2.8
Équilibres acido-basiques : définition du pH basée sur l'activité
Pour une solution idéale, le pH est défini comme le logarithme négatif de la concentration en ions hydrogène. Pour une solution non idéale, une mesure…
La définition du pH d’une solution comme le logarithme négatif de la concentration en ions hydrogène n’est valable que pour une solution idéale.
En pratique, la mesure du pH prend en compte le logarithme négatif de l’activité de l’ion hydrogène plutôt que la concentration.
Ainsi, le pH peut être redéfini plus précisément comme le logarithme négatif du produit de la concentration en ions hydrogène et de son coefficient d’activité.
Dans le cas d’une eau pure à 25 degrés C, la force ionique extrêmement faible suggère que les coefficients d’activité des ions sont proches de un.
Cependant, l’ajout d’un sel comme le chlorure de potassium à l’eau augmente la force ionique de la solution, diminuant ainsi les coefficients d’activité.
Il s’ensuit ici que l’activité des ions hydrogène est légèrement augmentée, ce qui correspond à une diminution du pH de la solution.
Notamment, l’ajout de chlorure de potassium provoque un changement négligeable du pH mais une augmentation significative de la concentration en ions hydrogène.
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Q1: Why is the traditional pH definition based on hydrogen ion concentration considered incomplete?
The negative logarithm of hydrogen ion concentration applies only to ideal solutions. In real solutions, pH must account for hydrogen ion activity, which reflects the effective concentration of hydrogen ions. Activity incorporates both concentration and an activity coefficient, providing a more accurate representation of how hydrogen ions behave in solution.
Q2: How does adding salt to pure water affect pH and hydrogen ion concentration?
Adding salt like potassium chloride increases ionic strength, which decreases activity coefficients and increases hydrogen ion activity. This causes a slight pH decrease. However, the hydrogen ion concentration increases significantly, demonstrating that activity and concentration respond differently to ionic strength changes.
Q3: What is the relationship between activity coefficient and ionic strength in solutions?
Activity coefficients decrease as ionic strength increases. In pure water with extremely low ionic strength, activity coefficients approach one, making activity nearly equal to concentration. As ionic strength rises through electrolyte addition, activity coefficients drop further, reducing the activity coefficient's value and affecting the solution's effective hydrogen ion concentration.
Q4: How can pH be mathematically expressed using activity and concentration?
pH is defined as the negative logarithm of hydrogen ion activity. This can be expressed as the negative logarithm of the product of hydrogen ion concentration and its activity coefficient. This formulation accounts for non-ideal behavior and provides accurate pH measurements in real solutions where activity differs from concentration.
Q5: Why do activity coefficients remain close to one in pure water?
Pure water has extremely low ionic strength because few ions are present to interact with hydrogen ions. When ionic strength is very low, activity coefficients approach one, meaning hydrogen ion activity nearly equals its concentration. This approximation breaks down when electrolytes are added, increasing ionic strength and decreasing activity coefficients.
Q6: What distinguishes ideal solutions from non-ideal solutions in pH measurement?
Ideal solutions assume activity equals concentration, so pH depends solely on hydrogen ion concentration. Non-ideal solutions require considering hydrogen ion activity, which accounts for ion interactions through the activity coefficient. Real solutions behave non-ideally, especially at higher ionic strengths, making activity-based pH definitions more accurate.
Q7: Why does potassium chloride cause negligible pH change despite increasing hydrogen ion concentration?
Potassium chloride increases ionic strength, which decreases activity coefficients and increases hydrogen ion activity. Although hydrogen ion concentration rises significantly, the activity increase is slight because the activity coefficient decrease partially offsets the concentration increase. This demonstrates that pH depends on activity, not concentration alone.