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General
Information The meter consists of two main components, a handheld keypad and a sensor assembly. It is the electrodes and cell configurations which we will consider in detail. The sensor assembly is available in three versions, a grab sampler type for general residual measurement in city or pool water, a flow through cell suitable for drinking water outlets, taps, hydrants and water tanks and a float type suitable for swimming pool applications.
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| Measuring
Principle
The Measuring Principle is based on work performed by the technology research group of DKK-TOA Corporation in Japan using polarography and researching the resulting voltamograms that were produced. This innovation was then applied with their industry leading instrument manufacturing ability to produce a simple yet highly accurate meter for the lab and field. The critical measuring process is occuring at the tip of a miniature rotary gold electrode (cathode). There is also a silver counter electrode (anode) with a significantly larger surface area. There is a variable resistance between the anode and cathode. As the voltage is increased at the gold cathode, reduction will begin. In the case of Chlorine, (CL2), it is reduced to chloride ion. Chlorine reacts in water and exists in the form of hypochlorous acid (HOCL) and Hypochlorite ion (OCL-). The ratio of these species is dependent upon the pH of the water. When appropriate voltage is applied between the gold rotary electrode and the silver counter electrode in solution under equilibrium a few reactions occur. At the silver electrode, silver is oxidized to form Ag+ and free electrons. In the presence of hypochlorite ion, the silver ion will form silver chloride and oxygen. At the gold electrode, hypochlorous acid (HOCL) will be reduced by the available electrons to form hypochlorite ion and liberate hydrogen gas. Proportional current will flow to the existing free residual chlorine from the counter electrode to the rotary electrode, therefore, the concentration of free residual chlorine can be measured by measuring the current (Amperometric detection). So, before electrolysis occurs, the concentration of hypochlorous acid is equal at both the adjacent area to the electrode as well as at a distance. At the beginning of electrolysis, the electrode potential of the miniature rotary gold electrode surpasses a crtical voltage at which point the concentration of hypochlorous acid is decreased as it is being reduced to hypochlorite ion. The resulting consequence is a difference of concentration of hypochlorous acid between the surface of the gold electrode and the rest of the sample. When continuing electrolysis, a diffusion layer appears and chlorine is supplied only through the increasing diffusion layer. Therefore, the current is regulated based on the speed of formation of this diffusion layer.The current at this point is called a diffusion current and it is proportional to the concentration of hypochlorous acid in the sample. The purpose of the gold electrode being rotated is to produce the formation of the diffused layer in equal thickness and make the layer as thin as possible to improve electrode sensitivity. Silver chloride is formed at the counter electrode. The
critical object of measure relates directly to the diffusion current which
can be expressed by a few parameters including the rotation of electrode,
the concentration of the measuring object, and a few know or measurable
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| Comparison
Data (RC-24P Amperometric versus colorimetric DPD Method)
A significant amount of R&D went into the development of this instrument. There was also a lot of data collecting to validate the instrument as a comparable method for measuring disinfection residuals in public drinking water and pool water. The flow characteristics at the surface of the electrode are critical for accurate measurement. Therefore, two types of sensors have been designed to eliminate or reduce these effects. The data below is grouped by the electrode type and sensor cell configuration. There are two main types which include the RC-24-DP used for grab sampling, and the RC-24P-Q type used in conjunction with the flow cell design. The
RC-24-DP uses a vibrating sensor so that the sample can be measured in
a static sampling vessel or configured as a floating cell probe. The electrode
that is uses is the FCL-221BA. Comparison data can be found below comparing
these two methods. The data presented in not exhaustive, but is was supplied
by DKK-TOA to indicate the validity and accuracy of the method being used.
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FCL-221BA Electrode Comparison Data
Data for FCL-221BA
Electrode System
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CLS-221AA Electrode Comparison Data
Sensor For Testing Flowing Tap Water (RC-24-Q using electrode CLS-221AA) Data for CLS-221AA
Electrode System
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RC-24 Comparison Data at public drinking water utility Data was compared
against the DPD colorimetric test procedure for a period of about 20
days at a public drinking water utility in Sayama, Japan. From the comparison
data provided, there is compelling evidence as to the accuracy and effectiveness
of this technique.
