DD259459A1 - AUTOMATIC, COULOMETRIC RECEPTACTION PROCEDURE FOR THE DETERMINATION OF INORGANIC PEROXIDES - Google Patents

Abstract

The invention relates to an automatic coulometric back titration method for the rapid quantitative determination of inorganic peroxides. The determination of inorganic peroxides or the active oxygen content is required, for example, in detergent bases, washing and bleaching liquors and in the electrolytes of water and peroxodisulfate electrolysis. According to the invention, the peroxides metered in as a solution with an injection valve integrated into the coulometric titration cell are catalyzed by catalyzed conversion with a precisely defined amount of excess reagent simultaneously metered by the same injection valve and by titration of the remaining excess reagent with titration agent electrochemically generated by oxidation of iodide ions in the electrolysis space divided by a diaphragm biamperometric, bipotentiometric or photometric flow detection of successful display of the titration endpoint determined.

Description

For this 1 page drawings

Field of application of the invention

The invention relates to a coulometric Rücktitrationsverfahren for rapid and quantitative determination of inorganic peroxides, eg. B. in detergent bases, washing or bleaching liquors and in the electrolyte of water and Peroxodisulfatelektrolyse.

Characterization of the known state of the art -

The titration methods for determining inorganic peroxides described in the specialist literature are mostly volumetric titrations / 1 /, which require the preparation and storage of suitable custom solutions. Coulometric titration methods for hydrogen peroxide and peroxodisulfate have also been described / 2 /. The coulometric titration of H ₂ O _{2 is} achieved by direct titration with electrolytically produced Ce (IV) and Mn (III) in acidic and OBr-in alkaline medium, the accuracies obtained are unsatisfactory.

The determination method based on the conversion of inorganic peroxides with Γ to I ₂ , subsequent iodine reduction with a known thiosulfate excess and back titration with electrochemically generated I ₂ provides the most accurate analytical results for a very costly technique of dosing sample solution and excess reagent solution with weighing pipettes or calibrated burettes, thereby extending analysis times. The method can be automated only with great effort. In addition, only one determination can be carried out in a base electrolyte solution, which causes high chemical costs.

Object of the invention

The aim of the invention is to determine inorganic peroxides quickly and quantitatively.

Explanation of the essence of the invention

The object of the invention is a fast and accurate automatic absolute method, which is independent of reference materials and can be transferred to the microscale.

According to the invention as a solution with an integrated in the coulometric titration cell injection valve metered, to be determined peroxide solution by conversion with a precisely defined and metered with the same injection valve excess amount of reducing agent and back titration of the remaining excess with electrochemically generated by oxidation of iodine ions contained in the base electrolyte titrated in by a diaphragm divided electrolytic cell space with biamperometric, bipotentiometric and photometric display of the titration endpoint determined. The peroxide or active oxygen content results from the difference between the amounts of charge converted in the titration of the metered excess reagent amount and the amounts of charge converted in the titration of the excess reagent remaining after conversion with the sample substance. Sample and excess reagent solutions are injected by horizontal swiveling of the injection valve into the base electrolyte circulated by a magnetic gyratory pump.

Perborates, perphosphates, percarbonates and urea peroxohydrates release H ₂ O ₂ when dissolved in aqueous solutions, which is dissolved in a 0.1-1 M Kl or Nal electrolyte, with a pH between 2-4, the Mo (Vl) or W (VI) was added, liberates an equivalent amount of iodine. The iodine is immediately reduced with a known excess amount of thioglycolic acid or Sn (II). Peroxodisulfate releases an equivalent amount of iodine in a 0.1-3 M Kl or Nal electrolyte with a pH between 1-7 to which Fe (III) or Cu (II) has been added and reacted with a excess of tiosulfate becomes.

