Name: acids, bases, and ph in titration
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 6 min 35 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

1.2.1 9L.4.2.2.1

Acid and Base Concentrations

Un ácido Arrhenius es una sustancia que produce iones de hidrógeno cuando se disuelve en agua, mientras que una base produce iones de hidróxido. Los iones de hidrógeno reaccionan inmediatamente con el agua para formar iones de hidronio, pero para simplificar, seguiremos pensando en ellos como iones de hidrógeno. Dependiendo de la cantidad de iones de hidrógeno o iones de hidróxido en la solución, se considera ácido o básico.
Medimos la cantidad de acidez o basicidad utilizando el pH, que se calcula como el logaritmo negativo de la concentración de iones de hidrógeno. Por lo tanto, los valores de pH inferiores a 7 son ácidos y los valores de pH superiores a 7 son básicos. pH 7 es neutro.
Los ácidos y las bases también se comparan en función de su fuerza, que es diferente de su pH. La fuerza de un ácido está relacionada con la facilidad con la que el ion de hidrógeno se disocia del anión, llamado base conjugada. La misma idea se sigue para una base en referencia al ion hidróxido y su ácido conjugado. Podemos asignar un valor a esa fuerza usando la constante de disociación ácida, o Ka.
Ka se define utilizando las concentraciones del ácido no disociado y los iones de hidrógeno disociados y la base conjugada. A menudo se puede ver esta relación representada como pKa, que es simplemente el logaritmo negativo de Ka. Cuanto más pequeño es el pKa, más fuerte es el ácido.
Algunos ácidos, como el ácido clorhídrico, son monopróticos, lo que significa que solo pueden disociar un ion de hidrógeno. Los ácidos polipróticos, al igual que el ácido fosfórico, pueden disociar varios iones de hidrógeno. Cada disociación tiene su propio pKa.
Entonces, ¿cómo podemos determinar pKa? Una forma es realizar una valoración ácido-base. La valoración se realiza añadiendo lentamente una solución de concentración conocida a una solución de concentración desconocida mientras se observa la reacción entre ellas. En este caso, el ácido reacciona con la base en una reacción de neutralización para formar una sal y agua.
Por lo tanto, si queremos medir la concentración de iones de hidrógeno en un ácido, simplemente podemos valorar con una base fuerte con una concentración de iones de hidróxido conocida hasta que el ácido se neutralice. Para realizar una valoración con precisión, la base debe estar estandarizada, lo que significa que conoce la concentración exacta de iones de hidróxido. Esto no siempre es sencillo.
Por ejemplo, el NaOH, que usarás en tu experimento, es muy higroscópico, lo que significa que absorbe agua de la atmósfera. Esto le sucede al NaOH tanto en forma sólida como en solución. Por lo tanto, la concentración real de una solución de NaOH puede ser más baja de lo que cabría esperar.
Para determinar la concentración exacta de NaOH, primero debemos realizar una valoración ácido-base. Para hacer esto, debe usar la base para valorar un ácido con una concentración conocida. El ftalato de hidrógeno de potasio, KHP, es un ácido no higroscópico, por lo que podemos calcular con precisión su concentración a partir de su masa.
Podemos ver cuándo se completa la valoración, lo que significa que el ácido se neutraliza, mediante el uso de un indicador de pH como la fenolftaleína. La fenolftaleína es neutra e incolora entre aproximadamente pH 0 y pH 8.
A medida que aumenta el pH, dos iones de hidrógeno se disocian. Esta forma aniónica es de color rosa. Entonces, cuando comenzamos la valoración, la solución de KHP es ácida y la fenolftaleína es incolora. A medida que añadimos NaOH y los iones de hidrógeno se neutralizan, el pH aumenta.
En esta reacción, la solución es neutra cuando se han mezclado cantidades iguales de ácido y base. Después de eso, la adición de un poco más de NaOH hace que el pH sea básico y la solución se vuelva rosada. Esto se conoce como punto de conexión. Si conocemos los moles de KHP y el volumen de NaOH utilizado para neutralizarlo, podemos calcular la concentración exacta de la base.
Una vez que tenemos una base estandarizada, podemos determinar el pKa de un ácido valorando una concentración conocida del ácido con nuestra base estandarizada mientras monitoreamos el pH. El gráfico del pH frente al volumen de base añadida se denomina curva de valoración. La curva suele seguir una forma S o sigmoidal, donde el punto de inflexión de la parte más empinada de la curva denota un punto de equivalencia.
Aquí, los moles de iones de hidróxido e iones de hidrógeno disociados son iguales. Al igual que pKa, veremos un punto de equivalencia para cada ion de hidrógeno disociado. Por lo tanto, un ácido monoprótico tiene solo un punto de equivalencia y un ácido triprótico tiene tres.
Cuando realicemos la valoración, sabremos que hemos pasado el punto de equivalencia cuando el indicador de pH apenas pasa de incoloro a rosa. Esto se denomina punto de conexión de valoración. Al igual que cuando estandarizamos la base, esto es cuando la solución tiene un pequeño exceso de iones de hidróxido y, por lo tanto, es ligeramente básica.
Otro punto de inflexión en el gráfico se produce a mitad de camino del punto de equivalencia. Aquí, las concentraciones de los ácidos disociados y no disociados son iguales. Por lo tanto, el pH en este punto es igual al pKa. Por lo tanto, si realizamos una valoración y determinamos el volumen del punto de equivalencia, entonces podemos calcular pKa como el pH a la mitad de este volumen.
En este laboratorio, primero estandarizará la base y, a continuación, realizará una valoración con esa base estandarizada para determinar dos pKa de un ácido poliprótico.

