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Файл:Optical methods of analysis. Educational aid
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31
Determination of copper concentration in the test solution. A sample
of the investigated copper solution is taken into a 50 ml volumetric flask
using a burette in the range of 5–7 ml. The sample volume VX must be
accurately measured. Add 5 ml of ammonia solution, dilute with distilled
water to the mark. After stirring the solution, measure its optical density
under the selected conditions at least three times and calculate the average
value. If the value of the optical density of the measured solution goes beyond
the interval covered by the constructed graph, the experiment should be
repeated, taking a correspondingly larger or smaller volume of the sample of
the initial test solution.
Concentration can be determined in two ways:
1. You can use a calibration curve. To do this, determine the average
value Ā for the measured solution and plot it on the calibration graph, draw
a line parallel to the concentration axis until it intersects with the line of
the calibration graph. You must to lower the perpendicular to
the concentration axis and determine the concentration, Cvf, of the copper
solution in the volumetric flask. After this calculate the concentration of
copper Cx in the original test solution;
2. You can use the calculation method. In this case, the obtained values
of Ā and dependence А
х
= ε・l・сх are used for each reference solution. Find
the value of the molar absorption coefficient of the copper ammonia solution ε. Calculate the average of them. Based on the average value of ε and
the optical density of the measured solution Ā, the concentration of copper
in the measured solution of С
measured
is determined. Recalculation is per-
formed to determine the concentration of Cх, taking into account dilution.
To compare the measurement results of Cx obtained by two methods, it
is necessary to bring them to the same dimension.
5 . 1 . 3 . P h o t o c o l o r i m e t r i c d e t e r m i n a t i o n
o f t i t a n i u m ( I V )
Aqueous solutions of titanium salts in the visible part of the spectrum
practically do not absorb incident radiation. This method for determining titanium is based on measuring the optical density of a solution of a bright
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yellow oxide complex of titanium (IV), which is formed in an acidic environment with an excess of hydrogen peroxide:
TiO2+ + H2O2 = [TiO(H2O2)]2+.
The optical density of solutions of titanium complexes will be proportional to the concentration of titanium (IV). Vanadium, cerium, molybdenum
ions, which also form colored compounds under these conditions, can interfere with the determination of titanium. In large quantities, fluorides and
phosphates interfere with the analysis. In this case, the results of the analysis
for titanium will be underestimated. In the presence of interfering ions,
the use of the addition method gives the best results.
Equipment and reagents:
1. Photometer with a set of cuvettes, measuring flasks for 50 ml, meas-
uring cylinder for 5 ml.
2. Standard titanium salt solution with a certain titanium concentration
of about 1·10
-3
g/ml.
3. 3 % hydrogen peroxide solution.
4. Solution (5 %) of sulfuric acid.
Construction of a calibration graph:
1. Prepare 5 reference solutions of Ti (IV) using volumes (MIC) 1.00;
3.00; 5.00; 8.00; 10.00 ml of standard titanium salt solution. At the beginning, a solution with the highest concentration is prepared, with the help of
which a light filter and a cuvette are selected. To prepare a reference solution,
a certain volume of a standard titanium salt solution is injected into a 50 ml
volumetric flask using a burette, then 5 ml of H2O2 (3 %) and brought to
the mark with a solution of sulfuric acid (5 %). After closing the measuring
flask with a stopper, the resulting solution is thoroughly mixed. The concentration of titanium in the prepared reference solutions is calculated taking
into account the dilution of the standard solution.
2. Prepare a comparison solution: 5 ml of H2O2 (3 %) is injected into
a 50 ml volumetric flask and brought to the mark with a solution of sulfuric
acid (5 %). A light filter and a cuvette with an optimal working length are
selected according to the procedure described in subsection 5.1.1.
3. Under the selected conditions, the optical density A of the reference
solutions is measured, repeating each definition 3 times. Find the average
value of A for each standard.
4. Calculate the value of ε for each solution.
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5. The values of the measured and calculated values are entered in
a table similar to Table 5.1.
6. Build a calibration graph in the coordinates A – CTi, g\ml.
Determination of the titanium concentration in the test solution.
