Chemist in Laboratory III (Re-run) SCIS2123A

Transcription

1 Department of Chemistry, HKUST The Hong Kong Academy For Gifted Education Programme of: Chemist in Laboratory III (Re-run) SCIS2123A Lab Manual Prepared by: Dr Joanne W T Tung Lab 7161 (Lift 33) th December, 2012 (10 am 5 pm) 1

2 Basic Safety Manual 1. Personal Safety 1.1. Safety goggles must be worn at all times in the labs. Contact lenses should not be worn in the lab because chemicals and particulates can get caught behind them, causing severe eye damage Lab gown must be worn in the labs where an unexpected chemical spill may expose you to the risk of injury. The following clothing is not permitted in the labs unless covered by protective clothing: Open-toed shoes, sandals or other uncovered footwear; clothes that expose above the ankles; Eating, chewing gum, and drinking in the lab. Untied long hair, dangling jewelry, loose clothing, and anything else that may get caught in equipment, or dipped in chemicals Never work alone or unsupervised in the labs. Work only during the scheduled laboratory periods and perform only authorized experiments. Wash your hands, arms, and then face, with soap and water as soon as possible after leaving the lab. If you are uncertain about any safety aspect of an experiment, please ask your TA. 2. Lab Safety 2.1. Make sure you know the exact locations of the safety features of the lab; e.g., eyewash fountains, safety showers, chemical spill kits, fire extinguishers, fire alarms, fire blankets Whenever possible, do not deal with incidents on your own. Your TA, the lab instructor and the technician are all trained to respond to the sort of incidents that may occur in this lab; e.g., chemical spills, cuts, burns, fires, medical emergencies, etc Keep your work area clean and organized to reduce the possibility of accidents. Know what you are doing and don't be careless. 2

3 2.4. Avoid unnecessary exposure to chemicals. Never pipette by mouth. Never taste or inhale a chemical on purpose. Wear gloves when directly working with hazardous chemicals. Use hoods when appropriate Take appropriate precautions. Keep flammables away from hot plates and open flames. Wear gloves when using toxic, carcinogenic, or other hazardous chemicals. Take care with corrosive acids and bases. Always pour concentrated acid slowly into water (never water into acid). Read the Safety Issues section at the beginning of each experiment Be informed. Material Safety Data Sheets (MSDS) summarize known hazards associated with every chemical are available from the lab. Chemicals & equipment may not be removed from the lab without permission from the Lab Instructor. 3

4 Experiment 1: Determination of Free Available Chlorine Content in Swimming Pool Water by Colorimetric Analysis This experiment is designed for 4 students in a group. Each group is given one sample. Reference Harris D C, Quantitative Chemical Analysis, 8 th Edition, Chapter 17-19, W. H. Freeman and Company, Objective To determine free available chlorine content in swimming pool water using a colorimeter sensor. To determine the precision of the result. Theory In disinfection, gaseous chlorine (Cl2) or liquid sodium hypochlorite (bleach; NaOCl) is added to, and reacts with, water to form hypochlorous acid (HOCl). Hypochlorous acid kills microorganisms by oxidation. When these chlorine compounds are added into swimming pool, water is sterilised by the formed hypochlorous acid. The disinfection power refers to the amount of free available chlorine in the water sample, this value takes into account of the contents of hypochlorous acid (HOCl) and hypochlorite ion (ClO ). After adding bleach in water, sodium hypochlorite rapidly hydrolyses to hypochlorous acid [in reaction (1)], also chlorine to hypochlorous acid [in reaction (2)]: NaOCl (aq) + H 2 O (l) Na + (aq) + HOCl (aq) + OH - (aq) (1) Cl 2 (g) + H 2 O (l) HOCl (aq) + HCl (aq) (2) The hypochlorous acid is a weak acid that can dissociate to hypochlorite ion at relatively high ph: HOCl (aq) H + (aq) + OCl - (aq) (3) Both HOCl and OCl - exist at the ph range between 6.8 and 8.5. The ph of swimming pool water is around 7.8. Oxidation of Iodide ions by Free Available Chlorine In this experiment, potassium iodide (KI) solution in the presence of acetic acid (ethanoic acid) is used to generate HOCl from the swimming pool water sample as below: NaOCl (aq) + H + (aq) HOCl (aq) + Na + (aq) (4) 4

