DE4440872A1 - Method for determining the equilibrium concentrations of the components halogen, halide, trihalide, hypohalide and / or hypohalous acid in aqueous solutions - Google Patents

Abstract

A process is disclosed for determining the equilibrium concentrations of halogen, halogenide, trihalogenide, hypohalogenite and/or hypohalogenic acid components of an aqueous solution obtained by oxidising a predetermined amount of halogenide or by dilution with halogenide. The process is characterised in that the redox potential of the oxidised or diluted solution is measured before and after diluting it. The analysis values to be determined are calculated from the following equations: (a) the halogenide weight assessment; (b) the Nernst equation; and (c) the terms of the mass action of the chemical equations for hydrolysis, dissociation and trihalogenide formation. The process is particularly suitable for determining the equilibrium concentrations in aqueous iodine solutions. A simplified weight assessment equation is disclosed that dispenses with pH-value measurements.

Description

The present invention relates to a method for Determination of the equilibrium concentrations of halogen, Halide, trihalide, hypohalite and / or hypo halogenitic acid components in aqueous solutions.

Halogens, especially chlorine and iodine, are large Scope used as a disinfectant. Since the bacteri zide effectiveness essentially by the same free halogen is important, are possible to determine these components of interest.

The sum of HOX + X₂ is called "free halogen". These two components are essentially for the bactericidal activity responsible.

The halogens, chlorine, Bromine and iodine go the same with the solvent water Reactions on:

X₂ + H₂O ↔ HOX + X ^- + H⁺ (hydrolysis, K₁) (I)

HOX ↔ OX ^- + H⁺ (dissociation, K₂) (II)

X₂ + X ^- ↔ X₃ ^- (trihalide formation, K₃) (III)

3 HOX ↔ XO₃ ^- + 2X ^- + 3H⁺ (disproportionation) (IV)

While the reactions I, II and III are very rapid, the disproportionation IV is a relatively slow process, which leads to non-bactericidal halogenate (XO₃ ^- ), but its formation by suitable reaction conditions (pH ≦ ωτ 6, finite halide concentrations ) can largely be prevented. The following species are therefore present in aqueous halogen solutions used for disinfection purposes:

X₂, HOX, OX ^- , X₃ ^- and X ^-

The percentage of each halogen depends on the pH, the initial halogen content cX₂ and the initial halide content cX ^- . Due to the different sizes of the individual equilibrium constants, there are naturally considerable differences within the individual halogens, as can easily be seen from the following compilation.

Halides have so far been determined using ions selective electrodes where a certain concentration tration of the ion to be determined has a certain potential setting causes. Such ion selective electrodes but tend to determine halide in the presence of Halogen for corrosion, equilibrium takes time relatively long, so that for more accurate measurements generally conditioning over several hours is required.

The object of the present invention is a method for determining the equilibrium concentrations of halo gene, halide, trihalide, hypohalite and / or halogen acid components to create a fast Measurement with simple non-ion selective electrodes enables.

This object is achieved according to the invention by a gat Proper procedure with the characteristics of the mark of the main claim. The subclaims give preferred Embodiments of the invention again.  

In aqueous halogen solutions, which contain finite concentrations of the species X ^- and X₂, a potential E _{X2 / X-} (= redox potential) is established on inert metal electrodes (Pt, Ir, Rh, etc.) very quickly and with excellent reproducibility Nernst's equation

E _{X2 / X-} = E _o - (RT / nF) _* ln (f² [X ^- ] ² / [X₂]) (V)

is determined.

According to Eq. V can from the redox potential Equilibrium concentration of free molecular halogen [X₂], can be calculated if the halide concentration is known is what is generally not the case. However, the redox potential, together with the pH value, can - without knowing the halide concentration - to calculate the Equilibrium concentrations in aqueous halogen solutions be used if

• - The halogen / halide solution by partial oxidation of one Halide solution of known concentration was prepared (Option A)
• - Redox potential and pH of a halogen / halide solution before and after adding a known amount of halide (or Dilution with a halide solution of known concentration) are known (variant B)

option A

A halide solution of concentration cX ^- is partially oxidized (electrical current or chemical):

