Name: buffer ph and the common ion effect
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 4 min 35 s
Channel: JoVE

缓冲液是能够维持体系 pH 值稳定的溶液，即使加入强酸或强碱也能保持 pH 基本不变。在没有缓冲液的情况下，向溶液中加入强酸或强碱会导致 pH 值发生显著变化。缓冲液在日常生活中至关重要。例如，血液具有缓冲作用，能够将 pH 值维持在 7.35 至 7.45 之间，因为 pH 值高于 7.8 或低于 6.8 均可导致死亡。
那么，缓冲液是如何起作用的呢？缓冲液本质上是由一种弱酸或弱碱与其共轭碱或共轭酸组成的平衡体系。例如，由乙酸和乙酸钠配制的缓冲液。该缓冲溶液中含有弱酸及其共轭盐，后者解离生成共轭碱乙酸根离子。该溶液呈酸性，因为乙酸的酸解离常数 Ka 高于其共轭碱的碱解离常数 Kb。
请记住，解离常数 Ka 或 Kb 分别定义了酸或碱的强度，因为它是化合物在水中解离反应的平衡常数。解离常数也可以用 pKa 表示，即 Ka 的负对数，pKa 越小，酸性越强。pKb 同理，pKb 越小，碱性越强。
回到乙酸盐缓冲液，如果加入强碱（如氢氧化钠），氢氧根离子会与溶液中已有的水合氢离子发生反应。这会导致更多的乙酸与水反应，以重新达到平衡，从而生成更多的水合氢离子。如果加入强酸（如盐酸），溶液中会形成更多的水合氢离子。这些离子会与乙酸根离子反应，生成更多的乙酸。在这两种情况下，溶液的 pH 值变化都非常小。
需要注意的是，每种缓冲液都有其特定的 pH 范围，在此范围内才能有效缓冲溶液。此处展示了乙酸缓冲液的滴定曲线，其缓冲范围已被标出。该范围由 Henderson-Hasselbalch 方程确定，该方程告诉我们缓冲液的 pH 取决于弱酸的 Ka 值以及共轭碱与弱酸的浓度比。
当远离该 pH 值时，缓冲液中的弱酸或弱碱将被耗尽，从而无法再缓冲 pH 值。因此，在为实验应用选择缓冲液时，应选择其 pKa 接近所需 pH 值的缓冲液。
最后，所有缓冲液都有其缓冲容量的限度，即在 pH 值发生显著变化之前，可加入缓冲溶液中的酸或碱的量。例如，如果向乙酸和乙酸钠缓冲溶液中加入过多的酸，将会使所有的乙酸根离子质子化，并积累大量水合氢离子，从而导致 pH 值下降。
同样，如果加入过多的碱，将会使所有的乙酸去质子化，并积累大量氢氧根离子，从而导致 pH 升高。因此，缓冲能力受到弱酸及其共轭盐浓度的影响。例如，1 摩尔的乙酸盐缓冲液比 0.1 摩尔的乙酸盐缓冲液具有更高的缓冲能力。
在本实验中，您将配制并检测多种不同 pH 范围的缓冲液。随后，您将使用这些缓冲液测定一种名为中性红的 pH 指示剂的 pKa，并探究当加入核黄素结合蛋白时，中性红 pKa 的变化情况。

