HT+Task+4B

=Water Test: Alkalinity Analyzing Results= Read the following background information and answer the questions that follow.

The alkalinity of water is due to the presence of weak anions which can accept and neutralize protons. In fresh waters, buffering is attributed mainly to several forms of inorganic carbon. The major anions in most fresh waters are bicarbonates (HCO3-). Carbonates (CO32-) are present if the pH is above 8.3. If the pH is extremely high, hydroxides (OH-) may contribute. Carbonates and bicarbonates are common to most waters because of the abundance of carbonate minerals in nature. The most important form of carbonate in aquatic systems is calcium carbonate (CaCO3).

Although nearly insoluble in water, CaCO3 will dissolve readily in carbonic acid (H2CO3), formed as carbon dioxide (CO2) reacts with water. CO2 enters the water directly from the atmosphere and as the product of aquatic respiration and decomposition. Rainwater, charged with CO2 as it falls, is further enriched as it percolates through organic soil, later entering a waterway through a subterranean source. As CO2 dissolves in water, some of it hydrates to form carbonic acid, as shown by the equilibrium equation:

CO2 + H2O ⇔ H2CO3

In soil, H2CO3 readily dissolves calcium carbonate from rock formations in the drainage basin, producing a bicarbonate solution.

H2CO3 + CaCO3 → Ca(HCO3 )2

In water, H2CO3 may dissociate twice, depending on the pH (Fig. 7.3). Between pH 4.5 and 8.3, H2CO3 dissociates into HCO3- and H+, as shown by the equilibrium equation:

H2CO3 ⇔ HCO-3 + H+

The concentration of bicarbonate ions is greatest at pH 8.3, when H2CO3 and dissolved CO2 are no longer analytically present. Above pH 8.3, the relative concentration of HCO3- declines as the second dissociation yields CO32- and H+. HCO-3 ⇔CO32- +H+

In testing waters for alkalinity, it is important to keep in mind that aquatic systems are dynamic. At any given time, inorganic carbon exists in several forms. The relative concentration of any one form is dependent on influences both in and out of the waterbody, including drainage basin characteristics, pH, biological activity (i.e., photosynthesis, respiration, and decomposition), and pollutants.

To determine which anions are contributing to alkalinity, two tests are performed - phenolphthalein alkalinity and total alkalinity. Phenolphthalein alkalinity is part of the total alkalinity. When phenolphthalein indicator is added to water, a pink color will appear if the pH is equal to or above 8.3, the carbonate endpoint. If this occurs, assume the presence of CO32-, probably HCO3-, and possibly OH-.

When the pH is lowered to 8.3 with sulfuric acid (H2SO4), the pink color will fade to clear, indicating that the following reactions have occurred:

H2SO4 +Ca(OH)2 →CaSO4 +2H2O H2SO4 +2CaCO3 →CaSO4 +Ca(HCO3)2

Below pH 8.3, only bicarbonates are present, since all CO3- have been acid neutralized. As acid titration continues, each bicarbonate ion takes up a hydrogen ion to form a molecule of H2CO3.

If BG-MR is used as an indicator, the color of the water will turn from blue-green to pink as the pH is lowered to about 4.5, the bicarbonate endpoint. At this point, H2CO3 no longer dissociates, the buffering capacity of the water is lost, and alkalinity is zero. The sum of the mls of titrant used to convert CO3- to H2CO3 is the total alkalinity measured in mg/l CaCO3.

1.) What type of acid forms when CO2 is introduced to the water?

2.) If you have phenolphthalein alkalinity, which anions are present?

3.) How does the alkalinity effect the environment?

4.) Using your labs and the internet as a resource, determine if the alakalinity of your water sample is healthy or not. Back it up with facts.