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Robert M. Hanson
Department of Chemistry, St. Olaf College, Northfield, MN 55057

Principal Species and pH in Acid/Base Solutions, allows you to determine the concentrations of the principal species in a variety of solutions or mixtures of solutions. Just select the solutions to mix from the drop-down lists and then click on the "mix" button. Alternatively, you can "virtually titrate" the first solution into the second by clicking the "start/stop titration" button. Finer control over the solution parameters volume and concentration than is available through the drop-down lists is available by clicking on the "design solutions..." link.

The program supports two methods of calculation: first-year method and mass/charge balance. The approximations used in first-year methods can lead to significant errors when working with fairly "strong" weak acids, such as HSO₄^–. In such cases, the mass/charge balance method is recommended. This method involves finding the pH at which mass (mmol) and charge are both balanced.

Principal Species and pH in Acid/Base Solutions has a database of 14 weak acids and their conjugate bases and four buffer solutions. Weak acids can be temporarily added to the database by clicking the "new acid..." link. All such additions remain in the database only during the current session and do not persist to future sessions with the program.

Figure. Titration curve: oxalic acid added to sodium hydroxide.

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