RU2784330C1 - Method for photometric determination of the quantity of iron - Google Patents

Abstract

FIELD: analytical chemistry.

SUBSTANCE: invention relates to the field of analytical chemistry and relates to a method for photometric determination of the amount of iron in an aqueous solution of the test sample. The method consists in adding an acid and an indicator to an aqueous solution of the test sample in an amount effective for the formation of a colored complex. After adding the acid, the mixture is heated to 37–40°C and maintained at this temperature for a day to obtain a ferric iron solution. After adding the indicator, the amount of ferric iron in the colored complex is determined by the spectrophotometric method. Hydrochloric acid is used as an acid, and an aqueous solution of ammonium thiocyanate is used as an indicator, and hydrochloric acid is added in excess relative to the solution of the test sample.

EFFECT: increasing the accuracy and reliability of determining low concentrations of iron in colloidal solutions, suspensions, and dispersions.

2 cl, 5 dwg, 5 tbl

Description

The invention relates to the field of analytical chemistry, namely the photometric method for determining the amount of iron, in particular iron oxide (II, III) (Fe ₃ O ₄ ) in colloidal solutions, suspensions, dispersions and can be used to determine the concentration of magnetite nanoparticles in aqueous media .

A known method for photometric determination of the concentration of iron oxide (II) ions in a sample (see US patent No. 5898002 according to IPC class G01N17 / 06, pub. 04/27/1999), which consists in the fact that the analyzed dispersion is subjected to preliminary magnetic filtration in order to exclude undissolved oxides. The method provides an increase in the accuracy of estimating the content of iron (II).

However, the method does not allow taking into account the contribution of the content of iron oxide (III) ions in the test sample. In addition, the method does not allow a complete assessment of the iron content in the analyzed sample, since the precipitated solid phase is not analyzed.

There are alternative approaches based on the photometric determination of iron (III).

A known method for the photometric determination of iron (III) (see RF patent for the invention 2664504, according to class IPC: G01N21 / 78, G01N31 / 22, pub. 08/17/2018), including the conversion of iron (III) into a complex compound with an organic reagent - xylenol orange and a surfactant in a slightly acidic medium, adding distilled water up to 10 ml volume with heating in a water bath at a temperature of 60-80°C for 10 minutes and subsequent photometry of the resulting solution at a wavelength of 440 nm and the thickness of the cuvette , ^equal to 1 cm. At the same time, 0, 5 ml of 1 x 10 ^-2 M xylenol orange solution and 0.1 ml of 4 x 10 ^-3 M solution of isodecyl alcohol ethoxylate surfactant.

However, this method makes it possible to determine iron (III) at a very low concentration in solutions of pure salts and also does not take into account the presence of iron in other oxidation states.

There are methods to determine the content of iron ions with different degrees of oxidation. For example, a method is known for determining iron (III) and iron (II) (see RF patent for invention 2563984C1, according to IPC class G01N31 / 22, pub. 09/27/2015), including the preparation of a solution containing iron (III) and iron ( II), creation of the required pH value, interaction of the solution with the sorbent, measurement of the diffuse reflectance at 500 nm, and determination of the iron content according to the calibration curve. To isolate iron in various oxidation states from a solution, a sorbent is used - silica, successively modified with polyhexamethylene guanidine and pyrocatechol-3,5-disulfonic acid, while iron (III) is sequentially isolated from a solution with pH = 3, and then iron (II) - from a solution with pH=6. Thus, the method consists in successive sorption of iron with different degrees of oxidation at different pH values, followed by determination of the iron content by the diffuse reflectance.

However, this method makes it possible to determine the content of iron ions only in solutions and is not suitable for the analysis of dispersions (suspensions/colloids) of solid iron-containing particles.

