Skip to main content

Sentier.dev Common Units Taxonomy adapted from QUDT 2.1

Search from vocabulary

Content language

Concept information

Acidity > Acidity

Preferred term

[pH]Acidity  

Definition

  • In chemistry the unit $\ extit{pH}$, also referred to as $\ extit{acidity}$ or $\ extit{basicity}$, is the negative logarithm (base 10) of the concentration of free protons (or hydronium ions). The definition of $pH$ in terms of hydrogen ions in solution is: $$\ ext{pH}=-\\log_{10}(a_{H^+})\\equiv-\\log_{10}\\left(\\left[H^+\ ight]\ ight)$$ Where $a_{H^+}$ is the equilibrium molar concentration of $H^+$ in the solution, the activity of the hydrogen ion in the solution. $$$$ This definition is appropriate for concentrations equal to, or less than $1\\ mol/l$, where $aH+ \\equiv [H+]$, that is, $1\\ mol/L\\ HCl$ has a $pH$ of zero. $$$$ To relate this to standard molality ($b^\\circ$), typically taken as $1 \\ mol/kg$, consideration is given to the activity ($a_{H^+}$) of the hydrogen ions. $$$$ The activity can be expressed as: $$a_{H^+} = \\gamma_{H^+} \ imes m_{H^+}$$ Where, $\\gamma_{H^+}$ is the activity coefficient, which adjusts the molality to account for non-ideal behavior due to interactions between ions in the solution. $m_{H^+}$ is the molality of hydrogen ions in the solution relative to the standard molality, expressed in $mol/kg$. $$$$ The expansion of $pH$ then becomes: $$\ ext{pH} = -log_{10}\\left(m_{H+}\ imes\\gamma_{H^+}\ ight)$$ $$$$ This definition is relevant in more concentrated solutions or when precise thermodynamic calculations are required. It reflects how the properties of the solution deviate from ideal behavior and provides a more accurate understanding of the $pH$ under various conditions. $$$$ While $pH$ is a universally recognized scale for expressing hydrogen ion activity, its appropriateness and accuracy can diminish under conditions of extremely high ionic strength, non-aqueous environments, high temperatures, or very high or low $pH$ values. In such cases, alternative measurement strategies might be required to obtain meaningful and accurate descriptions of acidity or basicity.

Broader concept

Note

  • Unsure about dimensionality of pH; conversion requires a log function not just a multiplier

Notation

  • [pH]
  • pH

URI

https://vocab.sentier.dev/units/unit/PH

Download this concept:

RDF/XML TURTLE JSON-LD