DE4440872C2 - Method for determining the equilibrium concentrations of the components halogen, halide, trihalide, hypohalite and / or hypohalous acid in aqueous solutions - Google Patents

Description

The present invention relates to a method for Determination of the equilibrium concentrations of the components Halogen, halide, trihalide, hypohalite and / or hypohalous Acid in aqueous solutions.

Halogens, especially chlorine and iodine, are large Scope used as a disinfectant. Because the bactericidal Effectiveness essentially through the in balance existing free halogen is due, are procedures for Determination of these components of interest.

The total [HOX] + [X₂] is called free halogen. These two components are essentially for the bactericidal activity responsible.

The halogens, chlorine, which are suitable for disinfection purposes 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 ^- + 2 X ^- + 3 H⁺ (disproportionation) (IV)

While reactions I, II and III are very rapid, disproportionation IV is a relatively slow process that leads to non-bactericidal halogenate (XO₃ ^- ), the formation of which however takes place through suitable reaction conditions (for iodine e.g. pH <6) can be largely prevented.

In aqueous, used for disinfection purposes Halogen solutions are therefore included:

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 ^- .

Within the individual halogens result from the different sizes of the individual equilibrium constants naturally considerable differences, as from the following compilation can be easily removed.

Halides have so far been determined using ion-selective methods Electrodes where a certain concentration a certain potential setting of the ion to be determined causes. Such ion selective electrodes but tend to determine halide in the presence of Halogen for corrosion.

The object of the present invention is a method to determine the equilibrium concentrations of the Components halogen, halide, trihalide, hypohalite and / or hypohalous acid in aqueous solutions create a quick measurement with simple not enables ion-selective electrodes.

This object is achieved according to the invention by a generic Procedure with the characteristics of the license plate 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 (V) is determined:

E _{X2 / X-} = E₀ - (RT / nF) ln (f² [X ^- ] ² / [X₂]). (V)

Here f is the activity coefficient (for calculation see below).

According to Eq. V can be calculated from the redox potential, the equilibrium concentration of free molecular halogen [X₂], if the halide concentration [X ^- ] is known, but this is generally not the case.

The redox potential, together with the pH, can - without knowledge of the halide equilibrium concentration [X ^- ] - be used to calculate the equilibrium concentration of the species X₂, HOX, OX ^- , X₃ ^- and X ^- in aqueous halogen solutions if

• - The halogen / halide solution by partial oxidation of one Halide solution of known concentration was prepared (Variant A) or
• - 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 the known concentration c (X ^- ) is partially oxidized (electrical current or chemical):

2X ^- → X₂ + 2e ^- (VI)

The mass balance then applies after the oxidation

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

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, E₀ ^* = corrected "standard potential" (see below) and S = Nernst factor (in the redox system X ^- / X₂ [n = 2] at 25 ° C Value S = 29,579) follows from the mass action expressions of equations I-III and equations V and VII the system of equations VIII-XII.

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

E = E₀ ^* - Slog (f²y² / x) (IX)

K₁ = yfpH / x (X)

K₂ = rfH / p (XI)

K₃ = z / (xy) (XII)

It has proven useful to use these five equations to resolve after y:

CA - y [A + K₁ / (Hf) + K₁K₂ / (H²f²)] = 2y² + 3K₃y³, (XIII)

wherein A means [₁₀ ((E₀ ^* -E) / S)] / f².

Equation XIII can be solved by the usual methods, e.g. B. done by iteration. The calculation of f will integrated into the same iteration process. The other equilibrium concentrations are with the calculated the following equations:

x = y² / A, p = K₁y / (fHA), r = K₁K₂y / (f²H²A), z = K₃y³ / A.

The following applies to the 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 ). (XIV)

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 ( y = [X ^- ], z = [X₃ ^- ], r = [OX ^- ]).

Variant B

The redox potential and the pH of one Halogen / halide solution, its equilibrium concentrations are not known, before and after adding a Halide solution of known concentration measured.

Using the definitions
C = total halogen concentration of the measurement solution (unknown)
D = total halide concentration of the measurement solution (unknown)
V = 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 + V _Δ ). . . Halide increment
Φ = V / (V + 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 equation system XV-XXI follows:

C = x₁ + z₁ + p₁ + r₁ = (x₂ + z₂ + p₂ + r₂) / Φ (XV)

D = y₁ + z₁ - p₁ - r₁ = (y₂ + z₂ - p₂ - r₂ - Δ) / Φ (XVI)

By substitution and dissolution after y₁ follows with

G₁ = 10exp ((E₀ ^* -E₁) / S) and
G₂ = 10exp ((E₀ ^* -E₂) / S)

γ₁ = f₁² / G₁
γ₂ = f₂² / G₂)
β₁ = 2B₁f₁² / G₁ - 1
β₂ = 2B₂f₂² / G₂ - 1
the equation XXII

Φγ₁y₁ (K₃y₁² + y₁ + B₁) = γ₂y₂ (K₃y₂² + y₂ + B₂), (XXII)

wherein
y₂ = -β₂ / (2γ₂) ± {[Φy₁ (γ₁y₁ + β₁) - Δ] / γ₂ + β₂² / (4γ₂²)} ^1/2 .

