|MadSci Network: Biochemistry|
This sounds like a very good science project for your grade level. The acid in the stomach is part of gastric juice and is HCl (hydrochloric acid) It has a pH range in the stomach of between 1 & 3 so it is a VERY strong acid. You should therefore be VERY careful when you are handling it as it can burn skin, clothes, etc. The amount that you use is probably not very important as long as it is consistent for all of your set ups.
The amount of gastric juice normally secreted per meal is about 700 mls. and about 1-3 liters per day. The juice is not made entirely of HCl, as gastric juice contains several other ingredients including water, electrolytes, digestive enzymes and some mucus. The amount of acid you use should be constant but the absolute amount, as long as it is enough for the antacid and not too much for it, is not critical. You may want to experiment a little with different amounts of acid before you start your actual project. Also, if possible, your results will be more accurate if you can use a pH meter or at least hydrion paper. Both of these methods will tell you a pH, not just whether it is acid or base like litmus paper will tell you. Hydrion paper works pretty much like litmus paper but will tell you a pH range, like 2-3 or 9-10. This is done by comparing its color with colors on the paper's container. If your school doesn't have any, perhaps the local high school or community college does. Be sure to do every step the same in each test. You may want to set up a control too, such as a tube of acid with no antacid added just to show the color or reading for its pH. Good luck with your project. I hope you continue your interest in Biology. Its a very interesting field to learn about.
Try the links in the MadSci Library for more information on Biochemistry.