Similar experiments were performed using water to which precise amounts of calcium chloride (CaCl2) and sodium bicarbonate (NaHCO3) were added to slightly buffer the water. This allows the FCE feature to correlate low ORP values to the typically low FAC concentrations of tap water after it has been in the municipal water system for several days.
The ability to include pH in the concentration calculation allows the FCE to compensate for the effect of the changing ratio of chlorine species as pH changes, resulting in a FAC concentration value germane to the actual sanitizing power of the source water. OCl- is measured as OCl-, and HOCl is measured as HOCl.
In addition, because the function displays both the FAC concentration and a predictive, stable ORP value, the user can, by comparing these two values from successive measurements, track how ORP (and disinfecting power) falls as pH rises and how ORP rises as pH is lowered when concentration is constant.
Another advantage of an ORP-based measurement is that, within the limits of its range, it can be used to measure the disinfection effectivity of any oxidizing germicide. The FCE measurement can be used with non-bleach oxidants, such as chloramines or even non-chlorine oxidants, such as peracetic acid, bromine or iodine.
While the concentration values reported by the function will not be absolutely correct for non-FAC oxidants, since they are based on a HOCl / OCl- model, FCE can still be an effective tool for monitoring relative changes in concentration levels. For absolute accuracy a correlative study should be performed to relate concentration levels of the oxidant in question to the ORP values displayed by the Predictive ORP feature and ppm values output by the FCE.
About the Author: Richard James Spahl is Quality Engineering Manager at Myron L Co.
"Myron L® is an exhibitor at the ACE® '12 event and can be found at Booth No. 1936"
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