Are limes and lime juice more acidic than lemons and lemon juice? I attended a business meeting in the Midwest and one of our clients took our management team and a few other business partners to dinner at an awesome Mexican restaurant. When the appetizers were served I commented that it was a wonderful presentation and that everything looked very fresh, including the avocado slices. 

Speaking to the woman seated across from me I said that even though I use a good amount of lemon juice, whenever I prepare avocados they still turned dark pretty quickly. This happens whether they are sliced, used in guacamole, etc. She responded that the secret is to use lime juice. She proceeded to tell me that she was born and raised in Central America and that it is customary there to use fresh limes or juice rather than lemon.

I have tried it and lime seems to work much better than lemon, so it would stand to reason that they are more acidic. However, I never thought there was that much difference. So, on to the questions.

Are limes more acidic than lemons? Are there any other differences in their properties that could make a difference?
I'm culling a lot of information from one of my favorite cocktail books for this one.

Yes, limes have a slightly higher acid content (on average) than lemons do - about 6% for limes, compared to 4.5% for lemons.  More importantly for their flavor, lemons have about 2% total sugar, while limes have somewhere between 0.5% and 0.75%.  Sugar/sweetness has quite a suppressive effect on the perception of sourness, so lemon juice will most likely taste a bit less sour than lime.

The composition of acids in the two also differ.  The acid in lemon juice is almost entirely citric acid, which also makes up most of the acid in limes.  However, limes include about 10% each of succinic acid and malic acid which have an effect on their flavor.  Malic acid is most recognizable as the sour flavor from tart apples and rhubarb.  About succinic acid, Wikipedia states:


  It lends to fermented beverages such as wine and beer a common taste that is a combination of saltiness, bitterness and acidity.


Personally, I think the difference is perceptible in beverages, but less so in cooking.

Culturally, lime would most certainly be the traditional choice for Latin American cuisine.  Lime is much more readily available in Central and South America (three of the biggest producers in the world are located there).  How the difference in acid content would contribute to the browning of avocados is more questionable.

EDIT: Here's an interesting paper, suggesting that ascorbic acid (vitamin C) is more effective at preventing avocado browning than citric acid.  Which is an interesting wrinkle, because lemons contain significantly more vitamin C than limes do...

EDIT 2: And along comes Jolenealaska, with an experiment that seems to undermine the entire assumption that acid reduces browning of avocado at all!  Seems like the real secret may just be keeping your avocados either uncut, or refrigerated and tightly wrapped if you have to cut them ahead of service.