Quantities of acid and base present in a sample, as well their pKa values can be determined from the analysis of titration curves. Three types of acid-base titrations are often used:
1) Titration of Strong Acid with Weak Base
2) Titration of Weak Acid and Strong Base
3) Tritration of Weak Base with Strong Acid
As bioanalytical chemists what we really care about is the activity of a substance. Activity is defined by Daniel C. Harris in Quantitative Chemical Analysis as "the value that replaces concentration in a thermodynamically correct equilibrium expression.
Where gc is the activity coefficient and [X] is the concentration.
The activity coefficient is the number by which the concentration must be multiplied to get the activity.
Using the above equation allows for the effect of ionic strength on the equilibrium because the activity depends on ionic strength.
Now you might be asking yourself, "How in the world am I ever going to find out what the activity coefficient, g, is?" Well, that's when we ask our good friends Debye and Huckel.
Where g is the activity coefficient of an ion of charge +/-z and a is the size of the ion in pm in aqueous solution of ionic strength m.
Knowledge of a substance's activity is key to bioanalytical chemists and should always be used in place of a substance's concentration. So, if you ever catch yourself finding the pH of a solution using the [H+], take a step back and remember your g !
Another very important part of acid-base chemistry for bioanalytical chemist is the notion of alpha fractions. For each species present in a titration, a fraction of it exists as either acid or base. This fraction will be helpful in several different titrations and equilibria.
In monoprotic systems-
Since much of bioanalytical chemistry involves diprotic acids as well as monprotic acids, a alpha fractions are integral to diprotic systems. The same pattern is used as was used above.
Much of acid-base chemistry revolves around the use titration curves. However, in order to really understand and use the information, one must know how much of the species exists in each form. This is when our trusty alpha fractions come to the rescue. In order to carry out effective amino acid chemistry, alpha fractions are a necessity.
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For interesting medical applications and interactive fun:
For an acid-base tutorial and links to other acid/base web material:
For a quick review of acids, bases and pH:
For a summary of acid-base chemistry:
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