I have a couple more very common terms for you today, as you get ready for Saturday’s exam. Remember, vocabulary is the key to scoring well on the non-math problems of the certification exams.
Sodium thiosulfate is a chemical that we use very often, but that we rarely see. It is added to sample bottles by the laboratory, and in such a small quantity that it’s hard to notice. Its purpose is simple: react with any residual chlorine or chloramines in the sample, so that those disinfectants will not kill any more microorganisms while we wait for the lab tests to get started. Simply stated, sodium thiosulfate is a dechlorinating agent. It can also be used to dechlorinate water that is being discharged from our system into storm drains or natural waters.
Alkalinity is the ability of water to resist a decrease in pH. In natural waters, the most common ions that constitute alkalinity are hydroxide, carbonate, and bicarbonate. Together, these are called total alkalinity. Alkalinity is needed for coagulation, and is an important factor in water corrosivity. It is normally present in sufficient quantities in our raw waters; however, if we need more, we can add alkalinity to the water. The most common chemicals for this purpose are lime and sodium hydroxide.
Please don’t confuse sodium hydroxide with sodium thiosulfate. That will cost you a couple points on Saturday!