Silicon

Location:  Group 14, Period 3
Atomic Weight:  28.0855
Coordination Number:  6
Chemical Form in Solution:  Si(OH)_x(F)_y^2-

^{Intended Use: For the calibration of analytical instruments and validation of analytical methods as appropriate.}

Storage & Handling:  Keep tightly sealed when not in use. Store and use at 20 ± 4°C. Do not pipet from container. Do not return portions removed for pipetting to container.

Chemical Compatibility:  Soluble in HCl, HF, H₃PO₄, H₂SO₄, and HNO₃ as the Si(OH)_x(F)_y^2-. Avoid neutral to basic media. Unstable at ppm levels with metals that would pull F^- away (i.e. - do not mix with Alkaline or Rare Earths, or high levels of transition elements unless they are fluorinated). Stable with most inorganic anions with a tendency to hydrolyze forming silicic acid (silicic acid is soluble up to ~ 100 ppm in water) in all dilute acids except HF.

Stability:  2-100 ppb levels - stability unknown - (alone or mixed with all other metals) as the Si(OH)_x(F)_y^2-. 1-10,000 ppm single element solutions as the Si(OH)_x(F)_y^2- chemically stable for years in 2-5% HNO₃ / trace HF in a LDPE container.

Si Containing Samples (Preparation & Solution):  Metal (soluble in 1:1:1 H₂O / HF / HNO₃); Oxide - SiO₂, amorphic (dissolve by heating in 1:1:1 H₂O / HF / HNO₃); Oxide-quartz (fuse in Pt⁰ with Na₂CO₃); Geological Samples (fuse in Pt⁰ with Na₂CO₃ followed by HCl solution of the fuseate); Organic Matrices containing silicates and non volatile silicon compounds (dry ash at 450°C in Pt⁰ and dissolve by gently warming with 1:1:1 H₂O / HF / H₂SO₄ or fuse / ash with Na₂CO₃ and dissolve fuseate with HCl / H₂O); Silicone Oils - dimethyl silicones depolymerize to form volatile monomer units when heated (measure directly in alcoholic KOH / xylene mixture where sample is treated first with the KOH at 60 - 100°C to "unzip" the Si-O-Si polymeric structure or digest with conc. H₂SO₄ / H₂O₂ followed by cooling and dissolution of the dehydrated silica with HF). Note that the direct analysis of silicone oils in an organic solvent will result in false high results due to high vapor pressure of volatile monomer units such as hexamethylcyclotrisiloxane. The KOH forms the K₂⁺Si(CH₃)₂O=NaCl, which is not volatile at room temperature.