Experiment: Iodometric Titration Analysis of Ascorbic Acid Chem251 modified 09/2018 Experiment. Iodometric Titration of Ascorbic Acid. Objective: The goal of this lab is to determine the concentration of vitamin C in an unknown solid. The analysis will be carried out using redox reaction of triiodide with ascorbic acid via iodometric titrimetry using starch indicator. Equipment Chemicals 400mL Beaker 600-mL Beaker 3M Sulfuric Acid, H 2 SO 4 Potassium iodate, KIO 3 50-mL graduated cylinder 500-mL Volumetric flask Potassium iodine, KI Vitamin-C Unknown 250mL Volumetric flask 500-mL graduated cylinder 1% Starch solution 50-mL Buret 50-mL Volumetric pipet 125mL Erlenmeyer flask 3" x 5" index card Safety and Waste Disposal Wear safety goggles and be cautions when working with concentrated acid. Background Information: Although most mammals can synthesize vitamin C, or ascorbic acid (C 6 H 8 O 6 ), from sugars, man must ingest considerable quantities of this substance. The National Academy of Sciences recommends the consumption of 60 mg of ascorbic acid per day. Vitamin C deficiency, which typically causes abnormalities in bones and teeth, was first characterized in sailors in the eighteenth century. Compelling sailors to eat limes, a source of vitamin-c, eliminated these abnormalities. Many vegetables also contain large quantities of vitamin C, but many cooking processes commonly destroy ascorbic acid, and hence citrus fruits are regarded as the most reliable source of vitamin C. Vitamin C can be determined in food by use of an oxidation-reduction reaction. The redox reaction is preferable to an acidbase titration because a number of other species in juice can act as acids, but relatively few interfere with the oxidation of ascorbic acid by iodine. The solubility of iodine is increased by complexation with iodide to form triiodide: I 2 (aq) + I! I 3 (1) Triiodide then oxidizes vitamin C to dehydroascorbic acid (not balanced): C 6 H 8 O 6 + I 3 VitaminC C 6 H 6 O 6 + I dehydroa scorbic acid (2) As long as vitamin C is present in the solution, the triiodide is converted to the iodide ion very quickly. However, when the all the vitamin C is oxidized, the triiodide excess will be present, which react with starch to form a blue-black complex. I - 3 + starch g iodine-starch complex (blackish-blue color) (3) Iodine solution is used to test for starch; a dark blue color indicates the presence of starch. The details of this reaction are not yet fully - - known, but it is thought that the iodine (I 3 and I 5 ions) fit inside the coils of amylose, the charge transfers between the iodine and the starch, and the energy level differences in the resulting complex correspond to the absorption spectrum in the visible region. The strength of the resulting blue color depends on the amount of amylose present. Waxy starches with little or no amylose present will color red. Starch indicator is biodegradable and so fresh starch indicator must be prepared after a week of storage. Ask the instructor or lab tech, when the indicator was prepared before use. Furthermore, although vitamin C is very stable when dry, it is readily oxidized by oxygen when in solution. Therefore, a solution of vitamin C should not be exposed to air for an extended period of time. Remember that the molar mass of vitamin-c is 176.12 g/mol.
Procedure Preparation of iodine solution. 1. Dissolve ~5.00 g potassium iodide (KI) and ~0.268 g potassium iodate (KIO 3 ) in 200 ml of distilled water in a 400 ml beaker. 2. Add 30 ml of 3 M sulfuric acid. Then pour the solution into a 500-mL graduated cylinder, and dilute to a final volume of 500 ml with distilled water. Mix thoroughly and transfer to a 600-mL beaker. Do not put this solution in a volumetric flask!!!! Please note: Iodine is very weakly soluble in the water, and can be easily lost from the solution due to its volatility. However, in the presence of excess iodides, iodine creates I 3 - ions. This lowers free iodine concentration and such solutions are stable enough to be used in lab as a titrant. Still, we should remember that their shelf life is relatively short (they should be kept tightly closed in dark brown bottles, and standardized every few weeks). If you take more than two weeks from when you prepared this solution to the analysis of your unknown, standardize the iodine solution with ascorbic acid again as discussed in the next procedure. Preparation of vitamin-c standard solution. For best accuracy, prepare this solution on the day you are to standardize your iodine solution. Vitamin-C will oxidize in air, so to reduce errors and improve the accuracy of your iodine standardization, you will need to limit the exposure of air to the Vitamin-C solution. 3. Weigh 0.2500 g (to.1mg) vitamin C using an analytical balance and place in 100 ml water. Dilute to volume in a 250-mL volumetric flask. 4. In your result page, calculate the formality of vitamin-c. Standardization of the iodine solution with the vitamin C standard solution. 5. Add 25.00 ml of vitamin C solution into a 125 ml Erlenmeyer flask. Add 10 drops of 1 % starch solution. 