.77 68.69 9.9134 9.9312 0.07 2 10.00 20.00 10.00 68.69 78.62 9.9330 9.9508 0.05 3 20.00 29.90 9.90 78.62 88.54 9.9224 9.9402 0.04 4 29.90 40.00 10.10 88.54 98.54 10.0009 10.0189 0.08 Table 2 . Example data from the buret calibration. The correction is calculated from the difference in volumes Vtrial VH2O. Prepare a calibration curve by plotting the correction vs. total delivered volume for both trials. To obtain the xaxis values, you will need to create an additional column that calculates the running total for the volume. For example, the third data point for Trial A appears at 29.90 mL, not 30 mL. Figure 1 . Plot of the error in predicting the volume dispensed as a function of the total volume. C. Volumetric pipette calibration On the toploading balance, r ecord the mass of the 50 mL beaker that you will use to weigh your dispensed volume of water. Use the same balance as often as possible. Dispense the 10 mL delivered volume into the beaker , and record the mass . Rep eat for a total of three trials, making sure to discard the water between trials and to record a new initial mass each time. Use the temperature of your water along with data in Table 1 to determine the corrected volume of water at each of the three masses that you determined. Then calculate the average, stand ard deviation, % RSD, and 95% confidence interval. Be sure these calculations are formatted with the appropriate equation in Excel.
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Spring 2021 CHE 3010 Palmer This wor k by Alycia Palme r is licensed under CC BY 4.0 4 D. Micropipette calibration Obtain about 100 mL of nanopure water and record the temperature to the correct number of significant figures. Determine the density for water at that temperature using your trendline from the data in Table 1. Check out a micropipette from the instructor/ TA and write the serial number in your notebook . Dial the pipette to 1000 L and transfer one pipetting to waste in order to wet your p ipette. Then take the mass of the next pipetting to four decimal places. Repeat this for a total of ten trials , making sure to use the technique demoed by the instructor . Check with the instructor to m ake sure your masses look correct, then repeat this process for the 100 L volume. Convert each mass to volume in units of L using the density you determined. Use Excel to calculate the A% , standard deviation, and CV% for the 1000 L and 100 L volumes. Compare your values to the ISO standards in Tables 2 and 3 . Do your values match the required criteria? You must have the following table s in your notebook (in addition to your data tables ) . If your tolera nces are not within th ese specifications, you should consult the instructor. Table 2 . Relative Inaccuracy %A Volume Tolerance Your value Is the c ondition met? 1000 L 0.8 100 L 8.0 Table 3. Coefficient of Variation %CV Volume Tolerance Your value Is the c ondition met? 1000 L 0.3 100 L 3.0 Spreadsheet Assignment Download the spreadsheet template from Canvas. Each piece of glassware should have a separ ate sheet (one for the beaker, one for the buret, and one for the volumetric pipette ). The completed spreadsheet should be submitted to Canvas before the start of the following lab period. You should also have a sheet to determine the %A and CV% for each volume tested . Conclusion Your conclusion should be printed and submitted to Canvas before the next lab period. Format your typed conclusion to tell the purpose of the lab and list your results. Be sure to discuss the following. Discuss the volume of greatest uncertainty for the buret . D id you have the most uncertainty when trying to dispense 10, 20, 30, 40, or 50 mL? For example, in Figure 1, the data suggest this buret is very inaccurate at dispensing 50 mL but is accurate at dispensing 30 mL. Give the measured volumes for the beaker and volumetric pipette ( report as average standard deviation) . Which had the lowest %RSD and does this make sense? Summarize your micropipette calibration results. Do your results conform to the manufacturerâ€™s specifications? You may wish to copy Tables 2 and 3 from above to su mmarize your results. Discuss your error by c ompar ing your results with the tolerances listed in the tables in Chapter 2 of the Harris textbook for the volumetric pipette and buret . R eferences should be cited at the end of your conclusion using ACS specifications.
