MINEQL+ Chemical Equilibrium Modeling System

Analytical Insight

Here's one last example. Many laboratories routinely analyze water samples for a whole array of ionic constituents. It is inherent within any type of measurement that a certain amount of uncertainty will be introduced. All that can ever be done is to try to minimize the uncertainty -- you can never eliminate it. Many questions come up in this type of work. For one, is the sample well characterized (i.e., did you measure all the important anions and cations)? Second, if you are sure that the system is well characterized, how can you check to see that your analytical error is within reason? Finally, are organic solutes present in your sample?

One of the first checks that researchers perform on a sample is for ion balance. This is basically just an accounting of all the positive (cationic) and negative (anionic) charges in the system. In the actual system, the positive and negative charges will always balance each other out. This is the principle of electroneutrality. But in measured samples the introduction of analytical error will make this hard to observe. Usually a charge discrepancy that less than 10% of the ionic strength of the solution is considered good.

Here's the problem: One can't just calculate charge discrepancy by adding up all the total metal concentrations and subtracting the total ligand concentrations. Remember the system described above includes metal complexes of all types. Each complex contributes its unique charge to the system. So this is another example where knowledge of chemical equilibrium would be useful. If you know the concentration of each dissolved complex, you can calculate the charge discrepancy fairly easily.

Not only can the charge discrepancy function as a screening criteria for chemical samples, but it can help flag when to include additional ions in a suite of analyses or to help identify the presence of organic ion in waters.

Previous 1 2 3 4 5 6 7 8 Next