Science Buddies: "Ask an Expert"

My kid is working on a project to test the corrosion rate with different PH values.

The acid used in the project is distilled vinegar with PH value around 2.5. In addition to that acid solution with PH value 3.5, 4.5 and 5.5 are prepared by mixing vinegar with PH-up (soda ashes, for swimming pool). The copper sheets of size 2x2 inch are put into the solutions.

We were thinking solution with lower PH value (more acidic) will corrode the copper the most and turn the solution more blue in color (copper acetate???). However, after a few days we observed the reverse effect. We observed that the solution with highest PH value had the most blue-ish color. I.e., Solution with PH 2.5 has lightest blue and PH 5.5 has the highest shade of blue. That seems to mean the less acidic solution react with copper more. Can somebody please help explain what happen and point us to a good direction to continue the experiment.

Thanks a lot in advance.

I don't know what is happening in this case, but I have a hypothesis (which, of course, could be tested with a proper experiment : copper acetate is a salt. It tends to neutralize acids, and is soluble in water. If you have less acid in the solution, it's possible that the copper acetate is neutralizing the acid faster, leaving more copper acetate dissolved in the water instead. A couple of ways you could test this would be to measure the pH of the solution *after* the experiment, and measure the total amount of copper in the solution. I don't know if there are any tests specifically for copper acetate, though.

In general, the way that science buddies recommends to do this kind of experiment is to use copper testing tablets to measure the amount of copper in the solution, rather than relying on color. There are a lot of factors that could affect color, but none of them would fool a copper test. See here for a similar experiment with a procedure: https://www.sciencebuddies.org/science- ... p090.shtml