[Ken]

Why would grape juice with a pH of 2.8 not be more corrosive to a nail than sprite which has a pH of 3.3? You would think in a closed bottle, the lower pH liquid would be more corrosive.

[Arkcon]

Why would grape juice with a pH of 2.8 not be more corrosive to a nail than sprite which has a pH of 3.3?

Presumably, you have a specific experiment that supports this statement, yes?  How do you quantify corrosiveness?

You would think in a closed bottle, the lower pH liquid would be more corrosive.

Umm ... the closed bottle has what effect now? 

A bit more of your procedure is needed.

[Ken]

My child and I are doing a science project. The question was, "Is grape juice more corrosive than sprite?". We took a pH of each and used a 10 penny nail as our way to measure corrosiveness by amount of weight lost. The pH of grape juice was 2.8 and the weight loss of the nail in the grape juice in a closed bottle was .089 grams. The pH of the sprite was 3.3 and the weight loss of the nail in the sprite in a closed bottle was .097 grams. You would think the lower pH liquid would have been more corrosive. Can anyone explain? 

[agrobert]

How are you quantifying the weight of the nail?  How is the nail cleaned before weighing?  It may be that some of you mass on the nail is due to inconsistent cleaning methods.  I would suggest redoing the experiment with several samples in each liquid (say 5 in Sprite and 5 in grape juice) and averaging the results.  The corrosion depends on surface area as well, averaging your results would provide more accuracy between analysis and methods.

[Borek]

And let's don't forget that pH is not the only factor responsible for the corrosion process, substances present in the solution can have their effects (slowing or speeding up corrosion) as well.

[irishnenglish]

more than the concentration of H+ ions in a  solution has corrosive effects, perhaps the grape juice contains some sort of enzyme which inhibs its corrosiveness?