(36e) On the Solubility of Mercury in Liquid Hydrocarbons

Summary. The solubility of Hg^o was determined in 34 pure hydrocarbons, hydrocarbon mixtures, crude oils, and water from -65^oC to +65^oC (where each solvent remained liquid). The solubility of HgCl₂ was also quantified, but in a limited subset of the solvents. The solubility of any Hg species in its respective solvent can be very accurately represented by fitting the data to an exponential function of temperature (E1). A similar volume-based exponential (E2) was best-fit to data for the saturation concentration of Hg^o in air.  By converting the Hg^o solubility equations from a mass basis to a volume basis, and then dividing into E2, temperature-dependent unitless ?Henry's Law constants? (E3) were also be calculated.

For many samples, measurements were made both of Hg^o and total Hg. These data show that previously reported solubility data, made by measuring just total Hg may be biased high by the presence of oxidized Hg compounds, especially if the solvents were not first pre-purified to remove trace amounts of Hg-oxidizing compounds.

Pre-Treatment of Solvents. At the outset of the study, we discovered that some reagent-grade organic solvents rapidly corroded the surface of clean Hg^o, sometimes to the point of leaving only a gritty black powder in its wake. Different brands and grades of the same compound differed widely in corrosivity, suggesting the presence of trace oxidizing compounds. This hypothesis was supported by observations that the corrosion was (a) strongly temperature dependent, and (b) could be eliminated by pre-extracting with a small volume of aqueous NH₂OH?HCl. We did not want to risk artifacts later by using solvents that might harbor traces of the reducing agent, however, so, an alternative pretreatment was employed. Fifty grams of Cu granules were added per litre of solvent, which was then shaken for 24 hours. The Cu reduced, but didn't eliminate the Hg^o corrosion. We never saw signs of corrosion on the Cu surfaces.

Obtaining a Clean Droplet of Hg^o. The best way to obtain clean droplets of Hg^o was to use a 100 mL gas-tight syringe with a fine gauge stainless steel needle to pull liquid from the center of a larger mass, where the mercury is more than a centimeter deep. A 20 mL borosilicate scintillation vial holding 10 mL of Hg^o is excellent for this. Since all of the corrosion and dirt particles are forced to the outside of the mass by the strong bouyancy effects, the Hg in the center is extremely clean.

Analytical Methods. Total Hg in hydrocarbons was determined after first digesting ~0.4 grams of sample with 3.0 mL of concentrated HNO₃ using the Anton Paar High Pressure Asher (HPA) (Bloom, et al. 2004). Samples were wet-combusted for two hours in ultra-clean quartz tubes at 300^oC and 125 atm pressure. After dilution with reagent water, aliquots were analyzed using SnCl₂ reduction, purge and trap on gold, electrothermal desorption of the mercury as Hg^o, and quantification via CVAFS. Hg^o was independently determined after dilution with oxidizer-free hexane. Diluted samples were purged directly after floating up to 200 ml of sample on the surface of 100 mL pre-purged 0.2% HCl in water contained in a bubbler that has never been in contact with Sn(ll). Purging the sample results in the evaporation of the hexane and release of Hg^o onto a gold trap (Bloom, 2000). See Table 1 for a QC summary.

Experimental Setup. Samples for analysis were made by completely filling 125 mL borosilicate bottles with solvent, injecting ~1.0 gram of clean liquid mercury, and tightly replacing the Teflon-lined caps.  Bottles were incubated at temperatures ranging from -65^oC to +65^oC with constant gentle agitation or periodic vigorous agitation depending upon the equipment available. Periodically, the agitation was stopped, and the samples allowed to settle at least four hours prior to aliquoting. As a buffer against temperature fluctuations, samples were kept in 400 mL plastic beakers 70% filled with a

temperature-appropriate inert liquid. This beaker was used to carry the sample from the incubator to and from the analytical balance. The entire handling process generally took less than 2 min, resulting in trivial changes in the temperature and so the Hg^o concentration of the sample. The Hg^o concentration in at least one of the samples per temperature was periodically monitored over days to months, until a constant concentration was observed. The actual quantitative data produced for the study were then collected in replicate digestions two or more days after equilibrium was verified. The solubility of HgCl₂ was also quantified in a smaller subset of the hydrocarbons. The set-up and procedures were analogous to the case with Hg^o, except that in place of the small amount of Hg^o, surprisingly large amounts of HgCl₂ were required to saturate some of the solvents?over 100 grams, in the case of methanol!

A special procedure was employed for the determination of the solubility of Hg^o in water, owing to the ease with which Hg^o is oxidized to Hg(ll) even in extremely carefully prepared water, giving results that can be many orders of magnitude too high. A droplet of clean Hg^o was placed into a bottle filled to the top with ultra-pure argon-purged water to which 100 ppm of stannous citrate (pH 7.0) was added, and the lid sealed on. By adding Sn(ll) to the aqueous sample, the redox potential was forced down to the point that only Hg^o can exist. Aliquots from this sample were then analyzed in the same manner as were the Hg(ll) standards--by SnCl₂ reduction, purge and trap, and CVAFS.

Multiple determinations at each of three or more temperatures were measured for each species/solvent pair, and the concentration expressed as an exponential function (E1) of temperature. The solubility of Hg^o in the entire range of hydrocarbons varies over about a factor of five at a given temperature. The lowest solubilities are found in low MW polar compounds such as water and methanol, and highest solubilities are found in aromatic compounds (Table 2). The solubility of HgCl₂, is generally much greater than Hg^o in any given solvent, and it varies over a factor of 100,000 across the range of solvents at any given temperature. Conversely to Hg^o, the greatest solubility for HgCl₂ is in the most polar in polar solvents (Table 3).

Many hydrocarbons react to form Hg(ll) corrosion products that produce total Hg levels far higher than the true Hg^o concentration (Table 4). Aromatic solvents tended to convert the mercury droplets to a heavy black powder, while most simple alcohols were seen to produce a fine pinkish powder?growing darker with increasing MW. The presence of corrosion products appears sometimes to be due to reaction between Hg^o and the solvent itself, rather than with trace impurities. For example, significant

levels of dissolved Hg(ll) compounds appear to result from almost every oxygen-containing solvent, suggesting that they, themselves, are reacting with the Hg^o, perhaps forming species analogous to Hg(ll) methate (Hg(OCH₃)₂).

References

1.   Bloom, N.S, Parker J, and Vondergeest, E. 2004. RMZ-Materials and Geoenvironment, 51: 590-593.

2.  Bloom, N.S. 2000. Fres J Anal Chem 366: 438-443.