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Файл:Optical methods of analysis. Educational aid
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7.2 .5. F l a m e p h o t o m e t r i c d e t e r m i n a t i o n o f s o d i u m
a n d p o t a s s i u m i o n s i n d r i n k i n g w a t e r
Instruments and reagents: flame photometer; standard solutions; 50 and
100 ml volumetric flasks; 1.0 and 5.0 ml graduated pipettes; distilled water.
Drinking water has a complex composition. It is possible to simplify
the determination of sodium and potassium in water by using the additive
method. Standard solutions of sodium chloride contain 500 and 100 mg/l of
sodium and potassium, respectively. To calibrate the device, it is necessary
to prepare solutions containing 2.5; 5.0; 10.0; 15.0; 20.0 mg/l of sodium and
2.5; 4.0; 6.0; 8.0 and 10.0 mg/l of potassium in 5-dimensional flasks with
a capacity of 100 ml. Set the measuring range 2 or 3 and profotometrirovat
standard solutions. Build calibration graphs based on the results. After making sure that the graphs are linear, you can analyze the water. Pour tap water
into three measuring flasks with a capacity of 100 ml each. In two flasks, add
potassium and sodium solutions and bring the solutions to the mark with water of the same sample. The composition of the solutions should be written
in the form of Table 7.4.
Table 7.4
Results of determinations
Element
№ solution
1 2 3
Additives of sodium and potassium, mg/l
Sodium
–
5.0
10.0
Photocurrent value, mA
Potassium
–
2.5
5.0
Photocurrent value, mA
Photometer the solutions under the same conditions under which the instrument was calibrated. Position “0” on the scale of the microammeter set
to distilled water. The maximum reading should be observed during photometry of the solution with the maximum addition of the element being determined. Repeat photometry of solutions twice and take the average result.
Determine the concentration of sodium and potassium in drinking water by
a graphical method in the coordinates i = f (C).
C o n t r o l q u e s t i o n s
1. Describe the essence of the flame photometry analysis method.
2. Give a description of the flames.
3. Describe the physical and chemical processes occurring in the flame.
4. How is the excitation of the elements in the flame?
5. How does the composition of the solution affect the radiation inten-
sity of the element in the flame?
6. How to determine the lack or excess of an oxidizer by the type of
flame?
7. What explains the coloring of the flame when salts of a number of
metals are introduced into it?
8. Consider different methods of flame photometry.
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8. R E F R A C T O M E T R I C A N A L Y S I S
8 . 1 . F u n d a m e n t a l s o f m e t h o d
Refractometry (from Latin refractus – refracted and Greek metreo –
I measure) is a method for studying substances based on determining the index (coefficient) of refraction (refraction) and some of its functions. Refractometry (refractometric method) is used to identify chemical compounds,
quantitative and structural analysis, and determine the physicochemical parameters of substances.
Refractometric analysis is based on measuring the refractive index (refraction) of substances, which should be used to judge the nature of the substance, its purity and content in solutions.
The refractive index (refractive index) is the ratio of the speed of light
in vacuum to the speed of light in the test substance (absolute refractive index). In practice, the so-called relative refractive index n is determined,
which is the ratio of the speed of light in air to the speed of light in the test
substance.
The refractive index n is the ratio of the speeds of light in adjacent media. For liquids and solids, n is usually defined relative to air, and for gases,
relative to vacuum. The values of n depend on the wavelength λ of light and
temperature, which are indicated respectively in superscript and subscript.
For example, the refractive index at 20 °C for D is the sodium spectrum line
(λ = 589 nm) – Dn.
In the case of gases, the dependence of n on pressure must also be taken
into account (specify it or reduce the data to normal pressure). In ideal systems (formed without changing the volume and polarizability of the components), the dependence of the refractive index on the composition is close to
linear if the composition is expressed in volume fractions (percentage):
n=n1V1+n2V2 ,
where n, n1, n2 are the refractive indices of the mixture and components;
V1 and V2 are the volume fractions of the components (V1 + V2 = 1).
For refractometry of solutions in wide ranges of concentrations, tables
or empirical formulas are used.
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74
The refractive index depends on the temperature and wavelength of
the light at which the determination is made. In solutions, the refractive index
also depends on the concentration of the substance and the nature of the solvent.
The refractive index is one of the few physical constants that can be measured with very high accuracy and in a short amount of time with only a small
amount of material. For this, instruments are used – refractometers. They allow
you to determine the refractive index with an accuracy of 0.01 % and even up
to 0.001 % of the measured value, this requires only (0.05–0.5) g of substance,
and the entire measurement procedure is reduced to reading on a scale.
The refractive index is usually measured with Abbe refractometers.
The principle of operation of the device is based on the phenomenon of total
internal reflection that occurs at the interface between two media when
a beam passes from an optically denser medium to an optically less dense
medium (media with different refractive indices).
