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As we had a question concerning the salifert ca tests, I take the opportunity to explain how Ca and Mg tests works. This understanding make it more easy to evaluate different tests. I am sure someone has already gone through this, so in that case apologize for a potential repetition.
The colour agent molecule has some hydrogen ions bound to it, and to work it must release some of them. Therefore we must always raise pH to a least 9 ish. At that pH the colour dye release some hydrogen ions, and now works as we want:The colour has a high affinity to Ca and Mg ions, so as long as those are present in solution, the colourdye will take one Ca or Mg ion, and when thats happen the colour molecule will turn red. Now we add EDTA, which has even higher affinity to Ca and Mg than the colour molecule. First EDTA absorb the free Ca and Mg ions, and after that the colour-bounded ones, and when last ion is released from the colour dye, colour molecule turns back to blue. EDTA "absorb" Ca and Mg in a molar ratio of 1/1 so if you used X molar to convert the colour, it was X molar Ca+Mg in the solution.
This EDTA titration gives the sum of the Mg and Ca. To get only Ca, we must first hide the Mg ions, and that is achieved by raise pH even further , to 11-12. That will create Magnesiumhydroxide immediately (cloudy) and now its only free Ca ions in the solution, and we do same titration with EDTA as above.
If we want to do opposite, only titrate Mg directly its not that easy to selectively "remove" the Ca ions, but possible with ca selective absorber, like oxalat for instance (red sea uses this). But its not 100% selective, so its easy to get some Mg bound aswell, OR not bind all Ca, giving easily both false low and high results. Thats why Mg tests are all of them more o less not so accurate.
One way to solve this is to make a total titration of Ca and Mg as I described in the beginning, thus increase pH to only 9, thus high enough to make the colour work, but not so high so we create Magnesium hydroxide. Such a titration will give Mg+Ca. After that we do a separate Ca titration , (those are very accurate as its easy to isolate Mg by just increase pH to 12). The diff between the sum and Ca is ofcourse Mg.
/Jonas
The colour agent molecule has some hydrogen ions bound to it, and to work it must release some of them. Therefore we must always raise pH to a least 9 ish. At that pH the colour dye release some hydrogen ions, and now works as we want:The colour has a high affinity to Ca and Mg ions, so as long as those are present in solution, the colourdye will take one Ca or Mg ion, and when thats happen the colour molecule will turn red. Now we add EDTA, which has even higher affinity to Ca and Mg than the colour molecule. First EDTA absorb the free Ca and Mg ions, and after that the colour-bounded ones, and when last ion is released from the colour dye, colour molecule turns back to blue. EDTA "absorb" Ca and Mg in a molar ratio of 1/1 so if you used X molar to convert the colour, it was X molar Ca+Mg in the solution.
This EDTA titration gives the sum of the Mg and Ca. To get only Ca, we must first hide the Mg ions, and that is achieved by raise pH even further , to 11-12. That will create Magnesiumhydroxide immediately (cloudy) and now its only free Ca ions in the solution, and we do same titration with EDTA as above.
If we want to do opposite, only titrate Mg directly its not that easy to selectively "remove" the Ca ions, but possible with ca selective absorber, like oxalat for instance (red sea uses this). But its not 100% selective, so its easy to get some Mg bound aswell, OR not bind all Ca, giving easily both false low and high results. Thats why Mg tests are all of them more o less not so accurate.
One way to solve this is to make a total titration of Ca and Mg as I described in the beginning, thus increase pH to only 9, thus high enough to make the colour work, but not so high so we create Magnesium hydroxide. Such a titration will give Mg+Ca. After that we do a separate Ca titration , (those are very accurate as its easy to isolate Mg by just increase pH to 12). The diff between the sum and Ca is ofcourse Mg.
/Jonas