5 Then, the HOCl and OCl - oxidise KI and form iodine, as below: HOCl (aq) + 2 KI (aq) + H + (aq) I 2 (s) + KCl (aq) + H 2 O (l) + K + (aq) (5) OCl - (aq) + 2 KI (aq) + 2 H + (aq) I 2 (s) + KCl (aq) + H 2 O (l) + K + (aq) (6) Stochiometrically, one mole of HOCl oxidises two moles of KI to form one mole of iodine [in reaction (5)], so does OCl - [in reaction (6)]. The formed iodine will further react with KI to form one mole of triiodide (I 3 - ) ion [in reaction (7)]. I 2 (s) + I - (aq) I 3 - (aq) (7) Therefore, the mole ratio of HOCl [in reaction (5)] or OCl - - [in reaction (6)] to I 3 ion [in reaction (7)] is 1:1. KI added has to be in excess because it provides the iodide ion (I - ) in reaction (7). Also, triiodide ion itself is brown in colour. The intensity of the colour is directly proportional to the concentration of it higher the concentration of I - 3 ion formed, darker the intensity of brown colour it will be. Thus, by measuring the intensity of the brown colour, it can tell the concentration of triiodide ion formed, and that can reveal the concentration of OCl - ion present in the swimming pool water sample. Colorimetric Measurement The intensity of the colour of the iodine formed can be a measure of the concentration of the free available chlorine ([HOCl] + [OCl - ]) in the swimming pool water sample. The higher the intensity of triiodide ion formed, the higher the concentration of it will be. So, simply measuring the intensity of the resulting solution after reactions, the concentration of total content of free available chlorine ([HOCl] + [OCl - ]) would then be calculated via the calibration curve established using the standard solutions in known concentrations. A colorimeter mimics the way of human eyes to compare colours, it installs one (or a few) filter(s) to isolate wavelength(s) before the cuvette to measure the transmitted light intensity, as illustrated in the following diagram. The principle of calculating the concentration of the analyte from the light intensity is listed on the next section. The main difference between colorimeter and spectrophotometer is in the device used for the wavelength selection. For colorimeter, it uses absorption filter(s), while spectrophotometer uses grating, prism, or other sophisticated devices for wavelength selection. Spectrophotometer requires narrow wavelength selection at a specific time. 5

6 Figure 1. Basic component of colorimeter. Beer s Law To measure the amount of light that is absorbed by absorbing species in cuvette, we must compare the original power of light (P o ) with the power of light that is transmitted through the cuvette (P) (as in Figure 2). Figure 2. Illustration of transmittance (T). The fraction of transmitted light (transmittance) over the original power is of the relationship of the equation below. Such fraction is termed as transmittance. where the initial radiation power is P 0, and the power passes through the cuvette is P. The transmittance above is however in an exponential relationship with the concentration of the absorbing species at a particular wavelength. This does not allow quantitative determination. Another parameter was then established by Beer. Beer worked on the Lambert derivation on transmittance relationship and derived another term, absorbance (A), as shown below. This is called Beer s law and also called Beer-Lambert law. 6

7 A = log T Absorbance is in inverse relationship with transmittance but linearly relates with concentration of absorbing species in the cuvette at a particular wavelength, as follows: A = εbc where A = absorbance of measured solution ; ε = molar absorption coefficient (L mol -1 cm -1 ); b = pathlength of sample (cm); c = concentration absorbing species in the cuvette at a particular wavelength (mol L -1 ) By plotting a graph as absorbance (A) versus concentration of the absorbing species (c), one can obtain the concentration of the analyte in an unknown sample. Such a curve is called Calibration Curve. For details, please refer to lecture notes. Materials Chemicals 1. 2 wt% potassium iodide solution 2. 5 wt% ethanoic acid solution in water wt% sodium hypochlorite (NaOCl) solution (bleach solution), ppm (part per million) 4. Sample of swimming pool water 5. Deionised water Apparatus 1. Beaker (50 ml 4; 600 ml 1) 2. Pipettes (1 ml 2; 2 ml 2; 3 ml 1; 5 ml 1) 3. Volumetric flasks with stoppers (50 ml 7) 4. Datalogger (colorimeter sensor with 468 nm wavelength filter) (x10) 5. Desktop or notebook computers (x10) 6. Cuvettes (x10) 7

8 Chemical Hazards Chemicals Hazard Note(s) potassium iodide - harmful if swallowed - may cause serious eye and skin irritation ethanoic acid - flammable liquid and vapor - corrosive - may cause severe skin burns and eye damage sodium hypochlorite - corrosive - may cause severe skin burns and eye damage - very toxic to aquatic life Information source is: MSDS, Sigma-Aldrich, available from: 8

