Alkalinity

What is alkalinity?

Alkalinity is a measure of the capacity of water to neutralize acids. This is known as the buffering capacity of water or the ability of water to resist a change in pH when acid is added. Alkalinity in water is due primarily to the presence of bicarbonate, carbonate, and hydroxide ions. It relates to the balance of carbon dioxide in water and is a function of pH.

How is alkalinity measured?

Alkalinity is expressed as phenolphthalein alkalinity or total alkalinity. Both types can be determined by a titration with standard sulfuric acid to an endpoint pH. Indicators such as phenolphthalein and bromcresol green-methyl red define endpoints or a pH meter could be used for determination of endpoints. Phenolphthalein alkalinity is determined by titration to a pH of 8.3 and indicates the total hydroxide and half the carbonate present. Total alkalinity is determined by titration to a pH of 5.1, 4.8, 4.5 or 3.7 depending upon the amount of carbon dioxide present. The total alkalinity includes all carbonate-bicarbonate alkalinity and hydroxide alkalinity. Bicarbonate is the major form of alkalinity. The unit of measurement for alkalinity is mg/L CaCO3.

What is the significance of alkalinity?

Alkalinity is the measure of the resistance of water to the lowering of pH when acids are added to the water. Acid additions generally come from rain or snow, though soil sources are also important in some areas. Alkalinity increases as water dissolves rocks containing calcium carbonate such as calcite and limestone. Carbonates and hydroxide may be significant when algal activity is high and in industrial water.

When a lake or stream has too little alkalinity, typically below about 100 mg/L, a large influx of acids from a big rainfall or rapid snowmelt event could (at least temporarily) consume all of the alkalinity. This results in a drop the pH of the water to levels harmful for amphibians, fish or zooplankton. Lakes and streams in areas with little soil, such as in mountainous areas, are often low in alkalinity. These water bodies can be particularly sensitive in the spring during periods of rapid snowmelt. Because pollutants tend to wash out of a snow pack during the first part of snowmelt, there is often a higher influx of acidic pollutants in spring, which is also a critical time for the growth of aquatic life. High alkalinity can mitigate metal toxicity by using available bicarbonates and carbonates to take metal out of solution. The metals would thus be unavailable to fish and other aquatic organisms.

Instructions for measuring alkalinity:

1. On the vessels, alkalinity is currently measured using a LaMotte test kit and the test is not part of a basic trip. If this is a part of your trip, obtain a kit and two beakers from the aquatic science instructor.
2. Using marked beakers, obtain samples of water from the water sampling devices (Van Dorn bottles) located on the rear deck. Use the beaker marked T to obtain 50 ml of the top water sample from the Van Dorn bottle marked "T". Use the beaker marked B to obtain 50 ml of the bottom water sample from the Van Dorn bottle marked "B". Be sure to match the symbols on the beakers with the same symbol on the Van Dorn bottle (the symbol "T" for top and "B" for bottom).
3. Bring the beakers containing the water samples back to the alkalinity lab station and place them in the appropriate places in the rack. Measure the top water sample first then measure the bottom water sample.
4. Obtain the titration tube from the LaMotte alkalinity kit and fill it to the 5 mL line with the water sample from the top.
5. Add two drops of alkalinity indicator #2 to the titration tube. Cap the tube and swirl to mix. A blue color will appear.
6. Fill the syringe titrator (syringe) to the zero mark with reagent B.
7. Insert the titrator into the center hole of the cap of the titration tube.
8. While gently swirling the tube, slowly depress the titrator plunger to add reagent B until the blue color changes to green. Record the number that the bottom of the plunger reaches. Note that each minor division on the titrator is equal to 4 ppm. Record this number as total alkalinity in ppm CaC03.
9. Rinse the titration tube with deionized water and re-fill the syringe with reagent B to the zero mark. Repeat steps 4 through 8 with the bottom sample.
10. For GLOBE trips repeat steps 4 through 9 two more times for the top and for the bottom. Calculate an average of the readings for the top sample and the readings for the bottom sample. Record the averages on the data board.
    

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