Analytical Chemistry Laboratory 2
Spectrophotometric Analysis of a Two Component Mixture Mr. *****
Department of Chemical Engineering and Chemistry, Mapua Institute of Technology
Chromium and Cobalt ions both absorb visible light though their absorbance maxima are fairly well separated. By measuring the absorbance at two different wavelengths of a solution containing both ions, it is possible to simultaneously determine the concentration of each ion in the the solu soluti tion on.. An unkn unknow own n solu soluti tion on cont contai aini ning ng thes thesee spec specie iess was was anal analyz yzed ed usin using g a spectrophotometer. The concentration of cobalt in the mixture calculated using Beer’s law was 0.0131 M, while chromium was 0.0185 M.
Objective
The purpose of this experiment is to intr introd oduc ucee the the pro proper per ope operat ration ion of a spectrophotometer and how its use relates to chemical analysis. This will be achieved by recordi recording ng absorb absorbanc ancee measur measureme ements nts of a two component mixtures, and calculating its concentrations using Beer’s Law. Discussion
Overview When a beam of parallel radiation passes through a layer of solution having a thickn thickness ess,, b (cm) (cm) and a concen concentra tratio tion, n, c (mole (moles/ s/li lite ter) r) of an abso absorb rbin ing g spec specie ies, s, a consequence of interactions of the photons and the absorbing particles is attenuation of the the beam beam.. The The tran transm smit itta tanc ncee (T) (T) of the the solu soluti tion on is the the frac fracti tion on of the the inci incide dent nt radiation radiation transmitted transmitted by the solution solution.. The absorbance (A) of a solution is defined as the negative log of the transmittance (T) of the soluti solution. on. The absorb absorbanc ancee is direct directly ly proportional to the path length through the solu soluti tion on and and the the conc concen entr trat atio ion n of the the absorbing species. That is, A= εbc where ε
is a proportionality constant called the molar absorptivity. ε has units of M -1 cm-1 when b and c are expressed in cm and moles per liter respectively. This relationship between absorbance (A) and εbc is known as Beer's Law. Beer's Law is successful in describing the absorption behavior of dilute solutions only. only. At high concentrations concentrations,, the average average distance between the species responsible for absorption is diminished to the point where each each effect effectss the charge charge distr distribu ibutio tion n of its neighbors. This interaction, in turn, can alter their ability to absorb a given wavelength of radiation. Because the extent of interaction depends upon concentration, the occurrence of this phenomenon causes deviations from the linear relationship between absorbance and and conc concen entr trat atio ion. n. A simi simila larr effe effect ct is sometimes observed in solutions containing high high concen concentra tratio tions ns of electr electroly olytes tes.. The The pro proxi ximi mity ty of ions ions (in (in addi additi tion on to othe other r factors such as temperature) alters the molar absorptivity of the absorbing species.
Methodology The spect spectrop rophot hotome ometer ter was set to the the 575 575 nm chro chromi mium um maxi maximu mum. m. The The absorbance reading of a 0.02 M solution of cobalt nitrate at this wavelength was taken,
including the absorbance of the unknown chromium-cobalt mixture. The wavelength of the spectrophotometer was adjusted again to about 510 nm and the maximum was located for cobalt absorption using a 0.06 M solution of cobalt nitrate. After the maximum was located, the wavelength value was recorded. The absorbance of the 0.06 M cobalt nitrate solution, the absorbance of the 0.04 M chromium nitrate solution, and the absorbance of the unknown chromiumcobalt mixture was read.
Instrumentation
A spectrophotometer measures the amount of radiant energy absorbed by a species. It consists of a source (incandescent filament) that emits a continuous range of wavelengths. The radiant energy is focused by optics which includes a prism or grating; these define a beam of radiant energy of a specific wavelength range. The detector determines the intensity of the radiation before and after the beam is passed through the sample in a cell:
species are chromium ion and cobalt ion, both of which absorb strongly in the visible spectrum. The following data was obtained from the experiment: 0.06 M Cr soln.
0.02 M Co soln.
Sample
AλCr
0.8017
0.0158
0.2575
AλCo
0.314
0.0984
0.1661
In simultaneous determinations of two species it is necessary to generate two equations in order to determine the two unknown concentrations. In a spectrophotometric analysis these equations can be developed from the Beer-Lambert Law. Absorbance = A = e b C To solve the simultaneous equations that are generated when applying Beer's law to mixtures of absorbing species, one must know the molar absorptivity of each component at each wavelength. The molar absorbtivity of each compound at a given wavelength can be determined from the relationship between A and c in Beer’s law. Beer's Law requires the use of monochromatic radiation and it is under this restraint that the linear dependence of absorption and concentration occurs. If two or more species in a sample absorb at a specific wavelength, the instrument cannot distinguish between the individual species; it can only determine the total absorbance of the sample. In the mixture of the two species, Cr and Co, absorb at the same wavelength the total absorbance at that wavelength is: Atotal = ACr + ACo
and, Atotal = e1 bCCr + e2 bCCo
Interpretation
In this experiment, a two component solution will be studied. The absorbing
The most convenient way to construct two equations is to measure the total absorbance of the solution at two different wavelengths, 510 nm and 575 nm. Solving simultaneously: C Co = 0.0131 M and CCr = 0.0185 M.
Conclusion
The spectrophotometer was used in the analysis of the two components in the mixture which is cobalt and chromium. The calculation basically focuses on one formula – the Beer’s law. Beer's law states that absorbance of electromagnetic radiation is directly proportional to concentration: if there is more than one absorbing species in solution, the total absorbance is the sum of the individual absorbance of all the absorbing species, provided there is no interaction among the various species. By measuring the absorbance at different wavelengths (510 and 575 nm), different absorbing components could theoretically be measured by this technique. Hence calculating concentration from absorbance using Beer’s law: C Co = 0.0131 M and C Cr = 0.0185 M References
1. Christian, G.D., Analytical Chemistry, 6 th edition. New Jersey. John Wiley, 2004. 2. Englis, D. T. and D. A. Skoog, Ind. Eng. Chem. Anal. Ed., l5, 748.