缓冲液

Buffers

<h4>缓冲液</h4>
当一种阿伦尼乌斯酸（HA）加入水中时，它会解离为其共轭碱（A-）和一个氢阳离子（H+）。
HA + H2O → H+(aq) + A-(aq)
溶液中氢离子的数量决定了溶液的酸度，氢离子越多，表示 pH 值越低或酸性越强。同样，当向水中加入强阿伦尼乌斯碱（BOH）时，它会解离为其共轭酸（B+）和一个氢氧根离子（OH-）。
BOH + H2O → B+(aq) + OH-(aq)
通常，向溶液中加入强酸或强碱会显著改变其 pH 值，因为酸或碱会与溶液中的水分子发生反应，从而增加氢离子或氢氧根离子的浓度。然而，使用缓冲液可以减轻这种 pH 值的变化。缓冲液是一类能够维持体系 pH 值相对恒定的溶液，即使在加入强酸或强碱的情况下也能发挥作用。
缓冲液的组分通常是一种弱酸或弱碱的共轭酸碱对。因此，能够在水中完全解离的强酸或强碱非常不适合作为缓冲液，而部分解离的弱酸或弱碱则更适合作为缓冲液。当缓冲液存在时，强酸或强碱不会与溶液中的水分子发生反应，而是与弱酸/共轭碱发生反应，从而导致溶液的 pH 值几乎不发生变化。
<h4>同离子效应</h4>
缓冲液通过一种称为同离子效应的现象发挥作用。当向一个已含有某种离子的平衡混合物中加入该离子时，就会发生同离子效应。此时，平衡会向减少该离子生成的方向移动。
例如，乙酸（CH3COOH）在水中轻微解离，生成乙酸根离子（CH3COO-）和氢离子。
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
如果从可溶性乙酸钠中加入更多的乙酸根离子，平衡位置将向左移动，生成更多未解离的乙酸，氢离子浓度随之降低。在此情况下，共同离子——乙酸根——抑制了乙酸的解离。
缓冲液必须含有高浓度的酸性（HA）和碱性（A-）组分，才能对溶液起到缓冲作用。如果向缓冲液中加入的氢离子或氢氧根离子量较少，它们将导致少量的一种缓冲组分转化为另一种。只要所加入离子的浓度远小于缓冲液中已存在的 HA 和 A- 的浓度，这些加入的离子对 pH 值的影响就很小，因为它们会被其中一种缓冲组分消耗掉。当氢离子或氢氧根离子的浓度超过弱酸及其共轭碱的浓度时，缓冲作用便会丧失，pH 值将发生改变。
<h4>亨德森-哈塞尔巴尔赫方程</h4>
弱酸的解离常数 Ka 可通过未解离酸 HA 的浓度，以及氢离子和共轭碱 A- 的浓度来计算。
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="酸解离常数公式 Ka=[H+][A-]/[HA]，化学公式。" data-altupdatedat="1754904545000" data-alt="酸解离常数公式，Ka=[H+][A-]/[HA]，化学公式。" />
较高的 Ka 值代表较强的酸，而较小的 Ka 值代表较弱的酸。为了确定氢离子的浓度，需对公式进行重新整理。在此形式中可以明确看出，酸组分与碱组分的比值在决定氢离子浓度以及相应 pH 值方面具有重要作用。
<img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" alt="酸解离方程，[H+] = Ka × [HA]/[A-]，化学平衡概念。" data-altupdatedat="1754904545000" data-alt="酸解离平衡公式，用于计算水合氢离子浓度的方程。" />
对两边取负的常用对数，得到 Henderson-Hasselbalch 方程。
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="计算pH值的亨德森-哈塞尔巴尔赫方程，化学公式示意图。" data-altupdatedat="1754904545000" data-alt="计算pH值的亨德森-哈塞尔巴尔赫方程，化学公式示意图。" />
亨德森-哈塞尔巴尔赫方程能够直接计算缓冲液的 pH 值，而无需先计算氢离子的浓度。
例如，可以利用该方法计算在向 1 L 缓冲液中加入 0.02 摩尔强碱后的 pH 值。强碱完全解离，因此所加入的氢氧根离子浓度为 0.02 M。这将使酸的浓度降低 0.02 M。假设缓冲液中酸（HA）和碱（A-）组分的初始浓度均为 0.5 M，则碱的新浓度增加至 0.52 M，而酸的浓度减少至 0.48 M。已知缓冲液中酸组分的 pKa 值后，可将这些新的组分浓度代入 Henderson-Hasselbalch 方程，从而计算出溶液的 pH 值。
这有助于确定缓冲容量，即在不显著影响 pH 值的情况下，缓冲溶液所能容纳的强酸或强碱的量。
<h4>缓冲容量</h4>
缓冲容量是衡量缓冲溶液抵抗pH变化能力的指标。这种能力取决于缓冲组分的浓度，即酸及其共轭碱的浓度。缓冲液浓度越高，其缓冲容量越大。这意味着必须加入更多的氢离子或更强的酸，才能破坏平衡并改变缓冲液的pH值。
缓冲容量还受到缓冲组分相对浓度的影响。当缓冲组分的浓度相近时，缓冲效果更佳。如果缓冲组分的比例相近，则在加入酸或碱时，组分的浓度比不会发生显著变化；必须加入大量酸或碱才能改变该比例，从而引起 pH 值的变化。
缓冲液的 pH 值与其缓冲容量不同。pH 范围是指缓冲液有效的区间。通常，缓冲液的可用范围在其酸性组分的 pKa 值上下 1 个 pH 单位内。
<h4>参考文献</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). 化学与化学反应性。加利福尼亚州贝尔蒙特：Brooks/Cole, Cengage Learning。</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). 化学：物质与变化的分子本质。马萨诸塞州波士顿：McGraw Hill。</li>
</ol>

