1. Free Residual Chlorine Measurement?
Why measure residual chlorine in drinking water?
Why Measure Residual Chlorine?
Why do we need to measure the amount of residual chlorine in drinking water? All public utilities that use surface or ground water are required to maintain a minimum disinfection level to control the level of bacteria in intake devices. In most cases, chlorine is used as a disinfectant. Chlorine when ingested by humans in high doses can be harmful. For this reason it is necessary to measure the amount of residual chlorine that is distributed to the public through the water supply.

2. Chlorine Chemistry Free Chlorine Combined Chlorine Total Chlorine
= HOCl + OCl--
= NH2Cl + NHCl2 + NCl3
= Combined Chlorine +
Free Chlorine
Chlorine Chemistry
In order to fully understand residual chlorine measurement, it is necessary to understand the chemistry behind the measurement. In a drinking water plant, we usually talk about three different kinds of residual chlorine. There is free chlorine, combined chlorine, and total chlorine.
Free chlorine is the result of hypochlorous acid plus hypochlorite ion. Free chlorine is used quite often in drinking water plants because of its disinfection power. It potentially has 100 times the disinfection power of other forms of chlorine.
Combined chlorine is formed when ammonia or ammonia compounds are introduced to drinking water that contains free chlorine. Combined chlorine is used because of its staying power. It is not as strong a disinfectant as free chlorine, however it will not dissipate as quickly as free chlorine and can offer additional disinfection.
Total chlorine can be thought of as the addition of combined chlorine and free chlorine. Drinking water plants that use combined chlorine typically have free chlorine in the drinking water as well, and therefore need to measure total chlorine. IC Controls deals with the measurement of free residual chlorine, and we will therefore limit our discussions to this measurement.

3. Chlorine Chemistry HOCl + H+ + Cl- Cl2 + H2O H+ + OCl-- HOCl
More Chlorine Chemistry
How does the chlorine get introduced into the drinking water? The most popular way of introducing the chlorine to the drinking water is through chlorine in the gaseous state. The chlorine is stored in large tanks under pressure. The chlorine is added to the drinking water, hypochlorus acid and hydrochloric acid are formed. The hydrochloric acid is a strong acid and therefore disassociates completely. Hypochlorous acid is a weak acid and will dissociate based upon pH. As the pH rises or increases, the hypochlorous acid concentration will decrease and the hypochlorite ion concentration will increase. Therefore, it is important to note, that the measurement of free residual chlorine is pH dependent.

4. % Distribution HOCl vs. pH
100 90 80 70 60
% HOCI 50
In order to illustrate how free residual chlorine measurement is dependent on pH, we must look at the dissociation curve of hypochlorous acid. From this curve, it is easy to see that the concentration of hypochlorous acid vs. hypochlorie ion is very much pH dependent. You will also notice that at a pH os 7.5 we have an equal amount of hypochlorous acid and hypochlorite ion. This curve will become more important when we begin to discuss how IC Controls measures free residual chlorine. 40 30 20 10 4 5 6 7 8 9 10 pH

5. Measurement Methods Colorimetric Open /Electrode Buffered
Amperometric/Polarographic
High Resolution Redox
pH/ORP Method
Galvanic Sensor
Comparison of Methods for Residual Chlorine Measurement
There are a number of technologies used in order to measure free residual chlorine in drinking water. There are a number of advantages and disadvantages associated with each technique. It is important to understand these advantages and disadvantages before chosing the type of instrument you want to use in order to monitor chlorine.

6. Colorimetric Systems Operating Principle
Add buffer and indicator solution to a known volume of water.
After mixing, measure wavelength of light transmitted through sample to determine color change and thus, amount of free residual chlorine in the sample.
Colorimetric Systems
The first type of instruments we would like to talk about are colorimetric systems. Colorimetric systems operate by adding a buffer and an indicator solution to a known volume of water. These are allowed to mix for approximately 2 1/2 minutes. The color of the water will change according to how much free residual chlorine is in the sample. The instrument measures the amount of light transmitted through a blank sample or sample before buffer and indicator solution are added, and then measures the wave length of light transmitted through the sample after the color change has taken place. Based upon this difference, the amount of free residual chlorine in the sample is determined.

