3.13
バッファ: バッファ容量
緩衝能は、pH の変化に耐える緩衝液の定量的な尺度です。次の方程式に示すように、「ベータ」で示される緩衝能は、1 リットルの緩衝液の pH を 1 単位変化させるのに必要な酸または塩基のモル数として表されます。ここで、Ca と Cb はそれぞれ酸と塩基のモル数を示す。 dpH は pH の変化を表す…
ベータとして表される緩衝液容量は、酸または塩基を添加したときのpH変化に対する緩衝液の耐性の定量的尺度です。これは、単位pH変化の前に1リットルの緩衝液に添加された酸(C a)または塩基(Cb)のモル数として表されます。
緩衝液の容量が高ければ高いほど、pH変化に対する緩衝液の抵抗能力が高くなります。
pKaが5に等しい酸に対して、添加された塩基のモル数がpHに対してプロットされます。得られた曲線の導関数は、pHがpKaに等しい場合、緩衝液中の弱酸とその共役塩基の濃度が等しい場合に、最大の緩衝容量を示します。したがって、バッファーの有効pH範囲は、pKaより1単位多いか小さいかです。
3-10 pH範囲外では、既存の高濃度のH+またはOH−イオンは、添加された酸または塩基のpH変化に抵抗します。
緩衝種の濃度は、緩衝容量を比例して決定します。
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.