6. Rinse your buret twice with 5-10 ml of iodine solution, and then fill it. Record your initial buret volume. 7. Titrate the solution until the endpoint is reached (the first sign of blue color that remains after at least 20 s of swirling). 8. Record the final volume. Repeat this titration at least four times. Results should agree to 0.1 ml. If you do more than four trials, be sure to label the four trials you will use for your calculations. 9. In your result page, calculate (see calculation section): i) the molarity of iodine solution for each trial. ii) the average molarity, the standard deviation, RSD, and 95% CL for the standard iodine solution. iii) turn in the molarity of your iodine solution to your instructor via an index card. Preparation of unknown solution. Again, remember that Vitamin-C is easily oxidized in air. Take the same precaution in preparing your unknown as you did in preparing the Vitamin-C solution in step 3 above. 10. You will be assigned a solid sample that contains ascorbic acid (record your unknown number in your notebook). 11. Take your entire unknown and weigh to the closes 0.1 mg using the analytical balance and report the mass in your lab notebook. turn in the total mass of your unknown via an index card. 12. Deoxygenate the deionized solution in which you will be diluting your Vitamin-C. Dilute to 500-mL using a 500-mL vol flask. Analysis of Unknown. 13. Using a 20.00 ml volumetric pipet, add 20.00mL of your unknown solution into a 125 ml Erlenmeyer flask. Minimize exposure to air by using parafilm wax paper to cover your flask. Add about 10 drops of starch indicator to the sample. 14. Titrate the solution until the endpoint is reached (the first sign of blue color that remains after at least 20 s of swirling). 15. Record the final volume. Repeat this titration at least four times. Results should agree to 0.1 ml. If you do more than four trials, be sure to label the four trials you will use for your calculations. 16. Use the Grubbs and Q-test (95% confidence level) to check for bad data. 17. In your result page, calculate (see calculation section) i) the mass of vitamin-c used for each trial ii) the initial volume of the iodine used each trial iii) the final volume of the iodine used for each trial. iv) the total volume of iodine used for each trial v) the moles of vitamin-c in each trial of the unknown vi) the mass of vitamin-c in the 250mL unknown solution prepared per trial calculated vii) the % m:m vitamin-c in your unknown for each trial of your unknown viii) the average % m:m vitamin-c in the unknown ix) the standard deviation, x) the RSD xi) the 95% CL for your unknown analysis. vii) Report your final result in the form x + s (n = ), see chapter 4 Turn in the Excel spreadsheet
Calculations- 1. What is the reaction to produce iodine from iodate and iodide? Draw the structures of the organic compounds given in Equation (2). 2. (a) Prepare tables of all your titration data with the following information. Unknown Information- Unknown #: Mass Unknown: Vitamin-C Preparation Mass KI (g) Mass KIO 3 (g) Mass Ascorbic Acid (g) Volume aliquot (ml) Vol initial (ml) Vol final (ml) Unknown Titration (4 Trials) Mass Unknown: Volume aliquot (ml) Vol initial (ml) Vol final (ml) Iodine Standardization (4 Trials) 2. (b) Preparation of vitamin C standard solution. i) Calculate the molarity of vitamin-c standard solution. 2. (c) Standardization of the iodine solution with the vitamin C standard solution. i) Calculate the molarity of iodine solution for each trial, the average molarity, standard deviation and the rsd. 2. (d) Analysis of Unknown. Calculate the followingi) the moles of the vitamin-c for each trial for your unknown and the average moles. ii) the mass of the vitamin-c for each trial for your unknown and the average mass. iii) the % vitamin-c in your unknown for each trial of your unknown and the average. iv) the standard deviation, RSD, CV and 95% CL for your unknown analysis. Statistic Analysis i) Report the average, standard deviation (s) and relative deviation (RSD, sr) and the coefficient of variation (CV) and the 95% CL for your result of the vitamin C content in the samples you analyzed. ii) Apply a Grubbs and Q-test (95% confidence level) for any suspected result. Discussion- The main goal of this experiment is to determine the amount of vitamin-c in an unknown solid. Discuss your results (for the vitamin-c in your unknown) and any source of error that may cause your result to deviate. Discuss the standard deviation of the result and how the error analysis. Prelab Questions (Not required for lab prelab but know how to solve for midterm) - 1. A standard iodine solution was standardized against a 0.4123 g primary standard As 4 O 6 by dissolving the As 4 O 6 in a small amount of acidic solution and adjusting the ph, see equation (i). If the resulting H 3 AsO 3 solution required 40.28 ml triiodine to reach the end point, what is the concentration of the triiodine solution? Reaction - (i) As 4 O 6 (s) + H 2 O g H 3 AsO 3 (ii) H 3 AsO 3 + I - 3 + H 2 O g H 3 AsO 4 + I- + H+ 