Name: tracking ph during acid-base titration
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 23 s
Channel: JoVE

K12 Standards

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Ohio’s Learning Standards and Model Curriculum

9-12

Chemistry

Interactions Of Matter

Chemical reactions

Acid-Base Titration Curves

산-염기 적정은 알려진 염기 농도를 사용하여 미지의 산 농도를 결정하기 위해 수행될 수 있으며 그 반대의 경우도 마찬가지입니다. 산을 염기로 적정할 때 적정제가 점차 산에 첨가되어 pH를 상승시킵니다. pH 변화가 베이스의 볼륨에 대해 플로팅되여 적정 또는 pH 곡선을 생성합니다.

적정 곡선은 S형이지만 산의 농도에 따라 차이가 있습니다. 적정 시작 시 pH는 산성이지만 초기 산의 농도가 동일하다고 가정할 때 약한 산의 경우 더 높습니다. 당량점은 염기의 몰이 용액의 산의 몰과 같아지는 적정 곡선의 점입니다.

강한 산의 경우 강한 염기가 강한 산에서 생성된 모든 하이드로늄 이온을 중화시킬 때 당량점에 도달합니다. 결국 항상 pH 7에서 발생합니다. 반대로 약한 산의 당량점은 7보다 큰 pH에서 발생하며 모든 약한 산이 짝염기로 전환될 때 도달합니다.

당량점 근처에서 용액의 pH가 갑자기 증가합니다. 그러나 약산 적정에서 이러한 급격한 pH 변화는 강산 적정만큼 가파르지 않습니다. 약산과 강산 모두에 대해 적정곡선의 당량점 이후의 pH는 과도한 강염기로 인해 점차 증가합니다.

약산 적정 곡선의 다른 특징으로는 강하지 않은 완충 영역과 반 등가점이 있습니다. 강염기가 첨가되면 약산은 짝염기를 생성하여 완충액을 만듭니다. 이 완충 영역 내에 반등가점이 있습니다.

이 점은 당량점에 도달하기 위해 필요한 염기 볼륨의 절반이 추가된 경우입니다. 여기서 약산의 농도는 그것의 짝염기와 같고 용액의 pH는 pKa와 같습니다 적정제로 강산을 사용하는 강염기 또는 약염기의 적정 곡선은 역 S자형이며 pH가 처음에는 높고 강산을 첨가하면 감소합니다.

<ol>
	<li>Ramette, Richard W. "적정 : 산 염기 적정을위한 학습 도구." (1994) : 238. <a href = "https://pubs.acs.org/doi/pdf/10.1021/ed071p238.2">https://pubs.acs.org/doi/pdf/10.1021/ed071p238.2</a></li>
</ol>

산-염기 적정 곡선

적정 곡선은 첨가된 적정제의 양에 대한 일부 용액 특성의 도표입니다. 산-염기 적정의 경우 용액 pH는 용액 조성에 따라 예측 가능하게 달라지므로 적정 진행 상황을 모니터링하고 엔드포인트를 감지하는 데 사용될 수 있으므로 모니터링하는 데 유용한 특성입니다. 산-염기 적정은 강산과 강염기, 강산과 약염기, 강염기와 약산을 사용하여 수행할 수 있습니다.

0.100M의 강염기 NaOH를 사용하여 25.00mL의 0.100M HCl(강산)에 대해 수행된 적정의 경우 적정 곡선은 그림 1a에서 빨간색으로 볼 수 있습니다. 0.100 M NaOH를 사용하여 0.100 M CH_3COOH(약산) 25.00 mL에 대해 수행된 적정의 경우 적정 곡선은 그림 1b에서 노란색으로 볼 수 있습니다.

<img alt="Image1" src="/files/ftp_upload/11453/11453_Image_1v2.png" style="text-align: center; height: 350px;" />

그림 1 (a) 0.100 M HCl(강산) 25.00mL를 0.100M NaOH(강염기)로 적정하는 적정 곡선의 당량점은 pH 7.00입니다. (b) 0.100M NaOH(강염기)를 사용하여 0.100M 아세트산(약산) 25.00mL의 적정에 대한 적정 곡선의 당량점은 pH 8.72입니다.

강산을 이용한 강염기 또는 약염기의 적정은 유사한 S자형 곡선을 갖습니다. 그러나 pH가 염기성 영역에서 시작하여 강산을 첨가하면 감소하므로 곡선이 반전됩니다. 약산이나 염기의 적정을 사용하여 각각 K_a 또는 K_b를 결정할 수도 있습니다.

