The Free Chlorine Equivalent
It's fast, accurate and easier to use than colorimeters and DPD test kits
By Richard Spahl, Myron L Company
Chlorine tests based on the measurement of free available chlorine (FAC) concentration under forced conditions can give a misleading picture of the effects of the chlorine. Changing pH or the addition of chemicals like cyanuric acid can radically alter its power as a disinfectant and its ability to destroy or degrade membranes. Accurately determining these effects therefore requires a test method based on the chemical reactivity of the chlorine, rather than on its concentration.
The Myron L Company has developed an innovative new way to report FAC quickly and accurately based on the measurement of ORP, the chemical characteristic of chlorine that directly reflects its effectiveness, cross-referenced with pH:
- How and Why it Works
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.
- pH Included, Not Ignored
In addition, because the FCE function displays both the FAC concentration and a predicted, 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.
- Chemistry Measured
The Myron L Company FCE method avoids these pitfalls and inaccuracies. FCE measures the real, unaltered chemistry of source water, including moment-to-moment changes in that chemistry.
The following controlled study shows exactly how differently the two methods respond, particularly at the high end where the effects of changes in pH are the greatest. In this study measurements were made with a digital colorimeter and a Myron L Company Ultrameter II 6P equipped with FCE. The solutions tested were made with various known concentrations of NaOCl in de-ionized water. The water was heated to above 80°C to remove any CO2 and, therefore, avoid interference from REDOX reactions between HOCL, OCl- and carbonates (HCO3).
In this study (See Table 1) as the pH rises and the ratio of OCl- to HOCL rises dramatically, the FCE is able to accurately track the changing concentration of FAC. The colorimeter's results do not.
- Handheld ORP Accuracy
The Predictive ORP feature's calculations are based on a model of sensor behavior developed through a series of experiments that measured the response time of a representative sample of ORP sensors over a range of controlled chlorine concentrations. The results of this set of experiments revealed that the shape of the curve is very similar for various ORP levels differing only in the initial starting point and the final stabilized reading. (See Figure 2).
Using a proprietary curve-matching algorithm, the Predictive ORP feature determines what point along the typical sensor response curve a measurement occurred and extrapolates an appropriate final reading. This extrapolated value is used to calculate the FCE ppm value without having to wait for the sensor to stabilize and is also reported directly to the instrument's display.