Alternatives to Hydroflouric Acid for Quartz Analysis

Dust in the workplace contains quartz that causes silicosis.

Quartz comprises 20 percent of the Earth’s crust. Crystalline silica is mostly quartz with small additions of crystobalite and tridymite, other crystalline versions of silica. It is a human carcinogen and a serious health hazard to workers in mining, foundry works, blasting and agricultural industries among others. Inhalation of silica causes silicosis, a disabling and sometimes fatal respiratory disease. Quartz analysis is the standard name for crystalline silica analysis. It is a scientific procedure that developed from the need to assess airborne concentrations of quartz in the workplace and the quartz content of bulk materials. The method aims for an accurate quantification of small quartz concentrations in samples from a working environment.

Sample Preparation

The first step in quartz analysis is to collect a sample of airborne dust from the required location. The dust collects on a mounted filter that is protected from the wind. The sample will be a mixture of crystalline quartz, silicate minerals, other minerals such as calcite and amorphous silica. This last compound is a version of silicon dioxide that can coat around crystalline quartz and affect the analysis results. It removal prior to the analysis is necessary.

Hydrofluoric Acid

Hydrofluoric acid is a highly corrosive acid that penetrates human skin and damages bones. Burns from small amounts of this acid can be lethal. It dissolves glass and all versions of quartz. A dilute solution of hydrofluoric acid can dissolve silicate compounds such as amorphous silica surrounding crystalline quartz in a dust sample. It leaves a pure quartz residue for analysis under a microscope. This method is mostly obsolete and has been replaced by safer procedures.

Infrared Spectroscopy

Infrared spectroscopy is a method of determining the chemical nature of a compound through its reactions to infrared radiation. A compound is identified by the wavelength of its response to radiation. It can quantify quartz concentrations in a sample even if contaminants such as calcite, clay or amorphous silica are present. This avoids the use of hydrofluoric acid for sample preparation.

X-Ray Diffraction

X-ray diffraction is the most popular, precise and fastest method for quartz analysis that also avoids the use of hydrofluoric acid. It relies on the large scale crystalline structure of quartz that differentiates it from other silicate compounds. Diffraction is the bending and spreading of radiation as it impinges on an obstacle. The crystalline structure of quartz is different from that of other compounds and contaminants in the dust sample. This property diffracts the X-rays in a different manner and permits its concentration to be quantified.

References

• Occupational Safety and Health Administration: OSHA Fact Sheet; What is Crystalline Silica?
• Fun Science Gallery: Dust
• Occupational Safety and Health Administration: Silica Advisor; Frequently Asked Questions
• Occupational Safety and Health Administration: Review of Quartz Analytical Methodologies; F.A. Madsen et.al; 1995
• University of Washington: Quartz Separation by Selective Etching with HF

Resources

• National Coal Ash Board Israel: Occupational Health Aspects of Quartz; Y. Nathan et.al.
• Berghof: Theory of Sample Preparation Using Acid Digestion
• Centers for Disease Control and Prevention: Determination of Airborne Crystalline Silica; R.J. Key-Schwartz et.al.