적정의 4단계:

<ol>
	<li>초기 상태(추가된 적정제 용량 = 0mL): pH는 적정되는 산에 의해 결정됩니다. 두 개의 산성 샘플이 동일하게 농축되어 있기 때문에 약산은 더 큰 초기 pH를 나타냅니다.</li>
	<li>전당량점(0mL < V < 25mL): 용액 pH가 점차 증가하고 첨가된 적정제와 반응하여 산이 소모됩니다. 조성에는 미반응 산과 반응 생성물, 그 짝염기가 포함됩니다.</li>
	<li>당량점(V = 25 mL): 용액 조성이 산성에서 중성(강산 시료의 경우) 또는 염기성(약산 시료의 경우)으로 전환됨에 따라 pH의 급격한 상승이 관찰되며 이때 pH는 산의 짝염기의 이온화에 의해 결정됩니다.</li>
	<li>후당량점(V > 25 mL): pH는 첨가된 과잉 강염기 적정제의 양에 따라 결정됩니다. 두 샘플 모두 동일한 적정제로 적정되므로 이 단계에서는 두 적정 곡선이 모두 유사하게 나타납니다.</li>
</ol>

이 문서는 에서 발췌되었습니다 <a href="https://openstax.org/books/chemistry-2e/pages/14-7-acid-base-titrations"> Openstax, Chemistry 2e, Section 14.7: Acid-base Titrations.</a>

적정 곡선은 첨가된 적정제의 양에 대한 일부 용액 특성의 플롯입니다. 산-염기 적정의 경우, 용액 pH는 ...

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JoVE Core

화학

하나

Acid-base and Solubility Equilibria

산-염기 및 용해도 평형

KEY TERMS AND CONCEPTS

주요 용어 및 개념

tracking ph during acid-base titration

Acid-base titration uses pH changes to track how a solution responds as titrant is added. A titration curve is a graph of a solution property versus the amount of added titrant. In acid-base titrations, pH is the main property to follow because it changes in a predictable way with the solution makeup and helps show the endpoint.

This pattern can be seen with a strong acid and strong base, a strong acid and weak base, or a strong base and weak acid. For example, 25.00 mL of 0.100 M HCl titrated with 0.100 M NaOH gives one curve, while 25.00 mL of 0.100 M acetic acid titrated with 0.100 M NaOH gives another. The strong acid curve reaches an equivalence point at pH 7.00, while the weak acid curve reaches an equivalence point at pH 8.72.

A titration of a base with a strong acid follows a similar S-shaped pattern, but the curve is flipped. The pH starts in the basic region and drops as the strong acid is added. Weak acids and weak bases can also be used to find Ka or Kb, respectively.

The first stage is the initial state, when no titrant has been added. The pH depends on the acid being titrated. Because the two acid samples in the example have the same concentration, the weak acid starts at a higher pH than the strong acid.

The next stage is the pre-equivalence point, which is 0 mL 25 mL, the pH is set by the extra strong base that has been added. Since both example solutions are titrated with the same base, their curves look similar in this final stage.

Tracking pH During Acid-Base Titration

common ion effect

Common Ion Effect on Solubility

The common ion effect reduces the solubility of an ionic compound in a solution that already contains one of its ions. In aqueous solutions, this means less of the solid dissolves than it would in pure water. The effect follows the law of mass action and can be explained with Le Châtelier’s principle. Silver iodide shows this equilibrium clearly. When silver(I) ions or iodide ions are added, the dissolution equilibrium shifts left. More AgI precipitates, and the concentrations of dissolved Ag+...

buffers

Buffer Solutions and pH Stability

Buffer solutions are mixtures that help keep pH steady. A buffer contains an appreciable amount of a weak conjugate acid-base pair. It resists a change in pH when small amounts of a strong acid or a strong base are added. One common buffer uses a weak acid and its salt. Acetic acid and sodium acetate form this type of system, written as CH3COOH(aq) + CH3COONa(aq). Another buffer uses a weak base and its salt. A solution of ammonia and ammonium chloride, NH3(aq) + NH4Cl(aq), is an example. The...