7. Colorimetric Systems Advantages Price.
High pH is not a limiting factor.
EPA approved DPD measurement.
Disadvantages
Uses reagents.
Must use pumps, valves and capillary tubes.
Can only generate a new residual reading every 2 1/2 minutes.
High maintenance.
No pH or temperature output.
Colorometric Systems- Advantages and Disadvantages
Colorometric systems have three main advantages. This first advantage is price. These systems can be manufactured and are sold at a very inexpensive price.
Another advantage of colorometric systems is the fact that high pH is not a limiting factor. A buffer is added to colorometric systems in order to lower the pH, therefore a strong signal is always generated.
The third advantage to colorometric systems is that they are using an EPA approved DPD measurement. The indicator solution which is added to the drinking water is known as DPD. It is important to note that there is no approved measurement technique for continuous measuring instruments for residual chlorine. However, the approved measurement for chlorine in laboratories uses DPD. Therefore companies who manufacture colorometric systems often emphasize the fact that they are using a DPD measurement which is EPA approved.
Colorometric systems have many disadvantages. The first disadvantage is that they use reagents. Reagents need to be replenished monthly. This adds cost of ownership to the instrument and also adds a lot of maintenance because you must constantly monitor the level of reagents in order to assure that you are getting a good measurement. The second disadvantage to colorometric systems is that they use pumps, valves, and small capillary tubes. These accessories often wear out and need to be replaced. The third disadvantage is that a colorometric system can only generate a new residual reading every two and one-half minutes. It takes time to mix the buffer and indicator solution with the drinking water in order to create the color change necessary in order to get a good colorometric reading.
A fourth disadvantage is that the colormetric system generates a lot of maintenance. You must monitor the reagents constantly, you have to change pumps, valves, and capillary tubes and you are constantly having to clean the instruments. The final disadvantage of colorometric system is that they have no pH or temperature output. Many drinking water plants monitor these two parameters and like to see this as an added feature with the instrument.

8. Open Electrode/Buffered Systems
Operating Principle
A known volume of sample water is pumped into a chamber and buffered to a pH of 4.
Two bare electrodes are in contact with the sample inside the chamber and based upon mV readings, a free residual chlorine value is derived.
Open Electrode Buffer Systems
The next type of instrument we are going to talk about is the open electrode buffered system. In the open electrode buffered system, a know volume of sample water is pumped into a chamber and buffered to a pH of 4. Two bare electrodes are in contact with the sample inside the chamber and based upon changes in the mV output, a free residual chlorine value is derived.

9. Open Electrode/Buffered Systems
Advantages
High pH is not a limiting factor.
Disadvantages
Use of buffers.
Must use pumps and valves.
High maintenance.
Difficult start-up.
No pH or temperature output.
Open Electrode Buffered Systems-Advantages and Disadvantages
These systems basically have one advantage and that is that high pH is not a limiting factor. The change which these systems are measuring is generated by the concentration of hypochlorous acid in the sample. Therefore, as pH increases there is less hypochlorous acid and therefore you are going to have a weak signal and eventually you will reach a point where the signal to noise ratio is indistinguishable. By buffering the system to a pH of 4, the open electrode buffered system eliminates any error created by high pH.
The open electrode buffered system has many disadvantages that the colorometric system has. Once again you are using buffers. You have to watch these buffers and make sure that they don’t run out otherwise you will get an inaccurate signal or measurement. These systems use pumps and valves. Once again, you have to watch the pumps and valves. They wear out and you have to replace them over time. You are adding cost of ownership to the instrument. You also have high maintenance with these systems because you are monitoring the buffers, you have moving parts that wear out and you have to make sure that the chamber stays clean so that you are getting an accurate measurement. These systems are difficult to start up. You have to hook up the pumps and valves. You have to plumb the buffers. And once again, these systems have no pH or temperature output.

