EXPERIMENT 23. Determination of the Formula of a Complex Ion INTRODUCTION

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1 EXPERIMENT 23 Determination of the Formula of a Complex Ion INTRODUCTION Metal ions, especially transition metal ions, possess the ability to form complexes (as shown below) with ions, organic and inorganic molecules or ions called ligands. Transition metal ions in aqueous solutions generally exist as complex ions in which water molecules, acting as Lewis bases, coordinate or bond with the small cation (which acts as a Lewis acid). The water molecules in these structures are known as ligands. Historically this kind of attachment has been called either a coordinate covalent bond or a dative bond. The distinguishing characteristic of such bonds is that the shared electron pairs which constitute the bonds come from only one of the bonded species. In normal covalent bonding the assumption is that each atom donates one electron to the shared pair that is the bond. The number of ligand attachments to the metal ion is called the coordination number. Common coordination numbers are 2, 3, 4, 5, and 6. Ligands which can make only one bond with a metal ion are called monodentate ligands ( one tooth ). Bidentate ligands are generally larger structures which can attach twice to an ion (e.g. ethylenediamine and 1,10-phenanthroline). A few ligands are polydentate (such as EDTA). The existence of metal ion-water complexes is mainly due to the attraction of the lone pairs of the water molecules for the high, concentrated, positive charge on the metal cations. The silver ion, for example, is typically coordinated with two water molecules. Although it is usual to write the aqueous silver ions as Ag + (aq), a more accurate representation would be [Ag(H 2 O) 2 ] + (diaquasilver ion). Similarly, aqueous copper (II) ions are generally coordinated with four water molecules resulting in the species [Cu(H 2 O) 4 ] 2+ (tetraaquacopper(ii) ion). Although these examples include only water molecules as ligands, other neutral molecules, anions, and even some cations are also possible. 193

2 The formula of a metal ion/ligand complex in the solid state can be determined by direct analysis of the stoichiometric amounts of each element that make up the complex. Once in solution however, determination of the complex formula is not quite as direct. A Job Plot, using the Method of Continuous Variation, allows us to find the formula for the complex in solution. In this method, several solutions are prepared in which the concentrations of the metal ion and the ligand are varied but the sum of the concentrations is kept constant. Using these solutions, the light absorption or the conductivity of the solutions is measured and plotted versus the mole fraction of the ligand. Mole fraction is the ratio of the number of moles of one component in a mixture to the total number of moles of all substances in that mixture. The symbol for mole fraction is χ. For example, in a mixture of A and B, the mole fraction of component B (χ B ) would be calculated according to the following formula: B moles B moles A + moles B From our hypothetical metal ion/ligand complex shown above, a plot of absorbance versus mole fraction of ligand would have a graph as shown here From the graph, note the maximum absorbance for the absorbing species occurs at 0.80 mole fraction of ligand. If the mole fraction of ligand in the complex ion is 0.80, then the mole fraction of the metal ion in the complex must be = By calculating the ratio of ligands to metal: 0.80 / 0.20 = 4.0 it can be determined that there are four moles of ligands to every one mole of metal, and the formula must be M 1 (ligand) 4. Since the ligand is monodentate, the coordination number is

3 In today s experiment, you will find the find the formula for an iron (II)-phenanthroline complex (meaning determining the number of phenanthroline ligands attached to an Fe 2+ ion) and the coordination number (meaning the number of ligand attachments). Both the iron (II) ion and the free phenanthroline molecules are colorless in solution; however, an iron ion with at least one phenanthroline molecule attached will be a reddish-orange color in solution. Fe y phen Fe(phen) y Colorless Red-Orange where phen is the bidentate ligand 1,10-phenanthroline, C 12 H 8 N 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 DETERMINING THE ABSORPTION OF THE COMPEX ION SOLUTIONS 2. Obtain thirteen, clean, dry small test tubes and a test tube holder. Label the twelve tubes with 1-12 and label the last tube blank. 3. To measure out the desired quantities of reagents, use a clean 5-mL graduated pipet. When pipetting any reagents, do not pipet directly from the container with the reagent. To avoid contamination, pour the desired amount into a clean, dry beaker, and pipet from there the necessary amount. Be sure to rinse the pipet at least once with the solution you will be pipetting to replace any water or other solution that may be in the pipet with the desired solution before actually measuring your samples. 195

4 4. Obtain 50 ml of the 3.0 x 10-4 M iron (II) solution in a clean 100 ml beaker, and obtain 75 ml of the 3.0 x 10-4 M 1,10-phenanthroline solution in a separate, clean 100 ml beaker. Use these stock solutions to prepare the following diluted solutions in your test tubes: Tube # Volume of Fe 2+ stock solution (ml) Blank Volume of 1,10- phenanthroline solution (ml) Calculate the mole fraction of the phenanthroline in each tube and record the values in your Data Table. 5. Mix each solution well by using a vortex mixer. 6. Fill your clean, dry cuvet ¾ full with the solution in the tube labeled Blank. Use a grease pencil (found in a top drawer below the balances near the instructor desk) to make a small mark on the top of the cuvet so that you can place it into the spectrometer in the same orientation throughout the experiment. From the Experiment menu, select Calibrate Spectrometer. The calibration dialog box will display the message: Waiting... seconds for lamp to warm up. The minimum warm-up time is one minute. When the dialogue box appears that says Place a blank cuvet in the device, wipe the cuvet with a KimWipe and place it in the spectrometer so that the beam of light passes through to two clear sides. Click Finish Calibration, and when the calibration is complete click OK. 196

