Measurement of pKa by Potentiometry

Summary

This educational video delves into the significance of the pKa value in determining the retention time of ionizing compounds in reverse-phase chromatography. The presenter introduces potentiometry as a method for measuring pKa, explaining the acid dissociation constant (Ka) and its logarithmic transformation to pKa. The Henderson-Hasselbalch equation is discussed to demonstrate how pH equals pKa at the half-equivalence point during an acid-base titration. The video guides viewers through the process of identifying this critical point and measuring pH to ascertain the pKa, emphasizing the importance of pH monitoring throughout the titration.

Takeaways

• 🔍 The video discusses the significance of pKa, the acid dissociation constant, in determining the retention time of ionizing compounds in reverse phase chromatography.
• 📊 pKa measures the degree of dissociation of an acid into its ions, and it is calculated as the negative logarithm of the acid dissociation constant (Ka).
• 🧪 The Henderson-Hasselbalch equation is introduced as a fundamental tool for understanding and calculating pKa, which relates pH, pKa, and the concentrations of the acid and its conjugate base.
• 🌡 The video explains that at the half equivalence point during an acid-base titration, the pH of the solution will be equal to the pKa of the acid, provided that the concentrations of the conjugate base and the acid are equal.
• 📝 The script outlines a step-by-step method for measuring pKa using potentiometry, which involves conducting an acid-base titration and measuring the pH at the half equivalence point.
• 🧬 The video emphasizes the importance of understanding the pH at the half equivalence point, as this is where the concentrations of the conjugate base and the acid are equal, leading to the determination of pKa.
• 🔬 Potentiometric titration is highlighted as a practical technique for measuring pKa, which involves adding a base to the acidic solution and monitoring the pH changes.
• 💡 The video provides a clear example of how to determine the endpoint of a titration and how to calculate the half equivalence point, which is crucial for pKa measurement.
• 📉 The script describes the use of a pH measurement system with a glass electrode to accurately measure the pH at the half equivalence point during the titration.
• 👨‍🏫 The presenter concludes by summarizing the process of measuring pKa through potentiometric titration and encourages viewers to apply this knowledge in their own experiments.

Q & A

What is the significance of pKa in chromatography?

pKa, or the acid dissociation constant, is crucial in controlling the retention time of ionizing compounds in reverse phase chromatography. The pH of the mobile phase significantly impacts the retention of the compound.

What is the definition of pKa?

pKa is the negative logarithm to the base 10 of the acid dissociation constant (Ka), which represents the degree of dissociation of an acid into its ions.

How does the Henderson-Hasselbalch equation relate to pKa measurement?

The Henderson-Hasselbalch equation, pH = pKa + log ([A-]/[HA]), is used to understand pKa measurement. When the concentrations of the conjugate base (A-) and the acid (HA) are equal, the pH equals the pKa.

What is the importance of the half equivalence point in pKa determination?

The half equivalence point is critical in pKa determination because at this point, the concentrations of the conjugate base (A-) and the acid (HA) are equal, which corresponds to the pH being equal to the pKa.

How can one measure the pKa of a compound using potentiometry?

One can measure the pKa of a compound using potentiometry by conducting an acid-base titration and measuring the pH at the half equivalence point, where the concentrations of the conjugate base and the acid are equal.

What is the role of sodium hydroxide in the titration process described in the script?

Sodium hydroxide, a strong base, is used in the titration process to neutralize the weak acid. The endpoint of the titration is determined when all the conjugate base (A-) has been neutralized by the sodium hydroxide.

Why is it necessary to measure the pH during a potentiometric titration?

Measuring the pH during a potentiometric titration is necessary to determine the exact point at which the concentrations of the conjugate base and the acid are equal, which is essential for accurately measuring the pKa.

What is the difference between the endpoint and the half equivalence point in an acid-base titration?

The endpoint of an acid-base titration is when all the acid has been neutralized, while the half equivalence point is when exactly half of the acid has been neutralized, resulting in equal concentrations of the conjugate base and the acid.

How can one identify when the concentrations of A- and HA are equal during titration?

One can identify when the concentrations of A- and HA are equal by measuring the pH at the half equivalence point during the titration. At this point, the pH will be equal to the pKa of the compound.

What equipment is needed to measure the pH during a potentiometric titration?

A pH measurement system, typically involving a suitable glass electrode, is needed to measure the pH during a potentiometric titration to determine the pKa accurately.

Why is it important to know the pKa value of a compound in pharmaceutical analysis?

