Alkalinity & Hardness in Water

WHY STUDY ALKALINITY?

The alkalinity of water is a measure of how much acid it can neutralize. If any changes are made to the water that could raise or lower the pH value, alkalinity acts as a

buffer, protecting the water and its life forms from sudden shifts in pH. This ability to neutralize acid, or H⁺ ions, is particularly important in regions affected by acid rain. In the diagram below, for example, the lake on the right has low alkalinity. When acid rain falls, it is not neutralized, so the pH of the water decreases. This drop in the pH level can harm or even kill some of the aquatic organisms in the lake. The lake on the left, however, has high alkalinity. When acid rain falls in this lake, the acid is partially neutralized and the pH of the water remains fairly constant. In this way, a high alkalinity level helps maintain the health of the water and the organisms that live there.

Alkalinity should not be confused with pH. The pH of a solution is a measure of the concentration of acid, or H⁺ ions, in the water. Alkalinity is a measure of the water’s capacity to neutralize an acid, or H⁺ ions, thereby keeping the pH at a fairly constant level.

The alkalinity of surface water is primarily due to the presence of hydroxide, OH^–, carbonate, CO₃^2–, and bicarbonate, HCO₃^–, ions. These ions react with H⁺ ions by means of the following chemical reactions:

OH^– + H⁺ → H₂O

CO₃^2– + H⁺ → HCO₃^–

HCO₃^– + H⁺ → CO₂ + H₂O

Most alkalinity in surface water comes from calcium carbonate, CaCO₃, being leached from rocks and soil. This process is enhanced if the rocks and soil have been broken up for any reason, such as mining or urban development. Limestone contains especially high levels of calcium carbonate.

Alkalinity is significant in the treatment of wastewater and drinking water, because it will influence treatment processes that use bacteria (anaerobic digestion) to help remove wastes. Water may also be unsuitable for use in irrigation if the alkalinity level in the water is higher than the natural level of alkalinity in the soil.

Expected Results

Alkalinity is reported in units of mg/L CaCO₃, because the carbonate ion, CO₃^2–, is its primary constituent. Alkalinity levels will vary across the country. Some sample data are shown in Table 1. In general, water in the eastern half of the United States will have a higher alkalinity than water in the west because of a higher occurrence of limestone.

In Wisconsin, areas in the northern and northwestern part of the state that have had the limestone scoured away by the last ice age or have granite as the primary bedrock will often have a lower alkalinity measurements. Areas with bogs, and cranberry marshes tend to also have lower alkalinity measurements. In southern and far eastern Wisconsin (especially the southeast and close to Lake Michigan) alkalinity readings tend to be higher due to presence of limestone bedrock close to the surface.

Summary of Method

Alkalinity is measured by adding sulfuric acid to a water sample. The acid is added until the equivalence point is reached. This is point where the acid has neutralized all the CO₃in the water sample. To calculate the alkalinity we must find the equivalence point and how much acid had to be added to the sample to get to the equivalence point.

Look at the graph below. When about 20 mL of the acid have been added the pH starts to drop quickly. The pH goes from 6.1 with 19.0 mL of acid to 5.8 when 2 mL more are added. All neutralization reactions have this type of quick change right before the equivalence point is reached.

Be aware of this quick change so when you are adding the acid you do not add too much, too fast and overshoot the equivalence point. The equivalence point will be at a pH of approximately 4.5, but will vary slightly, depending on what the alkalinity of the water is. When the pH starts to change very quickly you will reduce the amount of sulfuric acid you add to the water sample to 1 or 2 drops. This is done so that you are able to see the exact point where the acid has completely neutralized the CO₃ (the equivalence point). By getting a precise and accurate measurement of the amount of acid we then can calculate the alkalinity of the water sample.

NOTE: If the equivalence point is missed, it can be estimated. By finding the where the largest pH change occurred between two points and then using a slope calculation (y = mx + b) we calculate the equivalence point. By first finding the slope of the line between the 2 points (the “m” in the formula), then plugging in 4.5 for y, and then plugging in the y-intercept for b we can solve for x (the amount of acid used at the equivalence point).

We will use a pH probe with the Labpro system to monitor pH during the titration. When measuring pH realize that it may take up to 10-15 seconds for the pH to change after you have added the sulfuric acid. It takes that much time for the acid to evenly spread through the water sample when stirred. Your pH may initially drop because the pH immediately next to the probe is low. As the acid is stirred and spreads evenly through the sample the pH will rebound back up. This is normal.

Page updated

Report abuse