2.8
酸碱平衡：基于活性的 pH 定义
对于理想的溶液，其 pH 值能够定义为氢离子浓度的负对数。对于非理想溶液，在对pH 值进行准确的测量时，则必须考虑其中氢离子活度的负对数而不是其浓度。在这样的溶液中，pH值可以更准确地定义为氢离子浓度与其活度系数乘积的负对数。
在离子强度非常低的溶液中（例如纯水），当溶液的离子强度是因为添加了不提供…
将溶液的 pH 定义为氢离子浓度的负对数，仅适用于理想溶液。
在实际操作中，pH 值的测量考虑的是氢离子活度的负对数，而非其浓度。
因此，pH 可以更准确地重新定义为氢离子浓度与其活度系数乘积的负对数。
在25摄氏度的纯水中，极低的离子强度表明离子的活度系数接近于1。
然而，向水中加入氯化钾等盐类会增加溶液的离子强度，从而降低活度系数。
由此可见，氢离子活度略有增加，相当于溶液的 pH 值下降。
值得注意的是，添加氯化钾会引起pH值的微小变化，但会导致氢离子浓度显著增加。
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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.