Knowing the pKa value of a compound is important in pharmaceutical analysis because it helps in understanding the compound's ionization behavior, which influences its solubility, absorption, and distribution in the body.

Outlines

00:00

🔍 Introduction to Measuring pKa

This paragraph introduces the topic of measuring the pKa of a compound, emphasizing its importance in determining the retention time of ionizing compounds in reverse-phase chromatography. The presenter outlines the use of potentiometry as a technique to measure pKa, explaining that pKa is the negative logarithm of the acid dissociation constant (Ka), which quantifies the degree of dissociation of an acid. The Henderson-Hasselbalch equation is introduced as a fundamental tool for understanding pKa measurements, stating that pH equals pKa plus the log of the ratio of conjugate base to acid concentration. The concept is clarified by explaining that at the equivalence point in an acid-base titration, where the concentrations of the conjugate base and acid are equal, the pH will be equal to the pKa.

05:02

🧪 Conducting Acid-Base Titration for pKa Measurement

The second paragraph delves into the practical aspect of measuring pKa through acid-base titration. It uses the example of titrating a weak acid with a standard sodium hydroxide solution, explaining that the endpoint, or equivalence point, is reached when all the conjugate base (A-) is neutralized by the base. The critical point for pKa determination is identified as the half-equivalence point, where the concentrations of A- and HA are equal, which corresponds to the pH being equal to the pKa. The paragraph describes the process of adding half the volume of the base required to reach the endpoint and then measuring the pH at this point to determine the pKa. It also highlights the importance of using a pH measurement system during the titration to track the pH changes and accurately pinpoint the half-equivalence point.

10:03

📊 Determining pKa at the Half Equivalence Point

The final paragraph wraps up the explanation by focusing on the significance of the half equivalence point in pKa measurement. It reiterates that at this point, the pH of the solution will be equal to the pKa of the compound. The presenter provides a visual aid, describing a graph that shows the pH measurement at the half equivalence point, which is key for determining the pKa. The paragraph concludes by summarizing the method: by conducting a potentiometric titration and measuring the pH at the half equivalence point, one can accurately determine the pKa of a compound. The presenter thanks the viewers for watching and teases upcoming informative videos.

Keywords

💡pKa

pKa stands for the acid dissociation constant and is a measure of how acidic a substance is. It is defined as the pH at which half of the acid has dissociated into its conjugate base and half remains as the undissociated acid. In the video, pKa is central to understanding how the pH of the mobile phase in chromatography impacts the retention time of ionizing compounds. The script explains that by measuring the pH at the half equivalence point during a titration, one can determine the pKa of a compound.

💡Acid Dissociation Constant (Ka)

The acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for the dissociation of an acid into its conjugate base and a hydrogen ion (H+). The video emphasizes the importance of Ka in determining the degree of dissociation of an acid, which is crucial for understanding its behavior in chemical reactions and its retention in chromatography.

💡Potentiometry

Potentiometry is an analytical technique that measures the potential difference (voltage) between two electrodes in an electrochemical cell. In the context of the video, potentiometry is used to measure the pH changes during an acid-base titration, which helps in determining the pKa of a compound. The technique is highlighted as a simple method for pKa measurement, emphasizing its practical application in the lab.

💡Retention Time

Retention time in chromatography refers to the time it takes for a compound to pass through the system. The video discusses how the pKa of a compound, which is influenced by the pH of the mobile phase, significantly affects its retention time. This is particularly important in reverse-phase chromatography, where understanding pKa can help in optimizing the separation and analysis of compounds.

💡Conjugate Base

A conjugate base is a species formed when an acid loses a proton (H+). In the video, the concept of the conjugate base is used to explain the dissociation of an acid into its conjugate base and a hydrogen ion. The script uses the example of hydrochloric acid (HCl) dissociating into H+ and Cl- to illustrate this concept.

💡Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical equation that relates the pH of a solution to the pKa of the acid and the ratio of the concentrations of the conjugate base to the acid. The video uses this equation to explain how to calculate the pH at the half equivalence point, which is essential for determining the pKa of a compound.

💡Acid-Base Titration

An acid-base titration is a common laboratory procedure used to determine the concentration of an unknown acid by reacting it with a solution of known concentration of a base. The video describes how to conduct a potentiometric titration to measure the pKa of a compound, with a focus on identifying the half equivalence point where the concentrations of the conjugate base and the acid are equal.

💡Equivalence Point

The equivalence point in a titration is the point at which the amount of titrant added is stoichiometrically equivalent to the amount of analyte present. The video explains that at the equivalence point, all of the analyte (the acid in this case) has reacted with the titrant (the base), and no excess acid or base remains.

