Sample Preparation Guide: Part 35 By Paul Gaines, Ph.D
General Information
Occurrence — Gallium is the third Group IIIA element, with an atomic number of 31. The average abundance of Ga is 19 ppm in the earth's crust, 1.9-29 ppm in soils, 0.09 ppb in streams, and <0.1 ppm in groundwater.³ In nature, gallium typically exists in the +3 oxidation state. Ga occurs in zinc ores, and was discovered by Paul-Émile Lecoq de Boisbaudran while observing spectra of sphalerite.²
Chemical Properties — Gallium is quite soft, and has a low melting point of about 30 degrees Celsius. It exists as a mixture of 69Ga (60.4%) and 71Ga (39.6%). Chemically, its behavior is similar to aluminum.
Uses — Gallium is used in semiconducting devices and for neutrino detection, and has also been considered for heat-exchange use in nuclear reactors.
Ga Chemistry as Practiced & Observed at IV
Inorganic Ventures uses a gallium metal starting material. Dissolution is performed in-house, and the starting material purity is found to be 99.9999% using ICP-OES and ICP-MS. The trace metallic impurity data can be found in Section 5.0 of the Certificate of Analysis. Our stock offerings, CGGA1 and CGGA10, are preserved in 7% v/v HNO3.
Sampling and Handling
Stability — We have found that 2-100 ppb levels of Ga are stable for months in 1% HNO3, and 1-10,000 ppm Ga solutions are chemically stable for years in 2-5% HNO3. All stability studies were conducted in low-density polyethylene (LDPE) containers.
Ga is soluble in HCl, HNO3, and H2SO4, and stable with most metals and inorganic anions. Neutral media should be avoided. The fluoride is insoluble in water, but soluble in HF. Gallium will expand upon solidification; therefore, the starting material should not be stored in glass or metal containers, due to risk of rupture.
Contamination Risks — Gallium is not a common contaminant in labware or reagents. However, analysts should always take precautions against cross-contamination between samples containing Ga. For more information about common contamination concerns, please see Chapters 8, 9, and 10 of the Inorganic Ventures Trace Analysis Guide.
The Metal, Oxides, Ores and Organic Matrices
Metal — Ga metal is best dissolved in HCl/HNO3. If Ga metal is exposed directly to HNO3, a protective gallium oxide barrier will form.
Oxides — For Ga2O3, we typically recommend a Na2CO3 fusion in platinum crucibles. For more information on sample preparation by fusion, please see Chapter 13 of the Inorganic Ventures Trace Analysis Guide.
Ores — A carbonate fusion in platinum crucibles, followed by HCl dissolution, is common.
Organic Matrices — We recommend a H2SO4/H2O2 digestion, or a HNO3/H2SO4/HClO4 decomposition, or dry ash followed by dissolution in dilute HCl.¹ For more information on acid digestions of inorganic samples, please see Chapter 11 of the Inorganic Ventures Trace Analysis Guide.
Testing Methods
Electrothermal Atomic Absorption methods are typically recommended for Ga. While Ga is not mentioned in methods for Inductively Coupled Plasma, it can be used and successfully tested by ICP-OES and ICP-MS in most cases. The user should take note of the following interferences:
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