1. Chlorine can be measured several ways

Chlorine is most commonly referred to in parts per million (ppm) or free available chlorine content (FAC). Free available chlorine is the chlorine that remains in a solution after all existing pathogens have been eradicated. Parts per million can be used as an indicator of the FAC that is present in the solution. You can calculate the free available chlorine by measuring the original ppm of your chlorine before you add it to a solution, and then taking another measurement after it has been added. This will show you how much chlorine was immediately used up, and can act as a indicator of whether or not you need to add more.

2. A high ppm does not always mean a more effective disinfectant

Bleach (sodium hypochlorite) is the perfect example of this. Bleach is very high in chlorine, but quite low in hypochlorous acid, which is the active sanitizing agent. For example, 2000 ppm of bleach will only contain about 2% of hypochlorous acid, making it much less powerful as a disinfectant. A high ppm bleach will take longer to inactivate bacteria and require larger quantities of disinfectant than a much smaller ppm of hypochlorous. For reference, 4 ppm is the maximum limit of FAC that can be present in drinking water in the United States. However, for sanitizing purposes hypochlorous acid is usually between 200-500 ppm, while bleach can be as high as 5,000. 

3. The pH is critical for chlorine concentration

The pH of a chlorine solution is crucial in determining its efficacy. At pH 5-6, the solution will be nearly 100% hypochlorous acid. As the pH increases above 6, hypochlorite (bleach) begins to increase. In acidic pH conditions, Cl- (chlorine gas) is present. The neutral pH of hypochlorous acid is what allows it to be odorless, non-toxic, and much less volatile and corrosive than other forms of chlorine. 

Photo by Clark Tai on Unsplash

References

White's Handbook of Chlorination and Alternative Disinfectants 

The Fundamentals of Chlorine Chemistry and Disinfection (December, 2007)