EP0896669A1

EP0896669A1 - Measuring chlorine concentration

Info

Publication number: EP0896669A1
Application number: EP97914457A
Authority: EP
Inventors: Stephen Dennison; Kevin Anthony Foster; David Bonnick
Original assignee: Wallace and Tiernan Ltd
Current assignee: CHEMFEED Ltd
Priority date: 1996-05-03
Filing date: 1997-03-26
Publication date: 1999-02-17
Also published as: GB9609301D0; AU2170097A; WO1997042497A1

Abstract

The concentration of a dissolved chlorine-containing species is monitored using an amperometric cell having a platinum working electrode, a counter electrode and a reference electrode. Water the chlorine concentration of which is to be monitored is tested to determine the current (i1) through the cell associated with the dischargeable chlorine-containing species, and the pH (pH1) of the water. The values of i1, pH1 and a calibration factor, ν (representative of the current generated in the cell per unit concentration of the said electrochemically dischargeable species), are used to calculate the total concentration (C11tot) of dissolved chlorine as a function of the fraction (f1) (formula (10)) where x is 2.50 to 7.53 and y is 3.00 to 5.00.

Description

MEASURING CHLORINE CONCENTRATION

The present invention relates to a method of measuring the concentration of residual chlorine in water.

Water from reservoirs is generally treated with a disinfecting agent before being supplied to the public as drinking water. The disinfecting agent serves to control or kill bacteria present in the reservoir water and therefore render it safe for human consumption.

One commonly used disinfecting agent is chlorine which may be added to the water as elemental chlorine or sodium hypochlorite. The treated water contains an amount of residual chlorine which should be between predetermined upper and lower limits, the former (i.e. upper limit) being to ensure that the water is safe for consumption and the latter (i.e. lower limit) being to ensure that the chlorine is present in an amount sufficient to provide effective bacteriostatic or bacteriocidal control.

Thus, tests on the treated water are carried out to ensure that the chlorine is within the predetermined limits. If not, then the amount of sodium hypochlorite or elemental chlorine added to the water may be adjusted accordingly to ensure that the residual chlorine content of the treated water is between said limits.

The residual chlorine is considered to be present as HOC1 and OC1^" which co¬ exist in accordance with the equation (1 )

HOC1 ► OC1^" + H" (1 )

Numerous methods for measuring the total residual chlorine content of water are known. One such method utilises an amperometric cell which comprises a platinum working electrode, a counter electrode (e.g. platinum), and a reference electrode (e.g. silver/silver chloride). An example of such a cell is the DEPOLOX 4 as manufactured by Wallace & Tiernan Limited.

Such amperometric cells make use of the fact that 1. The HOCl component of the residual chlorine may be discharged at the working electrode of the cell in accordance with the equation (2)

HOCl + 2 ε > OH^" + CI^" (2)

2. This discharge provides a current, /, which is proportional to the concentration (Clnoci) °f chlorine present as HOCl in the water. Thus

C1HOCI ^{= ~} (3) μ where μ is a calibration factor representative of the current generated in the cell per unit amount of HOCl.

In practice, the water to be monitored by the cell is buffered to a pH of about 6. This ensures that the equilibrium of equation 1 is shifted almost wholly to the left so that substantially all residual chlorine is present as HOCl, i.e. the electrochemically dischargeable species. The current measured by the cell is therefore directly representative of the total amount (Cl_to() of dissolved chlorine in the buffered water.

There are however two disadvantages with this method of operation.

Firstly, the need to add a buffer to the water represents a significant extra cost to the measurement operation both in terms of the cost of the buffering chemicals and the cost of disposing of the buffered water, of which large quantities are generated.

Secondly, the measurement provides no information on the relative amount of HOCl and OC1^' in the unbuffered sample. This information may be required as HOCl is considered to be the main disinfecting species and therefore its actual concentration in the unbuffered water may be of importance. To obtain this information it is necessary to make a separate measurement of the pH of the unbuffered water and then to calculate the actual amount of HOCl (in the unbuffered sample) on the basis of the following. For a given pH, the concentration of HOCl in the water is a particular fraction. /, of the total amount of dissolved chlorine (some of which is provided by the OCl^" species - see equation (1) above), i.e.

