Editing PH (section)

=== Extremes of pH measurements ===
{{Redirect|Negative pH|the band| Negative pH (band)}}
The measurement of pH can become difficult at extremely acidic or alkaline conditions, such as below pH 2.5 (ca. 0.003&nbsp;[[Mole (unit)|mol]]/dm<sup>3</sup> acid) or above pH 10.5 (above ca. 0.0003 mol/dm<sup>3</sup> alkaline). This is due to the breakdown of the [[Nernst equation]] in such conditions when using a glass electrode. Several factors contribute to this problem. First, [[liquid junction potential]]s may not be independent of pH.<ref name="Feldman2">{{cite journal |author=Feldman, Isaac |year=1956 |title=Use and Abuse of pH measurements |journal=Analytical Chemistry |volume=28 |issue=12 |pages=1859–1866 |doi=10.1021/ac60120a014}}</ref> Second, the high [[ionic strength]] of concentrated solutions can affect the electrode potentials. At high pH the glass electrode may be affected by "alkaline error", because the electrode becomes sensitive to the concentration of cations such as {{chem2|Na+}} and {{chem2|K+}} in the solution.<ref>{{VogelQuantitative}}, Section 13.19 The glass electrode</ref> To overcome these problems, specially constructed electrodes are available.

Runoff from mines or mine tailings can produce some extremely low pH values, down to −3.6.<ref>{{cite journal |last1=Nordstrom |first1=D. Kirk |last2=Alpers |first2=Charles N. |date=March 1999 |title=Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1495&context=usgsstaffpub |url-status=live |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=96 |issue=7 |pages=3455–62 |bibcode=1999PNAS...96.3455N |doi=10.1073/pnas.96.7.3455 |pmc=34288 |pmid=10097057 |archive-url=https://web.archive.org/web/20170923012227/http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1495&context=usgsstaffpub |archive-date=23 September 2017 |access-date=4 November 2018 |doi-access=free}}</ref>