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How do I pick up the difference between barium, calcium, and magnesium ions? I had to do a quantitative analysis practical exam: I was given an aqueous solution of a salt, and when adding separately dilute sulphuric acid, magnesium chloride, and sodium carbonate to the solution, a white precipitate formed in each tube. The notes say that all three ions make insoluble precipitates, so I don’t know how to get the difference.

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2 Answers 2

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The unfortunate thing is that salts of these ions are white and can't be differentiated by just looking. But you can take solubility to your advantage.

The solubility of the sulfates of alkaline earth metal decreases down the group. So, you can form their sulfates and differentiate based on their solubility.

\begin{array}{c|c} \mathbf{Salt} & \mathbf{Solubility~at~r.t.} & \mathbf{Inference}\\\hline \text{Magnesium sulfate} & \text{$\ce{35.1 g/100 ml}$} & \text{Very Soluble} \\ \text{Calcium sulfate} & \text{$\ce{0.26 g/100ml}$} & \text{$\ce{Slightly Soluble}$}\\ \text{Strontium sulfate} & \text{$\ce{97 mg/l}$} & \text{$\ce{Almost Insoluble but not negligible}$}\\ \text{Barium sulfate} & \text{$\ce{2.5 mg/l}$}& \text{Insoluble}\end{array}

You can make oxalates of ions and then dissolve in hot acetic acid. Barium oxalate is readily dissolved in acetic acid while calcium and magnesium oxalate are practically insoluble in acetic acid (calcium and magnesium oxalates can still be separated as described here)

Their chromates can be considered but I would not recommend as chromates are toxic and carcinogenic. Barium chromate is practically insoluble in water and acetic acid while calcium and magnesium chromate are soluble in water and acetic acid (the latter is more soluble).

But the most trustworthy test is flame test. Barium - pale green ; Calcium - Brick red ; Magnesium - Salts are colorless due to formation of oxide layer

Previous discussions:

  1. How to separate and analyse a sample of cations of alkaline and earth alkaline cations and ammonium?
  2. In qualitative analysis for basic radicals, why are group 2 cations not precipitated along with group 4?
  3. How to test for magnesium and calcium oxide? (how to separate calcium and magnesium salts -- easiest way is turning them to their respective oxide and check its solubility in water)
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Magnesium sulfate $\ce{MgSO4}$ is soluble in water. So when adding some dilute sulphuric acid $\ce{H2SO4}$ to the solution of magnesium sulphate, no precipitate should occur. It should occur with barium ions $\ce{Ba^{2+}}$ for sure, and with calcium $\ce{Ca^{2+}}$ if the solution is concentrated enough. So sulphuric acid helps deciding if the solution contains magnesium or not.

Solutions of sodium carbonate $\ce{Na2CO3}$ and magnesium chloride $\ce{MgCl2}$ are no use in your exam. Adding sodium carbonate will produce a precipitate with all ions. Adding magnesium chloride will never produce any precipitate.

To distinguish between calcium $\ce{Ca^{2+}}$ and baryum $\ce{Ba^{2+}}$ ions, the traditional method is to add some drops of potassium dichromate $\ce{K2Cr2O7}$ plus $2$ mL sodium acetate $\ce{CH3COONa}$ as a pH buffer. If there is barium in solution, it will produce a precipitate $\ce{BaCrO4}$. Calcium and magnesium do not produce a precipitate in this mixture.

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  • $\begingroup$ Magnesium chloride could form a precipitate from some calcium or barium bearing solutions, especially if the solution is basic. $\endgroup$ Sep 21, 2022 at 14:55
  • $\begingroup$ @ Oscar Lanzi. Yes, if the solution is basic. Is it basic ? $\endgroup$
    – Maurice
    Sep 21, 2022 at 16:10
  • $\begingroup$ Unknown from the information given. $\endgroup$ Sep 21, 2022 at 16:11

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