In my analytical chemistry lab course my lab partner and I are trying to determine the percent calcium by mass of an antacid tablet by EDTA (mixed with 0.001M MgCl2) titration using an EBT inidcator. We have ran into a problem which is we've got poor results which we refuse to accept. Long story short we calculated the percent calcium to be 30% higher than it actually is. We think the magnesium in the antacid tablet is throwing things off. Since the endpoint we observe through color change is cause by magnesium replacing the calcium bound to the EBT the color change wont occur exactly when all of the calcium is titrated by the EDTA. Basically we think the magnesium in the tablet is causing the sample to 'titrate itself,' for lack of a better term. We know how much magnesium and calcium actually is in the tablet so if that assumption is true is there a way we can factor that in? Any help would be greatly appreciated. Thanks!