2. The purity of a hydrazine (N 2 H 4 ) sample can be determined by titration with triiodide. A 1.4286 g of the oily liquid sample is dissolved in water and diluted to 100 ml in a volumetric flask. A 50.00 ml aliquot is titrated against the standard triiodine solution in question 1 (previous problem) requiring 42.41 ml to reach the end point. What is the percent purity by weight of the hydrazine? Reaction - (iii) N 2 H 4 + I 3 g N 2 + I- 3. A 0.200 g sample containing copper is analyzed iodometrically. The titration analysis is prepared by taking copper(ii) ion and reducing it to copper(i) by iodide ions according to the following reaction: (iv) Cu2+ + I- g CuI (s) + I 2 If the liberated I 2 from this reaction is titrated against 20.0 ml of 0.100 M sodium thiosulfate (Na 2 S 2 O 3 ), what is the percent copper in the sample? The reaction for the titration is- (v) I 2 + S 2 O 3 2- g I- + S 4 O 6 2-4. Triiodide ions are generated in solution by the following reaction: IO 3 - + I- g I 3 - If a 25.00 ml sample of 0.0100 M KIO 3 is added to excess of KI and the product, triiodide, requires 32.04 ml of Na 2 S 2 O 3 to reach the equivalent point, what is the molarity of the Na 2 S 2 O 3? Use the equation: I 3 - + S 2 O 3 2- D I- + S 4 O 6 2-
Turn in the following information to you instructor: Analysis of Vitamin-C by Iodometric Titration Name: Mass Unknown Unk # Vitamin-C Preparation Mass KI, g Mass, KIO3, M Mass Vit-C, g, Standardization reagent Molarity 250mL Stock Vit-C Standard Standardization of Iodine Titration of Vit-C Standards Trial1 Trial2 Trial3 Trial4 Vol Iodine (final) ml Vol of Iodine, initial (ml) Vol Iodine, Final (ml) Vol of Iodine, ml Concentration Iodine solution (M) Average concentration iodine solution (M) Titration of Unknown Vitamin-C Titration of Unknown Trial1 Trial2 Trial3 Trial4 Mass Vitamin-C Unknown (g) Vol Iodine (initial) ml Vol Iodine (final) ml Vol Iodine, ml Mol Vit-C Unknown Mass Vit C in 250ml Unk % Conc. Vit C Unknown Avg. % Vit-C Unknown standard deviation Relative Standard Deviation (RSD) or CV 95% CL +/- (Avg. +/- CL) Final Results in form of x + s ( n = ) % Error Analysis: No not fill g of Vit C g of Total Mix % Vit C % error #1 Instructor Calculations Student Calculations
Iodometric Titration of Vitamin-C Experiment Lab Report Write-up Criteria Analytical Chemistry 251 # CRITERIA % pts pts 1 Introduction and Procedures A. Introduction Objective of Expt. Background information. Math relationship used in study. B. Procedures Outline of procedures in Expt. Flow chart pictorial of procedures. Procedural changes. Information (data) to be recorded during expt. (to be presented in Table form.) Safety and disposal information. This portion of the report should be turned in before the start of lab class (pre-lab discussion). 2 Data, Observe., Results and Calc. C. Data and Observation Data in table form. & detailed observation written in the table. All data entry should contain the proper number of significant figures and units. Data should always be recorded in an organize fashion. Balance chemical equations; all chemical reaction which occurred during an experiment should be written in this section. Then it should also be written in the discussion portion of the report. This portion of the report should be turned in before you leave the laboratory. Calculations & Results D. Calculations Sample calculation shown Statistical analysis of data and result (if applicable) E. Results (Complete analysis with Excel spreadsheet) Result(s) in table form. Statistical analysis In this section accuracy of results is very important as well as detailed calculation showing how the result was obtain. "Unknown" will also be included in this section. 3 Discussion / Conclusions and Post-Lab Questions F. Discussion A complete discussion should be written in this section. Topics to be discuss can be found at the end of each experimental procedure from the lab manual. Each discussion should include the significance of the result(s) and the meaning of the result of the experiment. All chemical reactions that occurred during the experiment should also be included here. G. Conclusion Summary of the goal of the experiment and how that goal was achieved in the experiment. This portion (Calculation and Discussion) is turned in at the beginning of class of the due-date 4 Overall Presentation (of lab notebook) Lab technique during experiment; example are: class preparation, safety glasses precautions and leaving the laboratory clean. Report presentation: examples are the headings of each report that includes name, title, lab partner, date and section #. Legibility of report. Is the report easy to read or is important information jotted down by small print in the corners of the lab report. The overall impression is important. Lab Technique Safety: wear goggles, handle chemicals with caution, proper handling of lab equipment Leave lab clean and tidy Time management 20% 10% 10% Total (This total may be adjusted depending on lab technique and student conduct in the experiment) Unknown # ; % Mass Vitamin-C Unknown % Vitamin-C