In the range of 1-10 ^ g of active oxygen content of aqueous solutions of said inorganic peroxides, relative standard deviations of 0.5-0.8% (n = 5-7) are found for errors of ± 0.15-0.5%. Between 10 and 100μg active oxygen content, the relative standard deviations are below 0.5% (n = 5-7), the errors below ± 0.3%. The determination of the titration end point is carried out with a biamperometric or bipotentiometric flow detector which operates with a Pt, Au or glassy carbon electrode pair to which a polarization voltage or a polarization current is impressed. The resulting indicator current or the indicator potential regulates the titration current via an appropriate circuit and causes the automatic end point shutdown if it exceeds or falls below a certain threshold value.

The titration end point can advantageously be determined using a built-in titration cell photometric flow detector consisting of a transparent tube as a cuvette, a light emitting diode as a light source and a phototransistor as a light receiver, wherein the measured during the titration photocurrent controls the titration and causes the automatic Endpunktabschaltung.

In 10-12 ml of the above-mentioned basic electrolyte solutions up to 60 titrations for active oxygen determination can be carried out at a sample volume of 50-60μΙ, the continuous series titration is applied to a preselected Abschaltschwellenwert the respective indicator signal, optionally using an autozero circuit. · -

embodiments

For active oxygen determination is a compact coulometric titration cell (Fig. 1), which consists of 4 functional elements: K - the head part, which carries the counter electrode space and the electrodes of the biamperometric or bipotentiometric end point determination, D - the detector segment, I - the injection valve and M - The magnetic centrifugal pump, generator and counter electrode space are separated from each other either by a sealingly clamped cation exchange membrane or a water glass reinforced G-4 glass frit. The basic electrolyte, approximately 10-12 ml, filling the whole cell internal volume, is forced through the main circulation bore, the sample introduction and reagent injection wells to the cell head, flows past the diaphragm (4) and back through the cuvette tube (5) to the vacuum inlet of the centrifugal pump , By means of a 54 ° turn of the injection valve I, precisely defined amounts of excess reagent solution (55μ.Ι) and sample solution (55μΙ) can be coupled into the circulating base electrolyte simultaneously.

A Pt sheet (A = 5.2 cm ² ) covering the bottom of the cell acts as a generator anode for producing the titrant iodine. A platinum wire spiral (A = 0.8 cm ² ) serves as a counter electrode. For biamperometric indication, a Pt wire cathode and a Pt wire anode are inserted into the main circulation well. For photometric indication, the detector segment carries the type SP201 D phototransistor (2) acting as a light receiver and the VQA23 D light emitting diode (3). The change in the intensity of the light radiating through the base electrolyte is measured during the titration as a change in the photocurrent ,

The coulometric titrations are carried out using the universal coulometer 04-404 developed by RADELKIS, Budapest, or a coulometric autotitrator designed for amperometric and photometric end point indications. For coulometric determination of the active oxygen content in solutions of the substances hydrogen peroxide, percarbonate and urea peroxyhydrate, the auxiliary electrode space separated from the cell interior with a water glass impregnated G-4 frit diaphragm is filled with 0.1 M KCl. To the base electrolyte of composition 0.1 N Cl, 0.01 N HCl, 10 ~ ⁴ mol / l ~ ¹ Mo (VI) catalyst is added. Based on the amount of active oxygen to be determined, a 40-20% thioglycolic excess is used as the excess reagent.

After switching on the titration current, the base electrolyte is pretreated to a preselected switch-off threshold value of 0.1-0.2μ.Α of the biamperometric end point indication. The coulometer is then reset to the charge Q = Q. Subsequently, precisely defined volumes of excess reagent and bidistilled water are metered into the cell with the injection valve and the amount of charge Q corresponding to the amount of reagent is determined by titration up to the same switch-off threshold value.

