Measurement of pKa by Potentiometry
Summary
TLDRThis 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
- π The video discusses the significance of pKa, the acid dissociation constant, in determining the retention time of ionizing compounds in reverse phase chromatography.
- π 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).
- π§ͺ 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.
- π‘ 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.
- π 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.
- 𧬠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.
- π¬ 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.
- π‘ 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.
- π 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.
- π¨βπ« 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
π 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.
π§ͺ 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.
π 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.
Mindmap
Keywords
π‘pKa
π‘Acid Dissociation Constant (Ka)
π‘Potentiometry
π‘Retention Time
π‘Conjugate Base
π‘Henderson-Hasselbalch Equation
π‘Acid-Base Titration
π‘Equivalence Point
π‘Half Equivalence Point
π‘Glass Electrode
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
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