The alkali metals, lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs), are all extremely light in weight, compressible, and relatively, very soft. Lithium is the hardest of the alkali metals with a hardness of 0.6 on Mohs' scale. Freshly cut surfaces of the alkali metals show a silvery white luster, which tarnishes quickly and becomes dull in air.
All of the alkali elements are univalent (+1), electropositive, and form strong bases. They react with and decompose water, the reaction increasing in vigor with atomic number. Potassium, rubidium and cesium ignite when placed on water. The resulting alkali hydroxides are very soluble in water and are essentially completely dissociated in aqueous solution. Typically, analysts prefer to introduce neutral or acidic solutions of these elements to avoid attack of the glass and quartz introduction components of their introduction systems. The use of nitric acid is most common in the neutralization of the hydroxides but HCl will work equally well. Other acids are less commonly used for matrix adjustment and perchloric acid is avoided due to the relative insolubility of its salts with K, Rb, and Cs.
The alkali group elements have a chemistry that is more similar than any other group in the Periodic Table. For this reason, the preparation chemistry will be presented as a group rather than for each individual element.
Minerals — Natural and Manufactured
This procedure assumes silicate minerals and rocks, glass, ceramic material, etcetera.
- First, make certain that the sample is finely ground. Grind to pass 200 mesh if possible. An Agate mortar is recommended.
- If organic matter is suspected, weigh 0.5 to 1.0 grams of sample into a Pt crucible and muffle at 450 °C overnight.
- The fundamental approach is to dissolve the mineral in a mixture of a high boiling strong mineral acid and HF. The use of sulfuric acid as the 'high boiling mineral acid' is most common. The purpose of the high boiling strong acid is to catalyze the action of the HF and to allow for the HF to be boiled off (as H2SiF6 and excess HF) if an HF resistant introduction system is not available. Otherwise, the mineral is simply heated with the acid mixture until complete. I have found that adding sufficient water to thoroughly wet the mineral by forming a slurry speeds up the reaction and that the use of nitric or HCl is acceptable, provided that the HF is not to be boiled off. This reaction can be carried out in Teflon or Pt with the understanding that the Teflon does not allow the analyst to bring the sulfuric acid to fumes and the Pt does not allow the analyst to use HCl/nitric acid combinations.
I prefer to use sulfuric and HF only in Pt if the HF is to be boiled off. For a 0.5 gram sample ~2 mL water, ~5 mL of the mineral acid and ~10 mL of HF are used. For high surface are minerals (~400 m2/g) such as zeolites, the dissolution is within a few seconds and no heating is required. For lower surface area minerals (~4 m2/g) heating is required. Teflon screw cap bombs and a water bath are useful provided the HF does not have to be boiled off.
- For an alternate method of eliminating the excess HF see the following article: Elemental Analysis of Zeolites
- For the analyst who prefers a fusion over acid digestion, it is recommended that Li ores and minerals be fused with high purity 99.999% sodium carbonate (available from EM Science) and that the Na, and/or K, and/or Rb and/or Cs ores and minerals be fused with high purity lithium carbonate. Please see the following article for more detailed information concerning carbonate fusions: Sample Preparation by Fusion
Water and Acid Soluble Material
This category includes oxides/hydroxides, salts, brines, saline residues, mineral waters, carbonates, etc.
Dissolve the sample in water using a slight excess of nitric acid and dilute to a specified volume. In the case of alkali carbonates, the solution should be boiled after acidification to expel carbon dioxide before making to volume. The presence of dissolved carbon dioxide or other gases can cause poor precision due to gas bubble formation in some nebulizers during sample introduction.
This includes a wide variety of materials including fertilizers, agricultural material, organic plant material, biological material, synthetic organics, etc.
The crucible can be a variety of materials including Pt, quartz, fused silica, porcelain, or glassy carbon. Samples high in Li or K will attack Pt and all of the alkali metal oxides attack silica, quartz and the silica glaze on porcelain. It is always suggested that the sample be treated with sulfuric acid either before charring or just after charring and before muffling, which eliminates alkali oxide formation during muffling and consequently alkali attack on the crucible. It is best to use a minimum of sulfuric since the goal is to encourage sulfate formation but rid the sample of all excess sulfuric acid before placing in a muffle furnace, otherwise the formation of SO3 will contaminate the air. Muffling (ashing) temperatures of 450 to 500 °C are suggested. The alkali sulfates can tolerate higher temperatures, but there is no need to push limits. Temperatures approaching 700 °C risk formation of the alkali oxide (decomposition is catalyzed by certain elements/compounds) with release of SO3 and serves no real purpose.
