View source for The pH scale

[[Category:Done 2018-07-20]] 
[[File:ph.jpg|300px|thumb|Figure 1. Various substances arranged by their pH.<ref>Wikimedia Commons [Online], Available: https://upload.wikimedia.org/wikipedia/commons/2/23/216_pH_Scale-01.jpg</ref>]]

<onlyinclude>The term '''pH''' is an abbreviation for '''power of Hydrogen''', which is a measure of how acidic or basic a [[chemical]] [[solution]] is. </onlyinclude>  Most solutions are either acidic or basic (also called "[[alkaline]]") - a substance that is neither acidic nor a basic is "neutral".<Ref name=epa>US EPA. (July 8, 2015). ''What is pH?'' [Online], Available: http://www.epa.gov/acidrain/measure/ph.html</ref>

Acids and bases are very important chemically, and are found almost everywhere. One example in the context of energy is [[acid rain]], which is formed when water interacts with various [[pollutant]]s in the [[atmosphere]]. Pollutants dissolved in the rain water cause it to be acidic, which can cause damage to the [[environment]] and man-made structures.  Read more about acid rain [[acid rain|here]].

==The pH scale==

A pH value is used to describe a water-based solution. In general, a small pH value describes a solution that is [[acid]]ic, and a larger pH value describing solutions that are less acidic (more [[base|basic]]). The pH scale is centered on 7 - meaning that a solution with a pH of 7 is perfectly neutral (neither acidic nor basic). Some examples of common substances and their place on the pH scale is shown in Figure 1. 

The pH value of a solution directly measures the concentration of [[hydrogen]] [[ion]]s (H<sup>+</sup>) in the solution. Acids have a large H<sup>+</sup> concentration, which translates to a small pH value. Bases have very little H<sup>+</sup>. Basic solutions have large pH values - greater than pH&nbsp;7. 

While pH values have been measured as low as -3.6<Ref name=pnas>Nordstrom and Alpers, ''Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California'' <i>Proc Natl Acad Sci U S A</i>, vol. 96, no. 7, pp 3455-3462, 1999 [Online], Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC34288/</ref> and as high as 17.6 <ref name=ACS>Licht, "pH Measurement in Concentrated Alkaline Solutions" <i>Anal. Chem.</i>, vol. 57, pp. 514-519, 1985 [Online], Available, http://pubs.acs.org.ezproxy.lib.ucalgary.ca/doi/pdf/10.1021/ac50001a045 </ref>, most solutions encountered in nature range between pH&nbsp;0 to pH&nbsp;14. 

pH is a [[logarithmic scale]]. This means that for each one-digit change in pH, the acidity (H<sup>+</sup> concentration) changes by 10 times. For example, a solution with a pH of 4 has 10&nbsp;times more H<sup>+</sup> than a solution with a pH of 5. A solution of pH&nbsp;3 will have 100 times more H<sup>+</sup> (it is 100 times more acidic) than a solution of pH&nbsp;5. 

==Hydroxide and Hydrogen Ions==
Since pH is measured in water-based solutions, there is always a balance between H<sup>+</sup> (hydrogen ions) and OH<sup>&ndash;</sup> (hydroxide ions). This is because water can break down into these two ions: 

<center><math>H_{2}O \rightarrow H^{+} + OH^{-} </math></center>

At room temperature, the balance between the H<sup>+</sup> and OH<sup>&ndash;</sup> concentrations in water are related by this expression ([H<sup>+</sup>] means "concentration of H<sup>+</sup>"): 

<center><math> 10^{-14} = [H^+] \times [OH^-] </math></center>

This equation shows that the more hydrogen ions a solution has, the fewer hydroxide ions it must have. So: an acidic solution that has a large amount of H<sup>+</sup>  will have very little OH<sup>&ndash;</sup>. And a basic solution that has very little H<sup>+</sup> will have more OH<sup>&ndash;</sup>. A solution that is exactly neutral will have the same amount of H<sup>+</sup> and OH<sup>&ndash;</sup>.

==Calculating pH==
To calculate pH, you should know the concentration of hydrogen ions (H<sup>+</sup>) in your solution. From there, the pH can be found by: 

<center><math> pH = -\log_{10}([H^{+}]) </math></center>

Since this is a logarithmic calculation, in order to change the pH by one unit (say, from 2 to 3), the concentration of H<sup>+</sup> must be changed by 10&nbsp;times (say, from 0.01&nbsp;mol/L to 0.001&nbsp;mol/L). 

==Discover More==
The PhET simulation below was graciously provided by the University of Colorado. This simulation can help explore how pH changes as [[liquid]]s mix.

<html>
<iframe src="https://phet.colorado.edu/sims/html/ph-scale/latest/ph-scale_en.html" width="800" height="600"></iframe>
</html>

==For Further Reading==
*[[Acid]]
*[[Base]]
*[[Chemical]]
*[[Water]]
*[[Acid rain]]
*[[Water cycle]]
*Or explore a [[Special:Random|random page]]

To explore pH further please see the [https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Acids_and_Bases/Acids_and_Bases_in_Aqueous_Solutions/The_pH_Scale Chemistry LibreTexts page]. 

==References==
{{reflist}}
[[Category:Uploaded]]