A sample of the investigated titanium solution within 5–7 ml is taken into
a 50 ml measuring flask using a burette (the volume of the MX sample must
be accurately calculated). Add 5 ml of H2O2 and bring it to the mark with
a solution of sulfuric acid. After mixing the solution, its optical density is
measured at least three times. If the value of the optical density of
the measured solution goes beyond the range covered by the constructed
graph, the experiment should be repeated by taking a correspondingly larger
or smaller sample volume of the initial test solution. Then the desired
concentration is determined in two ways:
1. According to the calibration schedule. The average value of A for
the test solution is calculated. With the help of a calibration graph,
the concentration in a measuring flask is found, С
V.f
., then the concentration
of titanium CX in the initial test solution is calculated
2. According to the calculation method. For each reference solution, get
the values of . Find the value of the molar absorption coefficient of the solution of the titanium peroxide complex ε. Calculate the average value. In ac-
cordance with it and the average value of the optical density of the measured
solution, the concentration of titanium in the measured CVf solution is calculated. To determine the concentration of CX, Cvf is recalculated taking into
account dilution. To compare the CX measurement results obtained by
the two methods, it is necessary to bring them to the same dimension.
5 . 2 . L a b o r a t o r y w o r k o n s p e c t r o p h o t o m e t r y
5 . 2 . 1 . O b t a i n i n g a b s o r p t i o n s p e c t r a o f a m e t h y l
o r a n g e s o l u t i o n a t d i f f e r e n t p H o f t h e s o l u t i o n
Determination of methylorange concentration. The methyl orange in-
dicator (n-dimethylamino benzenesulfonic acid sodium) changes its structure
34
depending on the pH of the solution. At the same time, the color of the solution and the position of the maxima on the light absorption curve change, this
is associated with the appearance of new chromophore groups in the indicator
molecule:
Reagents:
1. Standard solution of methyl orange 110-3 m (0.327 g of methyl orange per 1 liter of distilled water).
2. Buffer solutions: pH = 1.68 and 9.18.
Obtaining absorption spectra. On a spectrophotometer, the absorption
spectra of the methyl orange solution are recorded in the visible region at two
different pH values. To do this, 2.5 ml of the initial standard solution of methyl orange are taken into two 25 ml volumetric flasks and the first flask is
brought to the mark with buffer solutions with pH = 1.68, the second with
buffer solution with pH = 9.18. The solutions are thoroughly mixed.
The resulting solutions are filled into two rectangular cuvettes with
a layer thickness of 1 cm. A reference solution is poured into one cuvette,
and distilled water into the other.
The cuvettes are installed in the cell holder and the optical density is
measured every 10 nm in the range of 380–700 nm.
Build light absorption curves for the studied solutions, determine
the position of the maxima of the absorption bands. According to the obtained curves, the optimal conditions (pH and λ
max
) for the quantitative de-
termination of methyl orange are chosen.
Determination of the concentration of methyl orange. At the selected
pH and λ
max
values, the optical density of a number of methyl orange solu-
tions of various concentrations is measured. To obtain standard solutions 0.5;
0.75; 1.75; 2.5; 3.75 ml of methyl orange standard solution transfer into
25 ml volumetric flasks and dilute to the mark with the appropriate buffer
35
solution. One need to calculate the concentration of the resulting solutions,
measure their optical density. Build a calibration curve A = f (C
ref
) for a refrence's solutions of methyl orange. Then, for a solution of unknown concentration under the same conditions, the optical density is measured and
the concentration is determined from the graph.
The test solution with a volume of 2.5 ml is also placed in a 25 ml
volumetric flask and adjusted to the mark with the same buffer solutions.
The optical density is measured at the selected values of λ
max
and pH,
the concentration of methyl orange is found from the graph. Calculate
the concentration of methyl orange in the studied solution.
5 . 2 . 2 . D e t e r m i n a t i o n o f t h e d i s s o c i a t i o n c o n s t a n t
of m e t h y l o r a n g e
The dissociation of colored organic compounds having the character of
weak acids or bases can be studied by the spectrophotometric method in cases
where the color of the undissociated and dissociated forms of the compound
differs. The dissociation constant can be found graphically. For this, the dependences of the optical density of the solution, measured at two wavelengths
corresponding to the absorption of different forms of the indicator, on the pH
of the solution are studied. This dependence may have the form shown in
Fig. 5.1. The point of intersection of these curves is close to the point corresponding to 50 % dissociation of this compound. The projection of this point
onto the pH axis gives the inverse logarithm of the dissociation constant.