calculating ph changes in a buffer solution

calculating-ph-changes-in-a-buffer-solution

Buffer pH After Acid or Base Addition

A buffer solution can resist sudden pH changes when a strong acid or strong base is added. It does not stop the pH from changing completely. The shift is usually small, as long as the addition stays within the buffer’s capacity. To find the new pH, first determine how the added acid or base changes the amounts of the buffer components. A buffer contains a weak acid and its conjugate base, or the reverse pair. These amounts are found with stoichiometric calculations, which track how much of...

buffer effectiveness

Buffer Capacity and pH Stability

Buffer solutions help keep pH fairly constant, but they do not work forever. Their resistance to pH change depends on having enough of a weak acid and its conjugate base, or a weak base and its conjugate acid, in the mixture. When too much strong acid or strong base is added, one part of the buffer pair is used up and the buffer action becomes weaker. Buffer capacity describes how much acid or base can be added to a fixed volume of buffer before the pH changes noticeably, usually by about one...

titration calculations: strong acid - strong base

titration-calculations-strong-acid-strong-base

pH at Key Points in Acid-Base Titration

pH at key points in a strong acid and strong base titration can be calculated from the amounts of acid and base present. The example uses 25.00 mL of 0.100 M HCl, a strong acid, and 0.100 M NaOH, a strong base. The pH changes as base is added, and each stage follows a different calculation. At 0 mL of titrant, the pH comes from the ionization of HCl. Because HCl is a strong acid, its ionization is complete. The hydronium ion molarity is 0.100 M, and the pH is found from that value. At 12.50...

titration calculations: weak acid - strong base

titration-calculations-weak-acid-strong-base

pH at Key Points in a Weak Acid Titration

pH at key points in a weak acid titration can be calculated for an acetic acid solution as sodium hydroxide is added. The example uses 25.00 mL of 0.100 M CH3CO2H titrated with 0.100 M NaOH. The reaction is shown as the weak acid reacts with the strong base. The initial pH is found from the acetic acid solution using the usual ICE approach. ICE means initial, change, and equilibrium, and it is a common way to set up equilibrium problems. The solution pH is calculated before any titrant is...

indicators

Acid-Base Color Changes

Acid-base indicators show color changes that help identify the pH of a solution. These are often weak organic acids or weak organic bases, and they respond to changes in hydronium ion concentration in dilute aqueous solution. Phenolphthalein is one example. It is colorless when the hydronium ion concentration is greater than 5.0 × 10^-9 M, which means the pH is below 8.3. In more basic solutions, where the hydronium ion concentration is lower, it turns red or pink. Methyl orange shows how an...

titration of a polyprotic acid

Polyprotic Acid Equivalence Points

Polyprotic acids have more than one ionizable hydrogen, so they lose protons in steps during titration. When the acid dissociation constants for those protons are far enough apart, the titration curve shows a separate equivalence point for each hydrogen that can ionize. Diprotic acids have two ionizable hydrogens and produce two equivalence points. Triprotic acids have three ionizable hydrogens and produce three equivalence points. Carbonic acid, H2CO3, is a weak diprotic acid. Carbonic acid...

solubility equilibria

Predicting Precipitation with Ksp

Solubility equilibria explain how a dissolved ionic compound and its solid form can exist in balance. In a saturated solution, the solute dissolves and the ions precipitate at equal rates. These ideas matter in real settings such as tooth decay and water purification, where compound solubility affects the outcome. A substance with finite solubility can form a saturated solution when more solute is added than the solvent can dissolve. The result is a heterogeneous mixture of the saturated...

factors affecting solubility

Common Ion Effects on Solubility

The common ion effect lowers the solubility of an ionic compound in water when the solution already contains one of the ions made by that compound. It follows the law of mass action and can be explained with Le Châtelier’s principle. When a common ion is added, the dissolution equilibrium shifts toward the solid, so less of the salt stays dissolved. Silver iodide shows this effect clearly. If silver ions or iodide ions are added to a solution of AgI, the equilibrium shifts left and more AgI...

formation of complex ions

Ligands and Complex Ion Stability

Complex ions form when a central atom, often a transition metal cation, binds to ligands. Ligands are ions or molecules around the metal ion. They can be neutral molecules such as H2O or NH3, or ions such as CN− or OH−. In many cases, the ligands act as Lewis bases by donating a pair of electrons to the central atom. These reactions are part of coordination chemistry, which studies coordination complexes. When a metal ion reacts with one or more ligands to form a complex, the equilibrium...