2X ^- → X₂ + 2e ^- VI

The following then applies after oxidation:

cX ^- = [X ^- ] + 2 [X₂] + 3 [X₃ ^- ] + [HOX] + [OX ^- ] VII

Derivative

Using the definitions: C = cX ^- , x = [X₂], y = [X ^- ], z = [X₃ ^- ], p = [HOX], r = [OX ^- ], H = hydrogen ion activity, f = Activity coefficient, E = measured potential difference of the platinum / reference electrode chain, E _o * = corrected "standard potential" (see 2.3.3.) And S = Nernst factor (has X ^- / X₂ [n = 2] in the redox system 25 ° C the value S = 29,579) follows from the mass effect expressions of equations I-III and equations V and VII the system of equations VIII-XII.

C = y + 2x + 3z + p + r VIII

E = E _o * - S _* log (f²y² / x) IX

K₁ = y _* f _* p _* H / x X

K₂ = r _* f _* H / p XI

K₃ = z / (xy) XII

Depending on the type of substitution, the five equations can be with the unknowns x, y, z, p and r in 3rd degree polynomials convert. It has proven useful according to y dissolve:

CA - y (A + K₁ / Hf + K₁K₂ / H²f²) = 2y² + 3K₃y³ (XII _a )

A = [10ˆ ((E _o * - E) / S)] / f²

Solving the equation

The root of equation XIIa can be e.g. B. be solved by iteration. Here y is inside of the iteration limits C ≦ λτ y ≦ λτ 0 varies until the Equation XIII is satisfied. The calculation of f (see below) is done in the same iteration process integrated. The other equilibrium concentrations are with the calculated the following equations:

x = y² / A, e = K₁y / fHA, r = K₁K₂y / f²H²A, z = K₃y³ / A

Calculation of the activity coefficient f

According to Kielland [1], for monovalent ions such as chloride, Bromide and iodide at 25 ° C

-logf = 0.509 _* I ^1/2 / (1 + 0.984 _* I ^1/2 ) XI

The ionic strength (I) is calculated as I = y + z + r + I _(add) , where I _{(add) is} the concentration of any electrolytes (e.g. buffers) that do not interfere with the equilibria I-IV.

Variant B

The redox potential of a halogen / halide solution whose Equilibrium concentrations are not known before and after adding a halide solution of known concentration measured.

Derivative

Using the definitions

C₁ = total halogen concentration of the measurement solution (unknown)
D₁ = total halide concentration of the measurement solution (unknown)
V₁ = volume of the measurement solution
V _Δ = volume of the halide solution
C _Δ = concentration of the halide solution
E₁ = potential difference Pt / Ref of the measurement solution
H₁ = hydrogen ion activity of the measuring solution
Δ = C _Δ V _Δ / V 1. . . Halide increment
Φ = V₁ / (V₁ + V _Δ ). . . Dilution factor
E₂ = potential difference Pt / Ref after adding the halide solution
H₂ = hydrogen ion activity after adding the halide solution
x₁, y₁, z₁, p₁ and r₁ or x₂, y₂, z₂, p₂ and r₂ = equilibrium concentrations of X₂, X ^- , X₃ ^- , HOX and OX ^- before or after adding the halide increment the system of equations XIII-XIX follows:

By substitution and dissolution after y₁ follows with

G₁ = 10ˆ ((E _o * - E₁) / S) and
G₂ = 10ˆ ((E _o * - E₂) / S)
B₁ = K₁ / f₁ / H₁ _* (1 + K₂ / f₁ / H₁)
B₂ = K₁ / f₂ / H₂ _* (1 + K₂ / f₂ / H₂)
γ₁ = f₁ˆ2 / G₁
γ₂ = f₂ˆ2 / G₂
β₁ = 2 _* B₁ _* f₁ˆ2 / G₁ - 1
β₂ = 2 _* B₂ _* f₂ˆ2 / G₂ - 1
the polynomial
F _I (y₁) = F _II (y₁)
in which
F _I (y₁) = Φγ₁y₁ (K₃y₁² + y₁ + B₁)
and
F _II (y₁) = Φγ₂y₂ (K₃y₂² + y₂ + B₂)
y₂ = -β₂ / 2 / γ₂ ± {1 / γ₂ [Φy₁ (γ₁y₁ + β₁) - Δ] + β₂² / 4 / γ₂²} ^1/2