10. Amperometric/Polarographic Membrane Sensor
Operating Principle
A sensor consisting of a membrane (which allows HOCL to migrate through it), two dissimilar metal electrodes, and an electrolyte are submersed into the water sample.
A free residual chlorine reading is derived based on mV changes experienced by the sensor.
Amperometric/Polarographic Systems
Systems which implement the use of a membrane sensor operate much like a dissolved oxygen sensor. A sensor which consists of a membrane that allows hypochlorous acid to migrate through it, two dissimilar metal electrodes and an electrolyte is submersed into the water sample. A free residual chlorine reading is derived based upon mV changes experienced by the sensor. Systems which use a membrane system in order to monitor residual chlorine are perhaps the simplest of all instruments used for this measurement.

11. Amperometric/Polarographic Membrane Sensor
Advantages
Ease of use
No reagents or buffers added
Disadvantages
No pH or temperature compensation (OPTIONAL)
No flow regulation
No pH or temperature output (OPTIONAL)
Amperometric/Polarographic Membrane Systems-Advantages and Disadvantages
There are three advantages to using an amperometric/polarographic system. The first advantage is price. Because the system consists of a membrane sensor and an analyzer to receive the signal, the system is inexpensive. Under ideal conditions, this system works just fine. Another advantage of this system is its ease of use. Setup and startup are very simple. You only need to determine where you want to place the sensor and you can either have submersion mount or an in-line mount. The third advantage of this system is that there are no reagents or buffers to add. And of course this is saving you on cost of ownership and also on maintenance.
There are three main disadvantages to this system. The first is there is no pH or temperature compensation. You would have to have a constant pH and temperature in order to receive an accurate residual chlorine reading. As we said before residual chlorine measurement is pH dependent. Amperometric/polarographic systems assume that pH is constant in the application. In most cases this is not true. You are going to get a variance in pH and therefore your signal will not be accurate. The second disadvantage to this system is there is no flow regulation. As with all membrane systems, the accuracy of the instrument depends upon proper flow across the membrane. Error is introduced with varying flow rate. The third disadvantage of this system is once again, there is no pH or temperature output.

12. High Resolution Redox Operating Principle
Characterize free residual chlorine demand based upon ORP values of the water sample.
High Resolution Redox Systems
These systems are an indirect way to measure the residual chlorine concentration in a drinking water plant. Operation is very simple. Basically you characterize the residual chlorine demand based upon ORP values of the water sample. In many cases these systems are pH compensated in order to get a more accurate reading.

13. High Resolution Redox Advantages
In some instances, does a better job of determining residual chlorine demand.
Disadvantages
Does not measure free residual chlorine directly.
Any oxidant present in water sample will be read as an increase in free residual chlorine.
High Resolution Redox-Advantages and Disadvantages
These systems have one main advantage: in some instances, they do a better job of determining residual chlorine demand. This is because they work on the principal that viruses are killed by chlorine because of its oxidizing power. High resolution readout systems directly measure the oxidation potential in the drinking water at the time of the measurement. The main selling point is that it is the oxidizing reaction in the drinking water which kills the pathogens, not the free residual chlorine.
Of course, this method of measurement has disadvantages: the main disadvantage being that free residual chlorine is not measured directly. For reporting purposes to the EPA or to any other type of regulatory agency, you cannot use this type of measurement. Another main disadvantage of this types of system is that any oxidant present in the water sample will be read as an increase in free residual chlorine. Of course, any oxidant introduced to the water or any type of reducing agent introduced to the water will cause a change in the ORP value and therefore you may be seeing a swing in your free residual chlorine concentration, when in fact it is a foreign agent which is causing this change.

