Stoichiometry of Ligand Binding and the Stability Constant of a Complex - Assignment Example

Experiment report Stoichiometry of Ligand binding and the stability constant of a complex Introduction A complex compound of Ni (NH3) n (H2O) 2+6-n is produce when aquo-Ni2+ ion, Ni (H2O) 2+6 interact with ammonia in water, where n=1 to 6. Apart from (NH3) (H2O) 2+5 no other complexes have been isolated. Investigation by use of spectrophotometer and potentiometer indicate the existence of these complexes in the solutions. The interactions in the complexes can be strong; for example, transition metals ions may react with ligands resulting in strong exothermic reaction. If it not however possible to crystallize all the species present in the solution and other technique can be used to establish their composition. In a similar manner, Ni (H2O) 2+6 cations react with bidentate amine ligand 1, 2-diamino-ethane producing other three complexes which are Ni(en)2+3, Ni(en)(H2O)2+4 and Ni(en)2(H2O)2+2 . The aim of this experiment was to the existence of the three possible complexes using spectrophotometric technique. It will use Job’s method or method of continuous variation to determine the solution of Ni2+-ethylenediamine complexes. In summary, it will examine n in the equiliobrium below. Where M is Ni2+ and L is ethylenediamine. Procedure After drying the flasks and cleaning the burettes, two standards aqueous solution, 100ML, one of them had 0.4 ethylenediamine and the other 0.4M nickel sulfate (NiSO4.6H2O) were prepared. A mixture of 10ml was then prepared using 50mL burettes, whereby the mole fraction of ethylenediamine (X) were 0.3, 0.4, 0.5, 0.6 0.7, 0.8 and 0.9. The mole fraction was prepared by mixing 3mL of ethylenediamine solution with 7mL of nickel sulfate. Other solutions were mixed through the same process. The absorbance of pure 0.4 nickel sulfate for the mixtures were determine in the range of 470nm and 700 nm of UV-visible spectrophotometers, with the absorbance of between 0 to 1. Cleanliness was carried out to avoid contamination, for example by rinsing the cell with a little solution each time before filling it. The cursor function was used to find the absorbance values for the wavelengths of 640, 622, 578, 545 and 530nm, and each absorbance values were read manually as the spectrum was recorded. Calculation and discusssion Adiff at each wavelength X 530nm 545nm 578nm 622nm 640nm 0.3 0.07728 0.1226385 0.28025 0.340925 0.256098 0.4 0.130503 0.191443 0.385666 0.44021 0.326294 0.5 0.186119 0.2658675 0.47269 0.486075 0.35074 0.6 0.3399256 0.442912 0.580106 0.42097 0.268596 0.7 0.542332 0.6435465 0.620832 0.248895 0.100992 0.8 0.554928 0.574031 0.389658 0.05968 -0.037292 0.9 0.257554 0.2670255 0.179684 0.014765 -0.02398 The value of n = X/1-X 530nm 545nm 578nm 622nm 640nm n 0.5/1-0.5 = 1 0.67/1-0.67 = 2 0.67/1-0.67 = 2 0.5/1-0.5 = 1 0.5/1-0.5 = 1 As it can be seen from the graph, there is uncertainty in deciding the best line in all the points. This shows that there are errors introduced to the experiment not only through contamination but it also shows the present of other compounds which alter the results, especially the maxima. The errors may also be due to assumptions, and random errors. The spectra of wavelength verses absorbance is shown in the figure below. It can be seen from the graph that the absorption depends on the wavelength. This produces absorption spectrum. The spectrum shows eight different concentrations. Part b: Stability of a complex This section dealt with the composition of a complex compound, that are produced by mixing ferric ions with salicylic acid. The stability can be obtained by use of spectrophotometer. The presence of a complex compound can be comfirmed due to the presence of isosbestic point, which is the frequency where the total absorbance does not depends on the concentration ratio of the two absorbing bodies. The isosbestic point is shown in the figure below. The equilibrium for ferric salicylate complex is written as Fe3+ + n(sal) ↔Fe3+ (sal-)n Optical absorbance A, according to Beer-Lambart’s law. A = Ԑl[Complex] Whre Ԑ = molar absorbption coefficiency, and l is the optical path length. Procedure Using 250mL of 0.002M hydrochloric acid form standard