Fig. 5.1. Dependence of the optical density of the indicator solution on pH
at different wavelengths λ1 and λ2
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Reagents:
1. Methylorange, 0.5 % solution.
2. A number of buffer solutions (pH from 1 to 9).
Determination of the position of absorption bands of two forms of
methylorange. In two 25 ml volumetric flasks, 1.0 ml of the initial indicator
solution is taken, adjusted to the mark with buffer solutions with pH 1.68 and
9.18. In the resulting solutions, the indicator will be in various forms.
The flasks are stoppered and mixed. The solutions are placed in cuvettes,
distilled water is used as a reference solution. The optical densities of the solutions (A1 and A2) are measured by changing the wavelength from 380 to
700 nm after 10 nm. Graphs of the dependence of optical densities A1 and A2
on the wavelength are built. The values of λ1 and λ2 corresponding to
the maximum optical densities on each curve are found from the graphs.
Then the optical densities of a number of prepared solutions with intermediate pH values are measured at the found wavelengths λ1 and λ2.
One have to build graphs of the dependence of the optical densities of
A1 and A2 on the pH of the solutions according to the data obtained. The dissociation constant of methyl orange is to determine from the point of intersection of the curves (pKa = pH point of intersection). The resulting pKMo
value is compared with tabular data.
5 . 2 . 3 . O b t a i n i n g a n a b s o r p t i o n s p e c t r u m a n d
q u a n t i f y i n g a c e t o n e
In order to determine the concentration of acetone, you can use the absorption of its aqueous solution in the ultraviolet part of the spectrum
(220–300 nm). The appearance of an absorption band in the spectrum of acetone in this region is due to the n→π* electronic transition upon excitation
of its molecule.
Reagents:
1. Standard 0.1 M solution of acetone in water. The molecular weight
of acetone is 58.081, the specific weight is 0.791 g/cm3.
2. Distilled water.
To prepare a 0.1 M acetone solution, pipette 7.35 ml of acetone and
dilute to 1 liter with water in a volumetric flask. The solution is prepared in
advance.
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Obtaining an absorption spectrum. A working solution of acetone in
water is prepared, for which 10 ml of a standard solution of acetone (0.1 M)
is added to a 25.0 ml volumetric flask and brought to the mark with distilled
water. Distilled water is used as a reference solution.
Measurements are made in rectangular quartz cuvettes with a working
distance of 1 cm. Optical density is measured every 5 nm in the range of
230–300 nm. Build a light absorption curve, determine the position of the ab-
sorption band λ
max
. The value of λ
max
can be refined by measuring the optical
density in the region of the maximum after 1 nm.
Building a calibration graph and determining the concentration of acetone. In volumetric flasks of 150.0 ml are taken 2.5; 5.0; 7.5; 10.0; 12.5 ml
of acetone solution (0.1 M) and dilute with distilled water to the mark. It is
need to calculate the obtained concentrations of standard solutions.
The optical density of the prepared solutions with a known concentration of acetone at the selected value of λ
max
measure and build a calibration
graph A = f (C acetone).
10.0 ml of the investigated acetone solution is added to a 25.0 ml volumetric flask, brought to the mark with distilled water, mixed, poured into
a cuvette. Optical density is measured at the same value of λ
max
. Using
the calibration curve and the found value of optical density determine
the concentration of the test solution. One have to find the concentration of
acetone in the studied solution, while dilution must be taken into account.
5 . 2 . 4 . S p e c t r o p h o t o m e t r i c d e t e r m i n a t i o n
of p h o s p h a t e s
The determination of phosphates is based on the formation of phosphorvanadatomolybdic acid and the measurement of the optical density of
its aqueous solution. In the presence of vanadium V, the phosphate ion
forms a yellow complex compound with molybdate ions. Its composition
is P2O5V2O522MoO3nH2O. The reaction takes place in the presence of
an excess of nitric or hydrochloric acid. The reaction rate is low, so the optical density is measured 10–15 minutes after the preparation of the solution.
