Sample Preparation Guides

General Information

Occurrence – Tin is the fourth Group IVA element, has an atomic number of fifty, and an atomic mass of 118.70. Tin is most commonly found in nature in the form of tin (IV) oxide (SnO₂), also known as stannic oxide or cassiterite. Its average abundance in the earth’s crust is relatively low at only ~2ppm Sn, 10 ppm in soils, 0.1 ppb in streams and <0.1 ppm in groundwater.¹ The majority of tin is mined in Southeast Asia.

Uses – Tin has been used for centuries as an alloy with copper to produce bronze. Its most common use today is as a protective coating for steel or other metals to prevent corrosion. It is also used in solders, magnets, electronics, glass manufacturing, and to produce alloys with other metals.

Chemical Properties – Tin primarily forms six-coordinate compounds, although coordinations of four, five, seven, and eight have also been observed. It exists in the +2 and +4 oxidation states. Sn²⁺ can be oxidized to Sn⁴⁺ over time by the oxygen in the air. Complexes containing Sn⁴⁺ are much more readily hydrolyzed than those containing Sn²⁺, forming a semi-colloidal suspension of tin (IV) hydroxide.

Organotin compounds will adsorb to sediments. This is important for environmental sampling as much of the tin present may be found in the bed of the water systems. Additionally, tin can build up in soils and water systems over time, increasing the risk of toxicity to species in the area. Plants can also absorb tin from the soil.

Sn Chemistry as Practiced & Observed at IV

Inorganic Ventures uses metallic Sn° shot as the starting material for our Sn standards, with a documented purity that is confirmed to be 99.99+% using ICP-MS and ICP-OES.  The pure metal is dissolved in either concentrated HNO₃ and HF, or in concentrated HCl. Impurities found in each lot (measured using ICP-MS and ICP-OES) are reported on the Certificate of Analysis.

Sampling and Handling

Stability –  Sn is not stable in solution under basic pH conditions and should be preserved in acid. If HCl is used to stabilize Sn in solution, a large excess of HCl will be required to prevent oxidation/hydrolysis.

Stability is best maintained by adding a small amount of HF. Sn will not be stable in HNO₃ alone as it will oxidize to form insoluble SnO₂. Sn will be stable in solution with both HCl and HNO₃ if there is a small amount of HF present. For general information on sampling and sub-sampling see Part 3 of the Inorganic Ventures Trace Analysis Guide.

Contamination Risks - HF will cause leaching of contaminants if glass is used in the analysis (i.e. containers, reaction vessels, analytical instrument components, etc.) as HF is caustic to glass. If HF-resistant materials (plastics such as LDPE, HDPE, Teflon, etc) cannot be used, HF can be neutralized with triethanolamine (TEA) to avoid this issue. TEA should be added on a 1:1 molar basis to a slight excess, or until the pH increases to 7-8. The fluoride ion itself does not attack glass; HF does.

For more on sample contamination risks see chapters 8, 9 and 10 of the Inorganic Ventures Trace Analysis Guide.

The Metal, Alloys, Ores, Oxides, and Organic Matrices

Metal – Tin metal is soluble in a mixture of HNO₃ and HF or in HCl alone. However, if HCl is used as the digestion acid, it is recommended to perform the reaction in a closed vessel or heat under reflux to avoid loss of Sn. Tin (IV) chloride is volatile and has a boiling point of only 114^OC.

Alloys – A common digestion method for tin alloys uses H₂SO₄. The alloy is first treated with concentrated H₂SO₄ (~100mL per gram of sample), followed by boiling until the sample disintegrates and nearly all H₂SO₄ is expelled. The solution is then diluted with either HCl (2:1 HCl:H₂O), or concentrated HF (~10mL per gram of sample) and heated gently.

Ores – A fusion method with sodium carbonate (Na₂CO₃) is preferred for ores, followed by dissolution with HCl and a small amount of HF. Na₂CO₃ is clean, easy to use, and allows for use of Pt crucible. Other appropriate fusion mixtures include Na₂S with S (following oxidation with HNO3), and Na₂O₂ with NaOH. Please use extreme caution when using peroxides such as Na₂O₂ as they are explosive and can be very dangerous.

Oxides – Stannous oxide (SnO) is soluble in HCl. Stannic oxide (SnO₂), on the other hand, is insoluble and very resistant to all acids, including HF. The best option for stannic oxide is a fusion with Na₂CO₃, followed by dissolution in HCl or HCl and HF. Fusion with equal parts Na₂CO₃ and S is also soluble in water or dilute acid as the thiostannate.

Organic Matrices – Organotin compounds are toxic, and caution should be used when working with these materials. Dry ashing is NOT recommended to avoid errors with evaporation of SnCl₄ or reaction between SnO₂ and the crucible. One option we recommend involves and oxidation with H₂O₂ and H₂SO₄. The sample is first heated with concentrated H₂SO₄ to fumes, then 30% H₂O₂ is added dropwise to finish the oxidation. Make sure there is always an excess of H₂SO₄ in the reaction vessel. Be patient with this reaction and proceed slowly, taking extra precautions to avoid the build-up of H₂O₂ in the solution as this can lead to an explosion. This method works particularly well for PVC (polyvinyl chloride).

Testing Methods

Flame atomic absorption may be used, though the electrothermal method may be preferable due to Flame AA’s high detection limits.

ICP-MS and ICP-OES are also recommended, keeping the following interferences in mind.

The 189.926/189.989 nm line is the best with respect to sensitivity and freedom from spectral interferences. The ability to measure wavelength (189.926 nm or 189.989 nm) may be dependent on the optics system used in the instrument. Additional masses for ICP-MS include 117, 118, and 119 amu. Start out with 10-20% HCl for the rinse solution. If washout issues occur, adding a small amount of HF will help purge Sn from the system. No more than 0.1-2% HF for ICP-OES systems and no more than 0.05-0.5% HF for ICP-MS systems (glass systems should not exceed 0.2% HF). Replacing the sample tubing will also help with washout as this is where most of the “sticking” of Sn occurs.

1. Standard Methods for the Examination of Wastewater. (2012). Washington, DC: American Public Health Association.

2. Bock, R. (1979). A Handbook of Decomposition Methods in Analytical Chemistry. Weinheim/Bergstr: Verlag Chemie GmbH.