CIHOC. = /-Cl,o« (4)

The fraction / for a particular pH may be determined in accordance with equation (3):

/ = 1 + 10^" (5)

-pit

where pH is the pH of the unbuffered water and Ka is the dissociation constant at the temperature T°C of measurement as given by equation (6)

3000 ^ _r . _Λ-I

K, - 10.0686 + [0.0253. (T + 273)] (6)

T + 273 \

Typically Ka is about 7.53 for a pH of 7 and a temperature of 25°C.

The concentration of HOCl (Cl_Hoα) in the unbuffered water may then be determined from equation (4)

Therefore in order to obtain the value of C1_H0CI (in the unbuffered water) it is necessary (in accordance with conventional practice) to make separate measurements on buffered and unbuffered samples of the water and then to obtain the value of Cl_Hθα as described above.

It would in theory be possible to make the measurement in the amperometric cell using unbuffered water and then to calculate the values of Cl_lot and Cl,ι_0C| using the following equations I

CI HOCl (3) μ o

CI, HOCl to! (4) f

- Ka \

I

Cl„ 1 + - ,,H (7) μ

where pH is the pH of the (unbuffered) water whereof the chlorine concentration is being monitored. However we have established that equation (6) does not provide an accurate value of Cl_l0, on the entire range of pH values likely to be encountered. More particularly the bracketed expression (which is equivalent to 1//) does not accurately define the fraction / at pH values in the range 6 to 8.5 and in fact leads to excessively high values of Cl_tol

It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantages.

According to the present invention there is provided a method of monitoring the concentration of dissolved chlorine containing species using an amperometric cell having a platinum working electrode, a counter electrode and a reference electrode, the method comprising the steps of

(A) testing water whereof the chlorine concentration is to be monitored to determine

(a) the current (ii) through the cell associated with the discharge, at the working electrode, of electrochemically dischargeable chlorine containing species

(b) the pH (pH 1 ) of the water, and (B) using the values of i,, pHl and a calibration factor, μ, (representative of the current generated in the cell per unit concentration of said electrochemically dischargeable species) to calculate the total concentration (Cll _u„) of dissolved chlorine as a function of the fraction (f_t)

.A (10)

where x is 2.50 to 7.53 and y is 3.00 to 5.00

We have established that (for an amperometric cell having a platinum working electrode, a counter electrode and a reference electrode) equation 10 provides an accurate representation of the fraction, f_b of the amount of electrochemically dischargeable residual chlorine containing species relative to the total residual chlorine at any pH over a relatively wide pH range, e.g. 6 to 8.5. More particularly, equation 10 provides a much more accurate model than that provided by the conventionally used equation 5. The fraction f, may then be used in a determination of the total amount of dissolved chlorine or the amount of dissolved chlorine present as electrochemically dischargeable species.

In accordance with the invention it is possible to make pH and current measurements on unbuffered water samples and use the values and pHl of i, obtained to calculate the concentrations set out in the preceding sentence. The fact that accurate measurements may be made on unbuffered water is a significant advance over the prior art since the expense associated with addition of buffers and disposal of the buffered water are avoided. Furthermore the electrochemical measurement (i.e. using the cell of the kind defined) and the pH measurement may be effected on the same sample of water so that the need for separate measurements on buffered and unbuffered samples is avoided. The total amount (Cll_tot) of dissolved chlorine may be determined in accordance with the equation

C/l = μf

10^"

1 + pH\ (1 1 ) v y

As an equivalent of this calculation, it is possible separately to calculate (i) the value of f ₁ (the fraction of HOCl) from equation ( 1 ), (ii) the amount of dissolved chlorine Cll_dιs present as said electrochemically dischargeable species in accordance with the equation

C/l (12) μ

(iii) the total amount of dissolved chlorine in accordance with equation

which is the equivalent of

As a further alternative, it is possible initially to calculate a value I₁₀₀o_/o where

10^_Λ

^ 100% ~ 'l 1 + (14) y^pm J

and corresponds to the current which would have been generated in the cell if all of the residual chlorine had been present as electrochemically dischargeable species. The value of Cll_l0, may then be calculated in accordance with the equation

C/l, 100% (15) It will be appreciated that equation (15) is, in effect, equivalent to equation (1 1).

Equation (10) incoφorates the parameters x and y which are such that x is in the range 2.50 to 7.53 and y is in the range 3.00 to 5.00 The particular value of x used in the calculation of equation (10) will also generally be that used for equation (12) and similarly for y will be kept constant for the various calculations.

If the method of the invention is employed at high pH values then the values employed for x and y may be different from those employed at lower pH values, although generally only x will need to be varied.

