Question

I need to make a carbonic acid/limestone solution for the pH levels: 8.50, 8.25, 8.00, 7.75 and 7.50 to simulate the bicarbonate/carbonate buffer system in marine conditions. How do I find the molarity of carbonic acid to use?/?

At first I was going to make a carbonic acid solution of those levels and add it to a constant amount of powdered limestone but the levels are basic. So I need to incorporate the limestone in the solution to make is basic??