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Technical Questions and Issues There are a number of issues that need to be considered with the measurement when using this technique. Many questions have be asked about this unit and this section will attempt to answer many of these issues. What is the measuring range? It is approximately 0.01 to 2.0 mg/l (ppm) What working pH range is acceptable? The acceptable range is between 5.8 and 8.0 pH How does the pH affect the measurement? A low pH value (below 4.0) shifts the reaction to produce higher hypochlorite ion, increasing measured value. Generally the prescribed method for finished drinking water requires a pH of 6.0 to 8.0 to eliminate undesirable byproducts. How does this compare to the DPD or OT colorimetric methods? The approximate accuracy of the DPD method is +/-0.1 mg/l in range of .01 to 2.0 mg/l, OT (Ortho-Trigin) is approx. +/-0.05 mg/l in range of 0.01 to 2.0mg/l. Our data is also within the measurement error of these other techniques. Does temperature affect the measurement? Increased temperature reduces available chlorine residuals. The meter has a built in sensor and temperature correction table to report the concentration at 25C. The measuring range is 0 to 45C. What is the response time? The response time is most affected by temperature equilibrium. At 25C, it is almost immediate, if temperature deviates greatly from that of the probe, equilibrium with the probe sensor may take 40 to 90 seconds. Are there interferences? A high combined chlorine concentration (the chloramines, which have a reduced disinfection ability) can produce an error displaying a higher value. Iron(II) at 1.31mg/l may add up to a 3% error to the reading and Manganese(II) at 1.78mg/l may increase the reading by up to 7%. If combined chlorine is present in high concentrations, can the meter still be used? Yes, the instrument can be calibrated against the DPD method or other technique so that the effect can be eliminated by performing a calibration against any other accepted method. Does sample conductivity affect the reading? If the conductivity is very low, the measurement will be difficult. Range is > 8mS/m (80uS/cm). How is the flow rate controlled? The flow rate is controlled by the sensor configuration. The electrode used with the grab sample cup incorporates an internal vibrator to supply electrode surface with a continuous "fresh" supply of sample. The flow thru cell is designed to allow an optimum, steady, continuous flow rate at the electrode.
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RC-24-P Free Residual Chlorine Meter - Conclusion and General Comments According
to the Standard method on drinking water and wastewater, 20th edition,
and the technical notes on drinking water methods(EPA-600/R-94-173),
This technique has been approved for use for continuous monitoring of
chlorine residuals. Since our detection circuit uses the amperometric techniques (4500-Cl D), and can measure in a continuous mode with as good accuracy and precision as well as be calibrated against a secondary approved method, this is a viable and acceptable technique for free residual monitoring. This technique can be used in addition to the certified methods and has significant advantage when used as a parallel testing procedure or as a benchmark procedure to aid in the check of chlorine disinfection residuals. The technique is clean in that it requires no reagent or indicator such as the (DPD), N, N Diethyl-P-Phenylenediamine indicator method which must be disposed of properly after measurement. It is inexpensive in that it only costs pennies per analysis and it is portable, so that multiple points along the distribution network could be easily field tested. It is fast to measure residual chlorine, and it is currently available technology that is being used around the globe for measuring disinfection ability of potable drinking water. It also has a distinct advantage to be able to log the data in memory and then interface to our software or printer for simple download or subsequent calculation (ie: 2-log disinfection calculation, etc.) The proposed EPA mandates for Stage 2 and LT2 drinking water monitoring will require increased residual monitoring along with the coliform monitoring requirements. Currently, the cost to do so using the reagent based technique (DPD) is significantly higher than by using this technique. This portable technology also makes it easy to monitor the public drinking water supplies, since chlorine residuals are the main component to monitor when considering harmful bacterialogical, viral or chemical contaminents which may be added to the water supply. It is the most common component used to eliminate the threat of Giardia and Cryptosporydium. The DKK-TOA RC-24P
residual chlorine meter is a valuable tool to help our public water
utilities with this difficult, time consuming, somewhat expensive and
often tedious task. For additional information please contact Analyticon Instruments.
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Analyticon
Instruments Corporation |