The following is a general outline for performing a wet ash or sulfated ash:
- Weigh the sample into your crucible (typically porcelain) and then wet the sample with concentrated sulfuric acid. Only add enough sulfuric acid to just wet the sample. Excess sulfuric acid will require extra time in fuming off before placing in a muffle furnace to achieve ashing temperatures.
- Place the sample on a hot plate and heat until the fumes of sulfuric acid (dense white fumes) stop. Proceed with caution: sulfur oxides are very irritating to the respiratory system and the elimination of all excess sulfuric acid as well as muffling in a ventilated area is important. If the sample is an oil there will be foaming - be on guard and only fill the crucible to 10 % of capacity (for a 60 mL crucible add only ~6 grams of sample and 0.5 grams of sulfuric acid).
- Muffle the sulfated char at 450 °C overnight or until the residue appears free of carbonaceous material. Carbonaceous material is typically signaled by the formation of coke-like material that is black in color.
- The appearance of the ash should lighten and will approach white to tan over time. If certain transition elements are present, the color may appear to get darker with time due to the formation of the transition metal oxides (Ni and Co oxides are very dark). If this occurs, the sample will need to be "resulfated" - cooled and wetted with sulfuric acid - in order to avoid the difficulties encountered with the dissolution of ignited transition metal oxides.
- Weigh the sample in the crucible and char it on a hot plate. The samples will turn/appear as carbon black or coke.
- After charring, wet the char with sulfuric acid. This typically takes only 1-3 drops of the concentrated acid. Continue to heat the sulfated char until fumes of sulfuric acid cease to evolve and then muffle at 450 °C overnight.
Hydrolytic Stability, Preferred Matrices and More
All of the alkali metals are soluble over the entire pH range. In addition the use of any mineral acid, water or caustic, is acceptable with the following exceptions. For K, Rb and Cs, avoid chloroplatinate, acid Tartrate, fluorosilicate, picrate, phosphomolybdate, perchlorate, periodate, fluorotitanate, fluorozirconate. For Na, avoid the fluorosilicate. All Li salts are soluble.
Attempt to use LDPE that has been pre-leached with dilute nitric acid as the container material whenever possible. Avoid glass due to the possibility of contamination with Na. The alkali metals are chemically compatible with all other elements and most inorganic molecular ions. In addition, spectral interferences upon other elements and from other elements are minimal with the Rare Earth elements presenting the worst interference issues when using ICP-OES.
When using axial view ICP-OES, be wary of ionization effects similar to that observed in flame AAS. Enhancements in the signal as large as a factor of three in matrices high in other elements, such as Cu for example, are possible. This ionization effect gets worse as the atomic number of the alkali element increases. Cs is often used as an ionization buffer with axial view ICP-OES for the determination of the lighter alkalis (usually Li, Na, and K). However it must be very pure, as in Inorganic Ventures' 1% Cesium Ionization Buffer.
ICP-MS is often the recommended technique for the determination of Rb and Cs where detection limits in the ppq region are common. The alkalis are also quite suited to measurement using ion chromatography because their tendency to form complex species is minimal at best.
Inorganic Ventures' QC laboratory uses axial view ICP-OES for trace Li, Na, and K determinations and ICP-MS for trace Rb and Cs determinations. Ion Chromatography or axial view ICP-OES are used for solutions where the alkalis are predominate and have the same order of magnitude in concentration to one another and/or other elemental components.
The following table contains suggested lines for ICP-OES and ICP-MS and typical detection limits:
|Element||ICP-OES axial view emission line (nm)||ICP-OES DL (µg/mL)||ICP-MS Isotope (amu)||ICP-MS DL (ng/mL)|
Detailed Elemental Profiles
Chemical compatibility, stability, preparation, and atomic spectroscopic information is available by clicking an element below. For additional elements, visit our Interactive Periodic Table.