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pKa Value

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Last updated date: 23rd May 2024
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An Introduction to pKa

When we refer to acids and bases, two important terms pH and pKa are usually used. The pKa value is employed to determine the strength of an acid. The lower the pKa of a Bronsted acid, the more simply it provides up its proton. Also, the acidic strength is high as well. The more the pKa of a Bronsted acid, the more tightly the proton is present, and therefore the less simply the proton is given up which means basic strength is low. The pKa And pKb values are related to each other; when one increases the other decreases.


What Is pKa?

pKa is the negative base -10 log of the acid equilibrium constant (Ka) of a solution.

\[pK_a = -\log \left[ K_a \right]\]


The lower the pKa value, the stronger the acid, for instance, trifluoroacetic acid, benzoic acid and acetic acid are 0.23, 4.19 and 4.76, respectively. Using the pKa values, one can see that trifluoroacetic acid could be a stronger acid than benzoic acid and acetic acid.


The reason pKa is considered is because it describes acid dissociation with small decimal numbers. A similar form of information could also be obtained from Ka values; however, they are usually very low numbers given in scientific notation that are difficult for many individuals to grasp.


Calculation of pKa Value

When an acid (HA) dissolves in water, it donates a proton, and therefore the product of the reaction consists of \[{{H}_{3}}{{O}^{+}}\] and \[{{A}^{-}}\], which is the conjugate base of the reaction. Looking at the relative skills of HA to donate protons and \[{{A}^{-}}\] to simply accept them, the reaction may also proceed in the other way till eventually an equilibrium is achieved.


Chemists confirm the strength of an acid (Ka) by measuring the concentrations of HA, \[{{H}_{3}}{{O}^{+}}\] and \[{{A}^{-}}\] at equilibrium and dividing the concentrations of the product by the concentration of the initial acid because the concentration of water may be a constant, they leave it out of the equation.


\[K_a=\frac{\left[ {{H}_{3}}{{O}^{+}} \right]\left[ {{A}^{-}} \right]}{\left[ HA \right]}\]


Conversion to pKa

For example, the Ka value for hydrochloric acid (HCl) ≃ \[{{10}^{7}}\]

And, the Ka value for ascorbic acid (Vitamin C) ≃ \[1.6\times {{10}^{-12}}\]

Working with such numbers is inconvenient, therefore to create things comprehendible, chemists have outlined the pKa range as

\[pK_a=-\log \left[ K_a \right]\]


According to this definition, the pKa value for HCl is as follows:

\[pK{{a}_{\left( HCl \right)}}=-\log {{10}^{7}}=-7\]


Whereas the pKa for ascorbic acid is:

\[pK{{_a}_{\left( ascorbic \ acid \right)}}=-\log 6\times {{10}^{-12}}=11.80\]


As is clear, the smaller the pKa value, the stronger the acid.


pKa and pH of Buffer Solution

The pH scale could be a measure of the concentration of H ions in a solution. pKa and pH are connected. The relationship between pH and pKa is represented by the Henderson Hasselbalch equation.


If you recognise either pH or pKa, you can solve it with the help of an approximation known as the Henderson-Hasselbalch equation:


\[pH=pK_a+\log \frac{\left[ conjugate~base \right]}{\left[ weak~acid \right]}\]


\[pH=pK_a+\log \frac{\left[ {{A}^{-}} \right]}{\left[ HA \right]}\]


pH is the total of the pKa value and the log of the concentration of the conjugate base divided by the concentration of the weak acid.


At half the equivalence point:

\[pH=pK_a\]


Typically this equation is written for the Ka value instead of pKa,

\[pK_a=-\log \left[ K_a \right]\]


List of pKa Values

Some of the pKa values of acids are listed in the following table.


Table: List of Few pKa Values

Compound

pKa Value

Formic acid

3.75

Acetic acid

4.75

Phenol

10

Water

14

4-Methoxy benzoic acid

4.46

Benzoic acid

4.19


Relation between pKa and pKb

pKa and pKb are customary terms in chemistry that are obtained from as dissociation constants. pKa is obtained from acid equilibrium constant, and pKb is obtained from base equilibrium constant. The “p” in these terms stands for “negative logarithm”. The key distinction between pKa and pKb is that pKa is the negative log of Ka whereas pKb is the negative log of kb.


We know that

\[pK_a=-\log \left[ K_a \right]\]


Similarly, \[pK_b=-\log \left[ K_b \right]\]


The relationship between Ka and Kb are given below.

\[K_w=K_a\cdot K_b\]


Then the connection between pKa and pKb is given as (at 25oC)

\[pK_a+pK_b=14\]


pKa and pKb are generally comparing the strength of acids and bases. pKa is given for acid dissociations, whereas pKb is given for dissociation of bases.


Conclusion

There are connected scales in chemistry usually measuring how acidic or basic a solution is and hence the strengths of acids and bases. pKa, Ka, pKb, and Kb are commonly used for the calculations that provide insight into acid-base reactions. The distinction between pKa and pKb is that pKa is the negative log of the Ka whereas pKb is the negative log of Kb. The higher the value of pKa, the lower the value of pKb and acidic strength.

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FAQs on pKa Value

1. What is pKb?

pKb is the negative log of Kb. Kb is the base equilibrium constant. It's used to confirm the strength of a base quantitatively. Once a base is dissolved in water, it dissociates into ions forming a basic solution. Strong bases dissociate fully. Weak bases dissociate in parts. A high Kb value is characterised by a low pKb value with small decimal places.

2. Which factors confirm the acidic strength?

Totally different acids have different acid strengths. An acid having a larger degree of dissociation behaves as a stronger acid. The degree of dissociation of an acid depends on the subsequent 2 factors. Strength of the H-A bond and the polarity of the H-A bond. In general, the weaker the strength of the H-A bond, the stronger the acid. Similarly, the larger the polarity of the H-A bond, the stronger the acid. Each of these factors creates the dissociation of acid molecules into H+ and A- easier thereby increasing the acidity.

3. What is pH and why is it important?

pH is a crucial value that reflects the chemical conditions of a solution. It will control the availability of nutrients, biological functions, microorganism activity, and also the behaviour of chemicals. Due to this, observation or controlling the pH of the soil, water, and food or other nutrients products is very important for a large type of application. In Agriculture and horticulture, soil may be an advanced system that involves many alternative factors that are affected by soil pHs, like microorganism activity, fungous growth, the convenience of nutrients, and root growth.