The range of measured refractive indices when measured in transmitted
light is 1.3–1.7. The measurement accuracy of the refractive index should not
be lower than ± 2·10-4. Other types of refractometers with the same or better
accuracy may be used.
Modern instruments are calibrated in such a way that the readings obtained on their scales correspond to the refractive indices for the D line of
the sodium spectrum (589.3 nm). When carrying out measurements, the instructions for the appropriate light source given in the instructions for the instrument must be observed. If white light is used, the refractometer is
equipped with a compensating system. The determination is carried out at
a temperature of (20 ± 0.5) °С. The division value of the thermometer should
not exceed 0.5 °С. The refractive index determined under these conditions is
denoted by the index n
20
D
.
Refractometry is used to establish the authenticity and purity of a substance. The method is also used to determine the concentration of a substance
in a solution, which is found from the graph of the dependence of the refractive index of the solution on concentration. On the graph, a concentration
interval is selected in which a linear relationship between the refractive index
and concentration is observed. In this interval, the concentration is calculated
by the formula:
С = (n − n
o
)/F,
where C is the concentration, in percent; n is the refractive index of the solution; no is the refractive index of the solvent at the same temperature; F is
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a factor equal to the increase in the refractive index with an increase in concentration by 1 %.
The value of the refractive index factors is set experimentally for each
substance and each percentage of concentration. For some substances (potassium iodide, magnesium sulfate, anhydrous glucose), the value of the factor
is constant and does not depend on the concentration of the solution. The factors of most substances in solutions of different concentrations are slightly
different from each other. The values of refractive indices and factors for
various concentrations of solutions of substances are given in refractometric
tables. Using tables greatly simplifies calculations.
For solutions containing two or more substances, the value of the refractive index is the sum of the refractive index of the solvent no and the refractive indices of all solutes, the formula is as follows:
n= no+n1+n2+…
To determine the content of one of the components, a modified formula
is used. For example, in three-component mixtures, the content of two substances is determined by the titrimetric method, and the content of the third
component (cx, %) is determined by measuring the refractive index n of a solution of a multicomponent mixture. The calculation is carried out according
to the formula:
,
where n is the refractive index of the analyzed solution; no is the refractive index
of the solvent at the same temperature; c1 and c2 – concentrations of the ingredients of the mixture, established titrimetrically, %; F1 and F2 are the refractive
index factors corresponding to them; Fx is the refractive index factor of
the component, the content of which is determined refractometrically.
8.2 . D e t e r m i n a t i o n o f c a r b o h y d r a t e c o n t e n t
in a q u e o u s s o l u t i o n s b y r e f r a c t o m e t r i c m e t h o d
The purpose of the work: to study the dependence of the refractive index of an aqueous solution of glucose or sucrose on its concentration; build
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a calibration graph and use it to determine the carbohydrate content in the test
solution.
The method is based on the dependence of the refractive index of
the solution on the content of carbohydrate in it.
Reagents and equipment: refractometer; pipettes; pieces of filter paper;
distilled water; an aqueous solution of sucrose with a mass fraction of a dissolved substance of 40 %
The order of the work:
1. Familiarize yourself with the refractometer device (using the exam-
ple of determining the refractive index of distilled water, n
D
20
=1.333).
2. Calculate the volumes and prepare a series of standard solutions using glucose (sucrose) as the initial solution and guided by the data in Table 8.1.
Table 8.1
Formulation of solutions
Indicators
Solution numbering
1 2 3 4 5 6 7
8
The volume of the initial solution, ml
–
Volume of water, ml
–
Total volume of solution after dilution with water
5.0
5.0
5.0
5.0
5.0
5.0
5.0
–
Solution concentration, % wt.
4.0
8.0
12.0
16.0
20.0
24.0
32.0
40.0
Refractive index, n
D
20
1. Measure the refractive indices of all solutions on a refractometer
(starting with the most diluted one). Each solution is applied to the prism of
the instrument with a pipette (do not touch the refractometer lens with your
hands with a pipette). After each sample, the prism is cleaned with distilled
water and filter paper. The refractive index is determined with an accuracy
of 0.0005.
2. On graph paper, build a calibration graph in the coordinates “refractive index – concentration of glucose (sucrose) (% wt.)”. Recommended
scale: along the y-axis – 1 mm = 0.001; along the abscissa – 20 mm = 10 %
of the mass.
Determining the concentration of carbohydrate in the test solution is
done as follows:
1. In the test solution, the refractive index is determined on a refractometer.
2. Using a calibration graph, determine the concentration of carbohydrate in the test solution.
Formulate conclusions on the work.