concentraciones de ácido y base

<h4>Ácidos y Bases</h4>
Un ácido Arrhenius produce iones de hidrógeno cuando se disuelve en agua:
HA + H2 O →H+(aq) + A-(aq)
Aquí, HA es el ácido no disociado, H+ es el catión hidrógeno y A- es el anión solvatado, llamado base conjugada. Una base de Arrhenius produce iones de hidróxido cuando se disuelve en agua:
BOH + H2O → B+(aq) + OH-(aq)
Aquí, BOH es la base no disociada, OH- es el ion hidróxido y B+ es el catión solvatado, llamado ácido conjugado. Una base conjugada se forma cuando un ácido pierde un ion de hidrógeno y tiene el potencial de ganar un hidrógeno. Lo mismo ocurre con un ácido conjugado, que se forma cuando una base pierde un grupo hidroxilo y tiene el potencial de recuperarlo. Cada ácido tiene una base conjugada, y cada base tiene un ácido conjugado.
<h4>pH</h4>
El pH es el grado de acidez de la solución y es una medida de la cantidad de iones de hidrógeno en una solución. La escala de pH es logarítmica y va de 0 a 14; Las soluciones acuosas con un pH inferior a 7 se describen como ácidas, y las soluciones acuosas con un pH superior a 7 se describen como alcalinas o básicas. Las soluciones a pH 7 se consideran neutras.
El pH de una solución es igual al logaritmo negativo en base diez de la concentración de iones de hidrógeno en la solución.
<img src="/CDNSource/lm/labs/44/44_Concepts_3.jpg">
El agua interactúa fuertemente con el ion hidrógeno porque su fuerte carga positiva atrae el polo negativo de las moléculas de agua circundantes. De hecho, interactúan tan fuertemente que forman un enlace covalente y el catiónH3O+, llamado hidronio. La ecuación anterior se reescribe para reflejar esto.
<img src="/CDNSource/lm/labs/44/44_Concepts_4.jpg">
Para simplificar, nos referiremos a la concentración de iones de hidrógeno en lugar de iones de hidronio cuando analicemos el pH. Cuanto más bajo es el valor de pH de una solución, más iones de hidrógeno están presentes y, por extensión, más ácida es la solución. Por ejemplo, el pH de 1 mM de ácido sulfúrico es 2,75, mientras que el pH de 1 mM de clorhídrico es 3,01. La concentración de iones de hidrógeno en la solución de ácido sulfúrico se calcula como 1 × 10-2.75, mientras que la concentración de iones de hidrógeno en la solución de ácido clorhídrico es 1 × 10-3.01. Por lo tanto, hay más iones de hidrógeno presentes en el ácido sulfúrico y es más ácido. Recuerde, aunque el pH de dos soluciones puede variar en tan solo medio valor de pH, debido a la naturaleza logarítmica de la escala de pH, la cantidad de hidrógeno varía mucho.
<h4>Fuerza de ácidos y bases</h4>
La fuerza de un ácido se ve afectada por la electronegatividad de la base conjugada y la polaridad del hidrógeno ácido. La fuerza, por lo tanto, se refiere a la facilidad con la que el catión de hidrógeno (H+) se disocia del anión. Los ácidos y bases fuertes se disocian completamente en soluciones acuosas, mientras que los ácidos y bases débiles solo se disocian parcialmente en sus iones conjugados.
La constante de disociación, Ka, representa la fuerza del ácido. Ka se calcula utilizando las concentraciones del ácido no disociado HA, y las concentraciones de los cationes de hidrógeno y la base conjugada, A-. Los valores más altos de Ka representan ácidos más fuertes, mientras que los valores más bajos de Ka representan ácidos más débiles.
<img src="/CDNSource/lm/labs/44/44_Concepts_5.jpg">
Ka es numéricamente muy pequeño, y a menudo se informa en forma de pKa, que es el logaritmo negativo en base diez de Ka. Los valores más bajos de pKa corresponden a un ácido más fuerte, mientras que los valores más altos de pKa corresponden a un ácido más débil.
<img src="/CDNSource/lm/labs/44/44_Concepts_6.jpg">
Algunos ácidos disocian solo un ion de hidrógeno y, por lo tanto, tienenun valor de pK a. Estos ácidos se denominan monopróticos. Sin embargo, algunos ácidos pueden disociar más de un ion de hidrógeno y se denominan polipróticos. Estos ácidos tienen un valorde pK a para cada disociación de iones de hidrógeno.
pKa también se puede utilizar para calcular el pH de equilibrio de una reacción ácido-base, como se muestra en la ecuación de Henderson-Hasselbalch.
<img src="/CDNSource/lm/labs/44/44_Concepts_7.jpg">
La ecuación de Henderson-Hasselbalch se utiliza para calcular el pH, cuando se conocen las concentraciones de la base conjugada y el ácido débil, o para calcular el pKa si se conocen el pH y las concentraciones.
<h4>Valoración</h4>
Las reacciones ácido-base se estudian cuantitativamente mediante valoración. En un experimento de valoración, se utiliza una solución de una concentración conocida, llamada solución estándar, para determinar la concentración de otra solución. Para las valoraciones ácido-base, se añade lentamente una solución estandarizada de base a un ácido de concentración desconocida (o el ácido se añade a la base). La reacción ácido-base es una reacción de neutralización, que forma una sal y agua. Cuando los moles de iones de hidrógeno en el ácido son iguales a los moles de iones hidroxilo agregados desde la base, la solución alcanza un pH neutro.
Para realizar una valoración ácido-base, la base estandarizada se añade lentamente a un matraz agitador del ácido desconocido utilizando una bureta, lo que permite la medición del volumen y la adición gota a gota de la base. El pH de la solución se controla de cerca durante toda la valoración utilizando un indicador de pH añadido al ácido. Por lo general, la fenolftaleína se usa ya que la solución permanece incolora hasta que se vuelve básica, volviéndose de un rosa claro.
A medida que la valoración se acerca al punto de equivalencia, que es cuando los moles de iones de hidrógeno son iguales a los moles de iones hidroxilo agregados, el indicador de pH cambia temporalmente de color debido a un exceso de iones hidroxilo. Cuando se agita el matraz, vuelve el color ácido del indicador de pH. La valoración se ha completado y ha alcanzado su punto final cuando un pequeño exceso de iones hidroxilo cambia el indicador permanentemente a su color básico.
La curva de valoración es un gráfico del pH de una solución frente al volumen de base estandarizada añadida. El punto de equivalencia se encuentra en el punto de inflexión de la curva y se calcula como la segunda derivada de la curva de valoración.
Si un ácido es poliprótico, tendrá múltiples puntos de equivalencia, uno para cada disociación de iones de hidrógeno. El pH en el punto medio del punto de equivalencia para los ácidos monopróticos, o entre los puntos de equivalencia en el caso de los ácidos polipróticos, es igual al pKa del ácido.
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química y reactividad química. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Química: La naturaleza molecular de la materia y el cambio. Boston, MA: McGraw-Hill.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análisis químico cuantitativo. Nueva York, NY: W.H. Freeman and Company.</li>
</ol>

