3.13
缓冲区：缓冲区容量
缓冲区容量是缓冲液用来抵抗 pH 值变化的定量指标。如下式所示，缓冲容量用“β”来进行表示，表示为将 1 升缓冲溶液的 pH 值改变 1 个单位所需的酸或碱的摩尔数。在这里，Ca和Cb分别用来表示酸和碱的摩尔数。请注意，dpH 的值代表了 pH 值所发生的变化。
在图中，pH 值被绘制为添加到 pK…
缓冲容量（用β表示）是衡量缓冲溶液在加入酸或碱时抵抗pH变化能力的定量指标。其定义为：向1升缓冲溶液中加入足以引起单位pH变化的酸（Ca）或碱（Cb）的摩尔数。
缓冲容量越高，缓冲液抵抗 pH 变化的能力就越强。
对于 pKa 等于 5 的一种酸，将加入的碱的物质的量对 pH 作图。所得曲线的导数表明，当 pH 等于 pKa 时，缓冲能力最强，此时缓冲溶液中弱酸与其共轭碱的浓度相等。因此，缓冲溶液的有效 pH 范围为 pKa 值上下各一个单位。
在 pH 3-10 范围之外，溶液中已存在的高浓度 H+ 或 OH− 离子会抵抗所加入酸或碱引起的 pH 变化。
缓冲液组分的浓度与缓冲容量成正比。
Q1: What is buffer capacity and how is it measured?
Buffer capacity, represented as beta, quantifies a buffer's resistance to pH change when acid or base is added. It is expressed as the number of moles of acid or base needed to change the pH of a one-liter buffer solution by one unit. Higher buffer capacity indicates stronger resistance to pH changes.
Q2: Why is buffer capacity highest when pH equals pKa?
Buffer capacity reaches its maximum when pH equals pKa because the buffer solution contains equal concentrations of weak acid and its conjugate base. At this point, the buffer has maximum resistance to pH changes. The effective pH range of a buffer extends one unit above and below the pKa value.
Q3: How does buffer species concentration affect buffer capacity?
The concentration of buffer species proportionally dictates buffer capacity. Higher concentrations of weak acid and conjugate base in the solution increase buffer capacity. Therefore, selecting a buffer with appropriate species concentration is essential for achieving desired pH resistance in analytical applications.
Q4: What pH range should a buffer maintain for effective buffering?
A buffer's effective pH range is one unit more or less than its pKa value. Outside this range, high concentrations of H+ or OH− ions resist pH change from added acid or base. Buffers should be selected with pKa values within plus or minus one unit of the desired pH.
Q5: How is buffer capacity determined graphically?
Buffer capacity is determined by plotting pH versus moles of acid or base added to a buffer solution. The derivative of this curve yields a plot showing buffer capacity versus pH. This graphical method reveals that maximum buffer capacity occurs at pH equal to pKa, where solution composition during acid base titrations is most balanced.
Q6: What factors determine the choice of an effective buffer?
An effective buffer requires a pKa value within plus or minus one unit of the desired pH and sufficient concentration of weak acid and conjugate base species. Both the pKa and buffer species concentration proportionally influence buffer capacity. These factors together ensure maximum resistance to pH changes in the target pH range.
Q7: How do weak acid and conjugate base concentrations relate to buffer effectiveness?
Buffer capacity depends directly on the concentration of weak acid and its conjugate base in solution. When these species are present in equal concentrations at pH equal to pKa, the buffer achieves maximum capacity. Increasing either species' concentration proportionally increases the buffer's ability to resist pH changes.