The Chemistry Section: Alkalinity

The Hach Alkalinity kit

Equipment List Chemicals
1) One bottle, Sulfuric Acid (0.03N)
2) One bottle, Phenolphthalein Indicator powder
3) One bottle, Bromo-creosol Green Methyl Red Indicator powder
4) One measuring spoon (0.05 g when measuring indicator powder)
Glassware
1) One Hach mixing bottle
2) One Hach plastic measuring tube

The Chemistry

Alkalinity is a measure of how much acid it takes to lower the pH below a certain level. In natural waters, most of this ability is due to the presence of dissolved species in the carbonate system. This system consists of an equilibrium between three chemical species:

		Low pH <----------------------------------------------> High pH
		H₂CO₃			HCO₃^-    			CO₃^2-

This system tends to be so dominant in its contributions to alkalinity, that we assume all of the alkalinity in the system is due to their presence. Alkalinity is thus measured in terms of the amount of CO3(2-) present in the system as a component of the CaCO3 molecule. This is chosen because Calcium Carbonate in solid form, known as limestone or the minerals calcite or aragonite, is often the primary contributor to the amount of dissolved carbonate in the water.

To measure the alkalinity in the water, we simply add acid to it, watching the pH and noting how much acid was added at the point where a certain pH is reached. Our chemistry kits make this easy by using a compound known as an indicator which changes color at a certain pH.

The test, explained!

If your sample is high in alkalinity, you need to follow the instructions for the high range test. However for the sake of brevity and clarity, in these instructions we will discuss the test as though the process was the same for both, since the theory is very similar.

Alkalinity Procedure

Step 1) Fill the sample bottle to the 15 mL line. We must know how much water we're testing, so in the first step, the Hach kit has you fill a bottle to a line marked on the side. (15 mL).

Step 2) Using the 0.05 g scoop, add a level scoop of BromCresol Green- Methyl Red to the sample bottle. This step adds a small amount of indicator powder to the solution, which will appear green at pH's above 4.3, and pink below 4.3.

Step 3) Add 0.03 N sulfuric acid solution dropwise to the sample bottle, counting drops as you put them in. Note the number of drops required to produce a full color change from green to pink. As the acid goes in, the alkalinity in the water will use it up, until it can't hold it any more. During this time, the pH will drop slowly, and when all of the acid-absorbing capacity is gone, the pH will begin to drop much more rapidly. Eventually the pH will reach 4.3, and the indicator will change colors. It is important to add the acid until all of the color change has taken place. If you are unsure that a full change has occured, add another drop and watch for further change. When no more is noted, record the number of drops required. The number of drops multiplied by 6.8 is the alkalinity in mg/L as CaCO₃ (Calcium Carbonate).

High End Alkalinity

When we measure a water with a high alkalinity, we simply use a modified method to save our chemicals. To do this we use a smaller initial volume of sample water and in the end, multiply our number of drops by a different conversion factor to account for the difference in initial volume. In this case, we say each drop of sulfuric acid is multiplied by 17 to get the alkalinity in mg/L as CaCO₃

I hope that this brief report has helped to answer questions you may have had about how our alkalinity kit works. Feel free to send email to me (roger@hwr.arizona.edu) or to my assistant, Chris Gutmann (cgutmann@hwr.arizona.edu).

Sincerely,

Roger Bales
Professor of Hydrology & Water Resources
University of Arizona