ENV6666_MOD3 - ENV 6666 AQUATIC CHEMISTRY Professor: Dr....

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© 2010 University of South Florida: Dr. Maya Trotz & Mr. Arlin Briley Page 1 of 28 ENV 6666 AQUATIC CHEMISTRY Professor: Dr. Maya Trotz Module 3 The “Proton Condition” Approach to Solving Acid-Base Systems
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© 2010 University of South Florida: Dr. Maya Trotz & Mr. Arlin Briley Page 2 of 28 Table of Contents Exposing the Flaw of LHS = RHS in Acid/Base System . .............................................................................. 3 Selection of “Reference Species” . ............................................................................................................ 7 Technique for Estimating Dominant Acid/Base Species . ........................................................................... 9 Second Example of Dominant Species Estimation Technique . ................................................................ 11 Mass Balance on H + and the Proton Condition . ...................................................................................... 13 Summary of the Proton Condition Approach . ........................................................................................ 21 Proton Condition Example Problem . ...................................................................................................... 23
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© 2010 University of South Florida: Dr. Maya Trotz & Mr. Arlin Briley Page 3 of 28 Exposing the Flaw of LHS = RHS in Acid/Base System In the previous module we introduced the three step methodology for solving multi-constituent acid- base equilibrium problems. That method typically led to making various substitutions into the charge balance equation, and solving via spreadsheet techniques. We also found that the equilibrium pH could be found graphically by relating the charge balance equation to an intersection of the curve of the left- hand-side (LHS) of the equation with the curve of the right-hand-side (RHS) of the equation. With this knowledge, we can quickly sketch a speciation plot, consider the positively charged species and negatively charged species that would be present at equilibrium, and draw the LHS = RHS intersection to identify the equilibrium pH. Now let’s look at a simple example for which the graphical method works well, then examine another system to learn why the LHS = RHS method does not always work well. We will also show why this flaw in the LHS = RHS approach is not limited to graphical methods. We will conclude with the “Proton Condition” method of solving problems, which is an expansion of the three- step methodology which works especially well for complicated problems such as open systems which must account for added chemicals, chemicals precipitated or evaporated out of the system, and so on. Suppose we add 0.001 moles acetic acid to one liter pure water. Consulting the reference sheet for acid
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This note was uploaded on 03/06/2011 for the course ENV 6666 taught by Professor Fuss during the Spring '11 term at Uni San Francisco de Quito.

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ENV6666_MOD3 - ENV 6666 AQUATIC CHEMISTRY Professor: Dr....

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