38
Equipment, reagents:
1. Spectrophotometer, 50 ml volumetric flasks.
2. Vanadium-molybdate solution. It is prepared as follows: 0.5 g of am-
monium vanadate is dissolved in hot water, 125 ml of HNO3 (density
1.42 g/cm3) are added. A solution of 10.0 g of ammonium molybdate in water
is added, the solution is made up to 1 liter with water. The solution is prepared in advance.
3. Nitric acid, 2N solution.
4. Phosphate standard solution. To prepare it, 0.2865 g of KH2PO4 is
dissolved in distilled water, the volume of the solution is adjusted to 100 ml.
A few hours before analysis, 1.00 ml of this solution is diluted to 50 ml, and
the concentration of phosphate ion in the resulting solution corresponds to
40 μg/ml.
Construction of a calibration graph. In volumetric flasks for 50 ml add
20.0; 10.0; 5.0; 2.5 ml phosphate standard solution (40 µg/ml), add water, 1 ml
2N HNO3 solution, 1.00 ml vanadium molybdate solution. Bring the volume
of solutions to 50 ml, mix and incubate for 15–20 minutes. Measure the optical
density of the solution with the highest concentration of phosphate in the range
of 300–420 nm. Measurements are made in centimeter quartz cuvettes through
10 nm. Reference solution – distilled water. An absorption spectrum is obtained from which λ
max
is determined. The optical density of the remaining
standard solutions is measured at the selected wavelength. Based on the obtained values of A and the calculated concentrations of phosphate in the reference solutions, a calibration curve A = f (C), µg/ml) is built.
Determination of phosphate content in solution. Take 20.0 ml of the analyzed solution into a 50 ml volumetric flask, add water, 1.0 ml of 2N HNO3
and 1.0 ml of vanadium-molybdate solution. Bring the volume of the solution
to 50 ml, after 15–20 min measure the optical density at λ
max
. According to
the graph, the concentration of phosphate in the resulting solution is found;
the phosphate content in the analyzed solution is calculated.
If the measured optical density lies outside the calibration line, you
should change the sampled volume of the analyzed solution Vorig in the desired direction.
C o n t r o l q u e s t i o n s
1. One have to name three spectral regions in which the absorption of
light by matter is studied, and the corresponding spectroscopy methods.
2. It is necessary to present the process of light absorption in terms of
changes in the internal energy of the substance under study.
3. Describe the method of atomic absorption spectroscopy.
4. Present the scheme of the spectrophotometer.
5. List the sources of radiation to obtain spectra in different wavelength
ranges of light.
6. Explain the need to use different materials for the manufacture of
optical equipment in the study of spectra in different regions of the spectrum.
7. List radiation receivers in the IR-region, the visible region and
the UV-region of the spectrum.
8. To present the conclusion of the Bouguer–Lambert–Beer law, to pre-
sent it in a power and linear form.
9. List the conditions for the applicability of the Bouguer–Lambert–
Beer law.
10. Substantiate the need to add reagents that color the test solution to
colorless solutions in the spectral study of the substance.
11. Describe the techniques for choosing a light filter in the colorimet-
ric study of a solution.
12. Give examples of constructing a calibration graph in photometry.
13. List methods for determining concentration in photometry.
14. It is necessary to compare the methods of the calibration curve and
standard additives and justify the area and conditions of their application in
photometry.
15. One have to compare the method of calibration curve and comparison method and justify the scope and conditions of their application in photometry.
16. Describe a method for the determination of two colored compounds
by spectrophotometry.
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6. I N F R A R E D S P E C T R O S C O P Y
a
b
Recall that spectroscopy is a set of research methods based on the study
of electromagnetic study that has passed through a substance.
Electromagnetic radiation or absorption, decomposed into wavelengths
or energy, forms a spectrum.
The absorption of light by a substance is studied by absorption spectroscopy, a variation of which is infrared (IR) spectroscopy. The IR-region
occupies the wavelength range 800–33000 nm (1 nm = 10
-3
μm = 10
In IR-spectroscopy, the wavelength is replaced by the wave number ,
which allows you to get a higher resolution in the short wavelength region
(Fig. 6.1):
-9
m).
Fig. 6.1. IR-spectrum of polystyrene: a – wavelength scale; b – frequency
scale (wave numbers)
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