14. IC Controls - pH/ORP Method
Operating Principle
Water flows through a Dual Flow Cell consisting of an ORP and a PH sensor.
Microprocessor in the analyzer calculates the free residual chlorine (HOCL+OCL-) concentration by using a polynomial formula. This calculation requires the analyzer to know the measured ORP, PH and temperature values.

15. IC Controls - pH/ORP Method
Advantages
No reagents or buffers added.
No moving parts.
pH and temperature compensated.
Easy to set up and calibrate.
Multiple measurement readouts/outputs.
Dedicated relay control for residual chlorine and pH.
Disadvantages
Does not measure free residual chlorine directly.
Any oxidant present in water sample will be read as an increase in free residual chlorine.
Advantages and Disadvantages
There are many advantages to theORP, pH and temperature compensated system. The primary advantage is that this system uses no reagents or buffers. This saves on both maintenance time and reagent and buffer costs. There are no moving parts, no pumps or valves to wear out. This once again cuts down on overall operating cost. This system is pH and temperature compensated. The IC Controls system will give an accurate reading for residual chlorine even when the pH and temperature vary.
Another advantage to this system is that it is easy to setup and calibrate. You only need to calibrate the ORP, & pH sensors.
The IC Controls system has multiple measurement readouts and outputs. It will display and has 4-20 mV outputs for pH, free residual chlorine, total free chlorine and temperature. You will notice that unlike all other systems, gives the operator pH and temperature outputs. Therefore, the IC Controls system is like multiple instruments.
The last advantage to the IC Controls system is that there is dedicated relay control for residual chlorine and pH. You can control both free residual chlorine concentration and your pH at the same time.

16. Model 876-25 Total Free Chlorine
Operating Principle
A sensor consisting of a membrane (which allows Cl2 to migrate through it), two dissimilar metal electrodes, and an electrolyte are submersed into the water sample.
A free residual chlorine reading is derived based on mV changes experienced by the sensor.

17. Model 876-25 Total Free Chlorine
Advantages
Ease of use
No reagents or buffers added
Absolute Zero
Simple Membrane Replacement
Galvanic Technology
pH compensation
Disadvantages
High pH can be a limiting factor

18. Polarographic vs IC Controls Galvanic
Model Total Free Chlorine
Polarographic vs IC Controls Galvanic
What's the difference?

19. V Polarographic Anode Cathode Membrane Cl2 Cl2 Cl2
Applied voltage from the Cathode to the Anode.
Reaction of Chlorine at Cathode generates a current.
Output can go below zero; a value that does not exist.
Therefore, decreased accuracy in calibration and measurement
Anode
Cathode
Membrane
Cl2
Cl2 V
Cl2

20. Galvanic No applied Voltage. Chlorine is the fuel for the battery.
Therefore, absolute zero and better accuracy.
Anode
Cathode
Membrane
Cl2
Cl2
Cl2

21. Recharging the Sensor
Over time the sensor internal will deplete and the membrane and fill solution will need replacement. Historically, this was a difficult process with creases being formed in the membrane and air bubbles in the fill solution which cause inaccurate readings. At times the process could take up to 1 hour to replace a membrane and put a sensor back on line.
The model 835 sensor provides ease of use which includes fast, reliable and simple maintenance.

22. Membrane Replacement Membrane Module Tip Retainer
Maintenance is made easy
with the Membrane Replacement device.
This system ensures there
are no air bubbles or creases
in the membrane cap which
will cause false inaccurate
readings.
An easy process that allows
recharging and re-installation
in less than 5 minutes.
Membrane
Module
Tip
Retainer

23. Model 876-25 Total Free Chlorine
Galvanic Technology
pH Compensation
Absolute Zero
No Reagents Required
Simple Membrane Replacement
Proven Technology
Superior Design
Better Results

24. Model 876-25 Total Free and 875 Free Chlorine
875 Analyzer
Panel
835 Galvanic Sensor