Editing PH (section)

=== p[H] ===
This was the original definition of Sørensen in 1909,<ref name="Sor2">{{cite web |title=Carlsberg Group Company History Page |url=http://www.carlsberggroup.com/Company/heritage/Research/Pages/pHValue.aspx |url-status=live |archive-url=https://web.archive.org/web/20140118043012/http://www.carlsberggroup.com/Company/heritage/Research/Pages/pHValue.aspx |archive-date=18 January 2014 |access-date=7 May 2013 |publisher=Carlsberggroup.com}}</ref> which was superseded in favor of pH in 1924. [H] is the concentration of hydrogen ions, denoted [{{chem2|H(+)}}] in modern chemistry. More correctly, the [[thermodynamic activity]] of {{chem2|H(+)}} (''a''<sub>H</sub>+) in dilute solution should be replaced by [{{chem2|H(+)}}]/''c''<sub>0</sub>, where the standard state concentration ''c''<sub>0</sub> = 1&nbsp;mol/L. This ratio is a pure number whose logarithm can be defined.

It is possible to measure the concentration of hydrogen cations directly using an electrode calibrated in terms of hydrogen ion concentrations. One common method is to [[Titration|titrate]] a solution of known concentration of a strong acid with a solution of known concentration of strong base in the presence of a relatively high concentration of background electrolyte. By knowing the concentrations of the acid and base, the concentration of hydrogen cations can be calculated and the measured potential can be correlated with concentrations. The calibration is usually carried out using a [[Gran plot#Electrode calibration|Gran plot]].<ref>{{cite journal |last=Rossotti |first=F.J.C. |author2=Rossotti, H. |year=1965 |title=Potentiometric titrations solution containing the background electrolyte. |journal=J. Chem. Educ. |volume=42 |doi=10.1021/ed042p375}}</ref> This procedure makes the activity of hydrogen cations equal to the numerical value of concentration of these ions.

The glass electrode (and other [[Ion selective electrode]]s) should be calibrated in a medium similar to the one being investigated. For instance, if one wishes to measure the pH of a seawater sample, the electrode should be calibrated in a solution resembling seawater in its chemical composition.

The difference between p[H] and pH is quite small, and it has been stated that pH = p[H] + 0.04.<ref>{{VogelQuantitative}}, Section 13.23, "Determination of pH"</ref> However, it is common practice to use the term "pH" for both types of measurement.