Free Chlorine and ORP
effective alternative to Free Chlorine control. In swimming pools, ORP is measured on a scale in millivolts using an an ORP electrode (probe). The ORP. Pulse Instruments specializing in Meters and Controls for pH, ORP, Oxidation indication of bactericidal properties of chlorine than PPM Free Chlorine values. ORP sensor response as a function of pH was studied active form of free chlorine, Hypochlorous Acid HOCl - . correlation between the values obtained by.
Typical reductants are the hypochlorite ion a component of free chlorinechloramines, cyanuric acid, organic matter dust and dirturine, perspiration, sputum, micro-organisms, cosmetics, and faecal material. A drop in the ORP indicates an increase in chlorine demand caused by reducing agents or contaminants entering the water.
A decrease in ORP indicates that chemical reactions are about to occur. Compared to amperometric control, ORP is considered to be a more accurate measure of disinfection rate.
Understanding ORP and PPM
Also, ORP controllers can automatically add disinfectant according to demand. They therefore anticipate the disinfecting and oxidising chemical reactions that are about to occur. ORP is an indicator of micro-organism inactivation.
Studies on specific micro-organisms have found a direct correlation between increasing ORP and micro-organism inactivation as shown in Graph 6. Drinking water is adequately disinfected at an ORP of mV. In swimming pools, an ORP of to mV allows for both a quick disinfection and for breakpoint chlorination destruction of chloramines where conditions permit.
Eutech Instruments Pty Ltd The ORP value required in swimming pools is higher than that required for water supplies because contamination is constantly being added to swimming pools. An increasing pH decreases the concentration of HOCl and hence its disinfection power. Similarly a decreasing pH increases ORP because the oxidative power of free chlorine increases.
ORP measurement correlates weakly with free chlorine measurement because they measure two different entities. ORP measures oxidative disinfection power not the concentration of free residual chlorine. Free chlorine measures the concentrations of hypochlorous acid HOCl and the hypochlorite ion OCl- not the oxidation disinfection power. Free chlorine is a variable component of ORP. Oxidative disinfection does not correlate well to free chlorine for two reasons: This will occur when the pH is high.
Therefore free chlorine could be high and the ORP low at a pH greater than 7. Therefore free chlorine could be high, but the ORP will be low if combined chlorine is high, cyanurate is present, contamination is high, etc. A pool with satisfactory ORP could have low free chlorine if the reductants are low. That is, there is low combined chlorine i. Many controllers use this form to provide a free chlorine measurement. But it does provide an idea of how much free chlorine is available.
This system uses a special membrane through which only free chlorine ions can pass and provide direct readings.
These are true readings of free chlorine and not indirect values derived from ORP and pH. Selective membrane sensors are good at measuring free chlorine but are the most expensive option. The term colorimetric is used to describe a measurement system where reaction-based chemical indicators are used to detect the presence of a specified chemical.
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The indicator reacts with the chemical and produces a visible color change in the solution. The darker the color, the more chlorine is present. DPD test kits, as an example, are familiar to those in the pool and spa industry. However, studies have shown that there can be significant variations in readings from these kits due to differences in color perception between individual users. Essentially, a colorimetric system is an automated hand check. DPD colorimetric chlorine measurement systems automatically test as often as every 5 minutes.
Reagents are automatically mixed with a sample of pool water and optically read to determine total chlorine, free chlorine and combined chlorine levels. A true PPM reading can then be transmitted through the controller to the internet-based data-management system.
Understanding ORP and PPM
The colorimetric system is connected directly to the controller, providing a true PPM reading not subject to problems associated with PPM sensors. ORP controllers provide the most cost-effective way to measure sanitizer, as long as operators understand how ORP measurements are obtained and remember that the measurements fluctuate. It is a very useful indicator of how the sanitizer is working, allowing operators to investigate factors that might be compromising its effectiveness.
By understanding how these readings are generated and the factors that affect them, operators can better adjust water chemistry and provide better reporting.