缓冲液  
当阿伦尼乌斯酸（HA）加入水中时，会解离为其共轭碱（A⁻）和一个氢阳离子（H⁺）。  
HA + H₂O → H⁺(aq) + ...

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buffer ph and the common ion effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-.

Buffers reduce those pH changes. A buffer usually contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Weak acids and weak bases are better buffers because they dissociate only partially in water. When a buffer is present, added strong acid or base reacts with the buffer components instead of causing a large shift in pH.

The way buffers work is called the common ion effect. A common ion is an ion already present in an equilibrium mixture. When more of that ion is added, the equilibrium shifts away from making more of it. A good example is acetic acid, CH3COOH, which dissociates slightly in water to form H+ and acetate, CH3COO-. If sodium acetate adds more acetate ion, the equilibrium shifts left and less hydrogen ion remains in solution.

A buffer needs high amounts of both the acidic form, HA, and the basic form, A-. Small amounts of added H+ or OH- are used up by one buffer component and converted into the other. If the added acid or base is much smaller than the amounts already in the buffer, the pH changes very little. Once the concentration of added H+ or OH- becomes larger than the buffer components, the buffer effect is lost and the pH shifts.

The Henderson-Hasselbalch equation helps calculate buffer pH. It comes from the acid dissociation constant, Ka, which uses the concentrations of HA, H+, and A-. A larger Ka means a stronger acid, while a smaller Ka means a weaker acid. After rearranging and taking the negative log, the equation shows that the ratio of acid to base controls pH, so buffer pH can be found without first calculating hydrogen ion concentration.

For example, a 1 L buffer with 0.5 M HA and 0.5 M A- can change after 0.02 moles of strong base are added. The strong base dissociates completely, so 0.02 M OH- is present. That amount lowers the acid concentration to 0.48 M and raises the base concentration to 0.52 M. The new values can be placed into the Henderson-Hasselbalch equation with the acid pKa to find the pH. This also helps estimate buffer capacity, or how much strong acid or strong base a buffer can absorb without a large pH change.

Buffer capacity depends on concentration. A more concentrated buffer has a greater ability to resist pH change. It also depends on how similar the acid and base concentrations are. Buffers work best when those concentrations are close, because the ratio changes only a little when acid or base is added. Buffer pH is not the same as buffer capacity. The effective pH range is usually within 1 pH unit of the acid component's pKa.

Buffer pH and the Common Ion Effect

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Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how buffers resist pH change through the common ion effect, the Henderson-Hasselbalch equation, and buffer capacity in high school chemistry.

Buffer pH and the Common Ion Effect | JoVE K12

通过这段 8 分钟的教师指南，准备教授 缓冲液。涵盖 JoVE Lab Manual 上 Chemistry 实验的关键准备步骤。

Chemistry 中的 缓冲液 教师准备 - 1.4K+ 观看次数 | JoVE Lab Manual

在这段 15 分钟的学生实验流程视频中，逐步学习 缓冲液 的操作步骤。属于 JoVE Lab Manual 上的 Chemistry 系列。

Chemistry 中的 缓冲液 学生实验流程 - 5.2K+ 观看次数 | JoVE Lab Manual

Buffer pH and the Common Ion Effect

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