Geochemistry

The Natural Lowdown on pH

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Science  11 Feb 2000:
Vol. 287, Issue 5455, pp. 933
DOI: 10.1126/science.287.5455.933d

Measurements of pH usually lie in the range of 0 to 14, and this scale normally covers variability in the laboratory and the natural world, where neutral pH is considered to fall in the middle of this scale. Solutions with negative pH are possible theoretically but beyond the range of most sensors and buffers.

Nordstrom et al. developed a pH sensor that operates under extremely acidic conditions and used it to identify water in the environment with a pH as low as -3.6. These low pH ground waters occur at Iron Mountain, California, in an abandoned mine that is now a Superfund site. The authors suggest that the interaction of ground water with pyrite (FeS2) produces sulfuric acid and that evaporation in the hot mine environment further concentrates hydrogen ion. Such acidic waters have extremely high levels of dissolved metals and sulfate.—BH

Environ. Sci. Technol.34, 254 (2000).

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