(aq) + 5e - Mn 2+ (aq) + 4H 2

Transcription

1 EXPERIMENT 20 Titrimetric Determination of Iron INTRODUCTION Potassium permanganate is widely used as an oxidizing agent in titrimetric analysis. In acidic solution, a permanganate ion undergoes reduction to manganese (II) ion as shown in the following equations: 8H + (aq) + MnO 4 - (aq) + 5e - Mn 2+ (aq) + 4H 2 O (l) Since the permanganate ion is violet and the manganese (II) ion is nearly colorless, the endpoint in titrations using potassium permanganate as the titrant can be taken as the first permanent pink color that appears in the solution. This experiment will utilize potassium permanganate to determine the molarity and mass percent of iron (II) ions in an unknown solution containing iron (II) ammonium sulfate hexahydrate, Fe(NH 4 ) 2 (SO 4 ) 2. 6H 2 O. The titration involves the oxidation of iron (II) to iron (III): Fe 2+ (aq) Fe 3+ (aq) + e- The titration is carried out in a sulfuric acid solution to prevent air-oxidation of iron (II). The overall oxidation-reduction reaction between the permanganate ion and the iron (II) ion is: 8H + (aq) + MnO 4 - (aq) + 5Fe 2+ (aq) Mn 2+ (aq) + 5Fe 3+ (aq) + 4H 2 O (l) The volume of the potassium permanganate solution required is noted, and using this volume and the molarity of the potassium permanganate solution, the moles of iron (II) can be calculated. By knowing the volume of the unknown solution used, the molarity of the iron (II) can be calculated. By knowing the mass of the unknown solution used, the mass percent of iron (II) can be calculated. In this experiment you will test both your accuracy and precision. Students with the same unknown sample will compile their data, and you will submit a mean molarity of iron (II) in the unknown. Before calculating the mean molarity of iron (II), a Q-test will be applied to the data to insure that all of the calculated molarities should be included in the calculation of the mean, and Q 95% values are given in the table below. Number of Data Points : Q 95% : Your group s mean molarity of iron (II) will then be checked against its accepted value by the instructor, and the agreement between the two will demonstrate your group s accuracy. Once the mean is determined, the standard deviation and relative standard deviation can be calculated. The relative standard deviation for the molarity of iron (II) will show the agreement between the multiple trials performed by your group, and will demonstrate your group s precision. 203

2 PROCEDURE 1. Students will work individually for this experiment. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. PART A - CALIBRATING THE BURET 2. Attach a support rod to a stirring plate, and attach a double buret clamp to the support rod. Obtain a buret (tolerance 0.03 ml), always carrying a buret in a vertical position. Attach a double buret clamp to a ring stand attached to a stirring plate. Attach the buret to the double buret clamp. 3. Prepare a buret card to be used every time you read your buret. Obtain a 3"x5" card from drawer 019 and using a black felt tip pen make a horizontal mark on your card, one centimeter thick and practically the length of the card. When the top of the black band is held just below the bottom of a meniscus you will see a reflection of the band in the meniscus against the white of the card behind. This offers you a repeatable method of determining the position of the meniscus. You must make sure during your readings that your line of sight is perpendicular to the buret so as to avoid parallax. If your line of sight is looking downward or looking upward, the meniscus will appear to be higher or lower, respectively, than its true value. 4. Using a funnel, and with the top of the funnel below eye level, fill the buret with deionized water. Run some of the deionized water through the buret tip into the waste beaker until you are sure that all the bubbles are removed from the buret tip. Add enough deionized water so that the water level is above the 0.00 ml mark, then drain the water slowly into the beaker until the meniscus is at the 0.00 ml mark (as read with your buret card). Touch the tip of the buret to the side of a beaker to remove the drop hanging from the tip. After about a minute, to allow for drainage, make an initial reading of the meniscus with your buret card, estimating the volume to the nearest 0.01 ml. Record the calibration reading in your Data Table. Allow the buret to stand for 5 minutes and recheck the reading. If the stopcock is tight, there should be no noticeable change in the reading. If the reading has changed tighten the locking nut on the stopcock and let stand for another 5 minutes. Check the reading again. If the buret continues to leak consult your instructor. 5. Empty the deionized water out of your buret. 204