Preferably x is in the range of 4.5 to 5.1 , more preferably 4.7 to 4.9, and most preferably about 4.8. Preferably y is in the range 3.5 to 4.5, more preferably 3.8 to 4.2 and most preferably about 4. We have found values of x = 4.83 and y = 4.00 to be particularly suitable for measurements effected at ambient temperature.

Prior to use in the method of the invention, the cell is calibrated to determine a calibration factor, μ, which represents the current generated in the cell per unit concentration of electrochemically dischargeable species. Generally i, is measured in microamps and μ has the units of microamps per ppm of the electrochemically dischargeable species in which case the value of Cll_dis is obtained in ppm. It will however be appreciated that, if desired, other units may be employed for i, and μ.

Conveniently the cell is calibrated by

(w) using a sample of water ("calibration water") containing a known total concentration (C12_tot) of dissolved residual chlorine containing species to measure the current flow (i₂) through the cell associated with the discharge, at the working electrode, of electrochemically dischargeable chlorine containing species,

(x) measuring the pH (pH2) of the calibration water sample, (y) determining the fraction (f₂) of the total dissolved chlorine which is present as said electrochemically dischargeable species by the equation,

Λ = - (15)

10^"

1 + -ι<ιn.

where x is 2.50 to 7.53 and y is 3.00 to 5.00,

(z) using the values of i₂, f₂ and C12_lot to determine a calibration factor (μ) for the cell representative of the amount of current per unit concentration of said electrochemically dischargeable species:

The value of μ may for example be calculated in accordance with the following equations

Cl2_ώs = f₂.Cl2_lnl ( 16)

μ (17)

C72,

Alternatively μ may be calculated from the following equations

(where l_cal corresponds to the current which would have been generated in the cell if all of the residual chlorine had been present as electrochemically dischargeable species) and ' cal (18)

C/2 ' m. i

The "calibration water" may be a water sample which has been "made up" so as to have a known total concentration of residual chlorine. It is however also possible for the "calibration water" to be a sample of the water whereof the chlorine concentration is being measured by the amperometric cell, in which case the "calibration sample" may have its total dissolved chlorine measured by a different technique (e.g. colorimetric analysis).

The values of x and y used in equation 13 will generally be the same as those employed in equation 10.

The method of the invention employs an amperometric cell having a platinum working electrode, a counter electrode and a reference electrode. The working electrode may be maintained at a fixed potential with respect to the reference electrode by means of a feedback "potentiostatic" electronic circuit. Preferably the platinum working electrode is maintained at a potential of 250 mV relative to the reference electrode. Preferably the counter electrode is of platinum.

Preferably the reference electrode is a silver/silver chloride electrode placed in a potassium chloride electrolyte which makes contact with the water to be monitored via a porous junction.

A particularly suitable cell for use in the method of the invention is the DEPOLOX 4 cell as manufactured by Wallace & Tiernan Ltd. Such a cell may be operated on a continuous basis to monitor the amount of dissolved chlorine in water.

It will be appreciated that a microprocessor may be used to convert current readings to concentration values, calibration factors etc.

The invention will be illustrated by reference to the following non-limiting Example which is to be read in conjunction with Figs. 1 and 2 of the accompanying drawings. Example

The current i, generated in a DEPOLOX 4 cell by a solution containing 0.4 ppm total dissolved chlorine (Cl_tol) was measured over a range of pH values. The results are plotted in Fig. 1.

The following calculations were then performed to compare the accuracy of results obtained using the method of the invention with those obtained using the prior art calculations. In the following calculations the calibration factor, μ for the cell was obtained using the current value (10.5μA) measured at pH = 7.5.

Invention

From Equation (10) and using x = 4.83 and y = 4.

/. = 10 -4.83

1 + -7 5

.-. f, = 0.674

Therefore (at pH = 7.5) 67.4% of the total dissolved chlorine is calculated to be present as electrochemically dischargeable species (considered to be HOCl).

Hence if all dissolved chlorine had been present as HOCl this would have provided a current I as given by the following equation

10 -4 87 λ

7 = 10.5 1 + -7 5

.-. 1 = 15.589 .-. Calibration factor μ for cell corresponds to 15.589 μA per 0.4 ppm present as HOCl.

