Potentiometric determination of «available chlorine» in electrochemically treated chlorinated electrolytes

Размещено на http://www.allbest.ru/

Размещено на http://www.allbest.ru/

Potentiometric determination of «available chlorine» in electrochemically treated chlorinated electrolytes

The water for different application is disinfected by chlorine to provide sanitary, hygienic indices of drinking, household, technical water, technological liquids and to solve environmental problems. During the last years and today in water purifying installations (both in Russia and other countries), liquefied chlorine for disinfection of water is being replaced by the other analogous methods that are more simplified and rather safe. According to our data and data obtained by the scientists of developed countries, the most promising way of disinfection of water is electrolytic sodium hypochlorite treatment. The basis of the technique is electrolysis of water solution of sodium chloride in flowing condition.

Control of residual available chlorine is obligatory after chlorination. The term of «available chlorine» means total content of strong chlorinated oxidants in water i.e. sodium hypochlorite, molecular dissolved chlorine and chloramines. The most of analyses of residual available chlorine are carried out by traditional chemical technique based in the methods of volumetric titration by means of working solution of sodium thiosulphate with visual indication of equivalence point on starch. The main disadvantages of the methods are low chemical stability of sodium thiosulphate solutions, necessity of periodic control of titre, high limit of available chlorine content and therefore low sensitivity of the technique and relatively significant error of determination [1, P. 224]. Systematic error of iodide-starch methods is about 8 relative% at C_Ach = 0.3-0.7 mg/l and it is reduced to 4 relative% at C_Ach = 1.5 mg/l. It is known that research work on developing instrumental methods of available chlorine control is carried out. Its purpose is to eliminate the disadvantages of classic volumetric analysis, to automatize it and reduce labour intensity.

Electrochemical and especially anodic treatment (activation) is widly used for sample preparation in algorithm of various electrochemical and other physics-chemical methods of analysis [2, P. 11].

The purpose of the paper is to develop the technique of determination of available chlorine in electrochemically treated liquid by indirect potentiometric method.

Electrochemical generation of available chlorine in sodium chloride solution (salt content is from 0.001 to 1.0 mole/dm³) was carried out in anodic chamber of two-chamber flowing electrolyzer with electrodes in the from of graphite disks. The experiment was made in condition of direct and pulse current (from 0.05 to 0.5 A; the interval of current density was from 8 to 70 A/m²). Formation of oxidation-reduction potential was controlled with potentiometric measurements (indicator-point-platinum electrode ЭПВ-1, chloride-silver reference electrode ЭВЛ 1 М3) in both anolyte flow and a sampler at mixing by means of magnetic mixer. A fragment of anolyte forms galvanic cell.

Analytical control of available chlorine content (m_Ach or C_Ach, mg/dm³ or mole/dm³) in electrochemically treated liquid (i.e. in anolyte) was carried out by iodometric titration technique with visual indication of the end point of titration according to ГОСТ 18190-72 [3, P. 3-6]. Unlike the standard methods we selected several times smaller volume of liquid sample to analyze.

Methods of determination of available chlorine by volume iodometric titration.

0.5-1.0 of dry potassium iodide is put into conic retort of 100 cm³, it is dissolved in 1-2 cm³ of distilled water, then 1 cm³ of buffer solution with pH = 4.5 and 25 cm³ of anolyte or proper model solution is added. Isolated iodine is titrated by means of thiosulfate solution of С(Na₂S₂O₃) = 0.005 mole/dm³concentration up to light-yellow colour. Then 1 cm³ of 0.5% starch solution is added and titrated until blue colour disappears.

Mass fraction of available chlorine (m_Ach, mg/dm³) is calculated according to the formula

thiosulfate sodium chlorine electrolyte

(1)

- the quantity of 0.005 (normal) sodium thiosulfate solution used for titration, cm³;

К - coefficient of correction of sodium thiosulfate solution normality;

0.177 - available chlorine content that is equivalent to 1 cm³ of 0.005 N sodium thiosulfate solution;

V - the volume of water sample selected for the analysis, cm³;

1000 - conversion coefficient to converse gramme of mass into milligramme.

Molar concentration of available chlorine in anolyte (C_Ach, mole/dm³) is calculated according to the formula

(2)

- total volume of used for titration (up to the end point of titration), cm³;

V_x - volume of anolyte for one analysis, cm³.

