Phosphoric acid is a good example o

Phosphoric acid is a good example of a titration where the first two equivalence points, corresponding to base reaction with the first and second protons, respectively, are clearly visible. By clearly visible, we mean that there is a large change in pH at the equivalence point.

The acid dissociation constants for phosphoric acid are quite different from each other with pKa's of 2.15, 7.2, and 12.15 Because the pKa are so different, the protons are reacted at different pH's. This is illustrated in the plot of the relative fraction as a function of pH shown below.

The pH at points where the relative fraction of two species are equal, e.g., where two relative concentration lines cross, have a simple relationship to the acid equilibration constants. For example, the first crossing occurs for [H3PO4] = [ H2PO4-]. The relationship to pH is most easily found by recognizing that all principle species are given in the first proton ionization equation

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磷酸是一种很好的例子，一个滴定在头两个等值点，对应的基本反应与第一和第二质子，分别，都清晰可见。通过清晰可见，我们的意思是说，有在对等点pH值的大的变化。

酸解离常数的磷酸是相当彼此不同的pKa值的2.15，7.2，12.15因为PKA是如此的不同，质子在不同pH值的。这说明在相对的部分情节作为pH的函数的反应如下所示。

pH值点处的两个物种的相对分数是相等的，例如，在两个相对集中的线交叉，有酸平衡常数的一个简单的关系。比如说呢。第一个十字路口发生[磷酸] = [ H2PO4- ]。通过认识到所有的物种都在第一原理质子电离方程给出最容易发现pH值的关系