Question

In this lab, you will be using 5-in-1 chemical test strips from Hach Company. Hach is a global

company based out of Loveland, CO and their products are widely used throughout the world to test

and analyze water samples. In your 5-in-1 test strips, two of the test pads analyze chlorine in water

(free chlorine and total chlorine). Using information from the handout by the Centers for Disease

Control and Prevention (CDC) which is posted on Canvas, provide a chemical explanation for what is

being tested in the free chlorine test versus the total chlorine test.

Answer 1

In chlorine tests, we check the amount of chlorine present in the water sample. Chlorine is used for cleaning or sanitizing the water. Chlorine exists as free chlorine and combined chlorine in water. These two forms of chlorine exists together in water and their sum gives total chlorine. Free chlorine are the amount of chlorine ions that are left freely or that didn't take part in sanitizing process. When we add chlorine to water, they form hypochlorous acid or hypochlorite ion. This is in free form and we call them as free chlorine. Combined chlorine is the chlorine that are found in compounds. So in uncleaned water, nitrogen and ammonia are present as contaminants. They react with this free chlorine and form compounds of nitrogen and ammonia (used chlorine). These are considered as combined chlorine and moreover they are the ones who took part in sanitizing. Hence they lose the ability to clean water. Now the total sum of free chlorine and combined chlorine gives total chlorine.

We have methods to measure free chlorine and total chlorine. So from those two values we an get the combined chlorine by subtraction.

ie Total chlorine = Free chlorine + combined chlorine

combined chlorine = total chlorine - free chlorine

your water can be considered as sanitized only when your free chlorine levels are higher than the combined chlorine levels.