Closest to the claimed is a method for the photometric determination of iron in a solution of the test sample (see US patent No. 3709662 according to class IPC G01N31 / 22, pub. 01/09/1973). The method includes adding to the sample solution a reagent consisting mainly of thioglycolic acid present in an amount sufficient to dissolve iron compounds in the sample solution and restore the iron content to its divalent form, a buffer to maintain the reagent solution and the sample solution at a given pH level and ferroic an amount of reagent in an amount effective to form a colored complex with ferrous iron in a reagent sample solution at a pH level that allows the complex to be colored for formation and subsequent colorimetric determination.

This method is intended for the photometric determination of the content of ferrous iron in the solution of the analyzed sample. However, it works at a strictly defined pH value and requires control of the pH level of the medium at each stage.

The technical problem of the claimed invention is the development of a method for the photometric determination of the iron content in colloidal solutions, suspensions, dispersions without controlling the pH level in them.

The technical result of the invention is to improve the accuracy and reliability of determining low concentrations of iron in colloidal solutions, suspensions, dispersions.

To achieve a technical result in the method of photometric determination of the amount of iron in an aqueous solution of the test sample, which consists in adding an acid and an indicator to the aqueous solution of the test sample in an amount effective for the formation of a colored complex, according to the invention, after adding the acid, the mixture is heated to 37-40 ° C and kept at this temperature for a day to obtain a solution of ferric iron, after adding the indicator, the amount of ferric iron in the colored complex is determined by the spectrophotometric method, while hydrochloric acid is used as an acid, and an aqueous solution of ammonium thiocyanate is used as an indicator, and hydrochloric acid is added to an aqueous solution of the test sample in excess.

As an aqueous solution of the test sample, either a colloidal solution, or a suspension, or a dispersion is used.

The invention is illustrated by illustrations, which show:

- in Fig. 1 - absorption spectra of calibration solutions (standard solutions of iron (III) chloride hexahydrate with concentrations of 0, 0.000258, 0.000516, 0.001032, 0.002065, 0.004129, 0.008258, 0.016516, 0.033032, 0.066064, 0.28);

- in Fig. 2 - calibration curve of the dependence of optical density (OD) on the concentration of standard solutions (mM);

- in Fig. 3 - absorption spectra of the analyzed colloidal solution of iron oxide nanoparticles (II, III), obtained as a result of three independent measurements;

- in Fig. 4 - absorption spectra of the analyzed suspension containing nanoparticles of iron oxide (II, III), obtained as a result of three independent measurements;

- in Fig. 5 - absorption spectra of the analyzed dispersion containing nanoparticles of iron oxide (II, III), obtained as a result of three independent measurements.

The method is carried out as follows.

An aqueous solution of iron (III) chloride hexahydrate is used as a standard solution for the calibration curve. To this solution is added a solution of concentrated hydrochloric acid HCl, stirred and the resulting reaction mixture is placed in a warm place (+37÷40°C) for 24 hours until complete discoloration.

After complete discoloration, deionized water and an aqueous solution of ammonium thiocyanate (concentration 50 mg/ml) are added to the reaction mixture. The intensity of the red color is measured spectrophotometrically (volume V of the _solution = 2 ml).

Receive absorption spectra of calibration solutions with concentrations of iron chloride (III) (see figure 1) and a calibration curve (see figure 2).

Then they take the analyzed aqueous system (colloidal solution, suspension, dispersion) with a sample of iron of unknown concentration

Also add a solution of concentrated hydrochloric acid HCl, stir and the resulting reaction mixture is placed in a warm place (+37÷40°C) for 24 hours until complete discoloration.

After complete discoloration, deionized water and an aqueous solution of ammonium thiocyanate (concentration 50 mg/ml) are added to the reaction mixture. The intensity of the red color is measured spectrophotometrically (volume V of the _solution = 2 ml).

The iron content is found from the calibration curve by the maximum optical density at a wavelength of 480 nm.

The method allows to determine the amount of iron in concentrations from 0.1 to 2.5 g/l (ie 0.1 to 2.5 mg/ml) in the analyzed system.

Example 1

Determination of the amount of iron oxide (II, III) in a colloidal solution.

