WaterCa_HTML_1.gif and pH

CO2 in pure water

Start with pure water in equilbrium with carbon dioxide gas  at partial pressure WaterCa_HTML_2.gif.  Henry’s law determines the amount of WaterCa_HTML_3.gif disolved in the water:


The dissolved WaterCa_HTML_5.gif combines with the water in a series of reaction with the following equilbrium constants. (In everything that follows, WaterCa_HTML_6.gif is taken to mean WaterCa_HTML_7.gifdissolved). )





In addition to these equilbrium equations, there is a charge neutrality condition:


Equations 1-6 are six equations for the unknowns  in terms of the WaterCa_HTML_13.gif--Once the partial pressure is established concentrations of the six quantities are determined.  They arre easily solved numerically by way of Mathematica or other software. Here is the concentration of the four species as a function of the WaterCa_HTML_14.gif partial presuure in units of atmosphere:


Figure 1.          CO2 dissolved in Pure Water-Concentration of components vs CO2 partial pressure(atm)

Here is chart of pH as function of CO2 partial pressure:


Figure 2.      CO2 dissolved in Pure Water--pH vs. partial pressure CO2(atm)

Note the pH is always less then 7.0. The pH is about 5.6 at WaterCa_HTML_17.gif concentration 380ppm; about the same as pure rainwater. Fresh water in lakes and streams acquires dissolved Calcium.  The Calcium in the form of Calcium Carbonate raises the pH, as we see in the next section.

Water with Calcium

Calcium carbonate is slightly soluble in water. From the Solubility Product value,


we obtain a solubility less than 3 milligrams per liter. This minute amount of Calcium makes a huge change in pH however. Assume the amount Calcium Caarbonate is at saturation level, include equation 7 into equations 1-6. get the following solutions:


Figure 3.          CO2 dissolved in Pure Water Saturated with WaterCa_HTML_20.gif--Concentration of components vs CO2 partial pressure(atm)

Here is plot of pH vs WaterCa_HTML_21.gif concentration for pure water and water saturated with WaterCa_HTML_22.gif;


Figure 4.          pH of wate with and without dissolved Ca; Horizontal units in WaterCa_HTML_24.gif atm)

The pH of about 8.3 is experimentally observed for water saturated with WaterCa_HTML_25.gif and exposed to air.  In nature, fresh water pH is lower:typicaly ranging 6.5 to 7.5. There are two reasons for this reduced pH : (1)  WaterCa_HTML_26.gif is not at saturation levels and (2) other ions sulfates for example will lower this pH. In the laboratory, pure water exposed to air and saturated with both WaterCa_HTML_27.gifand Gypsum WaterCa_HTML_28.gif)  is observed to have pH 7.75.  The attached calculation sheet shows how to calculate this value.

Next: Ocean pH

Spikey Created with Wolfram Mathematica 9.0