Water Technology - Jan 2012

FEATURE ing strength of the solution being tested Simply counting how much chlorine is present is misleading because certain changes in water chemistry such as pH or the addition of cyanuric acid dramatically alter the oxidizing power of chlorine and therefore its efficacy without changing how much chlorine is present When correlated with established disinfection control parameters measurements and bacterial plate counts this type of measurement gives a very accurate picture of the sanitizing activity For this correlation to be valid the water undergoing treatment must be characterized so that all chemical constituents are known The pH and temperature values should be reported and held constant ORP will report an empirical value or a hard number that indicates how active the sanitizer is However you have to make certain that microbial contamination is responding to the treatment Once a correlation is established in a stable system ORP is a very efficient and effective way to monitor microbial control ORP has long been used in bathing waters as the only means for automatic chemical dosing In fact the World Health Organization WHO suggests an ORP value of between 680 720 mV depending www watertechonline com 21 ecule thick by attracting or adsorbing hydrolyzed oxidant or reductant molecules to the surface of the electrode Unfortunately when the oxide layer becomes more than one molecule thick the resulting lowered exchange current density offsets this benefit Also the adsorbed molecules cause a memory effect If a sensor is placed in a less oxidizing solution after measuring a more oxidizing solution it can take a very long time for the sensor to equilibrate to the new sample Though the sensor response time is much slower the final ORP reading will be the same ORP electrodes never require recalibration because there is no drift in zero point as is the case with pH sensors Any deviation from expected readings is most likely due to surface contamination of the electrodes or buildup of the oxide layer both of which can easily be remedied by cleaning with a light abrasive such as Softscrub Exposing the sensor to an ORP conditioning solution will help reduce the memory effect due to adsorption Can ORP be used as a surrogate parameter for free chlorine Yes ORP measures the oxidizing power and therefore the actual residual sanitizchlorine an oxidant will strip electrons from the negatively charged cell walls of some bacteria Because ORP measures the total chemical activity of a solution ORP measures the total efficacy all oxidizing and reducing disinfectants in solution Hypochlorous acid monochloramine dichloramine hypobromous acid sodium hypochlorite UV ozone peracetic acid bromochlorodimethylhydantoin etc ORP indicates the effectiveness of only those disinfectants that work through oxidation and reduction ORP cannot be used to detect the presence of any one particular chemical or chemical species Nor can it alone be used to determine the concentration of a known species of chemical in solution This means that although ORP is the best way to know whether or not your sanitizer is working it cant tell you how much or what kind of sanitizer is working What factors affect ORP measurement While the accuracy of ORP sensors is relatively stable which is why they do not require calibration there are factors that affect their response time Changes in temperature can affect response times by altering the kinetic rates of the reactions being measured for example Low temperatures reduce the kinetic rates and lengthen sensor response times The condition of the electrode will also alter response times by changing the exchange current density the amount of electrons exchanged per unit area of exposed electrode The lower the exchange current density the more sluggish the sensor response The typical measurement electrode is made from pure platinum Pt because it is a noble metal and therefore highly unreactive i e the potential being measured is most likely due to the activity of the chemicals in the water and not reactions between the solution and the Pt itself Even though Pt is a noble metal it will form a thin oxide layer on the surface of the platinum when exposed to dissolved oxygen This oxide layer facilitates the ORP measurement when it is very thin one mol FIGURE 2 A simple handheld instrument that makes ORP measurements can be used to screen for tolerable levels of free chlorine and comply with membrane manufacturers specifications