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

缓冲液是可在系统中保持恒定 pH 值的溶液，无论是否添加强酸或强碱。在没有缓冲液的情况下，向溶液中添加强酸或强碱会显着改变 pH 值。缓冲器在日常生活中至关重要。例如，血液起到缓冲剂的作用，能够将 pH 值维持在 7.35 到 7.45 之间，因为 pH 值高于 7.8 或低于 6.8 会导致死亡。
那么，缓冲区是如何工作的呢？缓冲液本质上是一种弱酸或弱碱，其共轭碱或酸彼此平衡。以乙酸和乙酸钠制成的缓冲液为例。缓冲溶液含有弱酸及其共轭盐，其解离形成共轭碱乙酸盐。该溶液呈酸性，因为乙酸的 Ka 高于共轭碱的 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的平衡常数。解离常数也可以表示为 pKa 或 Ka 的负对数，其中 pKa 越小，酸越强。pKb 也是如此，其中 pKb 越小，碱基越强。
回到乙酸盐缓冲液，如果我们添加强碱，如氢氧化钠，氢氧根离子会与溶液中存在的水合氢离子反应。这会导致更多的乙酸与水反应以恢复平衡，从而形成更多的水合氢离子。如果我们添加强酸，如盐酸，溶液中会形成更多的水合氢离子。这些离子与乙酸根离子反应形成更多的乙酸。在这两种情况下，溶液 pH 值的变化都很小。
请记住，每种缓冲液都有一个特定的 pH 范围，它可以缓冲溶液。这里我们显示了乙酸缓冲液的滴定曲线。缓冲范围将突出显示。该区域由 Henderson-Hasselbalch 方程确定，该方程告诉我们缓冲液的 pH 值由弱酸的 Ka 和共轭碱与弱酸的比率决定。
远离此 pH 值，缓冲液的弱酸或弱碱已耗尽，无法缓冲 pH 值。因此，在为您的应用选择缓冲液时，请选择 pKa 接近所需 pH 值的缓冲液。
最后，所有缓冲液的缓冲能力都有限制，即在 pH 值发生显著变化之前可以添加到缓冲溶液中的酸或碱的量。例如，如果我们在乙酸和乙酸钠缓冲溶液中加入过多的酸，我们会使所有乙酸根离子质子化并积累大量水合氢离子，从而降低 pH 值。
同样，如果我们添加过多的碱，我们会使所有乙酸去质子化并积累大量氢氧根离子，从而提高 pH 值。因此，缓冲能力受弱酸和共轭盐浓度的影响。因此，1 摩尔乙酸盐缓冲液比 0.1 摩尔乙酸盐缓冲液具有更高的缓冲能力。
在本实验中，您将制备和检查具有各种 pH 值的缓冲液。然后，您将使用缓冲液测定称为中性红的 pH 指示剂的 pKa，并探索引入核黄素结合蛋白时中性红的 pKa 的变化。

