Sample Preparation Guide pt5: Samples Containing Calcium

• The Metal
• Ca Oxide, Hydroxide, Carbonate
• Minerals and Ores
• Alloys
• Organic Matrices
• Hydrolytic Stability and Preferred Matrices
• Detailed Elemental Profile
• Further Reading

The Metal

Calcium (Ca) is a silvery white metal resembling aluminium. The metal is a powerful reductant and is used to produce some metals by reduction of their oxides. The metal is stable in dry air but burns with incandescence in moist air. The metal decomposes water, evolving H₂ and forming Ca (OH)₂. The same reaction occurs more readily in acids—exercise caution.

Ca Oxide, Hydroxide, Carbonate

Calcium oxide combines with dilute acids to form the ions of the corresponding calcium salt. It absorbs CO₂ from the air, becoming CaCO₃. In moist air CaO becomes Ca(OH)₂, the reaction taking place rapidly, with increase in volume and generation of significant heat if sufficient water is present. The Ca(OH)₂ dissolves readily in dilute acids and is much less soluble in water than the corresponding Ba and Sr hydroxides.

The oxide, hydroxide and carbonate are all readily soluble in dilute acids. Most analysts prefer dilute (1:1) nitric acid.

Minerals and Ores

Calcium occurs combined as carbonate, sulfate, phosphate, fluoride, silicate, and in a large number of complex compounds associated with a number of elements, among which are silicon, iron, aluminium, boron, titanium, sodium and potassium. Calcium is found in all mineral springs, artesian and river water, primarily as bicarbonate. As oxalate, it occurs in plants, as phosphate in bones, and is an essential constituent of many rock-forming minerals.

Since the Ca minerals contain such a wide assortment of other elements, it is considered best to prepare Ca containing minerals/ores by fusion with either lithium carbonate in graphite crucibles or sodium carbonate in Pt crucibles.
The fuseate is dissolved in dilute HCl. The following method is for the preparation of limestone for Ca, Mg, Fe, Al, Mn and Si using a lithium carbonate fusion: Sample Preparation Procedure.

When Ca is associated with fluoride as CaF₂ (fluorite or fluor spar), the carbonate fusion will not work and it is suggested that the analyst fuse the sample with H₃BO₃ or B₂O₃ . This fusion is carried out in Pt crucibles at 1000 °C where the sample to flux ratio is at least 1:8 and can be as high as 1:30. The CaF ₂ is opened out by this fusion procedure and the fluoride is tied up with the boron making the dissolution of the fuseate in water or dilute acid possible without reformation of the CaF₂.

Lithium tetraborate (Li₂B₄O₇) has proven to be a very useful way of opening out any of the minerals associated with Ca including the fluorite. This fusion is carried out in Pt for ~ 10 minutes at 1000 °C and the fuseate is dissolved at room temperature by stirring with dilute (5 % v/v) nitric acid. When metal ions are present that are readily hydrolyzed, dissolution in the presence of EDTA in 0.01 M HCl is advantageous.

Alloys

Ca metal is not used to a great extent nor are it's alloys. Although commercial alloys are relatively rare, their dissolution can be effected using dilute nitric or nitric/HCl mixtures.

Organic Matrices

This includes a wide variety of materials including oil additives, petroleum matrices, coal, organic plant material, biological material, synthetic organics, and more. Samples can be digested with nitric/perchloric. For more detailed information about acid digestions of organics, please see the following article: Acid Digestions of Organic Samples.

It is also very acceptable to dry ash organic samples for Ca analysis in a Pt crucible and then bring the resulting CaO into solution using a sodium carbonate fusion. Or if Ca alone is sought, dissolution in dilute nitric or HCl. For more information, see the portion of our Trace Analysis Guide that discusses Ashing.

The following is a general guide for carbonate fusions:

1. Make certain that the sample is well mixed with the sodium carbonate.
2. A 5-9's pure sodium carbonate is recommended and available from EM Science.
3. Mix the sample with the flux at no more than a 1:20 ratio. Typical sample to flux ratios are in the 1:10 area.
4. If organic matter is present, either the sample is mixed with the flux initially and heated slowly to 500 °C for ~ 2 hours before bring up to full temperature, or the sample can be pre-ashed at 500 °C and then the ash mixed with the flux.
5. Use Pt as the crucible container material.
6. Perform the fusion at 1000 °C in a muffle furnace. Avoid flames since this fusion is difficult to perform in a flame due to the high melting point of the sodium carbonate.
7. Most fusions are complete in 15 minutes and some require up to 45 minutes.
8. Dissolve the fuseate in dilute HCl (1:1).

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.

Further Reading

• Part 6: Samples Containing Strontium or Barium
• Sample Preparation Guide: Table of Contents
• More Guides and Papers