RU2377557C2 - Method for tubidimetric detection of iodide ions - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to analytical chemistry, and enables determination of content of iodide ions in different objects, for example water (drinking, surface, artesian, packaged mineral water etc), in food products etc. The method is based on moving iodide ions into a partially soluble compound AgI (Conductance_AgI=1.1 10^-6). AgI is precipitated in the presence of a 25% ammonia solution (pH=9-11), which prevents precipitation of AgCl, which turns into a complex [Ag(NH₃)₂]⁺ ion. To stabilise the suspension, alcohol is added as a stabiliser. Optical density of the obtained suspension of silver iodide is then measured on a spectrometre. Measurements are taken at wavelength of 380 nm, which corresponds to maximum absorption of the given suspension. A calibration curve, obtained using standard KI solutions, is used to find content of iodide ions in the concentrate. Further, concentration of iodide ions in the analysed samples is then determined taking into account concentration level.

EFFECT: simple photometric method for determining concentration of iodide ions, cutting on the number of operations and time for determination, as well as non-use of toxic substances.

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Description

The invention relates to analytical chemistry, and in particular to methods for determining iodine, and can be used for its quantitative analysis in various objects, for example, in waters (drinking, surface, artesian, packaged mineral, etc.), in food products, food raw materials.

A variety of methods are known for determining iodine and iodide ions.

The Ministry of Health of the Russian Federation from 01.03.2002 recommended the titrimetric method for the analysis of water bodies to determine the iodine content in them in a concentration range of 0.01-1 mg / dm ³ [Methodical instructions. Determination of iodine in water. / Ministry of Health of the Russian Federation, March 1, 2002]. The method is based on the oxidation of iodides to iodates in an acidic medium with bromine water with the reduction of the latter to free iodine. The resulting iodine is titrated with sodium thiosulfate in the presence of starch as an indicator. The lower limit of measurement of iodine in the analyzed sample is 10 μg.

This method has certain disadvantages. For sample concentration, evaporation of samples and calcination of solids is recommended, which lengthens the analysis. To extract iodine from the calcined precipitate, extraction with iodine is required, which is carried out in 2 doses. The extract obtained is evaporated in a water bath, dried in an oven and calcined. After dissolving the residue, iodides are oxidized using bromine water, i.e. these operations are lengthy and involve the use of a toxic substance - bromine water. Other halogens interfere with the determination.

A technique has been developed for determining the valence forms of iodine in water in the form of iodide ions on mercury-film electrodes by cathodic inversion voltammetry [Noskova GN, Tolmacheva TP, Zaichko AV, Merta AN Determination of valence forms of iodine in water by cathodic inversion voltammetry. Abstracts of the VII conference "Analytics of Siberia and the Far East", 2004]. The total iodine content is determined against a background of 0.4 M formic acid (Ee = -0.10 V, te = 10-60 s). To restore iodate ions and organoiodine compounds to iodide ions and eliminate the interfering effect of dissolved organic substances adsorbed on the electrode surface, the sample is subjected to UV irradiation for 600 s.

When determining iodide ions by this method, preliminary removal of dissolved oxygen is necessary, for which an inert gas is used.

The disadvantages of this method are the complexity of the equipment necessary for UV irradiation of the analyzed samples, for deaeration of the samples with an inert gas, and for the inversion voltammetry method. Extraction-photometric methods are known for determining iodide ions in the form of colored ion pairs that are formed with basic dyes, for example, methyl violet (extraction with benzene or toluene) or ferroin (extraction with nitrobenzene) [Z. Marchenko. Photometric determination of elements (translation from Polish). M.: Mir, 1971, 502 p.].

The disadvantage of these methods is the work with toxic extractants such as benzene, nitrobenzene, toluene.

Known photometric method for the determination of iodide ions, based on a color reaction with starch; previously iodide ions are oxidized to iodine, using nitrates or iron (III) as oxidizing agents [Z. Marchenko. Photometric determination of elements (translation from Polish). M.: Mir, 1971, 502 p. 2].

The sensitivity of this method is small, but increases several times if the iodides are oxidized first to iodates in an acidic medium with bromine water, and then the latter is restored to free iodine using potassium iodide

I ^- + 3Br ₂ + 3H ₂ O = IO ₃ ^- + 6H ⁺ + 6Br ^-

KIO ₃ + 5KI + 3H ₂ SO ₄ = 3I ₂ + 3K ₂ SO ₄ + 3H ₂ O

Excess bromine is removed by boiling or bound with phenol to form tribromophenol. The molar absorption coefficient of the iodine starch complex is 1.08 · 10 ⁵ at λ = 590 nm.

The described method has certain disadvantages. They are associated with the use of highly toxic substances - bromine water and phenol. Removing excess bromine by boiling requires a lot of time.

The objective of the invention is to simplify the photometric method for determining iodide ions: reducing the number of operations and determination time, as well as the rejection of the use of toxic substances.

