My thanks today to Scott Hilburn, who writes the comic strip “Argyle Sweater.” You can find him at http://www.theargylesweater.com/
Scott humorously raises the question, “What is Alkalinity?” (Also, what does it have to do with batteries? Are alkaline batteries better? What is the alternative to an alkaline battery? Was Al Kaline a great ball player, or a chemical joke?)
Okay, so we’ll skip the last few questions. However, many of you commented to me following the last State Treatment Operator exam that alkalinity was featured in several test questions. So let’s take a look at this today.
We need to look at two other concepts first: pH and buffering. We measure the hydrogen ion concentration — the acid level of a water sample — using pH. The pH scale is kind of odd, but it makes perfect sense if you understand the fairly advanced mathematics that define the scale. We don’t need to go into that, but we do need to appreciate a few principles about pH. First, pure water has a “neutral” pH of 7.00. Lower values than 7 tell us that the substance is “acidic,” with strong acids having the lowest pH values, around 0 or 1 or 2. The opposite of acidic is “basic,” with strong bases having pH values at the opposite end of the scale, around 12 or 13 or 14. And the pH of a natural water will depend on the substances that are dissolved within it — not on the water itself (which would have a pH of 7.00).
When a strong base (like sodium hydroxide, also known as caustic soda or lye) is added to water, the pH will increase, becoming more basic. But how much will it change? This depends on those other substances that are present in the water. Substances that quickly neutralize a strong base so that the pH change is small are said to “buffer” the water. In short, “buffering” is the capacity of a water sample to resist a change in pH — in either direction: up or down on the pH scale.
The resistance to an increase in pH — through the addition of a strong base, as just described — is called “acidity.” The resistance to a decrease in pH — when a strong acid is added to the water — is called “alkalinity.” So both acidity and alkalinity are gauges of the buffering capacity of the water, but in opposite directions.
For whatever reason, we rarely examine a natural water’s acidity, but its alkalinity is often of interest. In natural waters, there are three chemicals — all anions — that contribute to alkalinity: hydroxide, carbonate, and bicarbonate. Together, these constitute the “Total Alkalinity.”
In conventional water treatment, we employ two chemical treatment processes: coagulation and disinfection. In both cases, we are adding acids to the water (alum or ferric chloride in coagulation, and hypochlorous acid and hydrochloric acid in the case of gaseous chlorine disinfection). These acids will cause the water to become more acidic, and thus lower the pH. But how much of a decrease we see will be due to the amount of hydroxide, carbonate, and bicarbonate ions — our alkalinity — that are present in our water.
A high alkalinity water will exhibit a minor decrease in pH during these processes. A water low in alkalinity will exhibit a much greater change.
And what any of this has to do with batteries is beyond me!