precipitation of ions

Solubility Limits and Selective Precipitation

Solubility limits and selective precipitation explain when ions form a solid in water. The key idea is the balance between an insoluble compound and its dissolved ions. In aqueous solutions that contain calcium ions and carbonate ions, this equilibrium can exist even if the solution was not made by saturating water with calcium carbonate. When two soluble salts are mixed, such as sodium carbonate and calcium nitrate, the ion concentrations determine whether a precipitate forms. If the reaction...

qualitative analysis

Cation Identification by Precipitation Tests

Cation identification by precipitation tests uses selective reactions to find metal ions in a mixture. In qualitative analysis, different reagents are added step by step. Each reaction can produce a characteristic precipitate for a specific group of cations. Other steps, such as changing pH, heating the mixture to dissolve a precipitate again, or adding reagents that form complex ions, help separate ions within the same group. Group IV cations such as Ba2+, Ca2+, and Mg2+ are identified...

henderson-hasselbalch equation

Buffer pH with the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation helps relate buffer pH to the weak acid in the solution. It comes from the ionization-constant expression for a weak acid. By rearranging that expression to solve for hydronium ion concentration, [H3O+], and then taking the negative logarithm of both sides, the equation can be written in terms of pH. In this form, pKa is defined as the negative logarithm of the weak acid ionization constant, or pKa = −log Ka. The equation shows the connection between pH, pKa,...

acid-base titration curves

Acid-base titration uses pH changes to track how a solution responds as titrant is added. A titration curve is a graph of a solution property versus the amount of added titrant. In acid-base titrations, pH is the main property to follow because it changes in a predictable way with the solution makeup and helps show the endpoint. This pattern can be seen with a strong acid and strong base, a strong acid and weak base, or a strong base and weak acid. For example, 25.00 mL of 0.100 M HCl titrated...

key terms and concepts

Learn how pH changes reveal the progress of acid-base titration, from the initial state to the equivalence point and beyond, using strong and weak acid examples.

Tracking pH During Acid-Base Titration | JoVE K12

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Homogeneous Equilibria for Gaseous Reactions

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Relative Strengths of Conjugate Acid-Base Pairs

Br&#248;nsted-Lowry acid-base chemistry is the transfer of protons; thus, logic suggests a relation between the relative strengths of conjugate acid-base ...

Calculating Equilibrium Concentrations

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Strong Acid and Base Solutions

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Mixtures of Acids

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Ions as Acids and Bases

Salts with Acidic Ions
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Bronsted-Lowry Acids and Bases

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Acid/Base Strengths and Dissociation Constants

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Polyprotic Acids

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Water: A Bronsted-Lowry Acid and Base

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Acid Strength and Molecular Structure

Binary Acids and Bases
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• Equilibrium
• Acids/bases