To 0.1 ml of a colloidal solution of iron (II, III) oxide (V _Fe3O4 ) of unknown concentration (X mol/l, M), add 0.9 ml of a solution of concentrated hydrochloric acid HCl (the analyzed solution is diluted 10 times), stirred and the resulting reaction mixture ( volume 1 ml, pH 1) placed in a warm place (+37÷40°C) for 24 hours until complete discoloration. After complete discoloration, 1.2 ml of deionized water and 0.6 ml of an aqueous solution of ammonium thiocyanate (concentration 50 mg/ml) are added to 0.3 ml of the reaction mixture (dilution by a further 7 times) and the intensity of the red color is measured using a spectrophotometer.

An aqueous solution of iron (III) chloride hexahydrate is used as a standard solution for the calibration curve.

The iron content is found from the calibration curve by the maximum optical density (OD) at a wavelength of 480 nm, taking into account the dilution of the original analyzed solution by 70 times. The initial concentrations of iron (III) in the standard solution (C _{Fe(III) ref_st} ) and the measured solution (C _{Fe(III) meas_st} ) are presented in Table 1. The iron content is found from the calibration curve with a maximum optical density at a wavelength of 480 nm.

Table 1. Information about the concentrations of iron (III) in the initial standard solution (C _{Fe(III) ref_st} , mg/ml) and in the measured standard solution (C _{Fe(III) rev_st} , mg/ml). Concentration of iron (III) chloride hexahydrate in the initial standard solution, C _{FeCl3ref_st} , mg/ml Concentration of iron (III) chloride hexahydrate in the measured standard solution, C _{FeCl3meas_st} , mg/ml Concentration of iron (III) chloride hexahydrate in the measured standard solution, C _{Fe(III)meas_st} , mM 0
0.004
0.009
0.019
0.039
0.078
0.156
0.312
0.625
1.250
2.500 0
6.97545*10 ^-5
0.000139509
0.000279018
0.000558036
0.001116071
0.002232143
0.004464286
0.008928571
0.017857143
0.035714286 0
0.000258
0.000516
0.001032
0.002065
0.004129
0.008258
0.016516
0.033032
0.066064
0.132128 C _{FeCl3meas_st} = C _{FeCl3ref_st} / 70
C _{Fe(III)meas_st} = C _{FeCl3meas_st} / M _FeCl3x6H2O
M _FeCl3x6H2O = 270.3 g/mol

The absorption spectra of the calibration solutions are presented in Fig.1. The data for constructing the calibration curve and the calibration curve itself are presented in Table 2 and in FIG. 2, respectively.

Table 2. Information about the concentrations of iron (III) in the measured standard solution (C _{Fe(III)meas_st} , mg/ml) and the corresponding optical density (OD) values at 480 nm on the absorption spectra of the measured standard solutions.

The concentration of iron (III) in the measured standard solution, C _{Fe(III)meas_st} , mM OD at 480 nm 0
0.000258
0.000516
0.001032
0.002065
0.004129
0.008258
0.016516
0.033032
0.066064
0.132128 0.001
0.002
0.004
0.009
0.017
0.034
0.065
0.138
0.282
0.55
1.065

In FIG. 3 shows the absorption spectra of the measured colloidal solution of iron oxide (II, III). The measurement was carried out in 3 repetitions. OD values at 480 nm for each measurement are presented in Table 3.

Table 3. (colloidal) Colloidal solution of Fe ₃ O ₄ OD at 480 nm Iron (III) concentration in measured solution, C _Fe(III)meas , mM The concentration of iron (III) in the initial solution, C _{Fe(III) ref} , mM Weight of Fe ₃ O ₄ in the initial colloidal solution, m _{Fe3O4_ref} , mg one
2
3 0.969
0.979
0.994 0.119213
0.120446
0.122296 8.344885
8.43122
8.43122 0.193
0.195
0.198 C _Fe(III)meas = (OD-0.0024)/8.1082
C _Fe(III)ref = C _Fe(III)meas * 70
m _{MNP_ref} , = C _Fe(III)ref * V _solution * M _Fe3O4
V _solution = 0.1 ml
M _Fe3O4 = 231.54 g/mol C _{Fe(III) ref} = 8.446 ± 0.109 mM
m _{Fe3O4_ref} = 0.195 ± 0.003 mg

The concentration of iron (III) in the measured solution is calculated from the equation of the calibration curve using the OD values (C _Fe(III)meas , mM), from which the concentration of iron (III) in the initial solution is calculated (C _{Fe(III) ref} , mM); the calculation results are presented in Table 3. And then the mass of iron oxide (II, III) nanoparticles in the initial colloidal solution (m _Fe3O4 ) is found (Table 3).