Concentraciones de ácido y base

Ácidos y Bases
Un ácido Arrhenius produce iones de hidrógeno cuando se disuelve en agua:
HA + H2O → H+(aq) + A-(aq)
Aquí, HA es el ...

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acids, bases, and ph in titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid.

pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14 and is logarithmic, so small changes in pH can mean large changes in hydrogen ion concentration. Solutions below pH 7 are acidic, solutions above pH 7 are basic or alkaline, and solutions at pH 7 are neutral. Water strongly attracts hydrogen ions and forms hydronium, H3O+, but pH is often discussed using hydrogen ion concentration for simplicity.

Lower pH means more hydrogen ions and greater acidity. For example, 1 mM sulfuric acid has a pH of 2.75, while 1 mM hydrochloric acid has a pH of 3.01. Because the pH scale is logarithmic, even a difference of about half a pH unit can reflect a large difference in hydrogen ion amount. In this example, sulfuric acid is more acidic because it contains more hydrogen ions.

Acid and base strength describes how readily hydrogen ions separate from the anion. Strong acids and bases dissociate completely in water, while weak acids and bases dissociate only partly into their conjugate ions. Acid strength is measured with the dissociation constant, Ka, which uses the concentrations of the non-dissociated acid HA, hydrogen cations, and conjugate base A-. Larger Ka values indicate stronger acids, and smaller values indicate weaker acids. Ka is often written as pKa, the negative base-10 log of Ka, so lower pKa means a stronger acid and higher pKa means a weaker acid.

