Lab 6. Report guideline to calibration lab

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Kennesaw State University *

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Chemistry

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Apr 3, 2024

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Lab 6. Calibration Methods and Introduction to UVVIS Name:_____________Avery Brothers_________________ Partner’s name:____________Anna Gray____________________ Prelab Questions 1. a. (5) You will prepare 1.000L of 100.0 ppm stock solution of Acetylsalicylic acid (ASA, from the pure powder) in a 2 (v/v) % HCl in this lab. (i) How many grams ASA will you weigh out? (ii) How many mL concentrated HCl will you measure out to prepare 200 ml of a 2% V/V HCl? Show your calculation. (i) Solute: 1 ppm = 1 mg 1 L 100 ppm = 100 mg 1 L x 1.00 L = 100.0 mg ASA 100.0 mg ASA = 0.1000g ASA (ii) HCl: 1.00L x 0.02 = 0.0200 L HCl 0.0200 LHCl x 1000 mL 1 L = 20.00 mL HCl b. (5) Describe exactly how you will make the 1.000 L of 100.0 ppm ASA. Dissolving the powder in small amounts of methanol sometimes helps to speed up dissolution. To make the 1.000L of 100.0 ppm ASA, rinse a 1.00L volumetric flask with deionized water and add 0.1g of acetylsalicylic acid to the flask. Add the deionized water until halfway full then add the 20.00mL of HCl. Mix until the solutions are dissolved completely. Once the solutions have dissolved, add deionized water to the calibration mark. Once all solution is added into the volumetric flask, invert the flask three times to mix properly. 2. (5) Generate dilution tables for your standards for the external calibration method using the stock standard solution in step (1), a 10.00 mL volumetric flask, and a micropipette with a range from 100.0 to 1000. µL. Use C 1 V 1 =C 2 V 2 . 1

0 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (0 ppm) (10.00mL) = 0.00 mL 4 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (4 ppm) (10.00mL) = 0.400 mL 6 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (6 ppm) (10.00mL) = 0.600 mL 8 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (8 ppm) (10.00mL) = 0.800 mL 10 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (10 ppm) (10.00mL) = 1.00 mL 15 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (15 ppm) (10.00mL) = 1.50 mL 20 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (20 ppm) (10.00mL) = 2.00 mL 100 ppm: (100 ppm) ( V ¿¿ 1 ) ¿ = (100 ppm) (10.00mL) = 10.00 mL 3. (5) Create a procedure you will use to prepare a 325 mg/tablet ASA in a 1 Liter volumetric flask and show how you will dilute it to end up with 10.00 ppm in a 10.00 mL volumetric using a micropipette. 325 mg 1 L = 325 ppm Unknown = (325 ppm) ( V ¿¿ 1 ) ¿ = (10 ppm) (10.00mL) = 0.308 mL 0.308 mL = 1000 mL 1 L = 308 L Break down the aspirin tablet and prepare the ASA and HCL solution in a 1.000L flask. We will substitute the aspirin tablet for the stock ASA solution. Add the 308  L of solution to a 10.00 mL volumetric flask using a micropipette. Add deionized water to the flask to the calibration line and mix well by inverting the flask three times. Experimental Results: Complete Table 1 by entering concentrations and absorbances of ASA (solutions 1 to 8) based on your stock standard solution and absorbance readings from the calibration spectra. State at what wavelengths the absorbances were read. Table 1. Calibration Data. All solutions are made up of 10 mL volumetric flasks in 2 % v/v HCl. Stock standard ASA solution = _____100____ ppm. 2

Solution # mL ASA used Concentration Absorbance 1 0.00 0 ppm 2 0.40 4 ppm 3 0.60 6 ppm 4 0.80 8 ppm 5 1.00 10 ppm 6 1.50 15 ppm 7 2.00 20 ppm 8 10.0 100 ppm 9 0.308 325 ppm 10 1.308 425 ppm Mg/tablet (ESM) Mg/tablet (SAM) Unknown: (325 ppm)(V) = (10 ppm)(10mL)  V = ____0.308_____ mL. Answer the following questions based on Table 1: 1. Plot the calibration curve using solutions 1 – 8. Remove any irrelevant points so you can obtain a good linear range (R 2 not less than 0.9700). See an example of a calibration curve in the handout. Graph of External Standard Calibration of ASA solution: 2. The vector Y = [0.0184, 0.0198, 0.0211, 0.0228, 0.0243, 0.0257, 0.0290, 0.0314] was taken from the 320 – 310 nm region of the background spectrum. Use these points to calculate the LOD of the instrument for ASA. Consult your textbook for LOD calculations using the calibration curve and the background points given above. (Show sample calculation here) 3. For each group, calculate the concentration of the unknown by the method of external standards using your calibration equation. (Sample calculation of unknown here, based on solution # 9, Table 1) 3

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4. For each group, calculate the concentration of the unknown by the method of standard addition using the equation: A o A t = C o ppm C o ppm + C s ppm In this equation, A o = absorbance of “unknown” solution, A t = absorbance of “unknown” plus the standard ASA added, C o = concentration of “unknown”, C s = concentration of the standard ASA in the 10 mL volumetric flasks. Note: if you weighed out 0.1054 g standard ASA and dissolved in 1 Litre flask, your stock standard solution is 105.4 ppm in ASA, not 100 ppm. Thus Cs in equation 1 is 10.54 ppm. Why? (Show calculation here) 5. For each group, bring your completed results for solutions 9 and 10 into the lab. There are about 5-7 groups in your class. At the beginning of the lab, before you start Lab 7 , you will complete Table 2 below as groups. Table 2. Pooled data for further analysis using Paired t-test (page 77, 9 th edition) Group # Conc. of Unknown by External Standards Conc. of Unknown by Standard Addition 1 2 3 “ n Mean Sx %RSD 6. Enter the mean, standard deviation, and % relative standard deviation. This is not all. Using a paired t-test as described in your textbook, are the two methods (external standard, standard addition) statistically the same or not? (Copy and paste your Excel spreadsheet calculations here) 4

7. Discuss the use of each of all three different calibration methods (external standards, internal standards and standard addition). Provide specific examples of situations where they would be most useful. A complete lab report will be submitted the following lab session, along with Lab 7. 5