Equation XXII is solved as indicated above by Iteration. The other equilibrium concentrations will be calculated using the following equations:

The activity coefficients, f 1 and f 2, are determined as described in variant A, where I₁ = y₁ + z₁ + r₁ and I₂ = y₂ + z₂ + r₂.

Variants A and B can be used for the redox-potentiometric content determination of aqueous halogen solutions, preferably if the redox potential is established very quickly at the measuring electrode (Pt). This is the case when the species X₂ and X ^- are present in finite concentrations, which is definitely the case for iodine and bromine solutions under the conditions customary in disinfection. With chlorine solutions, however, this is only the case at pH values <3, since at higher pH values - due to the different equilibrium ratios (see Table 1) - the species X₂, ie molecular chlorine, is practically not present, which is a very slow and sometimes not reproducible adjustment of the redox potential in the Cl₂ / H₂O system results.

The analysis of purely aqueous iodine solutions is problematic in that, due to the high air / water distribution coefficient of D≈100 for molecular iodine, evaporation losses very easily occur, which can lead to a falsification of the results (values that are too low). While this is in systems with a high proportion of complex iodine, such as. B. PVP-iodine solutions does not lead to any noteworthy errors, this must be taken into account with purely aqueous iodine solutions. Due to the extremely rapid potential setting, evaporation losses can be kept very low with the redox potentiometric method without special precautions. The method of redox-potentiometric halogen determination is not 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. V).

Definition of E₀ *:
To simplify the calculations you can use

E₀ * = E₀ - E _Ref - E _Diff (XII)

a corrected "standard potential" can be defined (for Difference from the thermodynamic value E₀), which in addition to the Calibration correction, d. H. the experimentally determined Diffusion potential, also the potential of Reference electrode considered. That way they can values read on the measuring device can be used directly.

The invention is illustrated below with the aid of the examples explained. All of the features included in the examples are regarded as 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 as follows:

[J₂] in saturated iodine solutions

With regard to the temperature dependence of the solubility of According to Ramette and Sandford, molecular iodine in water applies The following 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 [J ^- ]

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

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 / f) / f, a = [J₂].

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 above.

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 (E₀ = 244.4 mv), the results listed below with Eq. XII calculated corrected "standard potentials", which are subject to an inaccuracy of ± 0.3 mV.

Example 2 Determination of an oxidized halide solution (variant 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 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.

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 stirring vessel, Halide solution of precisely known concentration, Precision pipette with variable volume setting (50-1000 µl).

Measuring instructions: The measuring solution (volume: V) is placed in the thermostated mixing vessel and after reaching the measuring temperature (25 ° C) the 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 iodide solution added should be adjusted so that a potential change of 10-20 mV entry.

Example 4 Simplified versions for iodine / iodide solutions

To ensure sufficient stability (ie no iodate formation), iodine solutions should have a pH <6 [5]. Since in this weakly acidic environment, especially in the presence of finite iodide concentrations, HOJ and OJ ^{- are} only present in negligible traces, their determination can be dispensed with, which eliminates the need to measure the pH, which reduces the instrumental and metrological effort means. The difference to variants A and B of the redox potentiometric halogen determination described above is that the pH electrode is dispensed with and K₁ = K₂ = 0 is set in the algorithms specified above, with the result that [HOJ] = [OJ ^- ] = 0 becomes.

Under the conditions
- pH <7
- cJ₂ <10 ^-3 mol / L
- cJ ^- <10 ^-4 mol / L
As is the case in aqueous iodine solutions suitable for disinfection purposes, the error when using this approximation in the worst case is not more than 0.3%.

literature

[1] Kielland JJ Am. Soc. 59 (1937), 1675-8,
[2] Ramette RW, and Sandford RW: J. Am. Soc. 87, 5002 (1965),
[3] Mussini T., Faiti G. Encycl. Electroch. Elem. 1 [1973] 57-90,
[4] Jones G., chaplain BBJ Am. Soc. 50 [1928] 2076,
[5] Gottardi W. Zbl. Bakt. Hyg., 1st Dept. Orig. B 172, 498-507 (1981).

Claims (3)

1. A method for determining the equilibrium concentrations of the components halogen (X₂), halide (X ^- ), trihalide (X₃ ^- ), hypohalite (OX ^- ) and hypohalous acid (HOX) in an aqueous halogen solution obtained by oxidation of a predetermined concentration of halide, thereby characterized in that the redox potential and the pH of the oxidized solution are measured and from the known equations: a) the mass balance of the halogen components,
b) the Nernst equation and
c) the mass action expressions of the chemical equations for hydrolysis, dissociation and trihalide formation are calculated to determine the equilibrium concentrations. 2. Method for determining the equilibrium concentrations of the components halogen (X₂), halide (X ^- ), trihalide (X₃ ^- ), hypohalite (OX ^- ) and / or hypohalous acid (HOX) in a by adding a known amount of halide or Dilution with an aqueous halogen solution obtained with a halide solution of known concentration, characterized in that the redox potentials and pH values of the solution containing or diluted with halide are measured before and after the addition or dilution and from the known equations: a) the mass balances of the halogen components,
b) the Nernst equations and
c) the mass action expressions of the chemical equations for hydrolysis, dissociation and trihalide formation are calculated to determine the equilibrium concentrations. 3. The method according to claims 1 or 2, characterized characterized in that for determining the equilibrium concentrations in iodine solutions in a weakly acidic environment alone one or two determination (s) of the redox potential (s) be performed.