9 Procedure A. Preparation of Standard Solutions 1. Pipette 1 ml of sodium hypochlorite (NaOCl) stock solution to a 50 ml volumetric flask and dilute to the mark with deionised water. Label it as diluted standard solution. 2. Label five 50 ml volumetric flasks as: Flask A, Flask B, Flask C, Flask D, and Flask E. Figure 2. Preparation of a five-point standard solution for calibration curve. 3. Pipette the volume of the dilute Standard solution into each of the volumetric flask according to the following table: Chemicals Flask A Flask B Flask C Flask D Flask E Diluted standard solution 0 ml 1 ml 2 ml 3 ml 5 ml 4. In a fume cupboard, individually add 5 ml of 5% (w/w) ethanoic acid into all volumetric flasks with dispenser. Chemicals Flask A Flask B Flask C Flask D Flask E 5% (w/w) Ethanoic acid 5 ml 5 ml 5 ml 5 ml 5 ml 5. In a fume cupboard, individually add 5 ml 2% (w/w) KI into all volumetric flasks with dispenser. Chemicals Flask A Flask B Flask C Flask D Flask E 2% (w/w) Potassium 5 ml 5 ml 5 ml 5 ml 5 ml iodide 6. Make up to the mark of each volumetric flask with deionised water. 7. Stopper all flasks and mix thoroughly. 9

10 8. Allow about five minutes for colour development. These solutions are the working standards. B. Preparation of Swimming pool water sample 1. Add 2 ml of the swimming pool water sample into a 50 ml volumetric flask, and label it as Sample. 2. In a fume cupboard, add 5 ml of 5% (w/w) ethanoic acid solution, and then add 5.0 ml of 2% (w/w) potassium iodide solution with dispenser. 3. Fill up to the mark of the volumetric flask with deionised water. Mix thoroughly. 4. Also, allow about five minutes for colour development. C. Colorimetric measurement 1. Rinse the cuvette a few times using a wash bottle, and then fill the cuvette with deionised water. 2. Place the filled cuvette in the colorimeter sensor. 10

11 3. Cover the lid of the sensor. 4. Switch on the colorimeter sensor by pressing the green button as below: 11

12 5. Switch on the computer, on Windows operating system, choose Chlorine Water icon as below: 6. Enter the software, wait until it finishes initialisation, as shown below: 12

13 7. After initialisation, the software looks like below: 8. Under Displays, click Graph. It shows a graph looks like below. In order to remove the wavelengths we don t want, click the symbol in the legend box, look at the wavelength shows on the left hand side of the y-axis of the graph. If it is not 468 nm, right click the symbol, then click Remove selected data. There are three wavelengths unwanted. Leave the wavelength 468 nm for measurement. 13

14 Department of Chemistry, HKUST 9. Click Digital for displaying absorbance data. 10. Then the digital reading of absorbance will display as below: 14

15 11. Rinse the cuvette a few times with the solution to be measured. Carefully dropper the solution into the cuvette. Pour the waste into the waste beaker provided for each group. 12. Place the cuvette in the colorimeter sensor. 13. Cover the lid of the sensor. 15

16 Department of Chemistry, HKUST 14. Press Start on the computer screen for measurement. 15. The reading that shows in the Digital box is the absorbance of the solution at 468 nm. If the reading is steady, record the absorbance on the datasheet provided. Then, press Stop. 16

17 16. Repeat step 11 to 15 for measuring another solution. Repeat the measurement steps until all the solutions (Flask A to Flask E) have finished measurements. Record your reading on the datasheet in Section B, (a). 17. For the Further dilute sample solution, repeat measurement for 10 times. Take all readings on the datasheet in Section B, (b). 18. Complete the datasheet and perform the calculations. 17

18 Datasheet and Report Sheet Experiment 1: Determination of free available chlorine in swimming pool water by colorimetric analysis Student Name: Mark: School /Institution: A. Standard Solutions 1. Given the concentration of stock sodium hypochlorite (NaOCl) solution is 6.25 wt%, convert the concentration of sodium hypochlorite [NaOCl] to hypochlorite ion [OCl - ]: Given: Atomic mass of Na = amu; O = amu; Cl = amu. (a) Mass of NaOCl in one litre, present the result in the unit of ppm: (b) Convert the concentration of sodium hypochlorite [NaOCl] into the concentration of hypochlorite ion [OCl - ] in one litre, present result in ppm: 2. Determine the concentration of [OCl - ] in dilute standard solution after dilution in the Experimental Procedure, Section A, step 1: (a) Dilution of stock sodium hypochlorite solution 18

