Experiment 7: Spectrophotometric Determination of the Stoichiometry of a Complex Dela Cruz, Richard Dean Clod C. Group 4, Chem 27.1, WAD1, Sir Jet Torres November 29, 2014 I.1 I.1Abstract Spectrophotometry has different methods in which the stoichiometry and composition of an unknown complex can be determined. In this particular experiment, the stoichiometric ratio of Iron (II) ion and 1,10phenanthroline was investigated by measuring absorbance of a 508 nm UV-vis light source. The different methods were: the a) continuous variation method, where absorbance is plotted aganst mole fraction, b) mole-ratio method where absorbance is plotted against mole-ratio and c) slope-ratio method where the two slopes of two different cases of concentrations are compared with each other. Through these three methods, the complex was found out to have a 1:3 ratio of moles, which is the theoretical number of moles, Fe(C12H8N2)32+. II. Keywords: spectroscopy, complex, mole-ratio, mole fraction, slope ratio, continous variation
III. Introduction Spectrophotometry is the quantitative measure of how a material or unknown solution transmits or reflects certain properties as a function of wavelength. Spectrophotometric tiration, is the use of the spectrophotometer as the “indicator” in said titration. After observing multiple instances of concentrations and variation in the solutions, one measures them in the spectrophotometer and plots the absorbance much like in a titration curve. In this experiment we tested Iron (II) and phenanthroline through different methods of spectrophotometric titration. This experiment aims to determine the stoichiometric ratio of a metal and a ligand in an unknown complex, and thus also determing its chemical formula. This is done by three methods: continuous variation, mole-ratio, and slope-ratio method. The continuous variaton methods makes use of different volumes of the metal and the ligand are used but the total volume remains constant. Here, the point where absorbance is the highest (in the plot of absorbanec vs mole fraction), is where the stoichiometric ratio lies. This is because the proper complex formed, has the highest absorbance.
constant, while the other reactant (usually ligand) is added in excess. When the graph of absorbance vs mole-ratio reaches a point where it plateaus, the point where the plateau started is the ideal stoichiometric ratio. This is because absorbance does not increase anymore, since a complex has already formed and excess ligands do not contribute to absorbance. Lastly, the slope-ratio forces the completion of the reaction of the complex by adding in both reactants in excess in two separate cases. One is kept constant while you add the other in excess, then do the same but with reversed roles; then use linear regression to find the slopes of their graphs vs absorbance. The ratio of these slopes determines the stoichiometric ratio.
IV. Experimental A. Continuous Variation Six 50-mL volumetric flasks were labeled A to F. In each flask, flask , add the following f ollowing solut ions in the exact same order: 0.0007 M Iron (II) solution, 5 mL acetate buffer, 1 mL 0.0007 M hydroxylamine hydrochloride, and 0.0007 M phenanthroline solution. These can be summarized in Table 1.1
In mole-ratio the concentration and volume of one reactant, usually the metal is kept Chemistry 27.1, Spectrophotometric Determination of the Stoichiometry of a Complex
Page 1 of 6
Flask mL Fe(II) soln mL 1,10phenanthroline
A 0 10
B 2 8
C 4 6
D 6 4
E 8 2
F 10 0
The solutions were then diluted to the mark and mixed thoroughly. They were left to stand for 10 minutes so that the reaction may completely finish and would make the complex develop the color. The flasks were then measured for their absorbance at 508 nm. Mole fraction of each flask was computed and was then plotted against absorbance. B. Mole-Ratio Method Six 50-mL volumetric flasks were labeled A to F. The following solutions wered added to each flask. To each flask, add 2 mL 0.0007 M iron (II) solution, 5 mL acetate buffer, 1 mL hydroxylamine hydrochloride and 0.0007 M phenanthroline in various volumes. A
B
C
D
E
F
1
3
5
8
12
15
Flask
mL Fe(II) soln mL 1,10phenanthrolin e
C. Slope-Ratio Method First part : Label five flasks A to E. In each, add 5 mL Iron (II) solution, 5 mL acetate buffer, 1 mL hydroxylamine hydrochloride, 0.0007 phenanthroline solution. D
E
1
2
3
4
5
E
0.5
1.0
1.5
2.0
2.5
0.7
B 2
C 4
D 6
E 8
F 10
1 0
8
6
4
2
0
Continuous Variation
0.5
-0.1 0
0.2
0.4
0.6
0.8
1
Graph 1.1 Data of varying c oncentraions and the mole fraction of Fe vs phenanthroline
B. Mole-Ratio method
Flas k
Table 1.3 Volume of 0.0007 M phenanthroline to be added to each flask.
