Lecture 9

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Distribution of Species in a System at Equilibrium, Carbonate Equilibria

Reading Assignment: Study the material in this lecture and review the information on equilibrium and acids and bases at the web sites linked to Lecture 8. Read chapter 3 in Manahan and read the material on carbonate equilibria at the web sites linked below.

Homework: HW-4 (Due Wednesday, February 13)

Links and Additional Resources:

Course notes on Carbonate Equilibria, R. J. Zasoski, UC Davis

Carbonate Equilibria in Natural Waters, Stephen Lower, Simon Fraser University


Distribution of species in solution










Likewise, it can be shown



Figure 9.1. Distribution of Carbonate species as a function of pH


How do we use this and what good is it?

Measured Field Parameters

Alkalinity and pH

Alkalinity is the ability of a natural water to neutralize acid.

Determined from a titration:

Moles of H+ = (Volume need to reach end point) x (concentration of acid)

From the composition of the natural water, we also know:

Moles of H+ = Moles HCO3- + Moles OH- + 2 [Moles CO32-]

This is total alkalinity

We also have Bicarbonate Alkalinity (Moles HCO3-))

Hydroxide Alkalinity (Moles OH-)

Carbonate Alkalinity (2x Moles CO32-)

Titration

Show addition of acid to a natural water sample

Strongest Base neutralized first

Next strongest base neutralized second

Next strongest base neutralized third, etc.

Figure 9.2. Hypothetical titration curve for a solution containing hydroxide, carbonate and bicarbonate ions.




Let's finish the discussion today by reviewing some terms you are familiar with, but have not used recently.

Molar means moles of substance per liter of solution.

One mole of a substance is also one gram formula weight (GFW) of that substance

For instance, one mole of CaCO3 would be 100.09 g of CaCO3. This is because the GFW of CaCO3 is : 40.08 + 12.011 + (3) (15.9994) = 100.09.

Likewise, a normal solution contains one equivalent weight of a substance per liter of solution.

In acid base chemistry, an equivalent weight of a substance is the GFW of the substance divided by the charge of the aqueous ion.

For example, calcium carbonate dissociates to form Ca2+ and CO32-. The equivalent weight of calcium carbonate is therefore: 100.09/2 = 50.045. Since several common water quality measurements are expressed as meq/L, it is necessary to learn to work with these units.

Example 1

100 mL of a water sample required 20.0 mL of 0.0100 M HCl to reach an end point of pH = 4.2 Express the alkalinity of this samlple as mg CaCO3/L

Since 1 Mole of HCl = 1 Equivalent of HCl

Since there were 10.01 mg of CaCO3 in 100 mL of Sample, the amount per liter would be:

10.01mgof CaCO3/0.1 L = 100.1 mg CaCO3/L

 


ENV 440 - Course Topics

Environmental Chemistry 440
Last Updated:  02/06/2007