The other method of quantity determination of available chlorine in electrochemically treated solution (i.e. in anolute) with various content of sodium chloride is indirect potentiometric method without titration (one-point oxidation-reduction titration), based on measurement of two values of oxidation-reduction potential  in reversible (balance) electrochemical iodine-iodide system with Pt-indicator electrode: after mixing the sample and reagent (KI) - (E₁) and after adding the dose of standard (fiksanalny) solution of iodine (E₂). Primary isolation in the mix tested (E₁) is conditioned by the reaction of available chlorine (oxidizer) with iodide-ions [4, P. 24], [5, P. 1951].

Various techniques of similar determination for control of residual quantity of available chlorine in drinking, industrial water and sewage, have been published. Apparently, potentiometric determination of available chlorine in electrochemically treated chlorinated electrolytes on the base of potentials  have never been carried out. Fs it was mentioned above, reversible potentials  are an important characteristic of oxidation-reduction properties of electrochemically treated liquid [6, P. 1739], [7, P. 30].

According to the theoretical analysis in the previous works [8, P. 724-726] the following dominating volumetric equilibrium reaction proceed in I₂ - KI - NaCl system:

(3)

(4)

Electrochemical reactions are localized on the surface of platinum electrode:

(5)

(6)

(7)

In the experimental conditions iodine is separated at considerable surplus of NaCl and KI, thus it is possible to describe concentration of iodine C(I₂) in the tested solution

С(I₂) = [I₂] + [I₃-] + [I₂Cl-] (8)

In consideration of equilibrium of all reactions (3-7) we can describe oxidation-reduction potential of the studied system in the terms of Nernst equation for electrochemical reaction (6). Expressing the concentration [I₃-] from the equation (8) at

(9)

(10)

we can obtain

(11)

- formal potential of the reaction (6);

 - concentration constant of formation of  [9, P. 1459];

- concentration constant of formation of I₂Cl particle [9, P. 1461].

Taking into consideration relatively high ionic strength of the tested solution from the equation (11)

We can obtain

(12)

The correlation (12) agree with experimental data: experimental dependence of electromotive force of voltic cell on is linear and it is withing the range C(I₂) = 2^. 0^-4 - 1^. 0^-2 mole/dm³. Therefore, the average experimental value of the slope of the electrode function is S = 29.9 mV. The results obtained confirm that it is possible to use indirect potentiometric method of determination of available chlorine without titration in electrochemically treated salt solution.

Methods of determination of available chlorine in electrochemically treated liquid by indirect potentiometric technique. 50 cm³ of anolyte solution is put into a beaker of 100 cm³, then 0.5 g of dry potassium iodide is added and dissolved; the firs value of oxidation-reduction potential E₁ is measured. Then 1 cm³ additive of standard (fiksanalny) solution of iodine of C(I₂) = 0.1 mole/dm³concentration is put in and the second value of oxidation-reduction potential E₂ is measured. The difference of potentials

(13)

The content of available chlotine C_Ach, mole/dm³ is calculated by means of the formula

(14)

C(I₂) - concentration of fiksanalny solution of iodine mle/dm³;

 - the volume of standard iodine additive, cm³

V_x - the volume of anolyte for one experiment, cm³;

S - the slope of the electrode function, mV.

C_Ach is multiplied by 0.0355 to evaluate the content of available chlorine according to mass fraction m_Ach, mg/dm³.

The results of available chlorine determination in analyte of flowing two-chamber electrolyzer at various modes of electrolysis by iodometric titration and indirect potentiometric techniques are shown in tables 1 and 2.

The obtained results are considered to be quite reliable as there is satisfactory accordance between techniques of iodometric titration and indirect potentiometry: more then half results of anolyte samples by criterion F in tables 1 and 2; by t-criterion it has been established almost complete absence of significative systematic error [10. P. 47].

Table 1. The results determination of available chlorine mass fraction in the anolyte samples of flowing two-chamber electrolyzer by iodometric titration method (n=3; P=0.95; t_p=4.30)

Table 2. The results of determination of available chlorine mass fraction in anolyte samples of flowing two-chamber electrolyzer by the method of indirect potentiometry (without titration). The conditions of electrolysis - see table 1 (n=3; P=0.95; F (P, f₁, f₂)=19.00; t_p=4.30

At = 19.00 the value of t-criterion is using the formula

(15)

n₁=n₂=3, C₁ and C₂ are average values of according to the results of chemical and electrochemical analysis.

