How does ORP relate to ppm? What are the effective ORP ranges? How do contact times get involved? Does pH affect ORP? Should I still measure ppm? Does ORP behave differently for chlorine vs. ozone? What is pH and ORP? Water that is used for washing procedure can be a primary cause of cross contamination. Disinfection of the water occurs by the process of oxidation. Chlorine, Bromine, Ozone, Peroxides, and Peracetic Acid are all Oxidizers. Oxidation is a transfer of electrons, therefore whenever electrons are transferred from one location to another, oxidation occurs. The entity that loses the electron (microbe) is oxidized, and the one that gains the electron is reduced. The transfer of electrons creates an electro-chemical potential measured in millivolts (mV) known as ORP or Oxidation Reduction Potential. Oxidation occurs in the presence of an oxidizer because it is "hungry" for electrons. The stronger the oxidizer, the faster the electrons are transferred, and the faster the microbes are killed. Therefore it is ORP (mV) that measures disinfection rate and not ppm of oxidizer. Since ORP is the measure of disinfection, it does not matter what the oxidizer is, or what form of oxidizer is used. It may be chlorine, bromine, ozone, peroxide, or any other, and it may be in either solid, liquid or gas form. The most common form of oxidizer used in the industry is chlorine. It may be gas, liquid such as sodium hypochlorite, solid such as calcium hypochlorite, or chlorine dioxide. Chlorine, when dissolved in water, exists in two forms. Combined Chlorine is bound by the dirt, debris and organic matter in the water, and is not available for disinfection. Free Chlorine is the available chlorine, which again exists as Hypochlorous acid and Hypochlorite ion. The Hypochlorous form does all the oxidation in the water, whereas the ionic form does not. The two forms of free chlorine interchange in their proportion depending on the pH of the water. pH is the term indicating the hydrogen ion (positively charged atom) concentration of a solution, a measure of the solutions acidity. The typical range for measuring pH is 0 through 14. A reading of 0 through 7 is an acid. The lower the number, the stronger the acidity. A reading of 7 through 14 is an alkaline, or a base. At a pH of 6.5, 95% of the chlorine is in the Hypochlorous form, and at its maximum efficiency.