[purple121212]

In my lab that I designed (it's a requirement in my chemistry course) I reacted ground calcium carbonate supplements and various concentrations of hydrochloric acid. I got these results: basically, the pH drops a lot when I initially add the HCl, but then slowly increases. I think this has to be because of the presence of some sort of buffer in the tablets, but I'm having trouble figuring out what. The ingredients of the tablets are: Vitamin D3 (as cholecalciferol), calcium (as calcium carbonate from oyster shell), sodium, microcrystalline cellulose, vegetable glycerin, vegetable magnesium stearate, titanium dioxide (color), gelatin. Does anyone have an idea of what could be responsible for the results on my graph?

[AWK]

The reaction of CaCO₃ with HCl goes in two steps. Ca(HCO₃)₂ acts as a buffer solution.