Solving the equation

First, it is determined which of the two variants ("+" or "-") of F _II (y₁) lead to a solution. For this purpose, with gradual changes in y₁ (e.g. enlargement by a factor of 10ˆ (1 / W), with W = 10, 20..., Starting at y₁ = 10 ^-7 mol / l), the differences F _I (y₁) - F _II (y₁) "-" and F _I (y₁) - F _II (y₁) "+", where, depending on the parameters used, one of the two differences changes the sign, so that the variant of F leading to the solution _II (y₁) is found. In the area of the sign change there is also the exact solution, which is found by iteration between the two sign changing y 1 values. The other equilibrium concentrations are calculated using the following equations:

The determination of the activity coefficients, f₁ and f₂, is carried out as described in variant A, where I₁ = y₁ + z₁ + r₁ + I _Add and I₂ = y₂ + z₂ + r₂ + I _Add .

Simplified versions for iodine / iodide solutions

To ensure sufficient stability (ie no iodate formation), iodine solutions should have a pH ≦ ωτ 6 [2]. Since HOJ and OJ ^{- are} only present in negligible traces in this weakly acidic environment, especially in the presence of finite iodide concentrations (see Eqs. I and II), their determination can be dispensed with, which means that the pH value is measured superfluous, which means a reduction in the instrumental and metrological effort. The difference to that in 2.1. and 2.2. Variants A and B of the redox-potentiometric halogen determination described are that the pH electrode is dispensed with and K₁ = K₂ = 0 is set in the above-mentioned algorithms, with the result that [HOJ] = [OJ ^- ] = Becomes 0.

Under the conditions

- pH ≦ ωτ 7
- cJ₂ ≦ ωτ 10 ^-3 mol / L
- cJ ^- ≦ λτ 10 ^-4 mol / L

as in aqueous, suitable for disinfection purposes If iodine solutions are present, the error is Application of this approximation in the worst case no more than 0.3%.

For the other halogens, neglecting HOX / OX ^- incorrect values with - especially at low halide concentrations - up to 30% too low levels of "free halogen" can result.

Calibration of the measuring system

For the potential difference of the electrode to determine the Redox potential applies

E = E _{X2 / X-} - E _Ref - E _Diff XII

Further applies

E _{X2 / X-} = E _o - S _* log (f² _* [X ^- ] ² / (X₂]) XIII

E is the potential difference of the platinum / reference electrode chain read on the measuring device, E _{X2 / X-} the potential that arises at the platinum electrode, E _Ref that of the reference electrode, E _Diff its diffusion potential and E _o the (thermodynamic) standard potential of the reaction X₂ (aq) + 2e ^- ⇔ 2X ^- .

While the standard potential (e.g. iodine / iodide: E _o = 0.6224 V) and the potential of the reference electrode (e.g. E _Cal.ges = 0.2444 V at 25 ° C) are known, the diffusion potential, which is used by depends on the stirring speed and the geometry of the measuring vessel.

This is done by measuring solutions of known halogen and halide equilibrium concentration and application of Equation XIV (follows from XII and XIII):

E _Diff = E _o - E - E _Ref - S _* log (f² _* [X ^- ] ² / [X₂]) XIV

For this purpose, iodide solutions of a precisely known starting concentration (cJ ^- ), which are saturated with elemental iodine, are expediently used, as a result of which errors due to evaporation losses are avoided.

[J₂] in saturated iodine solutions: Regarding the Temperature dependence of the solubility of molecular iodine in According to Ramette and Sandford [3], water applies as follows Relationship:

log [J₂] _t = -2.8812 + 1,334 _* 10 ^-2 _* (t - 25) + 2,520 _* 10 ^-5 _* (t - 25) ²

where t is the temperature in ° C. In the solution saturated at 25 ° C is thus [J₂] = 1,315 _* 10 ^-3 mol / l.