19 Volume of stock solution added (ml) Final Volume (ml) Dilution Factor (b) Concentration of hypochlorite [OCl - ] in the 50mL dilute standard solution (ppm): 3. Calculate the concentration of OCl - ion (in ppm) for each of the working standard solutions prepared in Section A in the procedure. Fill up the following table. Chemicals Flask A Flask B Flask C Flask D Flask E Diluted Standard solution 0 ml 1 ml 2 ml 3 ml 5 ml 5% (w/w) Ethanoic acid 5 ml 5 ml 5 ml 5 ml 5 ml 2% (w/w) Potassium 5 ml 5 ml 5 ml 5 ml 5 ml iodide Final volume of the flask 50 ml 50 ml 50 ml 50 ml 50 ml Concentration of [OCl - ] solution (ppm) B. Colorimetric Measurement Data 1. Record the absorbance for standard solutions and the swimming pool water sample. (a) Measurement for Standard Solutions: Flask Standard Solution Flask A Blank Flask B Standard 1 Flask C Standard 2 Flask D Standard 3 Flask E Standard 4. Concentration (ppm) Absorbance (AU) 19

20 (b) Repeat Measurement for Sample: Sample Absorbance (AU) Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Run 7 Run 8 Run 9 Run 10 Mean Sample Standard Deviation Questions 1. Using the experimental data of standard solutions, plot the calibration curve as absorbance (y-axis) versus concentration of hypochlorite ion in ppm (x-axis) for all the standard solutions. With the calibration curve, calculate the concentration of hypochlorite [OCl - ] in the sample solution (in ppm). (Attach your calibration curve at the back of your report) 2. Determine the concentration of hypochlorite [OCl - ] in the original swimming pool water sample. 3. What is the purpose of adding ethanoic acid? 20

21 4. What is the purpose of adding potassium iodide? And why it has to be in excess? 5. Based on the experimental data, what is the sample standard deviation? Comment on it with respect to precision and uncertainty. 21

22 Experiment 2: Spectrophotometric Analysis of Iron in Vitamin Supplement Tablets This experiment is designed for 4 students in a group. Each group is given one sample. Reference Harris D C, Quantitative Chemical Analysis, 8 th Edition, Chapter 17-19, W. H. Freeman and Company, Objective To determine the iron content present in commercial vitamin supplement tablets using a spectrophotometer. To determine the accuracy of the result. Theory In this experiment, total iron content [iron (III) ions and iron (II) ions] in the tablet sample is determined. Iron (III) ion in the tablet sample is reduced to iron (II) ion with the reducing agent (hydroxylamine, NH 2 OH) as follows: 2 Fe NH 2 OH + 2 OH - 2 Fe 2+ + N H 2 O (1) The converted iron (II) ions together with the originally present iron (II) ions react with o-phen to form coloured complexes prior to the spectrophotometric measurement at a particular wavelength. 1,10-phenanthroline (C 12 H 8 N 2, ortho-phenanthroline, o-phen) is a ligand that can react with divalent metal ions to form strong red-coloured metal complexes. Figure 1. The structure of 1,10-phenanthroline. The colour intensity of the complex corresponds to the concentration of the total iron content [iron (III) ions and iron (II) ions] present in the sample. The complexation reaction is shown as below: 22

23 Fe o-phen [Fe (o-phen) 3 ] 2+ (2) Figure 2. Structure of Tris(1,10-phenanthroline)iron(II) ion complex Spectrophotometric Measurement In this experiment, spectrophotometer is used for measuring the absorbance of the solution. Standard solutions are prepared in a series of known concentrations of iron (II) ions. All solutions including sample solution will react with ortho-phenanthroline to form coloured complexes. The absorbances for the complexes are measured. Using Beer s law, the concentration of the complexes and then the total iron content can be determined. For details, please refer to lecture notes. Materials Chemicals mg/ml Iron(III) solution from Iron (III) chloride salt 2. 1 M ammonium acetate (ph = 3.5) 3. 10% (w/w) hydroxylamine HCl (NH 3 OHCl) solution, reducing agent % (w/w) 1,10-phenanthroline (o-phenanthroline) solution in water 5. Deionised water 6. Iron supplement tablet sample Apparatus ml centrifuge tube 2. Beakers (50 ml 4, 250 ml, 600 ml) 3. Pipette (1mL, 2mL, 5mL 2 and 10mL 2) ml volumetric flasks with stoppers 23

