Why is Mg complex less stable than the Ca complex when titrated with edta?

Hi Yama, The answer lies in the fact that Calcium is a nice little house cat and forms a more "stable" bond with EDTA when compared to Magnesium. Magnesium is like an independent alley cat and a little less stable with EDTA at what is called the coordination point. What happes is that EDTA has six sites available to share with another element. And Calcium is very happy to join with EDTA at those sites.. In my home area of Fresno (but I live in San Francisco) this is a very important issue because of the water "hardness" issues in the Great San Joaquin Valley. And EDTA is the most common method of determining "water hardness" Water "hardness" really describes how much "metals cations" are present in the water which can cause all kinds of interesting things; such as forming with soapy anions to form "scale" on refrigeration cooling towers (white muck). So how do you determine the hardness? Well, you look for "Calcium" and "Magnesium" ions! Because those two elements are the most common that exist in "hard" water. And "hardness" is most frequently measured through what is called "chelation". Chelation is simply the the abiltiy of available sites at the chemical structure level and how they combine with other available sites on another chemical structure. The problem is that the Calcium is such a happy house cat that it is very stable and difficult to get out when if forms a "white mucky scale" on cooling towers, piping systems, and other industrial equipment. So I hope I answered your questions. Its all about the details. Go to the references below for a technical discussion.