After setting the coulometer on the subtraction of electrical charge quantities, the same volumes of excess reagent - and dosed in place of the water sample solution. The amount of charge Q converted for the titration of the amount of excess reagent remaining after the completion of the determination reaction is now subtracted from the charge amount Q, whereby the amount of charge Q corresponding to the amount of the sample substance decreases

Q = Q - Q _T

(D

results. The active oxygen content of the sample solution can be calculated from the following relationship derived from Faraday's law:

c _m , O ₂ active = 0.1658 mg - A · s " ¹ (2)

c _m , O ₂ active ... active oxygen concentration " ¹ Q ... electrical charge in As

V ... sample dosing volume in

Table 1 gives an overview of the results for the analysis of peroxide-containing aqueous solutions. Each measured value represents the arithmetic mean of every 5 titrations.

For coulometric determination of peroxodisulfate, the counterelectrode space is filled with 0.5 M KCl. 2-10 " ⁴ M FeCl3 catalyst is added to the base electrolyte 0.5 M KCl Based on the amount of peroxodisulfate to be determined, a 100-200% thiosulfate excess is used as the excess reagent.

It is titrated to a preselected shutdown threshold of the photometric endpoint indication. The procedure of the determination method is analogous to that described above. The peroxodisulfate content of the sample solution can be calculated according to the following relationship:

Cm, S ₂ O ₈ = 0.9956 mg · A · s

(3)

Table 1

sample

Weighed value GemessenerWert mgi " ¹ mg! " ¹ O ₂ O ₂

Error in mgr ¹ O ₂

percarbonate 206.0 208.5 +2.5 450.5 449.5 -1.0 669.5 671.0 -0.5 perborate 126.0 · - 127.0 + 1.0 242.0 241.0 -1.0 478.5 476.5 -2.0 712.5 715.5 +3.0 Hindered peroxyhydrate 92.0 91.0 -1.0 179.0 180.5 + 1.5 354.5 354.5, ± 0.0 552.5 547.5 -5.0

Table 2 GemessenerWert error Weighed value ingr ¹ Value in gl " ¹ ingr ¹ peroxodisulfate peroxodisulfate peroxodisulfate 0.223 -0.001 0.224 0.853 +0.005 0.848 1,537 +0.005 1,532 2,727 -0,008 2,735 8.521 +0.044 8,477

Claims (7)

1. Automatic, coulometric Rücktitrationsverfahren for the rapid quantitative determination of inorganic peroxides, characterized in that the solution as a solution with an integrated in the coulometric titration cell injection valve peroxide solutions by catalyzed conversion with a well-defined and simultaneously metered with the same injection valve excess amount of reducing agent and by Rücktitratioi of remaining excess reagent with electrochemically by titration of iodide ions contained in the electrolyte contained in the base electrolyte in the divided by a diaphragm electrolysis space determined by biamperometric, bipotentiometric or photometric flow detection display. 2. The method according to item 1, characterized in that the Peroxidprobenlösung and the excess reagent solution are injected by a horizontal pivoting of the injection valve in the circulating base electrolyte. 3. The method according to item 1 and 2, characterized in that the oxidation of the iodine ions is carried out on platinum, gold or Edelmetalloxidbeschichteten titanium electrodes. 4. The method according to item 1 to 3, characterized in that a base electrolyte of the composition 0.05-3 M potassium iodide or sodium iodide having a pH of 2-7, to which a suitable catalyst is added, is used. 5. The method according to item 1 to 4, characterized in that the titration endpoint biamperometrically or bipotentiometrisch with one and the same indicator electrode pair (Pt-Pt, Au-Au, CC), housed in a flowed through arm of the titration cell and an impressed potential difference between 50 and 20OmV or a polarization current between 0.2 and 2μΑ using an electronic comparator circuit and an automatic Titrationsendpunktabschaltung the electrolysis current is determined. 6. The method according to item 1 to 4, characterized in that the titration end point is determined photometrically using a light emitting diode as a light source, a phototransistor as a light receiver and a glass tube cuvette and an electronic comparator circuit and an automatic Titrationspunktabschaltung the electrolysis current. 7. The method according to item 1 to 6, characterized in that a plurality of titrations are carried out in series to a preselected Abschaltschwellenwert the indicator signal in a base electrolyte solution.