Ecaldre, Alvek I. Chemistry 27.1- FEGCatalig, Antonio Mari P.
Experiment 7 Spectrophotometric Determination of the Stoichiometry
of a ComplexI. Abstract
The experiment aims to demonstrate, compare and contrast three
of the most common spectrophotometric methods used in determining
the stoichiometry of a complex, namely: (1) Continuous Variations
Method (CVM), (2) Mole-Ratio Method (MRM) and (3) Slope-Ratio
Method (SRM). These methods involve a play in the concentration of
the reagents in the reactionvarying them from a sample to
anotherall of which subjected to analysis under a UV-Vis
spectrophotometer. The recorded absorbance were plotted against the
mole fraction, mole ratio, and concentrations of the reagents for
CVM, MRM and SRM, respectively. After much consideration, a final
stoichiometry could not be concluded from the data gathered from
CVM, MRM and SRM for they were 50%, 25% and 50% erroneous,
respectively (comparing their 1:4, 1:6 and 1:2 metal to ligand
correspondence to the 1:3 theoretical). Keywords:
spectrophotometry, stoichiometry, absorbance,
continuous-variations, mole-ratio, slope-ratio
II. Introduction Spectrophotometry, a process involving the
interaction between molecules and light, is a useful technique in
understanding the behaviour/nature of complex ions in solution;
from its stoichiometry to even its formation constant. In this
technique, reagents are allowed to react, forming the complex.
Absorbance values are gathered, by analysing these solutions under
a spectrophotometer. These values, when analysed, can give valuable
information about the analyte.There are three most commonly used
methods for complex ion analyses, these are: (1) the method of
continuous variation, (2) the mole-ratio method and (3) the
slope-ratio method.The continuous variations method (CVM) deals
with solution samples with constant total volume and constant total
molar concentration, only varying the mole ratio of the reactants
(usually with one increasing and the other, decreasing). The
absorbance values are plotted against the mole fractionIII.
Methodology
The first major step of this experiment was the removal of
inhibitor from commercial styrene. In a small separatory funnel,
10-ml commercial styrene, 4-ml of 3M NaOH, and 15-ml water were
mixed thoroughly. Layers were allowed to separate to withdraw the
aqueous layer. The organic layers was sequentially washed with two
8ml portions of water to separate the organic layer thoroughly from
the aqueous layer. The styrene was dried in a small Erlenmeyer
flask containing a little anhydrous Calcium Chloride. The flask was
swirled, and the mixture was allowed to stand for 5-10 minutes.
Decantation was performed to separate the liquid from the drying
agent. The second major step of this experiment is the
polymerization of pure styrene. In a small soft-glass test tube,
2-3ml of dry styrene and 2-3 drops of tert-butylperoxybenzoate were
mixed. The test tube was clamped in a vertical position over wire
gauze, and a thermometer was inserted so that its bulb is in the
liquid. The mixture was heated with a small burner flame. The flame
was temporarily removed when the temperature reached 140 . Heating
was resumed to maintain gentle boiling. The temperature rose to
180-190 after the onset of polymerization, increasing the viscosity
of the mixture. The thermometer was removed and the styrene was
poured in a watch glass when the temperature started to decrease.
IV. Results and DiscussionsContinuous Variations MethodThis method
involves an absorbance versus mole fraction plot evaluated by
first, taking the intersection of the lines extrapolated from the
increasing and decreasing portions of the curve and then
identifying its corresponding ratio. Notice in Figure 1 that the
intersection lies at the maximum, implying that the complex has
higher absorbance at the given wavelength as compared to those of
the reagents.To obtain the complexs stoichiometry, the
intersections x-coordinate was calculated from the given trend
lines equations as in Solution 1. These calculations gave a 1:4,
metal to ligand correspondence (with the metal having a mole
franction of 0.2).
Mole-Ratio Method
This method, like the previous one, also involves taking the
intersection of two extrapolated lines (one from the increasing and
the other from the decreasing portion). Unlike the CVM, however,
this method uses an absorbance versus mole-ratio plot; directly
giving the stoichiometry of the complex.Solution 2 shows the
calculation for the intersections x-coordinate giving a value
approaching 6; meaning, there is a 1:6 correspondence between the
metal and the ligand, respectively.
Slope-Ratio Method
This last method is unlike the previous twographically, at
least. Instead of extracting the intersection of trend lines, SRM
shows the stoichiometry of the complex by comparing the slopes two
curves: one, for when the volume of the Fe solution is held
constant (i.e. Figure 3) and the other for when it Is the ligands
volume that is maintained (i.e. Figure 4). With the equations of
the curves displayed in the plots, the ratio was easily calculated
(as in Solution 3) giving a 1:2, metal to ligand
correspondence.
.
VI. Conclusion Free radical polymerization is a process wherein
a polymer forms by the consecutive addition of free radicals. Free
radicals are formed usually by splitting of the radical inhibitor.
Free radicals initiate the process of addition, thereby causing the
polymer chain to grow longer.
While in the process, several factors must be observed, such as
the boiling point of the reactant and viscosity of the product.
Free-radical polymerization is one of the easiest way and is also
very important in synthesizing various polymer products that are
widely used in the world. VII. References
[1] Figure 1. Mechanism of free radical polymerization. Image
retrieved from google.com. Bruice, P. (2006). The essentials of
organic chemistry. Pearson Education, inc. Chanda, Manas.
Introduction to Polymer Science and Chemistry. Boca Raton: Taylor
and Francis Group LLC, 2006 Chiu, H.H. & Villarante, N.R.
Laboratory manual in Organic Chemistry II. Klein, D. (2012).
Organic Chemistry. John Wiley & Sons, inc. Matyjaszewski, K.
& Davis, T. Handbook of radical polymerization. John Wiley
& Sons. Inc. Savin, D. A. (2008). Free-radical polymerization.
University of Southern Mississippi. Wnsch, J. R. Polystyrene:
Synthesis, production and applications. iSmithers Rapra Publishing.
Copyright.
