Name: ph and poh in water
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 41 s
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pH Scale

La concentration des ions hydronium dans une solution aqueuse est généralement écrite avec des exposants négatifs et peut être aussi petit que 1 10⁻¹⁴ molaire. Le pH a été développée par le chimiste Soren Sorenson en 1909 comme un moyen plus pratique pour comparer rapidement l'acidité de différentes solutions. Le pH d'une solution est le logarithme négatif de sa concentration en ions hydronium.Par exemple, une solution avec une concentration hydronium de 1 10⁻⁵ molaire a un pH de 5. Plus la concentration en ions hydronium est élevée d'une solution, plus son pH est bas. Le pH étant exprimé sur une échelle logarithmique, un changement d'une seule unité correspond à une multiplication par 10 ou diminution de la concentration en ions hydronium.Une solution avec un pH de 3 aura dix fois plus d'ions hydronium qu'une solution avec un pH 4 et cent fois plus qu'une solution avec un pH de 5. Une solution acide a une concentration plus élevée d'ions hydronium que les ions hydroxyde et un pH inférieur à 7, alors qu'une solution basique a une concentration plus faible d'ions hydronium que d'ions hydroxyde et un pH supérieur à 7. Une solution neutre à concentration égale d'ions hydronium et hydroxyde a un pH de 7.La concentration d'ions hydroxyde peut également être exprimée en pOH. pOH est le logarithme négatif de la concentration en ions hydroxyde. Plus la concentration en ions hydroxyde est élevée, plus sa valeur pOH diminue.La valeur du pH ou du pOH d'une solution aqueuse varie de 0 à 14. C'est parce que Kw, le constant équilibre pour l'auto-ionisation de l'eau, est égale à 1 10⁻¹⁴. Prendre le log négatif des deux côtés de l'équation donne une équation où pKw est égale à la somme du pH et du pOH.Puisque le log négatif de 1 10⁻¹⁴ est 14, la somme du pH et du pOH d'une solution aqueuse sera toujours 14. Cela peut être utilisé pour déterminer la valeur du pH lorsque la valeur du pOH est connue et vice versa. Par exemple, une solution avec un pOH de 10 a un pH de 4.

<ol>
	<li>van Lubeck, Henk. « Pourquoi ne pas remplacer le pH et le pOH par un seul véritable grade d’acidité, AG ? " Journal of Chemical Education 76, n° 7 (1999) : 892. <a href="https://pubs.acs.org/doi/pdf/10.1021/ed076p892">https://pubs.acs.org/doi/pdf/10.1021/ed076p892</a></li>
</ol>