:. μ = — ^{1 5 ■589} μA A per ppm u HnOπCl

.-. μ = 38.973 μA per ppm HOCl (μ may alternatively be calculated by the following) ( )

( μ = ) f CI

Now, for any particular pH, the measured current may be converted to a current I_|00% which would have been obtained if all of the dissolved chlorine had been present as HOCl in accordance with the following equation (based on equation (14) above)

Moreover

Ci. ' 100% (15)

Using the current i and pH readings plotted in Fig. 1, values of I|_00% were obtained for a range of pH values and subsequently the corresponding values of Cl_tol were obtained.

The results are plotted in Fig. 2 from which it will be seen that the mathematical model of the invention consistently gives a value of, or very close to, 0.4 ppm (i.e. the actual concentration of the solution).

Comparison

From equation (5) and using Ka = 7.53 at a pH = 7.5.

10⁷ "

1 + ιb-^7"5 = 0.517

Therefore 51.7% of the total dissolved chlorine is calculated to be present as electrochemically dischargeable species (considered to be HOCl).

Hence if all dissolved chlorine had been present as HOCl this would have provided a current I given by the following equations

10 -7 53 \

7 = 10.5 1 +

V 10 7 5

20.299 μA I

Now μ

:. μ = 50.748 μA per ppm HOCl.

On this comparative model

As previously, values of I₁₀₀»_/o were obtained to provide Cl_l0, values for a range of pH values. The results are also plotted in Fig. 2.

From a comparison of the two lines in Fig. 2, it will be seen that the comparative model is accurate at a pH of only about 7.5, below which the values of Cl_tot are significantly less than 0.4 ppm and above which the values of C1_I0, are significantly higher than 0.4ppm. Thus, at pH = 8, the comparative model provides a value of Cl_t0, of about 0.6 ppm, i.e. 50% greater than the true value. Hence the comparative model can only be used if the water has been buffered to a pH at which the model is accurate.

It will thus be appreciated from Fig. 2 that the method of the invention provides accurate values of Cl_t0, over a range of pH values thus avoiding the need to buffer the water.

Claims

1. A method of monitoring the concentration of dissolved chlorine containing species using an amperometric cell having a platinum working electrode, a counter electrode and a reference electrode, the method comprising the steps of

(a) the current (i,) through the cell associated with the discharge, at the working electrode, of electrochemically dischargeable chlorine containing species

(b) the pH (pH 1 ) of the water, and

(B) using the values of i_h pHl and a calibration factor, μ, (representative of the current generated in the cell per unit concentration of said electrochemically dischargeable species) to calculate the total concentration (Cl l_tot) of dissolved chlorine as a function the fraction (f , )

/> = ( 10)

1(T

where x is 2.50 to 7.53 and y is 3.00 to 5.00 2. A method as claimed in claim 1 wherein CI 1 _tol is calculated in accordance with the equation

10

Cll tot l + μ 10 pH\ (1 1 )

where x and y are as defined in claim 1

2. A method as claimed in claim 1 or 2 wherein x is in the range 4.5 to 5.1.

4. A method as claimed in claim 3 wherein x is in the range 4.7 to 4.9.

5. A method as claimed in claim 1 or 2 wherein x is about 4.8.

6. A method as claimed in any one of claims 1 to 5 wherein y is in the range 3.5 to 4.5.

7. A method as claimed in claim 6 wherein y is in the range 3.8 to 4.2.

8. A method as claimed in claim 7 wherein y is about 4.00.

9. A method as claimed in claim 1 or 2 wherein x is 4.83 and y = 4.00.

10. A method as claimed in any one of claims 1 to 8 wherein the value of μ is obtained by

(w) using a sample of water ("calibration water") containing a known total concentration (C12_tot) of dissolved residual chlorine containing species to measure the current flow (i₂) through the cell associated with the discharge, at the working electrode, of electrochemically dischargeable chlorine containing species, (x) measuring the pH (pH2) of the calibration water sample,

(y) determining the fraction (f₂) of the total dissolved chlorine which is present as said electrochemically dischargeable species by the equation,

Λ = (15) ι + 10^"

-pH2 y

where x is 2.50 to 7.53 and y is 3.00 to 5.00,

(z) using the values of i₂, f_> and C12_tot to determine a calibration factor (μ) for the cell representative of the amount of current per unit concentration of said electrochemically dischargeable species:

1 1. A method as claimed in any one of claims 1 to 10 wherein the platinum working electrode is maintained at a potential of 250 mV relative to the reference electrode.

12. A method as claimed in any one of claims 1 to 1 1 wherein the counter electrode is of platinum.

13. A method as claimed in any one of claims 1 to 12 wherein the reference electrode is a silver/silver chloride electrode.