C o n t r o l q u e s t i o n s
1. What is called refraction?
2. What is the basis of the refractometric method of analysis?
3. Describe the advantages and applications of refractometry.
4. What methods are used for refractometric analysis?
5. Tell us about the absolute and relative refractive indices.
6. What is the limiting angle of refraction?
7. Describe the refractometer, its purpose and device.
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C O N C L U S I O N
Physico-chemical methods, namely, optical methods, are based on
the use of the dependence of optical properties on the chemical composition
of substances. In most cases, optical methods are distinguished by their speed
of execution, selectivity, high sensitivity, and the possibility of unification
and automation. Therefore, this group of methods is becoming increasingly
important for an objective assessment of the quality of chemical products,
including for testing for authenticity, testing for purity and for quantitative
determination. Optical methods of analysis basically have their own theory,
different from the theory of other methods. The basis of this theory is the interaction of matter with the flow of energy. When using optical analysis
methods to obtain information about the chemical composition of a substance, the test sample is subjected to electromagnetic radiation. Depending
on the type of irradiation energy of a substance, a change in the energy state
of its constituent particles (molecules, ions, atoms) occurs in it, which is expressed in a change in one or another property (for example, color, etc.).
By registering a change in this property as an analytical signal, information
is obtained about the qualitative and quantitative composition of the object
under study or about its structure. Optical methods of analysis include physicochemical methods based on the interaction of electromagnetic radiation
with matter. This interaction leads to various energy transitions, which are registered experimentally in the form of radiation absorption, reflection and scattering of electromagnetic radiation. Optical methods include a large group of
spectral analysis methods. The experimentally simple methods of spectrophotometry and photometry have the greatest practical application for product quality control. According to the methods of determination, direct and indirect physico-chemical methods of analysis are distinguished. Photometric methods of
analysis are used to control a variety of manufacturing processes. These methods can be used to analyze large and small contents, but their most valuable
feature is the ability to determine impurities (up to 10-5...10
-6
%). Absorption
spectroscopy methods are used in the chemical, metallurgical, pharmaceutical and other industries, as well as in medicine and agricultural production.
The industry produces devices for absorption spectroscopy: colorimeters,
photometers, photoelectrocolorimeters, spectrophotometers, in which various combinations of illuminators, monochromators and light receivers are
used. In the methods of atomic spectroscopy, we deal with narrow line
spectra, and in the methods of molecular spectroscopy, with wide weakly
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structured spectra. This determines the possibility of their application in
quantitative analysis and the requirements for measuring equipment –
spectral instruments.
When using optical methods of analysis, reference samples are used,
i.e. samples of substances or materials with precisely known content of
the determined element (or several elements). During the analysis the sample
and the reference are measured, the data obtained are compared, and the content of this element in the analyzed sample is calculated from the known content of the element in the reference. Standards can be manufactured industrially or prepared in the laboratory immediately before analysis (comparison
samples). If chemically pure substances (impurities less than 0.05 %) are
used as standard samples, then they are called standard substances.
Thus, at present, optical methods of analysis are widely used in
the chemical industry. Modern optical analysis devices using computer technologies make it possible to automate the analytical process, increase its efficiency, and reduce energy consumption.
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G l o s s a r y
Absorbance − The attenuation of photons as they pass through a sample (A).
Absorbance spectrum − A graph of a sample’s absorbance of electro-
magnetic radiation versus wavelength (or frequency or wavenumber).
Atomization − The process of converting an analyte into a free atom.
Background correction − In atomic absorption spectroscopy, the cor-
rection of the net absorbance from that due to the sample matrix.
Beer’s law − The relationship between a sample’s absorbance and
the concentration of the absorbing species (A = Cl).
Calibration curves − Showing positive and negative deviations from
Beer’s law.
Characteristic concentration − The concentration of analyte giving
an absorbance of 0.00436
Chromophore − The specific bonds or functional groups in a molecule
responsible for the absorption of a particular wavelength of light.
Continuum source − A source that emits radiation over a wide range
of wavelengths.
Dark current − The background current present in a photon detector
in the absence of radiation from the source.
Effective bandwidth − The width of the band of radiation passing
through a wavelength selector measured at half the band’s height.
Electromagnetic spectrum − The division of electromagnetic radia-
tion on the basis of a photon’s energy.
Emission − The release of a photon when an analyte returns to a lower-
energy state from a higher-energy state.
Emission spectrum − A graph of emission intensity versus wavelength
(or frequency or wavenumber).
Frequency − The number of oscillations of an electromagnetic wave
per second (n).
Filter − A wavelength selector that uses either absorption, or construc-
tive and destructive interference to control the range of selected wavelengths.
Filter photometer − A simple instrument for measuring absorbance
that uses absorption or interference filters to select the wavelength.
Graphite furnace − An electrothermal atomizer that relies on resistive
heating to atomize samples.
Intensity − The flux of energy per unit time per area (I).
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