acid, X solution of 0.0024M in Fe3+ was made by reacting 0.289g ferric ammonium sulfate with 250mL of 0.002M hydrochloric acid. Solution Y of 0.0024M in salicylic acid was made by dissolving 0.083g salicylic acid in 250mL 0.002M hydrochloric acid. 150mL of solution Y was used in the experiment. This section had two parts. The first part produced a spectra of solutions X and Y. little absorption was seen below 435nm. Two 50 mL burettes were filled with the solutions with total volume of 10 mL in the ratios 1 mL: 9 mL, 2 mL: 8 mL, etc. up to 9 mL: 1 mL, before switching on the spectrum. The spectra of X and Y were obtained separately. The second part dealt with measurement of spectra of a series of 1:1 mixtures of X and Y. this was done using a burette as in the first method. Discussion and calculations The figure below shows the isosbestic point in the spectra. Calculations 1. The position of the isosbestic point for the series of spectra – is at a wavelength of approximately 800nm and 400nm. 2. Job’s method If the equation, aA + bB = dD, is devided by a, the results is: A + KB = mD, where K= b/a and m = d/a. Moles of Y Mole X Mole fraction of X Moles of Z 9 1 0.1 1 8 2 0.2 2 7 3 0.3 3 6 4 0.4 4 5 5 0.5 5 If the number of moles of Z is plotted against the mole fraction of X and Y, two straight lines are observed. The lines intersect at the maximum amount of product formed, 5 moles of Z. The corresponding mole fraction of X is 0.5. Thus the correct mole ratio is 0.5 mol of X: 0.5 mol of A or simply 1:1. 3. Molar Absorption Coefficient (Ԑ) Method A The table below shows the values for XY/A and A. XY/A for 530nm A XY/A for 800nm 9/0.26656 = 33.7635 0.26656 9/0.0077615 = 1159.56 16/0.67823 = 23.5908 0.67823 16/0.017428 = 918.06 21/1.2076 = 17.3898 1.2076 21/0.030953 = 678.45 24/1.4799 = 16.2173 1.4799 24/0.034914 = 687.403 25/1.6186 = 15.4454 1.6186 25/0.041115 = 608.05 24/1.4293 = 16.7914 1.4293 24/0.037060 = 647.60 21/1.1363 = 18.4810 1.1363 21/0.030591= 686.48 16/0.75538= 21.1814 0.75538 16/0.02025 = 790.12 9/0.42385 = 21.2339 0.42385 9/0.014337 = 627.75 4. The slope of the curve would yield = -10.04 Ԑl = 1/100.16 = 0.00998 Ԑ = 0.00988cm-1M-1 And the intercept = =30.49 K=-0.1031 From Beer Lambart law, A = Ԑbc 0.26 = (0.00988cm-1M-1) (1.00 cm) (c) Therefore, c = 26.316M Method B The table below shows the values of and c/√A √A 0.00968 0.51629 0.00607 0.823547 0.00454 1.0989085 0.00411 1.2165114 0.00393 1.272242 0.00418 1.1955333 0.00470 1.0659737 0.00575 0.86912599 0.00768 0.6510376 The slope =1/Ԑl = -0.007 Ԑl = -1/0.007 Ԑ = -1/0.007 = 142.85cm-1M-1 =0.012 K = Ԑl/0.0122 = -0.007/0.000144= - 48.61 The two graphs are not similar. The second method is more reliable due its more realistic figures. The two slopes are more similar. 5. Go = -RTlnK R = 8.314 J/moleK T = temp in Kelvin From Method A Go = 8.314 J/moleK x 298K x- ln 0.1031 =9591.17J = - 5.6292kJ Negative sign indicate forward reaction From method B Go = 8.314 J/moleK x 298K x- ln 48 =9591.17J = - 9.59117kJ Negative sign indicate forward reaction Conclusion This experiment has examined complexes compound present in a solution by use of spectrophotometer. In ideal conditions, Fe2+ forms a mole fraction with the correct stoichoimetry ratio. The colour indicates the presence of complex since complex products is coloured. The graphs produced have also indicated the presence of complex product at maxima. REFERENCES 1. W.C. Vosburgh and G.R. Cooper, J. Amer. Chem. Soc., 63, 437 (1941). 2. In Bailar, J. C. (1956). The chemistry of the coordination compounds. New York: Reinhold Pub. Corp. 3. Daniel C. Harris, (1987), Quantitative Chemical Analysis, 2nd ed, W.H. Freeman, New York, , p 512-519. 4. Skoog, D. A., West, D. M., & Holler, F. J. (1996). Fundamentals of analytical chemistry. Fort Worth: Saunders College Pub. 5. American Chemical Society. (1948). Analytical chemistry. Washington: American Chemical Society. 6. Kauffman, G. B. (1981). Inorganic coordination compounds. London: Heyden. 7. Kheir, A. A. E., Belal, S., Ayad, M., & Adl, S. M. E. (January 01, 1986). The Use of Charge Transfer Complexation in the Spectrophotometric Determination of Some Corticosteroid Drugs Through Intermediate Oxime Formation. Analytical Letters, 19, 9-10. Read More