Visual Minteq

Topics: Carbon dioxide, Carbonate, Bicarbonate Pages: 4 (868 words) Published: May 26, 2013
Visual MINTEQ -- Carbonate System


The carbonate system is extremely important, being central to respiration, photosynthesis, pH buffering, carbon budgets, and carbonate rock formation. CO2 is the primary cause of global warning, and carbonate rocks and dissolved species are the most important buffers of pH shifts in the natural environment. The carbonate system, however, can be challenging to predict without a full consideration of all species and the specific deviation from ideal conditions. Fortunately, with current computational power (and ease) this is a manageable problem. To make things even easier for us, there are a number of chemical speciation programs available. Visual MINTEQ is one such program and offers an easy means to fully speciate the carbonate or other systems. Our first assignment using MINTEQ will therefore be to further explore the carbonate system.

Open Visual MINTEQ and do the following exercises.

1. First the closed system without calcite present (the total carbonate is held constant).

Specify a total CO32- and Ca2+ concentration of 200 mg/L
(note: you have multiple options to choose from in the input file. When using mg/L, MINTEQ will convert your units to molarity and report this in the output) . Run MINTEQ
a)What is the resulting pH?
b)Are there species in the output that we had not considered in the carbonate equilibria earlier (e.g. ion pairs)? c)Is the system oversaturated with respect to calcite?

2.Now let’s look at a closed system with calcite present.

Specify CO32- = 1 mg/L and Ca2+ = 1 mg/L (you need to have them defined) Now go to the menu bar (at the top) and select “Solid phases and excluded species” From the drop-down menu choose “Specify infinite solid phases” Then scroll down until you find “Calcite” and select it. Run MINTEQ

a)What is the pH this time?
b)What is the saturation index for calcite?
c)What are the...
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