The Chemistry Section: Alkalinity

The Lamotte Alkalinity kit

Equipment List Chemicals
1) One bottle, Alkalinity titration reagent B (0.05N Sulfuric Acid
2) One bottle containing indicator tablets
Glassware
1) Titration Tube
2) Direct Reading Titrator

The Chemistry

Alkalinity is a measure of how much acid the impurities in a water can absorb before the pH drops below a certain level. In natural waters, most of this ability to absorb acid is due to the presence of dissolved species in the carbonate system. This system consists of an equilibrium between three chemical species: pH 4 <-----------------------------------------> pH 11

		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 procedure

Step 1) Fill titration tube to 5 mL line with sample water.
Step 2) Add one BCG-MR Tablet (BromCresol Green - Methyl Red). Cap and Shake until tablet dissolves. Solution will turn blue-green. This step simply adds an indicator (a pH sensitive colorant) to the water.
Step 3) Fill the direct reading titrator with Alkalinity Titration Reagent B. Insert the Titrator into the center hole of the titration tube cap.
Step 4) While gently swirling the tube, slowly depress the plunger to titrate the sample until the blue-green color changes to pink. Read the test result where the plunger tip meets the titrator scale. Record as Total Alkalinity in ppm CaCO₃. The Titration reagent is just a dilute solution of Sulfuric Acid. When it is added to the sample, it gradually lowers the pH of the sample. Samples with low alkalinities will drop in pH very quickly since there is very little to absorb the acid. Those with higher alkalinities will drop much more slowly, as the HCO₃^- absorbs some of the acid, converting to H₂CO₃. When the pH of the solution falls below the point of color change for the indicator, the solution will change colors. At this point, we assume that all of the alkalinity (in this case, HCO₃^-) has been used up.

I hope that this brief report has helped to answer questions you may have had about how our dissolved oxygen 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