Some acids release only one hydrogen ion, so they have one pKa and are called monoprotic. Others release more than one hydrogen ion and are polyprotic, with a pKa value for each step. The Henderson-Hasselbalch equation uses pKa to find the equilibrium pH of an acid-base reaction when the weak acid and conjugate base concentrations are known, or to find pKa when pH and concentrations are known.

Acid-base reactions can also be studied by titration. In a titration, a standard solution with a known concentration is used to find the concentration of another solution. For acid-base titrations, a standardized base is added slowly to an unknown acid, or the acid is added to the base. The reaction is a neutralization reaction that produces salt and water. When the moles of hydrogen ions match the moles of hydroxyl ions added, the solution reaches neutral pH.

During the experiment, the standardized base is added drop by drop from a burette to a stirring flask of acid. The pH is monitored with an indicator such as phenolphthalein, which stays colorless until the solution turns basic and becomes light pink. Near the equivalence point, the indicator may change color because of a temporary excess of hydroxyl ions, but swirling can return its acidic color. The endpoint is reached when a tiny extra amount of hydroxyl ions makes the basic color stay permanent.

A titration curve plots pH against the volume of standardized base added. The equivalence point appears at the inflection point of the curve and is found from the second derivative. Polyprotic acids have multiple equivalence points, one for each hydrogen ion dissociation. For monoprotic acids, the pH halfway to the equivalence point, or between equivalence points for polyprotic acids, is equal to the pKa.

Acids, Bases, and pH in Titration

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Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

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

Videos completos y enfocados en el plan de estudios, tanto para los instructores como para los estudiantes de los cursos de laboratorio de introducción a la química.

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Learn how acids, bases, pH, Ka, and pKa connect to titration, neutralization, and titration curves in high school chemistry.

Acids, Bases, and pH in Titration | JoVE K12

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Chemical Reactions

HS-PS1-11.

Georgia Standards of Excellence

SC3.

Indicators of Chemical Reactions

SC6.

Neutralization Reactions

Prepárese para enseñar Concentraciones de ácido y base con esta guía para instructores de 5 minutos. Cubre los pasos clave de preparación para el laboratorio Chemistry en JoVE Lab Manual.

Preparación del instructor para Concentraciones de ácido y base en Chemistry - Más de 6.2K visualizaciones | JoVE Lab Manual

Siga los procedimientos paso a paso de Concentraciones de ácido y base en este video de protocolo estudiantil de 15 minutos. Parte de la colección Chemistry en JoVE Lab Manual.

Protocolo estudiantil de Concentraciones de ácido y base en Chemistry - Más de 31K visualizaciones | JoVE Lab Manual

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Matter and Its Interactions: Plan and conduct an investigation of acid-base reactions to test ideas about the concentrations of the hydronium ion in an aqueous solution (pH). (P:3, CC: 3, CI: PS1)

9C.1.2.1.2

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Acids and Bases
An Arrhenius acid produces hydrogen ions when it is dissolved in water:
HA + H2O &#8594; H+(aq) + A-(aq)
Here, HA is the non-dissociated ...

Strong Acid and Base Solutions

A strong acid is a compound that dissociates completely in an aqueous solution and produces a concentration of hydronium ions equal to the initial ...

Polyprotic Acids

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ...

Water: A Bronsted-Lowry Acid and Base

The reaction between a Br&#248;nsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, ...

Weak Base Solutions

Some compounds produce hydroxide ions when dissolved by chemically reacting with water molecules. In all cases, these compounds react only partially and ...

Weak Acid Solutions

Few compounds act as strong acids. A far greater number of compounds behave as weak acids and only partially react with water, leaving a large majority of ...

The potential of hydrogen (pH) is a measure of the acidity or basicity of a water-based solution determined by the concentration of hydronium ions (H3O+). ...

water-quality-analysis-via-indicator-organisms

Water Quality Analysis via Indicator Organisms

Source: Laboratories of Dr. Ian Pepper and Dr. Charles Gerba -The University of Arizona
Demonstrating Author: Luisa Ikner
Water quality analysis monitors ...

Matter and Its Interactions: Obtain and communicate information about how Minnesota American Indian Tribes and communities and other cultures construct solutions to mitigate threats to biodiversity.* (P: 8, CC: 7, CI: LS2, ETS1)

1 Exploring phenomena or engineering problems
 1.2 Planning and carrying out investigations
  1.2.1 Students will be able to design and conduct investigations in the classroom, laboratory, and/or field to test students’ ideas and questions and will organize and collect data to provide evidence to support claims the students make about phenomena.

Acids, Bases, and pH in Titration

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