Calculation of aJ

From the mass balance sheets
cJ₂ = [J₂] + [HOJ] + [OJ ^- ] + [J ^- ]
2cJ₂ + cJ ^- = [J ^- ] + 2 [J₂] + 3 [J₃ ^- ] + [HOJ] + [OJ ^- ]
and follows the mass action expressions of Reactions I-III
cJ₂ = (G² / 4 / (2 - N) ² + S / (2 - N)) ^1/2 - G / 2 / (2 - N)

Here mean:
G = 2 (cJ ^- - a) (1 - N) - P, S = P (cJ ^- - a) + N (cJ ^- - a) ² + Q, P = 2a - cJ ^- , B = 2K3a + 1 , N = (3K₃a + 1) / B, Q = K₁aB (1 + K₂ / H), a = [J₂] = 1,315 _* 10 ^-3

With the help of cJ₂ and the known initial concentration of halide, cJ ^- , the other equilibrium concentrations can be calculated using the following equations:

[J ^- ] = (cJ₂ + cJ ^- - a) / (2K₃a + 1), [J₃ ^- ] = K₃ya, [HOX] = K₁a / f / [J ^- ] / H, [OJ ^- ] = K₂ [HOI ] / f / H

For the calculation of f see 2.1.3.

Definition of E _o *

To simplify the calculations,

E _o * = E _o - E _Ref - E _Diff XV

defines a corrected "standard potential" (in contrast to the thermodynamic value E _o ) which, in addition to the calibration correction, ie the experimentally determined diffusion potential, also takes the potential of the reference electrode into account. In this way, the values read on the measuring device can be used directly.

The invention is illustrated below with the aid of the examples explained. All features included in the examples are considered essential to the invention.

Example 1 Calibration of the measuring system

In a stirred vessel TTA 60, equipped with the Pt electrode P 1040, the pH electrode G 2040C and the Calomel reference electrode K 4018 (electrolyte: complete KCl solution), all from Radiometer, finely powdered iodine in iodide solutions of the concentration cJ _- = 0.008 - 0.04 mol / L, mixed with 0-0.058 mol / L NaNO₃ (additional ionic strength) suspended and stirred at 25.0 ° C until the potential difference E _{Pt / Ref} registered by a recorder was constant.

The diffusion potential was calculated from the latter using the method specified in the theoretical part. The result of five measurements within the concentration ranges given above is E _Diff = -2.4 ± 0.3 mV. With the known standard potentials for the reaction X₂ = 2e ^- ⇔ 2X ^- and the saturated calomel electrode (E _o = 244.4 mV), the results listed below with Eq. XV calculated corrected "standard potentials", which are subject to an inaccuracy of ± 0.3 mV.

Example 2 (Determination of an oxidized halide solution) (Option A)

Equipment expenditure: Two pH / mV meters with a resolution of 0.1 mV or 0.001 pH, one platinum and one reference electrode each, thermostatic stirring vessel. Measurement specification: The potential difference E _{Pt / Ref} registered by a recorder and the pH value of the partially oxidized halide solution, whose initial concentration C (ie before the oxidation) is known, is measured in a thermostatted stirring vessel after reaching the measurement temperature (25.0 ° C). Typically, the potential is constant after 2-3 minutes, which is mainly due to reaching the selected measuring temperature. Calculation: See 2.1.2.

Example 3 (Determination of an active halogen-containing solution by Dilution with halide solution (Variant B)

Equipment expenditure: Two pH / mV meters with one resolution of 0.1 mV or 0.001 pH, each a platinum, pH and Reference electrode, thermostatic mixing vessel, iodide solution Precisely known concentration, precision pipette with variable Volume setting (50-1000 µl).

Measuring instructions: The measuring solution (volume: V₁) is placed in the thermostated mixing vessel and after reaching the measuring temperature (25 ° C) redox potential and pH value are read (= E₁, pH₁). Then an exactly measured volume (V _Δ ) of the iodide solution (concentration: C _Δ ) is added and read again (= E₂, pH₂).

The whole measuring process requires due to the rapid Potential setting no more than 2-3 min. Volume and concentration of the halide solution added should be adjusted so that a potential change of 10-20 mV entry.  

Area of application of the redox potentiometric method

Variants A and B can then be used successfully to determine the redox potentiometric content of aqueous halogen solutions if the redox potential at the measuring electrode (Pt) is established very quickly. This is the case when the species X₂ and X ^- are present in finite concentrations, which applies in any case to iodine and bromine solutions under the conditions customary in disinfection.

