RU2333487C1 - Method of photometric iron (iii) detection in pure salt solutions - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention concerns analytical chemistry and iron (III) detection in extremely small concentrations. The method of photometric iron (III) detection in pure salt solutions involves transit of iron to complex compound with ammonium thiocyanate in weak acid medium. Note, 2 ml of buffer solution with pH 3.8-4.2, 0.3-0.6 ml of 0.2% surfactant solution obtained by solving 2% gelatin solution with water, 1.0-2.2 ml of 10^-2 mol ammonium thiocyanate solution, and up to 10 ml of water are added to the analysed iron (III) solution with further heating at 80-90°C for 4-6 minutes.

EFFECT: improved analysis sensitivity.

1 ex, 4 dwg

Description

The invention relates to the field of analytical chemistry, namely to the photometric method of analysis, and can be used to determine the content of iron (III) in solutions of pure salts containing iron (III) in a very low concentration.

A known method for the photometric determination of iron (III) in a slightly acidic medium using potassium thiocyanite (rhodanide) as a reagent (MI Bulatov, IP Kalinkin. A practical guide to photocolorimetric and spectrophotometric methods of analysis. - M .: Chemistry, 1972 . - 361 p.).

The disadvantage of this method is its low sensitivity: the molar extinction coefficient of this complex of the compound in the aqueous medium is 7.0 · 10 ³ , in the environment of acetone 1.5 · 10 ⁴ .

The technical result consists in increasing the sensitivity of the photometric determination of iron (III) during the implementation of the method.

The technical result is achieved by the fact that in the method for the photometric determination of iron (III) in a solution of pure salts, including its conversion to a complex compound with ammonium thiocyanate in a slightly acidic medium, 2 ml of a buffer solution with a pH of 3.8 - is added to the analyzed solution of iron (III) 4.2, 0.3-0.6 ml of a 0.2% solution of a surfactant obtained by diluting with water a 2% solution of gelatin, 1.0-2.2 ml of a 10 ^-2 M solution of ammonium thiocyanate and water up to 10 ml of volume followed by heating in a water bath at a temperature of 80-90 ° C for 4-6 m inut.

A surfactant solution is prepared as follows: a 2% gelatin solution is diluted with water to a 0.2% solution.

The pH range of the existence of the colored complex compound 1.8-5.0 is created by adding a buffer solution. The maximum color intensity of the complex is achieved at pH 3.8-4.2. Outside of these intervals, the complex breaks up.

Example. Dependence of optical density on the wavelength of incident light.

0.2 ml of a 10 ^-3 M solution of iron (III) are placed in test tubes, 2 ml of a buffer solution with a pH of 3.8-4.2, 0.3 ml of a 0.2% surfactant solution, 1 ml of 10 ^{- 3} M solution of ammonium thiocyanate, adjusted with water to 10 ml. Heated in a water bath for 5 minutes at a temperature of 80-90 ° C. In parallel, prepare comparison solutions. A pair of solutions with the largest difference in the intensities of the colors is photometered at wavelengths of 340-600 nm [Fig. 1 - light absorption spectrum of the iron (III) complex with ammonium thiocyanate (1) and ammonium thiocyanate (2) in the presence of a surfactant, l = 2 cm, solution comparisons are water] relative to water. According to the schedule, the optimal wavelength is found for working with this colored compound λ _max = 490 nm.

The prepared series of solutions is photometered against the comparison solutions at λ = 490 nm and a graph is plotted in coordinates A - pH [Fig. 2 is the dependence of the optical density on the pH of the iron (III) complex with ammonium thiocyanate in the presence of surfactant].

Find out the optimal amount of ammonium thiocyanate [figure 3 - the dependence of the optical density of the complex of iron (III) with ammonium thiocyanate in the presence of surfactants on the amount of ammonium thiocyanate. With _Fe = 5 · 10 ^-6 M; λ 490 nm; l = 1 cm] and a surfactant solution (figure 4 - dependence of the optical density of the complex of iron (III) with ammonium thiocyanate in the presence of surfactants on the amount of surfactant. With _Fe = 5 · 10 ^-6 M; C _HR = 2 · 10 ^-3 M ; λ 490 nm; l = 1 cm], as well as the temperature and duration of heating to obtain the maximum color intensity of the complex.

Under the chosen optimal conditions, a calibration graph is built, the linearity of which is observed in the range of iron (III) concentrations (1-9) -10 ^-6 M.

The equation of the calibration graph is calculated:

x 10 ⁶ 2 3 four 5 6 y 0.41 0.58 0.80 1.00 1.20

a = [90 · 10 ^-12 · 3.99-20 · 10 ^-6 · 17.9 · 10 ^-6 ]: [5 · 90 · 10 ^-12 - (20 · 10 ^-6 ) ² ] = 0.022

c = [5 · 17.9 · 10 ^-6 -20 · 10 ^-6 · 3.99]: [5 · 90 · 10 ^-12 - (20 · 10 ^-6 ) ² ] = 1.94 · 10 ⁵

y = 0.022 + 1.94 · 10 ⁵ x.

The molar absorption coefficient of the complex compound is equal to:

The advantages of the proposed method:

1) in the presence of a surfactant, the sensitivity of the photometric determination of iron (III) with ammonium thiocyanate increases 200,000 / 7,000 = 28.5 times in an aqueous medium, 200,000 / 15,000 = 13.3 times in an acetone medium;

2) the reaction proceeds in an aqueous medium, which eliminates the negative effect of an organic solvent on the human body;

3) reagents ammonium thiocyanate and gelatin are available in retail chains, cheap.

The sensitivity of the photometric determination of iron (III) with ammonium thiocyanate in the presence of a surfactant is many times higher than for the same reaction, but in the absence of a surfactant. By the method of intersection of the curves, it was calculated that the ratio of the components M: R = 1: 1 and the logarithm of the stability constant of the complex is logβ = 5.24 for the complex of iron (III) with ammonium thiocyanate in the absence of surfactant. For the same complex, but in the presence of a surfactant, M: R = 1: 2 and Igβ = 7.20. This is due to the fact that gelatin is likely to abruptly, energetically bring iron (III) together and the thiocyanate ion, pushing out the hydroxyl groups binding its activity in iron (III) in the complexation reaction. The Fe (III) + HR + surfactant complex is formed at pH 4, and the Fe (III) + HR complex is formed at pH 6. The exposed metal ion releases additional bonds to the thiocyanate ion by a large coordination number, and gelatin prepares a close, closed cell for them. which increases Igβ.

Claims (1)

A method for the photometric determination of iron (III) in solutions of pure salts, including its conversion to a complex compound with ammonium thiocyanate in a slightly acidic medium, characterized in that 2 ml of a buffer solution with a pH of 3.8-4.2 is added to the analyzed solution of iron (III) 0.3-0.6 ml of a 0.2% solution of a surfactant obtained by diluting with water a 2% solution of gelatin, 1.0-2.2 ml of a 10 ^-2 M solution of ammonium thiocyanate and water up to 10 ml of volume c subsequent heating at a temperature of 80-90 ° C for 4-6 minutes

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