3 PART B - DETERMINATION OF IRON (II) 6. Obtain a mL pipet (tolerance 0.02 ml), and an unknown solution containing iron (II) ions. Always carry pipets in a vertical position. Do not remove the code label from the unknown solution, but record the unknown number in the space provided in your Data Table. 7. Obtain a pipet bulb from drawer 014 or a pipet roller from drawer 013. Condition your pipet by drawing about 2 ml of the iron (II) solution into the pipet and, holding the pipet in a horizontal position, rolling it to make sure that the iron solution wets the entire inside surface. Drain the iron (II) solution through the pipet tip into the waste beaker. Do not use too much iron (II) solution for the conditioning process, or you may not have enough left for your titrations. Repeat this conditioning procedure two more times, each with about 2 ml of the iron (II) solution. All excess solutions in the waste beaker on your lab bench must be disposed of in the waste bottle in the fume hood. 8. Obtain a stopper from drawer 013. Measure the mass of a clean, dry, stoppered 125-mL Erlenmeyer flask and record it in the Data Table. Pipet a ml sample of the iron (II) solution into the flask, and record this volume in your Data Table. Measure the mass of the stoppered flask with the added sample of the iron (II) solution and record it in the Data Table. Calculate the mass of the sample and record it in the Data Table. Add about 5 ml of 6 M sulfuric acid to the flask. 9. Obtain 75 ml of the standardized potassium permanganate solution from the large container in the lab room. Read the container and record the concentration of the potassium permanganate solution in your Data Table. Condition your buret by rinsing it three times with 5 ml portions of the potassium permanganate solution. Drain the potassium permanganate solution through the buret tip into a waste beaker. Place all excess solutions in the waste beaker on your lab bench. Dispose of all excess solutions in the waste bottle in the Fume Hood A. Using a funnel, and with the top of the funnel below eye level, fill the buret with the potassium permanganate solution. Run some of the potassium permanganate solution through the buret tip into a waste beaker until (1) you are sure that all the bubbles are removed from the buret tip, and (2) the potassium permanganate meniscus is at or below the 0.00 ml mark on the buret. Remove the last drop from the tip of the buret, then remove the waste beaker 10. Obtain a clean and dry magnetic stirring bar from your locker, and carefully slide it into the Erlenmeyer flask containing the unknown iron (II) solution. Center the Erlenmeyer flask on the stirring plate, and adjust your buret so the tip is slightly inside the mouth of the flask. Read the buret and record it as the Initial Buret Reading. Have another student verify the reading. Turn on the stirring plate, slowly increasing the speed until that the stirring bar creates a vortex in the liquid, but does not collide with the sides of the vessel. 11. Add the potassium permanganate solution intermittently from the buret to the Erlenmeyer flask, noting the pink permanganate color that appears and disappears as the drops hit the liquid and mix with it. When the pink color begins to persist, slow down the rate of the addition of potassium permanganate until you are adding it drop by drop. In the final stages of the titration, rinse the inside wall of the flask with deionized water from your wash bottle, and add half drops until the entire solution just turns a pale pink, or salmon, color. If your solution turned from colorless to pale pink with the addition of only one drop of the potassium permanganate solution, use this titration to determine the molarity of iron (II) in the unknown solution. However, if your solution turned from colorless to pink with the addition of more than one drop of the potassium permanganate solution, this would be a gross error that caused the endpoint to be missed, and this titration should be discarded. 205