Content elaboration: In the Physical Science course, coefficients were used to balance simple equations. Other representations, including Lewis structures and three-dimensional models, were also used and manipulated to demonstrate the conservation of matter in chemical reactions. In this course, more complex reactions will be studied, classified and represented with balanced chemical equations and three-dimensional models. Classifying reactions into types can be a helpful organizational tool for recognizing patterns of what may happen when two substances are mixed. Teachers should be aware that the common reaction classifications that are often used in high school chemistry courses may lead to misconceptions because they are not based on the actual chemistry, but on surface features that can be similar from one system to another (e.g., exchanging partners), even though the underlying chemistry is not the same. However, these classifications may be useful in making predictions about what happens when two substances are mixed. Some general types of chemical reactions are oxidation/reduction, synthesis, decomposition, single replacement, double replacement (including precipitation reactions and some acid-base neutralizations) and combustion reactions. Some reactions can fit into more than one category. For example, a single replacement reaction can also be classified as an oxidation/reduction reaction. Identification of reactions involving oxidation and reduction as well as indicating what substance is being oxidized and what is being reduced are appropriate in this course. However, balancing complex oxidation/reduction reactions is reserved for more advanced study. Organic molecules release energy when undergoing combustion reactions and are used to meet the energy needs of society (e.g., oil, gasoline, natural gas) and to provide the energy needs of biological organisms (e.g., cellular respiration). When a reaction between two ionic compounds in aqueous solution results in the formation of a precipitate or molecular compound, the reaction often occurs because the new ionic or covalent bonds are stronger than the original ion-dipole interactions of the ions in solution. Laboratory experiences (3-D or virtual) with different types of chemical reactions should be provided. Reactions occur when reacting particles collide in an appropriate orientation and with sufficient energy. The rate of a chemical reaction is the change in the amount of the reactants or products in a specific period of time. Increasing the probability or effectiveness of the collisions between the particles increases the rate of the reaction. Therefore, changing the concentration of the reactants, changing the temperature or the pressure of gaseous reactants, or using a catalyst, can change the reaction rate. Likewise, the collision theory can be applied to dissolving solids in a liquid solvent and can be used to explain why reactions are more likely to occur between reactants in the aqueous or gaseous state than between solids. The rate at which a substance dissolves should not be confused with the amount of solute that can dissolve in a given amount of solvent (solubility). Mathematical treatment of reaction rates is reserved for more advanced study. Computer simulations can help visualize reactions from the perspective of the kinetic-molecular theory. In middle school, the differences between potential and kinetic energy and the particle nature of thermal energy were introduced. For chemical systems, potential energy is in the form of chemical energy and kinetic energy is in the form of thermal energy. The total amount of chemical energy and/or thermal energy in a system is impossible to measure. However, the energy change of a system can be calculated from measurements (mass and change in temperature) from calorimetry experiments in the laboratory. Conservation of energy is an important component of calorimetry equations. Thermal energy is the energy of a system due to the movement of its particles. The thermal energy of an object depends upon the temperature and chemical composition. Some materials require little energy to change their temperature and other materials require a great deal to change their temperature by the same amount. Specific heat is a measure of how much energy is needed to change the temperature of a specific mass of material a specific amount. Specific heat values can be used to calculate the thermal energy change, the temperature (initial, final or change in) or mass of a material in calorimetry. Water has a particularly high specific heat capacity, which is important in regulating Earth's temperature. As studied in middle school, chemical energy is the potential energy associated with chemical systems. Chemical reactions involve valence electrons forming bonds to yield more stable products with lower energies. Energy is required to break interactions and bonds between the reactant atoms and energy is released when an interaction or bond is formed between the atoms in the products. Molecules with weak bonds (e.g., ATP) are less stable and tend to react to produce more stable products, releasing energy in the process. Generally, energy is transferred out of the system (exothermic) when the products have stronger bonds than the reactants and is transferred into the system (endothermic) when the reactants have stronger bonds than the products. Predictions of the energy requirements (endothermic or exothermic) of a reaction can be made given a table of bond energies. Graphic representations can be drawn and interpreted to represent the energy changes during a reaction. The role of energy in determining the spontaneity of chemical reactions is dealt with conceptually in this course. Entropy and its influence on the spontaneity of reactions are reserved for more advanced study. All reactions are reversible to a degree and many reactions do not proceed completely toward products but appear to stop progressing before the reactants are all used up. At this point, the amounts of the reactants and the products appear to be constant and the reaction can be said to have reached dynamic equilibrium. Dynamic equilibrium means the rate of the reverse reaction is equal to the rate of the forward reaction so there is no apparent change in the reaction. If a chemical system at equilibrium is disturbed by a change in the conditions of the system (e.g., increase or decrease in the temperature, pressure on gaseous equilibrium systems, concentration of a reactant or product), then the equilibrium system will respond by shifting to a new equilibrium state, reducing the effect of the change (Le Chatelier's Principle). If products are removed as they are formed during a reaction, then the equilibrium position of the system is forced to shift to favor the products. In this way, an otherwise unfavorable reaction can be made to occur. Mathematical treatment of equilibrium reactions is reserved for advanced study. Computer simulations can help visualize the progression of a reaction to dynamic equilibrium and the continuation of both the forward and reverse reactions after equilibrium has been attained. Properties of acids and bases and the ranges of the pH scale were introduced in Physical Science. In this course, the structural features of molecules are explored to further understand acids and bases. Acids often result when hydrogen is covalently bonded to an electronegative element and is easily dissociated from the rest of the molecule to bind with water to form a hydronium ion (H3O+). The acidity of an aqueous solution can be expressed as pH, where pH can be calculated from the concentration of the hydronium ion. Bases are likely to dissociate in water to form a hydroxide ion. Acids can react with bases to form a salt and water. Such neutralization reactions can be studied quantitatively by performing titration experiments.

Tracking pH During Acid-Base Titration

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