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5.10 Total AlkalinityWhat is total alkalinity and why is it important?Alkalinity is a measure of the capacity of water to neutralize acids (see pH description). Alkaline compounds in the water such as bicarbonates (baking soda is one type), carbonates, and hydroxides remove H+ ions and lower the acidity of the water (which means increased pH). They usually do this by combining with the H+ ions to make new compounds. Without this acid-neutralizing capacity, any acid added to a stream would cause an immediate change in the pH. Measuring alkalinity is important in determining a stream's ability to neutralize acidic pollution from rainfall or wastewater. It's one of the best measures of the sensitivity of the stream to acid inputs. Alkalinity in streams is influenced by rocks and soils, salts, certain plant activities, and certain industrial wastewater discharges. Total alkalinity is measured by measuring the amount of acid (e.g., sulfuric acid) needed to bring the sample to a pH of 4.2. At this pH all the alkaline compounds in the sample are "used up." The result is reported as milligrams per liter of calcium carbonate (mg/L CaCO3).Analytical and equipment considerationsFor total alkalinity, a double endpoint titration using a pH meter (or pH "pocket pal") and a digital titrator or buret is recommended. This can be done in the field or in the lab. If you will analyze alkalinity in the field, it is recommended that you use a digital titrator instead of a buret because the buret is fragile and more difficult to set up and use in the field. The alkalinity method described below was developed by the Acid Rain Monitoring Project of the University of Massachusetts Water Resources Research Center.Burets, titrators, and digital titrators for measuring alkalinityThe total alkalinity analysis involves titration. In this test, titration is the addition of small, precise quantities of sulfuric acid (the reagent) to the sample until the sample reaches a certain pH (known as an endpoint). Th e amount of acid used corresponds to the total alkalinity of the sample. Alkalinity can be measured using a buret, titrator, or digital titrator (described below).
How to collect and analyze samplesThe field procedures for collecting and analyzing samples for pH and total alkalinity consist of the following tasks:TASK 1 Prepare the sample containersSample containers (and all glassware used in this procedure) must be cleaned and rinsed before the first run and after each sampling run by following the procedure described under Method A in Chapter 5 - Water Quality Conditions. Remember to wear latex gloves.TASK 2 Prepare before leaving for the sampling siteRefer to section 2.3 - Safety Considerations for details on confirming sampling date and time, safety considerations, checking supplies, and checking weather and directions. In addition to the standard sampling equipment and apparel, when sampling for pH and alkalinity include the following equipment:
TASK 3 Collect the sampleRefer to Task 2 in Chapter 5 - Water Quality Conditions for details on how to collect water samples using screw-cap bottles or Whirl-pak® bags.TASK 4 Measure total alkalinity (field or lab)The following steps are for use of a digital titrator in the field or the lab. If you are using a buret, consult Standard Methods (APHA, 1992). Alkalinity is usually measured using sulfuric acid with a digital titrator. Sulfuric acid is added to the water sample in measured amounts until the three main forms of alkalinity (bicarbonate, carbonate, and hydroxide) are converted to carbonic acid. At pH 10, hydroxide (if present) reacts to form water. At pH 8.3, carbonate is converted to bicarbonate. At pH 4.5, it is certain that all carbonate and bicarbonate are converted to carbonic aci d. Below this pH, the water is unable to neutralize the sulfuric acid and there is a linear relationship between the amount of sulfuric acid added to the sample and the change in the pH of the sample. So, additional sulfuric acid is added to the sample to reduce the pH of 4.5 by exactly 0.3 pH units (which corresponds to an exact doubling of the pH) to a pH of 4.2. However, the exact pH at which the conversion of these bases might have happened, or total alkalinity, is still unknown. This procedure uses an equation derived from the slope of the line described above to extrapolate back to the amount of sulfuric acid that was added to actually convert all the bases to carbonic acid. The multiplier (0.1) then converts this to total alkalinity as mg/L CaCO3. The following steps outline the procedures necessary to determine the alkalinity of your sample.
Initial pH of sample is 4.3. Enter "0" in the 4.5 column on the lab sheet. Titrate to a pH of 0.3 units less than the initial pH in this cas 4.0. It takes 10 digits to get to 4.0. Enter this in the 4.2 column on the lab sheet and note that the pH endpoint is 4.0. Alkalinity = (0 - 10) x 0.1 = -1.0. TASK 5 Perform an accuracy checkThis accuracy check should be performed on the first field sample titrated, again about halfway through the field samples, and at the final field sample.
TASK 6 Return the field data sheets and samples to the lab or drop-off pointAlkalinity samples must be analyzed within 24 hours of their collection. If the samples cannot be analyzed in the field, keep the samples on ice and take them to the lab or drop-off point as soon as possible.ReferencesAPHA. 1992. Standard methods for the examination of water and wastewater. 18th ed. American Public Health Association, Washington, DC. Godfrey, P.J. 1988. Acid rain in Massachusetts. University of Massachusetts Water Resources Research Center, Amherst, MA. River Watch Network. 1992. Total alkalinity and pH field and laboratory procedures (based on University of Massachusetts Acid Rain Monitoring Project). July 1.
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