Second part : Label five flasks from A to E. In each, add: Iron (II) solution, 5 mL acetate buffer, 1 mL hydroxylamine hydrochloride, 5 mL 0.0021 M phenanthroline.
A 0
Table 2.1 Data of varying concentrations of Fe and phenanthroline
0.1
C
D
A. Continous Variation
Dilute to mark and measure absorbance at 508 nm for each flask. Compute for mole ratio of Fe vs phenanthroline and then plot it against absorbance.
B
C
V. Results
0.3
A
B
Measure the absorbance in each case at 508 nm, and then using linear regression, compute for distinctive slopes for each. The resulting ratio would be the stoichiometric ratio.
Table 1.2 Volumes of 0.0007 M phenanthroline added to each flask.
Flask mL 1,10phenanthroline
A
Table 1.4 Volume of 0.0007 M Iron (II) solution to be added to each flask
Table 1.1 Volumes of standard Iron (II) solution and phenanthroline in each flask
Flask mL 1,10phenanthroline
Flask mL Fe(II) solution
A
Number of moles Phen Fe
7 x 107
B
2.1 x 10-6
1.4 x 10-6 1.4 x 10-6
Mole ratio (Phe n/Fe ) 0.5
Absorban ce
1.5
0.156
Chemistry 27.1, Spectrophotometric Determination of the Stoichiometry of a Complex
0.037
Page 2 of 6
1.2
C
3.5 x 10-6 5.6 x 10-6 8.4 x 10-6 1.05 x 10-5
D E F
1.4 x 10-6 1.4 x 10-6 1.4 x 10-6 1.4 x 10-6
2.5
0.224
4
0.325
6
0.322
7.5
0.320
mL Fe(II) 0.5 1.0 1.5 2.0 2.5
Flask A B C D E
[Fe(II)] in M
Absorbance
7.0 x 10-6 1.4 x 10-5 2.1 x 10-5 2.8 x 10-5 3.5 x 10-5
0.084 0.155 0.224 0.314 0.384
Table 2.3 Data when phenanthroline is constant
Table 2.2 Data of Mole-ratio method of spectrophotometry (Mole-ratio vs absorbance)
Slope Ratio (phen constant)
Absorbance 0.35
0.5
0.3
0.4
0.25
e c n 0.3 a b r o s 0.2 b A
0.2 0.15 0.1
y = 10843x + 0.0045 R² = 0.998
0.1
0.05
0
0 0
2
4
6
8
0
0.00001
0.00002
0.00003
[Iron(II)] (M)
Graph 1.1 Plot of Absorbance vs Mole-Ratio of complex
Graph 1.3 Plot absorbance vs varying iron
C. Slope-Ratio Method Flask A B C D E
mL C12H8N2 1 2 3 4 5
[C12H8N2] in M 1.4 x 10-5 2.8 x 10-5 4.2 x 10-5 5.6 x 10-5 7.0 x 10-5
Absorbance 0.048 0.092 0.142 0.195 0.246
Table 2.3 Data when Iron (II) solution is constant
VI. Discussion The complex used in the experiment has the iron ion as the metal, and 1,10 phenanthroline as the ligand. Fe2+(aq) + 3C12H8N2(aq) → [Fe(C12H8N2)3]2+(aq)
The phenanthroline complex has a theoretical stoichiometric ratio of 1:3 and has a deep red-orange color. Specific solutions were Slope-ratio method (constant also added to the mixture, to make sure the Fe(II)) complex is form without any problems. The acetate buffer was added in order to maintain a 0.3 pH of 2 to 9. Anything higher and/or lower than y = 3564.3x - 0.0051 e c R² = 0.9989 this range, results to the ferrous ions n 0.2 a b precipitating. The addition of 0.0007 M r o s 0.1 hydroxlamine hydrochloride was added to ensure b A that the ferrous ions (Fe2+) don’t oxidize into ferric 0 ions (Fe3+). If ferric ion is formed, it will generate 0.00E+00 2.00E-05 4.00E-05 6.00E-05 8.00E-05 a different colored complex with phenanthroline. [C12H8N2] (M) Also, 508 nm wavelength was used because this is the optimum level of wavelength the complex Graph 1.2 Plot absorbance vs varying C12H8N2 absorbs. Chemistry 27.1, Spectrophotometric Determination of the Stoichiometry of a Complex Page 3 of 6
0.00004
Three different methods were used: (1) continuous variation, (2) mole-ratio method, and (3) slope ratio method.
reactant, usually the metal, constant while adding the other, usually the ligand, in excess. The moleratio is then plotted against absorbance. Theoretically, same as continuous variation method, the right stoichiometric ratio of metal to ligand has the maximum absorbance. So when the reaction reaches the point where the metal to ligand mole ratio is right, the graph usually plateaus. This indicates that maximum concentration of complex is achieved, and all other components thereafter do not significantly contribute absorbance to the solution.