As a result, it turns out that 0.1 ml of the colloidal solution under study contains 0.195 ± 0.003 mg of iron(II, III) oxide nanoparticles, which corresponds to a concentration of 1.95 mg/ml.

Calculation example for OD = 0.969.

1) the concentration of iron (III) in the measured solution:

C _Fe(III) meas = (0.969-0.0024)/8.1082 = 0.119213 (mM)

2) the concentration of iron (III) in the initial solution:

C _Fe(III)meas = C _Fe(III) meas *7*10= 0.119213*7*10 = 8.344885 (mM) = 8.344885*10 ^-3 (M)

3) the mass of magnetite nanoparticles in the initial colloidal solution with a volume of 0.1 ml:

m _{MNP_ref} , = C _Fe(III)ref * V _solution * M _Fe3O4 = 8.344885*10 ^-3 *0.1*231.54 = 0.193 (mg)

where V _solution = 0.1 ml, M _Fe3O4 = 231.54 g/mol.

Example 2

Determination of the amount of iron oxide (II, III) in suspension.

To 0.1 ml of a suspension containing iron(II, III) oxide (VFe3O4) of an unknown concentration (X mol/l, M), add 0.9 ml of a solution of concentrated hydrochloric acid HCl, stir, and the resulting reaction mixture (volume 1 ml, pH 1) placed in a warm place (+37÷40 °C) for 24 hours until complete discoloration. After complete discoloration, 1.2 ml of deionized water and 0.6 ml of an aqueous solution of ammonium thiocyanate (concentration 50 mg/ml) are added to 0.3 ml of the reaction mixture, and the intensity of the red color is measured using a spectrophotometer. The iron content is found from the calibration curve with a maximum optical density at a wavelength of 480 nm.

The absorption spectra of the calibration solutions are presented in Fig.1. The data for constructing the calibration curve and the calibration curve itself are presented in Table 2 and in FIG. 2, respectively.

In FIG. Figure 4 shows the absorption spectra of the measured suspension containing nanoparticles of iron oxide (II, III). The measurement was carried out in 3 repetitions. The OD values at 480 nm for each measurement are presented in Table 4.

Table 4. (Suspension)

Suspension containing Fe ₃ O ₄ nanoparticles OD at 480 nm Iron (III) concentration in measured solution, C _Fe(III)meas , mM The concentration of iron (III) in the initial solution, C _{Fe(III) ref} , mM Mass of Fe ₃ O ₄ in the initial suspension, m _{Fe3O4_ref} , mg one
2
3 0.991
0.973
0.955 0.121926
0.119706
0.117486 8.534817
8.379418
8.224020 0.198
0.194
0.190 C _Fe(III)meas = (OD-0.0024)/8.1082
C _Fe(III)ref = C _Fe(III)meas * 70
m _{MNP_ref} , = C _Fe(III)ref * V _solution * M _Fe3O4
V _solution = 0.1 ml
M _Fe3O4 = 231.54 g/mol C _{Fe(III) ref} = 8.380 ± 0.155 mM
m _{Fe3O4_ref} = 0.194 ± 0.004 mg

The concentration of iron (III) in the measured solution is calculated from the equation of the calibration curve using the OD values (C _Fe(III)meas , mM), from which the concentration of iron (III) in the initial suspension is calculated (C _{Fe(III) ref} , mM); the calculation results are presented in Table 4. And then the mass of iron oxide (II, III) nanoparticles in the initial colloidal solution (m _Fe3O4 ) is found (Table 4).

