Experiment 2 Determination of the Composition of Iron-Phenanthroline Complex by Job s Method Objective Theory

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1 Experiment 2 Determination of the Composition of Iron-Phenanthroline Complex by Job s Method Objective To establish the formula of a colored complex by the spectrophotometric method Theory The formation of complex in solution is often accompanied by the color appearance. Measurement of the absorbance of such a solution will afford a measure of the amount of complex ion in solution. For a reaction of the type, M n+ + yl [ML y ] n+ the amount of complex ionic solution can be determined colorimetrically for various ratios of [M n+ ] to [L]; the total concentration of metal ion and ligand is kept constant. Measurements of the absorbance at a suitable wavelength will show a maximum when the ratio of ligand to metal is equal to that in the complex. The method is known as Job s Method, after the originator, and is often referred to as The Method of Continuous Variation. Measurement may be made at any wavelength where the complex shows appreciable absorption. A portion of maximum absorption is preferred. If the measurements are made at only one wavelength then the system must such that only one complex is formed. This may be verified by making measurements at a number of wavelength within the absorption spectrum of the complex(es). If measurements at all wavelengths give the same result, it may be concluded that only a single compound is formed. Note that concurrent formation of a colorless compound may be overlooked by this method. In general, transition metal ions form a large number of complex compounds. For example, the almost colorless iron(ii) cation reacts with 1,10-phenanthroline (o-phen, colorless) to form a red complex cation: xfe 2+ (aq) + y(o-phen) [Fe x (o-phen) y ] 2+ Job s method of continuous variation will be used in conjunction with visible spectroscopy to establish the formula of the complex formed by Fe(II) and 1,10-phenanthroline. Job s method gives accurate results only under this circumstances :

2 1. The reaction is a quantitative and complete. 2. A single complex species is formed, and the species is the stable under the condition of the reaction. 3. The max for the complex species is known and is that a different wavelength from either the ligand or the metal ion. 4. The ph and ionic strength of the solution remain constant. Using this method, one makes a series of absorbance measurements in which the concentration of Fe(II) and o-phen are varied, while the total number of moles remains constant. The requirement is most easily achieved by preparing solutions of Fe(II) and o-phen at identical concentration and then mixing them in various volume ratios, keeping the total volume constant. For example, the mole fraction of Fe(II) in a series of solution may be varied in increasing order (0, 0.10, 0.25, 0.40, 0.60, 0.75, 0.90 and 1.00) while the mole fraction of o-phen is varies in decreasing order (1.00, 0.90, 0.75, 0.60, 0.40, 0.25, 0.10 and 0). Note that the sum of the mole fractions of the metal ion and the ligand in each pair is constant. The mole fraction of the metal and the ligand are easily calculated. By definition, So that X M = n M / n T and X L = n L /n T X M + X L = 1 or X M = 1 - X L Where n M and n L = number of moles of the metal cation and the ligand, respectively; n T = total number of moles (= n M + n L ) and X M and X L = moles fraction of the metal and the ligand, respectively. Under ideal reaction conditions, the maximum amount of the complex [Fe x (o-phen) y ] 2+ will form when the mole fraction of Fe(II) and o-phen are in the correct stoichiometric ratio. All other combinations result in the formation of less amounts of the complex. Since the product complex is colored, the absorbance or the solution mixture indicates the amount of the complex that has formed. A plot of absorbance versus the mole fraction of the metal ion (X M ) and the ligand (X L ) generate a graph (Figure 2.1) whose maximum indicates the stoichiometric composition. In Figure 2.1, the absorbance maximum occurs at about X L = 0.75 and X M = The ratio between them X L /X M = 0.75/0.25 = 3, indicating that the ligand to metal mole ratio is 3 :1.

3 Figure 2.1 Absorption vs. mole fraction Procedures 2.1 Preparation of Stock Solutions The following solutions will be prepared and labeled as A, B, C, and D, corresponding as described: Solution A: 50 ml of 1.0 x 10-3 M Fe 2+ in 0.1 M HCl Solution B: 60 ml of 1.0 x 10-3 M o-phen Solution C: 10 ml of freshly prepared aqueous 1% hydroquinone solution Solution D: 10 ml of 1 M HCl solution 2. 2 Preparation of Complex Solution Using 10 ml graduate pipettes (use a separate pipette for each solution), add the amounts of solutions A through D shown for solution #1 (in the following table) to a 25 ml volumetric flask. Add 2.5 percent sodium citrate solution to the volumetric flask to adjust the ph to ~ 3.5 (test by applying a drop of solution to a piece of universal ph paper and discard the ph paper in a waste container No. 7). Finally, add distilled water to the 25 ml mark. Shake the solution well, and allow it to stand for 10 minutes. Transfer the solution to the 50 ml beaker labeled solution #1. Cover the beaker with a watch glass. Rinse the volumetric flask, and repeat the procedure for Solution #2 through Solution #7.

4 Beaker # A (Fe 2+ ) B (o-phen) C (hydroquinone) D (HCl) Solution Solution Solution Solution Solution Solution Solution Now, prepare seven blank solutions identical to the proceeding, except for leaving out solution B, in 25 ml volumetric flask. Transfer the solutions to labeled Erlenmeyer flasks. 2.3 Absorbance Measurement and Calculation Set the wavelength of the spectrophotometer to 508 nm. Fill one cuvette with the Blank # 1 solution and the other cuvette with Solution #1. Insert the Blank #1 cuvette in the cavity and set the absorbance to zero. Remove the blank cuvette and insert the cuvette with Solution #1. Record the absorbance, A. Repeat the procedure with all over sets of solutions (all sets of solutions discard af a waste container No. 4). Determine the number of millimoles of Fe 2+ (n M ) and o-phen (n L ) in each solution and obtain the total millimoles (n T ). Calculate the mole fractions of Fe 2+ (X M ) and the ligand (X L ) for each solutions. Make a plot of absorbance versus the mole fractions (X M and X L ) as shown in Figure 2.1. From the maximum of the curve, determine the mole ratio between Fe 2+ and o-phen and establish the formula of the complex [Fe(-phen) y ] 2+. Prelaboratory Problems 1. A student records a transmittance of 40 percent for a particular solution. What would the transmittance be if the solution concentration were doubled? 2. A metal cation M n+ reacts with a ligand L to form the complex am n+ + bl [ M a L b ] n+

5 Using a M solution of M n+ (A) and M solution of L (B), a series of solution was prepared as shown below. Each solution was diluted to 50 ml and the absorbance was measured. Sample # Solution A (ml) Solution B (ml) Absorbance Calculate the mole fractions of the metal cation and the ligand for each solution. Show your calculations. Make a plot of absorbance (y axis) versus mole fraction of the metal cation and the mole fraction of the ligand (x axis). Determine the composition of the complex. Postlaboratory Problems 1. What effect will a dirty cuvette have on the absorbance reading? 2. If the meter on a spectrophotometer read 30 percent transmittance, what is the absorbance? 3. A sample of 10 ml of 1.0 x 10-3 M Cu 2+ solution is mixed with 40 ml of M NH 3 solution. Calculate the mole fractions of Cu 2+ and NH 3 References 1. Sing, M.M., Pike, R.M., and Szafran, Z., Microscale & Selected Microscale Experiments for General & Advanced General Chemistry: An Innovative Approach. 1 st ed., John Wiley & Sons, USA, Pass, G. and Sutcliffe, H. Practical Inorganic Chemistry: Preparations, Reaction and Instrumental Methods, Chapman and Hall, London, 1974.

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