The Measurement of Free Chlorine
The methods for testing free chlorine levels in industry today fall into three distinct methodologies: colorimetric, potentiometric, and amperometric.
This methodology uses chemical reagents added to the sample which react with chlorine to produce a color. The intensity of the color is directly proportional to the amount of chlorine present in the sample. The analytical instrument used in this method measures the intensity of the color and converts this measurement into a concentration reading, typically parts per million (ppm).
The most prevalent colorimetric method used in industry today is called the DPD Colorimetric Method. In this method, N,N-diethyl-p-phenyldiamine (DPD) is the reagent added to the sample. Chlorine oxidizes this DPD to form two possible oxidation products: a magenta colored compound known as Wurster Dye, and a colorless imine compound:
When small amounts of chlorine react with DPD, then the magenta colored W?rster Dye is the favored product. The colorless imine compound is favored when high levels of chlorine is present, which, unfortunately, leads to the apparent “fading” of the solution.
While this established method does indeed measure total free chlorine, the principle drawback of this method is that it does not differentiate between the two forms of free chlorine: HOCl, and OCl-. Remember, that the amounts of HOCl and OCl- present in solution are pH dependent. If the process pH is allowed to rise too high, all free chlorine in solution will be present as OCl-, the less efficient sanitizer, yet the total free chlorine levels, as reported accurately by the DPD test, will remain the same.
An ORP (Oxidation Reduction Potential) electrode is used in this methodology. When placed in a solution containing an oxidizer, such as chlorine, a measureable potential develops, which varies with the concentration of the oxidizer. This Oxidation Reduction Potential is dependent on the ratio of oxidized to reduced species of chlorine in solution.
The half-cell oxidation-reduction potential, E0, for HOCl at 25°C is:
HOCl + H+ + 2e (electrons) à Cl- + H2O E0 = 1.49 Volts
Assuming HOCl is the oxidized form of chlorine, with Cl- being the reduced state of chlorine, the ORP value for HOCl can be expressed by the equation:
ORP = E0 + 0.059/N*log[(HOCl)/(Cl-)] - Eref
Where N = the number of electrons, and Eref = the potential of the reference electrode of the ORP sensor.
The half-cell oxidation-reduction potential, E0, for OCl- at 25°C is:
OCl- + H2O + 2e (electrons) à Cl- + OH E0 = 0.90 Volts
The ORP value for OCl- is therefore expressed by the equation:
ORP = E0 + 0.059/N*log[(OCl-)/(Cl-)] - Eref
Note that the E0 value for OCl- is much lower than that of HOCl. It is for this reason that ORP electrodes are said to read only the HOCl species of free chlorine – its higher ORP potential tends to mask the ORP potential of the OCl- species in solution.
Let us now take a look at the Oxidation Reduction Potentials of solutions of varying free chlorine concentrations. Notice how the ORP value changes with changes in pH:
Figure 2. Variation of ORP for Varying Free Chlorine Concentrations as a function of pH
Figure 2 clearly shows that as pH levels rise, the ORP value decreases. For further clarity, let us superimpose the Ionization curve of HOCl from Figure 1 (the dotted line) onto Figure 3:
Figure 3. Comparison of ORP values and HOCl concentrations as a function of pH
The ionization curve of HOCl reminds us that as pH rises, the equilibrium of free chlorine shifts towards the formation of hypochorite ion (OCl-). This equilibrium shift towards the formation of OCl- causes the HOCl concentration to proportionately decrease. Thus, we see a decrease in ORP values from the ORP electrode. Proper understanding of these ORP and HOCl curves is what allows the use of ORP sensors to successfully monitor HOCl levels in water.
For adequate disinfection properties, it has recommended to maintain a minimum ORP level of 750 mV.
The amperometric method is electrochemical in nature. Chlorine diffuses across a hydrophobic membrane and is electrolytically reduced at the sensor’s cathode. The current produced is directly proportional to the chlorine concentration. The analyzer measures this current and converts it into a concentration reading (typically ppm).
The main benefits of the traditional amperometric measurement are that no reagents need be added to the sample, and maintenance on the sensor itself is low. The only drawback to the typical amperometric method is its dependency on the pH of the sample. The amperometric sensor will only respond to the hypochlorous acid form of chlorine. If the sample’s pH is allowed to change, this will cause a change in the sensor’s current output, even if the total free chlorine levels have not actually changed.
There are two ways to handle the pH dependency. The first, is to add acid to the sample. Lowering the pH will ensure that all hypochlorite in the sample will be converted to hypochlorous acid. This may or may not be feasible, depending on the application. The second method is to measure the pH level of the sample with a pH sensor. Then, a pH-correction algorithm may be employed to calculate the actual free chlorine concentration.
Van London has developed an amperometric chlorine sensor which retains all the advantages of the traditional amperometric sensor (no reagents + low maintenance) and eliminates the pH dependency of the sample!
By filling the sensor with an acidic fill solution behind the hydrophobic membrane, all chlorine which diffuses across the membrane will always be in the hypochlorous acid form, thereby making the sensor’s response to chlorine independent of the sample’s pH. The only regular maintenance required is the replacement of the membrane and fill solution, which can be done simultaneously.
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