With chlorine solutions, however, this is only at pH values ≦ ωτ 3-4 the case because at higher pH values - due to the other Equilibrium ratios (see Table 1) - the species X₂, d. H. molecular chlorine, practically nonexistent, which is a very slow and sometimes not reproducible adjustment of the Redox potentials in the Cl₂ / H₂O system.

The analysis of purely aqueous iodine solutions is so far problematic than due to the high air / water Distribution coefficients of D ≈ 100 for molecular iodine very much Evaporation losses easily occur, leading to a Falsification of the results (values that are too low). While this is in systems with a high percentage of complex iodine, such as B. PVP iodine solutions to none leads to significant errors, this must be done with purely aqueous Iodine solutions are very well observed.

Due to the extremely quick potential setting, the redox potentiometric method without evaporation losses special arrangements are kept very low.  

The method of redox-potentiometric halide determination is not the use of an ion-selective, but an inert metal electrode whose potential in halogen / halide solutions does not depend on the halide activity aX ^- but on the ratio (aX ^- ) ² / [X₂] ( see Eq. II).

The method also allows a successful salary determination if ΔC = 0, i.e. H. if only diluted with water.

Claims (6)

1. A method for determining the equilibrium concentrations of the components halogen, halide, trihalide, hypohalide, and / or hypohalous acid in a known either by oxidation of a predetermined amount of halide or in a known by adding a known amount of halide or diluting with a halide solution Concentration obtained aqueous halogen solution, characterized in that the or the redox potentials or the or the pH values of the oxidized solution or the solution mixed or diluted with halide are measured before and after the mixing or dilution and from the following equations

• a) the mass balance of the halogen components,
• b) the Nernst equation and
• c) the mass effects of the chemical equations for hydrolysis, dissociation and tri halogenite formation

the equilibrium concentrations to be determined are calculated will. 2. The method according to claim 1, characterized in that in a solution which has been obtained by oxidation of a predetermined amount of halide cX ^- the redox potential and the pH of the aqueous solution is measured and from the equations (1) C = y + 2x + 3z + p + r (mass balance) (2) E = E _o * - S _* log (f²y² / x) (Nernst) (3) K₁ = y _* f _* p _* H / x (mass effect expression hydrolysis) ( 4) K₂ = r _* f _* H / p (mass action expression dissociat.) (5) K₃ = z / (xy) (mass action expression Trihal.bld.) In which:
H = hydrogen ion activity
C = cX ^- (halide concentration)
x = X₂ (halogen concentration)
y = X ^- (halide concentration)
z = X₃ ^- (trihalide concentration)
p = HOX (concentration of halogen acid)
r = OX ^- (concentration anion of p)
E = measured potential difference
E _o * = corrected standard potential
S = Nernst factor
the sizes x, y, z, p and r are determined. 3. The method according to claim 1, characterized in that the equation system (1) to (5) after the halide concentration y = [X ^- ] is solved. 4. The method according to claim 1, characterized in that the pH values and the redox potentials of the dilute solution are measured before and after dilution. Dilutions (E₁ and E₂) are measured and from the equations in which mean:
C₁ = total halogen concentration of the measurement solution (unknown)
D₁ = total halide concentration of the measurement solution (unknown)
V₁ = volume of the measurement solution
V _Δ = volume of the halide solution
C _Δ = concentration of the halide solution
E₁ = potential difference Pt / Ref of the measurement solution
H₁ = hydrogen ion activity of the measuring solution
Δ = C _Δ V _Δ / V 1. . . Halide increment
Φ = V₁ / (V₁ + V _Δ ). . . Dilution factor
E₂ = potential difference Pt / Ref after adding the halide solution
H₂ = hydrogen ion activity after adding the halide solution
x₁, y₁, z₁, p₁ and r₁ or x₂, y₂, z₂, p₂ and r₂ = equilibrium concentrations of X₂, X, X₃ ^- , HOX and OX ^- before or after adding the halide increment
be determined. 5. The method according to claims 1, 2 and 4, characterized characterized in that to determine the equilibrium concentrations in iodide solutions in equations (XII), (XIII), (3) and (4) the constants K₁ = K₂ = 0 are set.