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SOLUBLE AND INSOLUBLE METAL COMPOUNDS The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLVs) [4] list metals and metalloids having soluble TLVs; updates are published annually. Those from 2002 are listed in Table 1. For most of these elements, the TLVs for the soluble compounds are lower than for the corresponding insoluble forms (Aluminum [Al], Silver [Ag], Hexavalent Chromium [Cr VI ], Molybdenum [Mo], Nickel [Ni], Platinum [Pt], Rhodium [Rh], Tungsten [W]), while for the remainder (Barium [Ba], Iron [Fe], Thallium [TI]) the TLVs for soluble and insoluble compounds are the same [3]. Several countries (e.g., France, Germany, UK, USA) have also adopted workplace exposure standards for various soluble metal compounds.

TABLE 1. METALLIC ELEMENTS FOR WHICH SOLUBLE COMPOUNDS HAVE BEEN ASSIGNED TLVs.3 Element and Soluble Compounds Thereof

TLV, Time-Weighted Average (mg/m3 ) 2 0.5 0.05 1 0.5 0.1 0.002 0.01 0.01 0.1 1 0.2

Aluminum - Soluble salts, as Al Barium - Soluble compounds, as Ba Chromium - Water-soluble CrVI compounds Iron - Soluble salts, as Fe Molybdenum - Soluble compounds, as Mo Nickel - Soluble compounds, as Ni Platinum - Soluble salts, as Pt Rhodium - Soluble compounds, as Rh Silver - Soluble compounds, as Ag Thallium - Soluble compounds, as Tl Tungsten - Soluble compounds, as W Uranium - Soluble compounds, as U

Solvent. The term “solubility,” as used by analytical chemists, ordinarily pertains to the dissolution of a material in pure water [5]. The subject of water solubility of metal compounds is covered in several references [5-8]. According to Patty’s Industrial Hygiene and Toxicology [7] and the individual TLV documentation for these metals [6], the solubilities of metals and metal compounds are quite variable depending upon the solvent. Other pertinent references sustain the notion that solubility, regarding metals and their compounds, is generally identified in terms of their solubility in water [6]. The solubility and insolubility of numerous inorganic substances are presented in the Appendix. What is meant by “soluble” depends on the operational definition employed for the extraction conditions desired by the investigator. Temperature. Temperature is another variable that directly affects solubility. Most current analytical methods specify deionized water, but not water temperature (some procedures call for hot water (37°C), but others use water at room temperature). Some important questions thus arise:

a. 3/15/03

If using deionized water, should chemists assure that water temperature has been 168

NIOSH Manual of Analytical Methods