5 7. Empty the blank cuvet and rinse it twice with small amounts of the solution in test tube 5. Fill the cuvet ¾ full with the test tube 5 solution and place it in the spectrometer. Click. A full spectrum graph of the solution will be displayed. Note that one area of the graph contains a peak absorbance. Click to complete the analysis. 8. Click the Configure Spectrometer Data Collection button (the one with the colored graph on it) from the toolbar. A dialog box will appear. Under Set Collection Mode, select Abs vs. Concentration. The wavelength of peak absorbance (λ max ) should be automatically selected. If it is not, choose the λ max by clicking on the graph or by checking the box next to the desired wavelength. Record this value in your Data Table. Click OK, then click Yes to store the latest run. 9. Return solution 5 to its test tube. Starting with solution 1, pour a small amount into the cuvet. With this small amount, carefully coat the inside of the cuvet. Discard this rinse. Pour the remaining solution into the cuvet and then Click. When the absorbance reading stabilizes, click. Record the absorbance of this tube in your data. Because you will not be creating a graph with Logger Pro, click Cancel to close the dialogue box. 10. Return the solution back to its original tube in case you need to re-measure the sample. Continue with the next sample. Be sure to rinse the sample cuvet with a small portion of the solution each time before you fill it with the appropriate solution. Measure the absorbance of each of the standard solutions using this method. Record all of the absorbance readings in your Data Table. PART B PLOTTING THE DATA WITH EXCEL 11. Using Microsoft Excel, generate a graph of absorbance as a function of the mole fraction of the phenanthroline. You will want to graph your data as two sets: the set of solutions whose absorbances are ascending (the points producing a line of positive slope) and the set of solutions whose absorbances are descending (the points producing a line of negative slope). Input your data into 4 separate columns. The first set (the ascending absorbances) will go into the first 2 columns (mole fraction and ascending absorbance) and the second set (the descending absorbances) will go into the next two columns (mole fraction and descending absorbance). 12. Plot the first data set as a scatter plot. To create a regression line for the data points, click on one of the data points from the graph then right click the mouse and select Add a Trendline. When the Format Trendline dialogue box opens, select linear. Under Forecast, change the value of Forward from 0 to 0.2, and check the box marked Display Equation on chart. Select Close. 13. To add the second line (descending absorbance values) to the same plot, be sure that your chart, with its line, is selected. At the top of the screen under Chart Tools select the Design Tab and hit Select Data. A dialogue box should appear labeled Select Data Source, Select the Add button to add a new series. Another dialogue box should appear labeled Edit Series. Click in the box labeled Series x values. With your mouse, drag over the new column of data to be used as x-values (mole fraction of phenanthroline where the absorbance decreases column 3). Click in the box labeled Series y values, delete the +(1), and then drag over the new column of data to be used as y-values (absorbances column 4). Click OK. 197

6 14. To create a regression line for the second set of data points, click on one of these data points then right click the mouse and select Add a Trendline. When the Format Trendline dialogue box opens, select linear. Under Forecast, change the value of Backward from 0 to 0.2, and check the box marked Display Equation on chart. Select Close. 15. Write the two equations in Questions 1 and In Microsoft Excel, from the File menu, select Print. The selected printer should be ISCI321000A and the setting should be Print Selected Chart. Click Print and your graph will be sent to the printer in the lab room and printed there. Retrieve your graph and attach it to your lab report. 17. Dispose of all solutions in the waste bottle in the fume hood. Delete all data by going to the Data menu and selecting Clear All Data. 18. 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. 198

7 EXPERIMENT 23 LAB REPORT Name: Student Lab Score: Date/Lab Start Time: Lab Station Number: DATA TABLE PART A Wavelength of Maximum Absorbance. nm Tube # Volume of Volume of Mole Fraction Absorbance 3.0 x 10-4 M 3.0 x 10-4 M Phenanthroline Fe 2+ (ml) Phenanthroline (ml)

8 CALCULATIONS

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11 QUESTIONS 1. From your Excel graph, write the equation for the line generated by the ascending values. 2. From your Excel graph, write the equation for the line generated by the descending values. 3. By reading your graph, estimate the mole fraction of 1,10-phenanthroline in the complex ion (the x value at the intersection of the two lines). 4. Solve the two linear equations from your Data Table to determine the mole fraction of 1,10- phenanthroline in the complex ion (the x value at the intersection of the two lines). At the intersection of the two lines, the both the x and y values are the same for both linear equations. 203

12 5. Using the calculated mole fraction from question 4, determine the formula of the iron (II)- phenanthroline complex ion. 6. What is the coordination number of the iron (II) ion in this complex ion? 7. A certain metal ion, M 2+, forms a complex ion with ammonia molecules to form M(NH 3 ) x 2+. A plot of absorbance as a function of mole fraction of ligand was obtained, showing the maximum absorbance occurring at mole fraction of ligand equal to Determine the formula for the complex ion. 204