💡Half Equivalence Point

The half equivalence point is a term used in the video to describe the stage in an acid-base titration where half of the acid has been neutralized by the base. This is a critical point for determining the pKa because, at this stage, the concentrations of the conjugate base and the acid are equal, which allows for the calculation of the pKa using the Henderson-Hasselbalch equation.

💡Glass Electrode

A glass electrode is a type of pH electrode commonly used to measure the pH of a solution. The video mentions the use of a glass electrode to measure the pH during a potentiometric titration, which is essential for determining the pKa of a compound at the half equivalence point.

Highlights

Introduction to measuring the pKa of compounds, which is crucial for understanding retention time in ionizing compounds during reverse phase chromatography.

Explanation of pKa as the negative logarithm of the acid dissociation constant, which indicates the degree of dissociation of an acid.

Discussion on the impact of mobile phase pH on the retention of ionizing compounds in chromatography.

Introduction to potentiometry as a simple technique for measuring pKa values.

Overview of the Henderson-Hasselbalch equation, which is fundamental for understanding pKa measurements.

Description of the acid dissociation process and how it relates to the formation of the conjugate base.

Explanation of the significance of the equivalence point in acid-base titration for determining pKa.

The importance of identifying the half equivalence point where the concentration of the conjugate base equals the concentration of the acid.

Practical demonstration of conducting an acid-base titration to determine the pKa value.

Emphasis on measuring pH at the half equivalence point to find the pKa value.

The need for a pH measurement system during potentiometric titration to understand the acid's strength.

Guidance on how to measure pH at various time points during titration to accurately determine pKa.

The concept that at the half equivalence point, the pH will be equal to the pKa when the concentrations of the conjugate base and acid are equal.

Instructions on how to perform potentiometric titration to measure pKa, including the use of a glass electrode.

The conclusion that by measuring the pH at the half equivalence point, one can determine the pKa of a compound.

Final remarks summarizing the method for measuring pKa values through potentiometric titration.