24 ml volumetric flasks with stoppers 6. Filter funnel and filter papers 7. Ultrasonic bath 8. UV-Visible Spectrophotometer 9. Cuvettes Chemical Hazards Chemicals Hazard(s) Iron (III) chloride - it is toxic by ingestion - may cause irritation of the upper respiratory tract Ammonium acetate - may cause eye and skin irritation - may cause respiratory and digestive tract irritation hydroxylamine HCl - slightly hazardous in case of skin contact (corrosive, sensitizer), of eye contact (corrosive) Hydrochloric acid - Corrosive; can cause redness, pain, and severe skin burns. Concentrated solutions cause deep ulcers and discolor skin. 1,10-phenanthroline - may be harmful by inhalation, ingestion, or skin absorption - cause eye and skin irritation Information source is: MSDS, Sigma-Aldrich, available from: 24

25 Procedure A. Preparation of Standard Solutions 1. Label five 50 ml volumetric flasks as: Flask A, Flask B, Flask C, Flask D, and Flask E. 2. Pipette 0 ml, 1.0 ml, 2.0 ml, 5.0 ml and 10.0 ml of 0.05 mg/ml iron (III) standard solution to each of the 50 ml volumetric flask as the following table: Chemicals Flask A Flask B Flask C Flask D Flask E 0.05 mg/ml iron (III) 0 ml 1 ml 2 ml 5 ml 10 ml standard solution 1 M ammonium acetate 2 ml 2 ml 2 ml 2 ml 2 ml buffer 10% (w/v) hydroxylamine 2 ml 2 ml 2 ml 2 ml 2 ml 0.15% (w/w) 4 ml 4 ml 4 ml 4 ml 4 ml 1,10-phenanthroline solution Final volume of the flask 50 ml 50 ml 50 ml 50 ml 50 ml 3. Following above table, add the stated volumes of ammonium acetate buffer, hydroxylamine and 1,10-phenanthroline solution into the respective flask using dispensers, individually. 4. Fill up to the mark with deionised water. Mix thoroughly. 5. Allow all the solutions to stand for 10 minutes. B. Preparation of Sample Solution Weighing of Sample 1. Record the sample name on the datasheet provided. 2. Place a weighing paper on the balance, tare it zero. Put the original tablet sample as a whole on the weighing paper on the balance, jot down the mass of it on the datasheet in Section A, (a). 3. Place the weighed tablet into a mortar, crush it with a pestle. 4. Fold the weighing paper along the diagonal. 5. Tare the balance zero. Place the folded weighing paper again on the balance, open the weighing paper flat. Place the weighing paper on bench. 25

26 6. Then carefully transfer all the tablet powder from the mortar to the weighing paper with a spatula. Put the weighing paper back on the balance. Record the mass of tablet powder together with the weighing paper on the datasheet in Section A (b), in box (X). 7. Carefully decant the tablet powder from the weighing paper into a 50 ml centrifuge tube. 8. Weigh the weighing paper again. Take down the mass on datasheet in Section A (b), box (Y). 9. On datasheet, in Section A (b), subtract the residual mass of tablet powder on the weighing paper in step 8 (Y) from the mass of tablet powder and weighing paper in step 6 (X). The net mass (X - Y) is the mass of tablet powder actually used for the analysis. Preparation of Sample Solution 10. Add 1 ml ammonium acetate solution into the centrifuge tube, followed by 20 ml deionised water. 11. Screw the cap of the centrifuge tube. Place the centrifuge tube into a 250 ml beaker containing about 100 ml water. 12. Place the whole beaker, with the tube inside, into an ultrasonic bath. Sonicate the solution for 20 minutes. 13. During the waiting time, fold a piece of filter paper for filtration. 14. Put the folded filter paper into a funnel, and place the funnel inside a 100 ml volumetric flask. Slowly decant the sonicated solution into the filter paper using a glass rod. 15. Make up to the mark of the volumetric flask with deionised water. Label the flask as Sample Solution. Dilution of Sample Solution 16. Dilute the above Sample Solution by 20 times: Pipette 5 ml of the Sample Solution into another 100 ml volumetric flask, and make up to the mark with deionised water. Mix thoroughly. Label the flask as Dilute Sample. 26