A. Continuous Variation Method In the continuous variation method, the total number of moles of the solution were kept constant, and only mole ratio of each flask varied. Theoretically, the right metal to ligand ratio of the complex has the maximum aborbance. It means that the complex has the highest concentration at that specific ratio, and no other components contribute significantly to the solution.
Absorbance
0.7
Continuous Variation
0.5
0.6
0.4 0.5
0.3
0.4
0.2
0.3
0.1
0.2
0 0
0.1 0 0
0.2
0.4
0.6
0.8
1
1.2
Graph 2.1 Continuous Variation – mole fraction (X-axis) vs absorbance (Y-axis)
As seen in graph 2.1, when you extrapolate the two sections of the graph – descending and ascending, you get the correct combining ratio of the complex. The intersection lies on top of around 0.25 mole fraction of Iron (II) solution. Because iron (II) solution has 0.25 mole fraction, then phenenthroline has 0.75, and therefore 0.25/0.75 is equal to 1/3. This indicates that the metal to ligand ratio is 1:3. This method is best applied to ligands with only one complex. If more than one complex forms, the different peaks of the graphs would be more than one, and it would be difficult to determine the right stoichiometric ratio.
B. Mole-Ratio Method The mole-ratio method determines the correct stoichiometric ratio by keeping one
5
10
15
Graph 2.2 Mole-Ratio - mole ratio (X-axis) vs absorbance (Y-axis)
As seen in graph 2.2, the graph plateaus (almost constant) after a certain point. To find the plateau point, find the intersection of the extrapolated line of the increasing part of the graph, and the flat line of the plateau. The point of intersection is near 3, therefore the metal to ligand ratio is 1:3. This particular method is effective for large complexes where the ligand can accodomate more metal, like for example 1:3 is more favorable than 1:1. When this happens, the graph would look more identical to the extrapolated lines, suggesting a clearer point of plateau. This in turn implies that a there is a large formation constant and the complex is stable. C. Slope-Ratio Method Lastly, in this method the complex was forced into completion by adding excess amounts of either metal or ligand. When one reactant is in excess, the concentration of the product is limited by the other reactant (not excess). This method
Chemistry 27.1, Spectrophotometric Determination of the Stoichiometry of a Complex
Page 4 of 6
assumes the complex follows the Beer-Lambert law, and the reaction is complete. After getting the plot of the excess reactant vs absorbance for both cases, the slope of each is then computed through linear regression. And combined with Beer ’s law, the ratio of the slopes would be equal to: / /
=
=
1 2
Where m1 is the slope of constant metal, and m2 is the slope of constant ligand. And in that case, y would be the moles of m etal and x would be the moles of ligand. It would imply a stoichiometric ratio of y:x. The slopes of the constant metal vs constant ligand in graphs 1.3 and 1.4, is 3564.3 and 10843 respectively. Therefore, 10843/3546.3 is around 1:3. Thus, the stoichiometric ratio of metal to ligand is around 1:3.
VII. Conclusion and Recommendation Spectrophotometry can be use to determine the stoichiometric ratio of the metalligand complex. The three methods used in this experiment each have their own disadvantages and advatanges, and knowing the best suited one would achieve more accurate results, and more efficient methods. Through these methods, the metal-ligand ratio of Fe(C12H8N2)3 was found out to be1:3, which is the actual theoretical ratio. It is recommended that the proper handling of the spectrophotometer and the cuvettes must be practiced at all tim es, to achieve proper and accurate results. It is also recommended to make sure that the complex is formed properly, and to also prevent inconsistencies, that may affect the results, from happening.
VIII. References Harvey, D. (1999). Modern Analytical Chemistry . USA: McGraw Hill Companies. Skoog, D., West, D., Holler, F., & Crouch, S. (2004). Fundamentals of Analytical Chemistry . Canada: Brooks/Cole-Thomson Learning. Wear, J.O. (1968). Mathematics of the variation and mole ratio methods of complex determination. Retrieved from http://libinfo.uark.edu/aas/issues/1968v22/v22a1 7.pdf on November 27 2014.
Chemistry 27.1, Spectrophotometric Determination of the Stoichiometry of a Complex
Page 5 of 6
Chemistry 27.1, Spectrophotometric Determination of the Stoichiometry of a Complex
Page 6 of 6