4 12. Read the buret and record it as the Final Buret Reading. Have another student verify the reading. Pour the contents of the Erlenmeyer flask through your funnel into the waste beaker to recover the magnetic stirring bar, then clean and dry it for the next titration. Refill the buret with potassium permanganate solution, repeat step 8, and do a second trial, titrating to a pale pink endpoint. 13. Complete two titrations in which you obtain a pale pink endpoint. Calculate the mass percent of iron (II) and the molarity of iron (II) for both trials. Precise work will give you two mass percents and two molarities that are nearly identical. The expected precision limit for volumetric analysis is 1%. For your individual work this means that your two mass percents and your two molarities should agree within 1% of each other. 14. Find your unknown number on the white board, and write your two calculated molarities of iron (II) underneath it. Record all of the calculated molarities for your unknown in your Data Table. List the molarities from lowest to highest, then apply a Q-Test to the highest and lowest molarities to determine if one of them should be rejected because it is an outlier. 15. The statistical analysis can be done on a TI-30 calculator. Clear the calculator by pressing. If Error appears, press. Enter the first molarity, then press. Enter the second molarity, press, and continue this until all of the non-rejected molarities have been entered. At this point you will see n = x, where x is the number of data points you have entered. To find the mean of the entered data, press, and you will see the mean displayed. To find the standard deviation, press, and you will see the standard deviation displayed. You will need to calculate the relative standard deviation on your own. Record each of these calculated values in your Data Table. Precise work will give multiple molarities that are nearly identical. The expected precision limit for volumetric analysis is less than 1%. For your group this means that the mean molarity of iron (II) should have a relative standard deviation of less than 1%. Accurate work will give you a mean molarity that is nearly identical with the accepted molarity. The expected accuracy limit for volumetric analysis is less than 1%. For your group this means that the mean molarity of iron (II) should agree within 1% of the accepted molarity of iron (II) when graded by the instructor. 16. At the end of the experiment, clean and dry the magnetic stirring bar and return it to your lab locker. Empty the permanganate solution out of your buret, then rinse the buret thoroughly, including the tip, with your remaining unknown. Dispose of this waste solution in the waste bottle in the Fume Hood A. Next, rinse the buret several times with tap water, then three times with deionized water, disposing of these washings down the sink, and then dry off the outside. Rinse your unknown container several times with tap water, then three times with deionized water, and then dry off the outside. Return these items to the back of the lab room. 17. Clean and wipe dry your laboratory work area and all apparatus. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. 206

5 EXPERIMENT 20 LAB REPORT Name: Student Lab Score: Date/Lab Start Time: Lab Station Number: DATA TABLE TRIAL 1 TRIAL 2 TRIAL 3 Calibration Buret Reading. ml Unknown Code Number Mass of Stopper, Flask... g Volume of Sample... ml Mass of Stopper, Flask, Sample... g 1-3 Mass of Sample... g Concentration of KMnO 4... M Concentration of MnO M Initial Buret Reading... ml Final Buret Reading... ml 4-6 Volume of KMnO 4 Used... ml 7-9 Mass % Iron (II) in Sample... % Concentration of Iron (II)... M Concentration from 2 nd Experimentor.. M Concentration from 3 rd Experimentor.. M Concentration from 4 th Experimentor.. M 13 Mean Concentration of Iron (II). M Standard Deviation. M 14 Relative Standard Deviation. % 207

6 CALCULATIONS

7

8

9 (Q-test)

10 QUESTIONS 1. If today s first titration required only 15 ml of the standard KMnO 4 solution to neutralize the unknown iron (II) solution, what procedural change could be made to insure that subsequent titrations used at least 20 ml of the standard KMnO 4 solution? 2. When cleaning the pipet, if the pipet is rinsed with water but then is not rinsed several times with the iron (II) solution before being used to transfer the iron (II) solution into the 250-mL Erlenmeyer flask, is the calculated molarity of iron (II) in the unknown greater or less than the actual molarity of iron (II) in the unknown? Explain based upon your calculation in Box

11 3. A water supply was known to contain lead (II) ions. A ml aliquot of the solution was pipetted, and its mass determined to be g. The ml aliquot was acidified, and ml of a M potassium dichromate solution were required to oxidize all of the lead (II) ions. Calculate (1) the molarity of the lead (II) ions in the solution, and (2) the mass percent of the lead (II) ions in the solution. 213

12 4. A person s blood alcohol (C 2 H 5 OH) content can be determined by titrating a sample of blood plasma with potassium permanganate solution. A g sample of blood plasma from a suspected drunk driver was titrated, and ml of an acidified M potassium permanganate solution were required to convert all of the alcohol into carbon dioxide. Calculate the mass percent of alcohol in the blood, and determine if the driver was legally intoxicated. 214