Échelle de ph

Les ions hydronium et hydroxyde sont présents à la fois dans l'eau pure et dans toutes les solutions aqueuses, et leurs concentrations sont inversement proportionnelles, comme le détermine le produit ionique de l'eau (KE). Les concentrations de ces ions dans une solution sont souvent des déterminants critiques des propriétés de la solution et des comportements chimiques de ses autres solutés. Deux solutions différentes peuvent différer d'un million, d'un milliard, voire d'un billion de fois leurs concentrations en ions hydronium ou hydroxyde. Un moyen courant d'exprimer des quantités qui peuvent s'étendre sur de nombreux ordres de grandeur est d'utiliser une échelle logarithmique. Le pH d'une solution est donc défini comme indiqué ici, où [H3O+] est la concentration molaire de l'ion hydronium dans la solution : 
  <img alt="Eq1" src="/files/ftp_upload/11397/11397_Equation_1.png" style="height: 50px;" />
 La réorganisation de cette équation pour isoler la molarité de l'ion hydronium donne l'expression équivalente : 
  <img alt="Eq2" src="/files/ftp_upload/11397/11397_Equation_2.png" style="height: 50px;" />
 De même, la molarité de l'ion hydroxyde peut être exprimée comme une fonction-p ou pOH : 
  <img alt="Eq3" src="/files/ftp_upload/11397/11397_Equation_3.png" style="height: 50px;" />
 ou 
  <img alt="Eq4" src="/files/ftp_upload/11397/11397_Equation4.png" style="height: 50px;" />
 Enfin, la relation entre ces deux concentrations d'ions exprimées sous forme de fonctions-p est facilement dérivée de l'expression de KE : 
  <img alt="Eq5" src="/files/ftp_upload/11397/11397_Equation_5.png" style="height: 135px;" />
 À 25 °C, la valeur de KE est de 1,0 × 10−14, et donc : 
  <img alt="Eq6" src="/files/ftp_upload/11397/11397_Equation_6.png" style="height: 50px;" />
 La molarité de l'ion hydronium dans l'eau pure (ou toute solution neutre) est de 1,0 × 10−7 M à 25 °C. Le pH et le pOH d'une solution neutre à cette température sont donc : 
  <img alt="Eq7" src="/files/ftp_upload/11397/11397_Equation_7.png" style="height: 95px;" />
 Ainsi, à cette température, les solutions acides sont celles ayant des molarités en ions hydronium supérieures à 1,0 × 10−7 M et des molarités en ions hydroxyde inférieures à 1,0 × 10−7 M (correspondant à des valeurs de pH inférieures à 7,00 et à des valeurs de pOH supérieures à 7,00). Les solutions basiques sont celles dont les molarités en ions hydronium sont inférieures à 1,0 × 10−7 M et les molarités en ions hydroxyde supérieures à 1,0 × 10−7 M (correspondant à des valeurs de pH supérieures à 7,00 et à des valeurs de pOH inférieures à 7,00). 
 Comme la constante d'auto-ionisation KE dépend de la température, ces corrélations entre les valeurs de pH et les adjectifs acides/neutres/basiques seront différentes à des températures autres que 25 °C. Par exemple, la molarité en hydronium de l'eau pure à 80 °C est de 4,9 × 10−7 M, ce qui correspond aux valeurs de pH et de pOH de : 
  <img alt="Eq8" src="/files/ftp_upload/11397/11397_Equation_8.png" style="height: 95px;" />
 À cette température, les solutions neutres présentent un pH = pOH = 6,31 ; les solutions acides présentent un pH inférieur à 6,31 et un pOH supérieur à 6,31 alors que les solutions basiques présentent un pH supérieur à 6,31 et un pOH inférieur à 6,31. Cette distinction peut être importante lorsqu'on étudie certains processus qui se produisent à d'autres températures, comme les réactions enzymatiques dans les organismes à sang chaud à une température d'environ 36 – 40 °C. Sauf indication contraire, les références aux valeurs de pH sont supposées être celles à 25 °C. 
 Ce texte est adapté de<a href="https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh"> Openstax, Chimie 2e, Section 14.2 : pH et pOH. </a>

Échelle de pH

Les ions hydronium et hydroxyde sont présents à la fois dans l’eau pure et dans toutes les solutions aqueuses, et leurs concentrations sont inversement proportionnelles ...

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ph and poh in water

pH and pOH show how hydronium and hydroxide ions behave in water and other aqueous solutions. These ion concentrations are linked by the ion product of water, K w , so when one goes up, the other goes down.

Because ion concentrations can differ by huge amounts, a logarithmic scale is used to describe them. pH is defined from the hydronium ion concentration, [H3O+], and pOH is the matching p-function for hydroxide ion concentration. These two values are also connected through the K w relationship.

At 25 °C, K w is 1.0 × 10−14. In pure water, or any neutral solution at this temperature, the hydronium ion concentration is 1.0 × 10−7 M. That gives pH = 7.00 and pOH = 7.00.

At 25 °C, acidic solutions have hydronium ion concentrations greater than 1.0 × 10−7 M and hydroxide ion concentrations less than 1.0 × 10−7 M. This means acidic solutions have pH values below 7.00 and pOH values above 7.00. Basic solutions show the opposite pattern, with pH above 7.00 and pOH below 7.00.

The link between pH and acidity changes with temperature because K w changes with temperature. For example, pure water at 80 °C has a hydronium ion concentration of 4.9 × 10−7 M, which gives pH and pOH values of 6.31. At this temperature, neutral solutions have pH = pOH = 6.31, acidic solutions have pH below 6.31, and basic solutions have pH above 6.31.

This temperature effect matters in processes that happen away from 25 °C, including enzyme reactions in warm-blooded organisms at about 36–40 °C. Unless another temperature is stated, pH values are usually assumed to refer to 25 °C.

pH and pOH in Water

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ph scale

pH and pOH show how hydronium and hydroxide ions behave in water and other aqueous solutions. These ion concentrations are linked by the ion product of water, K w , so when one goes up, the other goes down. Because ion concentrations can differ by huge amounts, a logarithmic scale is used to describe them. pH is defined from the hydronium ion concentration, [H3O+], and pOH is the matching p-function for hydroxide ion concentration. These two values are also connected through the K w...

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Learn how hydronium and hydroxide ion levels define pH and pOH, why neutral water changes with temperature, and how acidic or basic solutions are identified.

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pH and pOH in Water

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