缓冲区

<h4>缓冲区</h4>
当阿伦尼乌斯酸 （HA） 加入水中时，它会解离成共轭碱 （A-） 和氢阳离子 （H+）。
HA + H2O → H+（aq） + A-（aq）
溶液中存在的氢离子量决定了溶液的酸度，其中氢离子越多表示 pH 值较低或酸性越高。同样，当将强 Arrhenius 碱 （BOH） 添加到水中时，它会解离成共轭酸 （B+） 和氢氧根离子 （OH-）。
BOH + H2O → B+（aq） + OH-（aq）
通常，在溶液中添加强酸或强碱会显着改变 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 pH 值的这种变化。缓冲液是可在系统中保持恒定 pH 值的溶液，无论是否添加强酸或强碱。
大多数情况下，缓冲液的组分是弱酸或弱碱的共轭酸碱对。因此，在水中完全解离的强酸或强碱会产生非常差的缓冲液，而部分解离的弱酸或碱会产生更好的缓冲剂。当存在缓冲液时，强酸或强碱不与溶液中存在的水分子反应，而是与弱酸/共轭碱反应。这导致溶液的 pH 值几乎没有变化。
<h4>共离子效应</h4>
缓冲液通过一种称为常见离子效应的现象起作用。当将给定离子添加到已经包含给定离子的平衡混合物中时，就会发生常见的离子效应。当这种情况发生时，平衡会从形成更多的离子中转移出来。
例如，乙酸 （CH3COOH） 在水中略微解离，形成乙酸根离子 （CH3COO-） 和氢离子。
 声道3COOH（aq） H2O ⇔ H+（aq） + 声道3COO-（aq）
如果从可溶性乙酸钠中加入更多的乙酸根离子，平衡位置向左移动，形成更多未解离的乙酸，氢离子的浓度降低。在这里，常见的离子 — 乙酸盐 — 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-） 组分才能缓冲溶液。如果添加到缓冲液中的氢离子或氢氧根离子量较少，它们会导致少量一种缓冲液成分转化为另一种缓冲液成分。只要添加的离子浓度远小于缓冲液中已经存在的 HA 和 A- 的浓度，那么添加的离子对 pH 值的影响很小，因为它们被一种缓冲液成分消耗掉。当氢气或氢氧化物的浓度超过酸及其共轭碱的浓度时，缓冲作用丧失，pH值会发生变化。
<h4>Henderson-Hasselbalch 方程</h4>
弱酸的解离常数 KA 是用非解离酸 HA 的浓度以及氢离子和共轭碱的浓度 A- 来计算的。
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="酸解离常数方程Ka=[H+][A-]/[HA]，化学式。" data-altupdatedat="1754907201000">
较高的 Ka 值表示较强的酸，而较小的 Ka 值表示较弱的酸。为了确定氢离子的浓度，该方程被重新排列。在这种形式中，很明显，酸物质与碱物质的比例在确定氢离子浓度以及 pH 值方面很重要。
<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="1754907201000">
取两侧的负公对数得到 Henderson-Hasselbalch 方程。
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="亨德森-哈塞尔巴尔赫方程;pH、pKa、缓冲溶液、酸碱平衡、化学式。" data-altupdatedat="1754907201000">
Henderson-Hasselbalch 方程可以直接计算缓冲液的 pH 值，而无需先计算氢离子的浓度。
例如，它可用于测定加入 0.02 摩尔强碱后的 1 L 缓冲液的 pH 值。强碱完全解离，因此添加的羟基离子浓度为 0.02 M。这将使酸的浓度降低 0.02。假设酸 （HA） 和碱 （A-） 组分的原始浓度均为 0.5 M，则新的碱浓度增加 0.02 M 至 0.52 M，而酸浓度降低 0.02 M 至 0.48 M。知道缓冲液酸组分的 pKa，我们可以将这些新组分浓度代入 Henderson-Hasselbalch 方程以确定 pH 值。
这可用于确定缓冲液容量，或可以在不显著影响 pH 值的情况下向缓冲液中添加多少强酸或强碱。
<h4>缓冲容量</h4>
缓冲液容量是衡量缓冲液抵抗 pH 值变化能力的指标。这种能力取决于缓冲液组分的浓度，即酸及其偶联碱。缓冲液浓度越高，缓冲容量越大。这意味着必须添加更多的氢离子或更强的酸，以破坏平衡并改变缓冲液的 pH 值。
缓冲液容量还受缓冲液组分的相对浓度的影响。当组分的浓度相似时，缓冲液更有效。如果缓冲液组分比相似，则加入酸或碱时组分浓度比不会发生显著变化;必须添加大量的酸或碱以抵消比率并改变 pH 值。
缓冲液的 pH 值与其缓冲液容量不同。pH 范围是缓冲液有效的范围。通常，缓冲液的可用范围在缓冲液酸成分 pKa 的 1 个 pH 单位范围内。
<h4>引用</h4>
<ol>
	<li style="font-size:17px">Kotz， J.C.， Treichel Jr， P.M.， Townsend， J.R. （2012）.化学和化学反应性。加利福尼亚州贝尔蒙特:Brooks/Cole，圣智学习。</li>
	<li style="font-size:17px">Silberberg， M.S. （2009）.化学:物质和变化的分子性质。马萨诸塞州波士顿:麦格劳·希尔。</li>
</ol>

缓冲区
当阿伦尼乌斯酸 （HA） 加入水中时，它会解离成共轭碱 （A-） 和氢阳离子 （H+）。
HA + H2O → H+（aq） + ...

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Buffers

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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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 缓冲区。涵盖 JoVE Lab Manual 上 Chemistry 实验的关键准备步骤。

Chemistry 中的 缓冲区 教师准备 - 8.0K+ 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 缓冲区 的操作步骤。属于 JoVE Lab Manual 上的 Chemistry 系列。

Chemistry 中的 缓冲区 学生实验流程 - 25K+ 观看次数 | JoVE Lab Manual

Buffer pH and the Common Ion Effect

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