The Molecular Weight of Aqueous Hydroxyl Ion

Calculating the Functional Molecular Weight of Aqueous Hydroxyl Ion

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In the 29 June 2002 issue of Science News was an article that said that while the hydronium ion (H₃O⁺) is pretty well understood, that of the hydroxyl ion (OH^-) seems to be totally wrong. Computer modelling indicates that hydroxyl does not act in a similar fashion to the hydronium ion.

Before continuing, you should be asking: "So what? Why should I do this project? What's so important about it?" You should never do a project that has no important implications. Knowing more about the hydroxyl ion is vital in many fields - membrane ion balances, for a more esoteric reason, but the diffusions in fuel cells is of immediate economic importance as our electricity supplies dwindle and become more and more expensive. We have fuel cells that do work - but not as well as we would like them to. We need to increase their efficience and flow and output rates. Thus this project does hold significant implications.

Perhaps a project can be designed that would have something to say about the relative molecular weights of hydronium and hydroxyl ions: What are their relative diffusion rates?

At first thought you might think to take a tray of water, and place a drop of HCl at one end and a drop of NaOH at the other end, and see where the two diffusing molecules meet. Unfortunately, convection currents will confuse your observations. So a means is needed for immobilizing the water so that convections don't occur.

This might be rather easy to set up - a petri plate filled with agar in which an immobile or non-diffusing pH indicator is present - perhaps something like litmus agar. Nope! Better use a longer, narrow tray - even a tube! Yes, that's it! Wow, follow this:

To each of at least six glass tubes that are about 6-inches long, tape securely closed one of each of their ends. (The figure shows only one of the tubes.)
Stand the tubes upright, taped ends down, in a glass of cool water. Any leaks will become evident.
Make up a concoction of molten litmus agar as follows: ---
Add molten litmus agar to each tube so that exactly one centimeter remains unfilled.
In the cool water, the agar ought to gel rather quickly.
Make up two solutions: 0.1 M NaOH and 0.1 M HCl as follows:---
Place a small amount of the NaOH in a PLASTIC tumbler, and the same amount of HCl into another tumbler. Remove the tape, and stand three of the tubes in each tumbler such that the previously taped ends are down in the solutions at the bottom of each tumbler.
Using an eye-dropper fill the empty space above the agar in each tube with the solution opposite that in the tube's tumbler.
Wait one hour for the acid and base to diffuse into the ends of the gel.
Remove the tubes from the tumblers, rinse both ends with water, dry and re-tape, but this time tape closed both ends to prevent them from drying out. How is it that you cannot mix up which tubes were sitting in base and which ones in acid? (Look at the colors!)
As time proceeds, the diffusion from both ends will continue and converge. At some point in time they will meet. MARK THE TUBE where they first meet. Measure how far each has diffused, and write that in your notebook.
Since we understand hydronium, then we should be able to calculate the molecular weight of the hydrated hydroxyl ion.

As said, this 'might be rather easy.' Maybe it has other problems, and solutions need to be found to overcome those. But this is a project for keen-thinking high schoolers, who should be able to think themselves creatively past any problems that arise. Furthermore, it is left up to you to search out references beyond those given in the abovementioned Science News web-page.

As a last thing to do before writing your report, contact THIS WEB SITE with a summary of your results.