Samples Containing Calcium

Hydrolytic Stability and Preferred Matrices

• Ca begins to precipitate (as the carbonate due to dissolved CO₂) from solution at a pH of between 6.7 (high conc. Ca) to 10 ( low conc. Ca).
• CaO, hydroxide, carbonate, fluoride, oxalate and sulfate (dilute solutions soluble) are all insoluble in water and neutral to basic media. Precipitates are formed with neutral solutions of arsenite and arsenate as well. The most difficulty with solubility comes with the fluoride salt (very insoluble) and to a lesser extent the sulfate salt (moderately soluble) which are insoluble in acidic as well as neutral or basic media.
• Due to the presence of CO₂ most solubility studies of the Ca⁺² ion appear as the carbonate.
• The following table shows the improvements in the hydrolytic stability of Ca⁺² with different complexing agents. The pH where precipitation of CaCO₃ begins is shown for 0.1 M solutions of each complexing agent:

• Ca can be mixed with any of the elements at high concentrations (200 to 2000 µg/mL) with the exception of the fluoride containing elements (Ti, Zr, Hf, Nb, Ta, W, Si, Ge, Sn, Sb, Mo). Moderate to low levels (= 100 µg/mL) can be mixed with all of the elements except moderate to high levels of fluoride (F^-).
• Ca has a relatively simple emission spectrum. A popular Ca emission line is the 183.801 nm line with a DL of ~ 0.005 µg/mL (axial view) because of it's relative freedom from spectral interference combined with high sensitivity.

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