27 17. Then, further dilute the Dilute Sample solution by 10 times: Pipette 10 ml of the Dilute Sample solution into another 100 ml volumetric flask, and DO NOT make up. Label the flask as Further Dilute Sample. 18. Into the Further Dilute Sample volumetric flask, add 2 ml of 1 M ammonium acetate buffer, followed by 2 ml of 10% (w/w) hydroxylamine, and 4 ml of 0.15% (w/w) 1,10-phenanthroline solution with measuring cylinders respectively. 19. Then, make up to the mark with deionised water. Mix thoroughly. 20. Allow all the solutions to stand for 10 minutes. C. Spectrophotometric Measurement 1. From the spectrophotometer, the following is shown on the main menu. Choose Photometry by pressing 1 on the keypad: 2. In Photometry, press 1 for Parameter Setup : 27

28 3. In Parameter Setup, press 2 for selecting wavelength for absorbance measurement. Key in 510 as the wavelength of 510 nm to be used in this analysis. Then press Enter on the keypad. 4. Press Clear Return on keypad for going back to the previous menu. Then, press 0 for going Forward. 5. Now the instrument is ready for measurement. 6. Rinse the cuvette a few times with deionised water. Fill the cuvette with deionised water. Place the cuvette in the sample compartment and press Autozero on the keypad. 28

29 7. Pour deionised water from the cuvette into a waste beaker. If there is any difficulty for the transfer, use a dropper. 8. Then rinse the cuvette a few times with the solution to be measured. Again, pour the solution from the cuvette into a waste beaker. 9. Carefully decant Flask A (Blank) into the cuvette for the first measurement. If there is any difficulty for the transfer, use a dropper. 10. Place the cuvette into the sample compartment, press Start on the keypad. The absorbance is then shown on the screen. Take down the reading on the datasheet, Section C, (1). 11. After each measurement, rinse the cuvette with deionised water, then rinse a few times with the solution being measured. 12. Repeat step 9 to 11 above until all solutions have finished measurement, including all standard solutions and Further Dilute Sample solution. Record all readings on your datasheet. For Further Dilute Sample solution, record the reading on datasheet, Section C, (2). 29

30 Datasheet and Report Sheet Experiment 2. Spectrophotometric analysis of Iron in Vitamin Supplement Tablets Student Name: Mark: School /Institution: A. Information about Iron Supplement Tablet Sample Sample Name: Mass of Fe stated on the package: mg of Fe (a) Mass of the whole tablet: Mass of the whole tablet sample Mass (g) (b) Mass of tablet sample actually used for the analysis: Mass of tablet sample + weighing paper (X) Mass of weighing paper after transferring the sample to centrifuge tube (Y) Net mass of the sample used= (X)-(Y) Mass (g) 30

31 B. Standard Solution 1. Given the concentration of stock iron (III) solution is 0.05 mg/ml, convert the concentration in the unit of ppm: 2. Calculate the concentration of Fe (in ppm) in the standard solutions prepared in Section A in the procedure. Fill up the following table. Chemicals Flask A Flask B Flask C Flask D Flask E iron (III) standard solution 0 ml 1 ml 2 ml 5 ml 10 ml ammonium acetate buffer 2 ml 2 ml 2 ml 2 ml 2 ml hydroxylamine 2 ml 2 ml 2 ml 2 ml 2 ml 1,10-phenanthroline 4 ml 4 ml 4 ml 4 ml 4 ml Final volume of the flask 50 ml 50 ml 50 ml 50 ml 50 ml Concentration of Fe (mg/l or ppm) C. Spectrophotometric Measurement Result 1. Fill in the table below, the absorbance is the reading of the spectrophotometer. Flask Standard Solution Concentration of Fe (mg/l) Absorbance (AU) Flask A Blank Flask B Standard 1 Flask C Standard 2 Flask D Standard 3 Flask E Standard 4 31

32 2. Sample Data Solution Absorbance (AU) Further Dilute Sample 3. What is the dilution factor (times) of the Further Dilute Sample? Questions 1. Establish the calibration curve using the experimental data as absorbance versus concentration of iron in ppm for all standard solutions. Write down the least-square fit equation below. (Attach your calibration curve at the back of your report) 2. Using the above least-square fit equation in the calibration curve, calculate the concentration of iron (in ppm) in the Further Dilute Sample solution. 32

33 3. Calculate the concentration of Fe in the original tablet sample, present your result in mg/kg. 4. Calculate your result in question (3) above in mg of Fe per tablet. 5. Compare the result obtained in question (4) with the stated content of Fe on the package of the sample product. Comment on the accuracy of your result. 33