The problem is solved in that the method of turbidimetric determination of iodite ions consists in photometric suspension of silver iodide isolated from the analyzed solution, the deposition is carried out in an ammonia medium (pH = 9-11) in the presence of chloride ions and the optical density is measured at a wavelength of 380 nm .

The essence of the method for determining iodide ions is to convert these ions to sparingly soluble AgI compounds ( _AgI = 1.1 · 10 ^-6 ). The presence of chloride ions in natural water, in concentrations much higher than iodide ions, interferes with the determination of the latter, because chloride ions also form an AgCl precipitate

(PP _AgCl = 1.8 · 10 ^-10 ), therefore, the precipitation of Agl was carried out in the presence of a 25% solution of ammonia, thereby preventing the simultaneous precipitation of AgCl. Silver chloride, unlike silver iodide, is soluble in ammonium hydroxide with the formation of the complex salt [Ag (NH ₃ ) ₂ ] Cl (KH [Ag (NH ₃ ) ₂ ] = 9.3 · 10 ^-8 ), therefore, it is envisaged to add to the analyzed solution 25% solution of NH ₄ OH (pH = 9-11) to dissolve AgCl and alcohol as a stabilizer.

The optical density of the obtained suspension of silver iodide is measured on a photocolorimeter or spectrophotometer.

To determine the maximum absorption wavelength, a suspension of silver iodide was prepared, and the optical density of this suspension was measured at different wavelengths. Based on the data obtained, a plot of the optical density versus wavelength is plotted. It was found that the maximum absorption of the suspension is observed at 380 nm; therefore, all further measurements were carried out at this wavelength.

Using a calibration curve constructed using a standard solution of potassium iodide, the content of iodide ions in the analyzed sample is calculated.

Calibration of potassium iodide

1.5 ml are placed in 50 ml volumetric flasks; 2.0; 3.0; 5.0 standard KI solution, with a concentration of 5 · 10 ^-5 mol / l, 15 ml of ethyl alcohol (as a suspension stabilizer), 5 ml of 25% ammonia solution (pH = 9-11) and 5 ml of silver nitrate solution , with a concentration of 5 · 10 ^-3 mol / l.

The resulting suspension can withstand 5-10 minutes (to obtain a stable system; a longer amount of time may cause the suspension of suspended particles to the bottom of the flask).

Next, the volume of the solution was adjusted to the mark with water. The optical density of the AgI suspension is measured on a photocolorimeter or spectrophotometer in a 1 cm thick cuvette at a wavelength of 380 nm (maximum light absorption of the AgI suspension). According to the data obtained, a graph of the dependence A = f (V) is constructed.

EXAMPLE 1. Determination of the content of iodide ions in mineral water

A 400 ml water sample is placed in a porcelain cup and evaporated to a volume reduction of about 10 times. The sample volume after evaporation is necessarily measured in order to take into account the degree of concentration of water in the final calculation.

5 ml of the obtained concentrate is placed in a 50 ml flask, add 15 ml of ethyl alcohol, 5 ml of 25% ammonia solution (pH = 9-11) and 5 ml of silver nitrate solution, with a concentration of 5 · 10 ^-3 mol / L. The suspension is incubated for 5-10 minutes and the volume is adjusted to the mark with water.

The optical density of the obtained AgI suspension is measured on a photocolorimeter or spectrophotometer in a 1 cm thick cuvette at a wavelength of 380 nm.

According to the calibration graph obtained using standard KI solutions, the iodide ion content in the concentrate is found, then, taking into account the degree of concentration, the iodide ion content in the analyzed sample is determined

where V ₁ is the initial volume of the water sample, (ml);

V ₂ - the volume of the solution, found according to the calibration graph, (ml);

127 - molar mass of iodide ions, (g / mol);

n is the degree of concentration of the sample;

5 · 10 ^-3 - the concentration of a standard solution KI, (mol / l).

where V ₁ is the initial volume of the water sample, (ml);

V ₃ - sample volume after concentration, (ml).

To compare the results, the iodide ion content in the test water was determined by potentiometry using an iodine-selective electrode.

The results of comparative determinations are shown in table 1.

Table 1 The results of the determination of iodide ions by various methods Try Content I ^- mg / l by turbidimetry Content I ^- mg / l by potentiometry Measurement error (%) Mineral water "ARKHYZ" 0,050 0,053 5,6 Mineral water "LIPETSKAYA" 0.152 0.152 0 Mineral water "BONAKVA" 0,110 0.105 4.8 Mineral water "NARZAN" 0.04 0,038 5

Based on the data obtained, it can be concluded that the proposed method for determining iodide ions is a fast, high-precision method, eliminates the use of toxic reagents. It allows you to determine the content of iodide ions in the presence of chloride ions in drinking and natural waters, as well as in food.

Claims (1)

A method for determining iodide ions, including photometry of a suspension of silver iodide isolated from the analyzed solution, characterized in that in the presence of chloride ions the precipitation is carried out in an ammonia medium (pH 9-11) and the absorbance is measured at a wavelength of 380 nm.