As a result, it turns out that 0.1 ml of the test suspension contains 0.194±0.004 mg of iron (II, III) oxide nanoparticles, which corresponds to a concentration of 1.94 mg/ml.

Example 3

Determination of the amount of iron oxide (II, III) in the dispersion.

To 0.1 ml of a dispersion containing iron oxide (II, III), (V _Fe3O4 ) of an unknown concentration (X mol/l, M), add 0.9 ml of a solution of concentrated hydrochloric acid HCl, stir, and the resulting reaction mixture (volume 1 ml, pH 1 ) is placed in a warm place (+37÷40°C) for 24 hours until complete discoloration. After complete discoloration, 1.2 ml of deionized water and 0.6 ml of an aqueous solution of ammonium thiocyanate (concentration 50 mg/ml) are added to 0.3 ml of the reaction mixture, and the intensity of the red color is measured using a spectrophotometer. The iron content is found from the calibration curve with a maximum optical density at a wavelength of 480 nm.

The absorption spectra of the calibration solutions are presented in Fig.1. The data for constructing the calibration curve and the calibration curve itself are presented in Table 2 and in FIG. 2, respectively.

In FIG. Figure 4 shows the absorption spectra of the measured dispersion containing nanoparticles of iron oxide (II, III). The measurement was carried out in 3 repetitions. OD values at 480 nm for each measurement are presented in Table 5.

Table 5. (Dispersion) Dispersion containing Fe ₃ O ₄ OD at 480 nm The concentration of iron (III) in the measured solution, C _Fe(III)meas , mM The concentration of iron (III) in the initial solution, C _{Fe(III) ref} , mM Weight of Fe ₃ O ₄ in the initial dispersion, m _{Fe3O4_ref} , mg one
2
3 0.067
0.076
0.083 0.007967
0.009077
0.009941 0.557707
0.635406
0.695839 0.013
0.015
0.016 C _Fe(III)meas = (OD-0.0024)/8.1082
C _Fe(III)ref = C _Fe(III)meas * 70
m _{MNP_ref} , = C _Fe(III)ref * V _solution * M _Fe3O4
V _solution = 0.1 ml
M _Fe3O4 = 231.54 g/mol C _Fe(III)ref = 0.630 ± 0.070 mM
m _{Fe3O4_ref} = 0.015 ± 0.002 mg

The concentration of iron (III) in the measured solution is calculated from the equation of the calibration curve using the OD values (C _Fe(III)meas , mM), from which the concentration of iron (III) in the initial dispersion is calculated (C _Fe(III)ref , mM); the calculation results are presented in Table 5. And then the mass of iron oxide (II, III) nanoparticles in the initial dispersion (m _Fe3O4 ) is found (Table 5).

As a result, it turns out that 0.1 ml of the studied dispersion contains 0.015 ± 0.002 mg of iron(II, III) oxide nanoparticles, which corresponds to a concentration of 0.15 mg/ml.

The ratio of hydrochloric acid to an aqueous solution of the test sample is selected in such a way that the amount of acid is in excess.

Thus, the proposed method allows you to accurately and reliably determine low concentrations of iron in colloidal solutions, suspensions and dispersions.

Claims (2)

1. A method for photometric determination of the amount of iron in an aqueous solution of the test sample, which consists in adding an acid and an indicator to the aqueous solution of the test sample in an amount effective for the formation of a colored complex, characterized in that after adding the acid, the mixture is heated to 37–40°C and kept at this temperature for a day to obtain a solution of ferric iron, after adding the indicator, the amount of ferric iron in the colored complex is determined by the spectrophotometric method, while hydrochloric acid is used as an acid, and an aqueous solution of ammonium thiocyanate is used as an indicator, and hydrochloric acid is added in excess with respect to the solution of the test sample. 2. The method according to claim 1, characterized in that either a colloidal solution, or a suspension, or a dispersion is used as an aqueous solution of the test sample.

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