Transcripts

okay so welcome to this video

let us talk about how one can measure

the pka for any given compound

because pk that is acid dissociation

constant

plays a very vital role in controlling

the retention time in case of ionizing

compound

in case if your compound undergoes

ionization the ph of the mobile phase

plays a very impact the very high impact

onto the retention of the compound as

far as reverse space chromatograph is

concerned

so in this video

we are going to talk about how one can

measure the pka value of the given

compound and i am going to explain you

measurement of the pka with the help of

a very simple technique called as the

potentiometry

so let us begin with the presentation

now

so how to measure the pk of the given

compound and if you look at the various

research articles published you will

find that there are more than 13

different techniques by which you can

measure the pka but in this video let us

understand how potentiometry will help

us in measuring the pka

so the measurement of the pka with the

help of potentiometry so what is mean by

k the ka stands for the acid

dissociation constant means how far your

acid can undergoes a dissociation

for example if it is scl hydrochloric

acid it will dissociate into h plus and

cl minus ions

so the degree of dissociation is called

as the acid dissociation constant

and the minus logarithm to the base

stain of the acid dissociation concept

is called as the pka

as like the ph okay so the henderson and

hazel badge

has established a very important

equation and by which equation we are

going to understand how to measure the

pka so the ph

equal to pka plus log of

a minus concentration to the h a

concentration now what is this a minus

and h so in this case the h a stands for

the acid or the weak acid so the acid

undergoes a dissociation of let us say h

plus a minus

right it will donate the proton and it

will form a conjugated base that is a

minus

for example in the case of scl you will

form h plus and cl minus ion

so let us understand you know

what will happen

if the a minus

and h a are exactly the same

and if you put this same value in the

above equation right you will find that

the ph equal to now pka plus log of let

us say a minus equal to 1 and h a is

also equal to 1

and then it will further get simplified

as ph equal to pka plus log 1 i hope you

must be able to easily understand it

now here is the important

term that log 1 is equal to 0

right the value of log 1 is equal to 0

and hence if you substitute the value of

log 1 in this equation

you will end up getting ph equal to pka

so what this equation talks about

this says that in case

if the concentration of your conjugated

base that is a minus

is equal to the conjugated of as the at

the concentration of h a

right then

you will end up

with the ph

whatever it may be but that will be

nothing but your pka value

right so in a sense we need to

understand what is that concentration

what is that ph

what is that ph value with a 3 or 4 or 1

at which

your conjugated concentration the

concentration of conjugated base will be

exactly equal to the concentration of an

acid

the 50 percentage of the conjugated base

and 50 percentage of the acid

that concentration you need to determine

first and measure the ph at that

particular concentration and you will

end up with the peak level so please

allow me to explain you how you can

easily do that

okay so how one can identify now the

situation when the a minus equal to h a

if you're able to identify this one you

have got the pk value so let us

understand you know this is simply a

acid base titration

we are going to conduct a simple

acid-base titration and how you

determine the end point our end point is

when the entire h a

right when entire a minus get consumed

by the base so at the equivalence point

or the end point all a minus would have

consumed by the base this is the

fair importance of conducting the

acid base titration

so let us assume

that the end point for weak acid is 24

ml of point over normal sodium hydroxide

solution

sodium hydroxide is a base washer and we

are titrating our sample the weak acid

against the 0.01 normal sodium hydroxide

solution and let us assume that the 24

ml is our endpoint

that is equivalence point

meaning

the 100 percentage of a minus

will get consumed by 24

ml of 0.01 normal sodium hydroxide

why you are needing 24 ml of 0.0 normal

naoh

to consume the entire that is 100

percentage of the a minus ions present

into a solution

now the next equation is very important

this means

when you have a 24 ml during a titration

when you have a 24 ml of the 0.01 normal

sodium hydroxide solution into a

solution

then you will left with zero percentage

of the a minus and zero percentage of

the h

because all your a minus

would have already consumed by 24 ml of

point over normal animation if there is

no a minus there will be no h also

now the second equation is very

important so

let us understand at 12 ml exactly 50

percentage of the entire titan consumed

that is in this case 12 ml of point over

normal in average what will be the

situation

or what would have been the situation

so

you will have the 50 percentage of the a

minus and 50 percentage of the ha

now this statement is very important

and once you understand that

then what is the situation when a minus

is equal to h a now

can you understand over here

just goes

one step back in this step of the when

you have added 12 ml of the point over

normal animation to the solution during

titration

you will let with equal amount of a

minus and h

and this is for the situation we are we

were talking about the pka is equal to

ph

when

when you will have the equal amount of

your a minus and ha so you need to now

you need to now just understand what is

the

ph what is the ph at the 12 ml of the

0.01 normal sodium hydroxide solution

so what is the situation when a minus is

equal to h it is actually

the the ph

whatever it may be

that will be equal to a pka

and this is called as this 12 ml of the

point over normal naoh addition

as the 24 ml is what our equivalence

point the 24 ml is our end point so this

12 ml which is 50

of the equivalence point

is called as the half equivalence point

so in case of the pka measurement

you need to understand you need to

understand what is the ph at the heart

equivalence point

because at the half equivalence point

your a minus will be equal to h a

right so let us understand so the value

of ph at the half equivalence point will

be your pka value so once you understand

what is the half equivalence point

during the titration then measure the ph

at the half equivalence point

so you need to have the the ph

measurement system

right ph measurement system during the

potentiometric titration because your

interest in acid-base titration is what

to understand let us say the potential

difference right when you add the sodium

hydroxide solution into the weak acidic

solution

but along with measuring the potential

you need to also understand what is the

ph across all the time points that you

have consumed your nav during the

titration

probably at the beginning of acid-base

titration

your ph of the solution will be highly

acidic because your sample is what

highly acidic or the acids or substance

the moment you start adding the energy

of the

alkali solutions you will have a now the

change into the potential and then you

measure the potential

that is required for understanding the

acid the strength of the acid but in

this case you need to also keep on

measuring the ph as well across all the

time points

because unless and until you understand

the ph you will not be able to measure

the pka

so at the half equivalence point right

at the 12 you may have the potential

measured but also measure the ph with

the suitable glass electrode when you

have added exactly 12 ml

of the point over normal energy in this

particular case

and your crop

could be looking this way so this is

your equivalence point almost 24 ml this

is your end point but you are not

interested in understanding the end

point as far as the pka measurement is

concerned you are looking for the half

equivalence point

you are looking for the half equivalence

point so what is the half equivalence

point if this is let us say 24 mm this

becomes 12 ml and then understand what

is the ph if you look at the y axis i

have measured the ph of the solution

over here

right and then i will realize that the

pk of the

uh as the ph is around let us say 4.5 or

around close to 5 my pk will be again

the 4.5

so just by measuring the ph

at a time point when my a minus is equal

to h a

i will have actually got the pka of the

compound

okay i hope you must have now understand

how to measure the pka value

just by

simply conducting the potentiometric

titration

thank you very much for watching this

video and i will meet you soon in

another such kind of informative and

useful video till then take care and bye

bye see you soon