At= 19,00 the value of t-criterion is using the formula

(16)

taking into account that the results of chemical analysis C₂ are the nearest tj the true value of m(ACh) concentration [10. P. 60]. According to tables 1 and 2 the conditions of electrolysis (amperage, velocity of electrolyte current) do not influence sensitively the results of determination of C_Ach.

Suggested in the paper variant of indirect potentiometric method of total content determination of available chlorine without using buffer system leads to quite reliable data about C_Ach in electrochemically (by anode) treated chlorinated water solutions with pH about 1.5 - 3.5 (HCl formation).

The method can be used by control-analytical services that makes it possible to reduce labour intensity of the analysis of residual available chlorine determination and to enhance its sensibility.

Reference

thiosulfate sodium chlorine electrolyte

1. Determination of chlorine oxides by amperometric titrator using current-integration method / Watanabe T., Tanaka M., Shu-ming Chen, et al // Bunseki Kagaku. - 1991. - Vol.40, №12. - P.221-226.

2. Svintsova L.D., Chernyshova N.N. Sample preparation by electrochemical pretreatment in the membranous electrolyzer // Int. Congress on Analyt. Chem.: Abstracts / Moscow. St. Univ. - M., 1997. - Vol.1. - P.10-12.

3. GOST 18190-72. Voda pit'evaja. Metody opredelenija soderzhanija ostatochnogo aktivnogo hlora. [Drinking water. Methods for determining the content of residual active chlorine]. Vved.1974-01-01. - M.: Izd-vo standartov, 2010. - 12 p. [in Russian]

4. Serikov U.A. Potenciometricheskoe opredelenie aktivnogo hlora v promyshlennyh i stochnyh vodah titanomagnievogo proizvodstva [Potentiometric determination of available chlorine in industrial water and sewage of titanium-magnesium production] / U.A. Serikov, T.V. Noskova // Zavodskaja laboratorija [Laboratory Manager]. - 1988. - №7. - P.23 - 26. [in Russian]

5. Vlasov M. Ju. Potenciometricheskoe opredelenie ostatochnogo aktivnogo hlora [Potentiometric determination of residual active chlorine] / M. Yu. Vlasov, Yu.I. Nikolayev, A.M. Pisarevsky and others // Zhurnal prikladnoj himii [Journal of Applied Chemistry]. - - Vol. 57, №. 9. - P.1949-1954. [in Russian]

6. Pisarevskij A.M. Potenciometricheskoe opredelenie aktivnogo hlora s ispol'zovaniem kulonometricheskogo vvedenija standartnoj dobavki ioda [Potentiometric determination of active chlorine using coulometric injection of a standard iodine supplement] / A.M. Pisarevskij, Yu.A. Serikov, Т.D. Shigaeva and others // Zhurnal prikladnoj himii [Journal of Applied Chemistry]. - Vol. 59, №. 8. - P.1737-1743. [in Russian]

7. Pisarevskij A.M. Vozmozhnosti potenciometricheskoj metodiki opredelenija ostatochnogo aktivnogo hlora v pit'evoj vode [Possibilities of potentiometric technique for determination of residual active chlorine in drinking water] / A.M. Pisarevskij, I.P. Polozova, Yu.I. Nikolaev and others // Vestnik SPbGU. Serija 4 [Bulletin of St. Petersburg State University. Series 4]. - - Vol. 1, №. 4. - P.29-35. [in Russian]

8. Turyan A.I. Okislitel'no-vosstanovitel'nye processy v sisteme I₂+KI+NaCl i potenciometricheskoe opredelenie iodida kalija v povarennoj soli [Oxidation-reduction processes in the system I₂+KI+NaCl and potentiometric determination of potassium iodide in sodium chloride] / A.I. Turyan, L.M. Maluka, T.R. Markova // Zhurnal analiticheskoj himii [Analytical chemistry journal]. - 1992. - Vol.47, №4. - P. 723-730. [in Russian]

9. Turyan A.I. Kosvennoe redoks-potenciometricheskoe opredelenie medi [Indirect redox-potentiometric determination of copper] / A.I. Turyan, L.M. Maluka, T.R. Markova // Zhurnal analiticheskoj himii [Analytical chemistry journal]. - 1992. - Vol.47, №8. - P. 1456-1463. [in Russian]

10. Doerfel K. Statistics in analytical chemistry / Transl. from German. - М.:Mir, 1969. - 248 p.

Размещено на Allbest.ru