pH and BufferspH and Buffers

The Whole StoryThe Whole Story

Acid & Base and pHAcid & Base and pHThis should be a review but we will go over it to help refresh This should be a review but we will go over it to help refresh

your memoryyour memory

00 7 7 1414

1 M1 M 10 10-7 -7 MM 10 10-14 -14 MMVeryVery Neutral Neutral Very VeryAcidicAcidic BasicBasic

Trivia time: Trivia time:

Why was the concept of pH Why was the concept of pH developed?developed?

Trivia time: Trivia time:

Why was the concept of pH Why was the concept of pH developed?developed?

By the Danish Biochemist By the Danish Biochemist Sorensen to test the Sorensen to test the acidity of the beer he acidity of the beer he was makingwas making

Who said biochemistry Who said biochemistry isn’t cool?isn’t cool?

pH - pouvoir hydrogene (the pH - pouvoir hydrogene (the power of hydrogen)power of hydrogen)

Water undergoes ionizationWater undergoes ionization

Water ionizes to form the hydronium (hydroxyl) ion and Water ionizes to form the hydronium (hydroxyl) ion and hydroxide ionshydroxide ions

Water can act as both an acid and baseWater can act as both an acid and baseThe equilibrium constant for the ionization of water is:The equilibrium constant for the ionization of water is:

The concentration of pure water The concentration of pure water 1 liter = 1000g MW of water is 10.0151 liter = 1000g MW of water is 10.015 the final concentration of water is 55M and H+ the final concentration of water is 55M and H+ concentration is about 1.8 x 10concentration is about 1.8 x 10-9-9

Very little water actually dissociates Very little water actually dissociates So Keq is very small – not easily measured or easy So Keq is very small – not easily measured or easy to useto use

pH - pouvoir hydrogene (the pH - pouvoir hydrogene (the power of hydrogen)power of hydrogen)

Water undergoes ionizationWater undergoes ionization

Water ionizes to form the hydronium (hydroxyl) ion and Water ionizes to form the hydronium (hydroxyl) ion and hydroxide ionshydroxide ions

Water can act as both an acid and baseWater can act as both an acid and baseThe equilibrium constant for the ionization of water is:The equilibrium constant for the ionization of water is:

The concentration of pure water The concentration of pure water 1 liter = 1000g MW of water is 10.0151 liter = 1000g MW of water is 10.015 the final concentration of water is 55M and H+ the final concentration of water is 55M and H+ concentration is about 1.8 x 10concentration is about 1.8 x 10-9-9

Very little water actually dissociates Very little water actually dissociates So Keq is very small – not easily measured or easy So Keq is very small – not easily measured or easy to useto use

HH22 OO22 HH OO33 ++ OHOH__++

pH - pouvoir hydrogene (the pH - pouvoir hydrogene (the power of hydrogen)power of hydrogen)

Water undergoes ionizationWater undergoes ionization

Water ionizes to form the hydronium (hydroxyl) ion and Water ionizes to form the hydronium (hydroxyl) ion and hydroxide ionshydroxide ions

Water can act as both an acid and baseWater can act as both an acid and baseThe equilibrium constant for the ionization of water is:The equilibrium constant for the ionization of water is:

The concentration of pure water The concentration of pure water 1 liter = 1000g MW of water is 10.0151 liter = 1000g MW of water is 10.015 the final concentration of water is 55M and H+ the final concentration of water is 55M and H+ concentration is about 1.8 x 10concentration is about 1.8 x 10-9-9

Very little water actually dissociates Very little water actually dissociates So Keq is very small – not easily measured or easy So Keq is very small – not easily measured or easy to useto use

HH22 OO22 HH OO33 ++ OHOH__++

KKeqeq==[products][products]

[reactants][reactants]== [H[H++ ] ][OH[OH-- ] ]

[H[H22O]O]22

pH - pouvoir hydrogene (the pH - pouvoir hydrogene (the power of hydrogen)power of hydrogen)

Water undergoes ionizationWater undergoes ionization

Water ionizes to form the hydronium (hydroxyl) ion and Water ionizes to form the hydronium (hydroxyl) ion and hydroxide ionshydroxide ions

Water can act as both an acid and baseWater can act as both an acid and baseThe equilibrium constant for the ionization of water is:The equilibrium constant for the ionization of water is:

The concentration of pure water The concentration of pure water 1 liter = 1000g MW of water is 10.0151 liter = 1000g MW of water is 10.015 the final concentration of water is 55M and H+ the final concentration of water is 55M and H+ concentration is about 1.8 x 10concentration is about 1.8 x 10-9-9

Very little water actually dissociates Very little water actually dissociates So Keq is very small – not easily measured or easy So Keq is very small – not easily measured or easy to useto use

HH22 OO22 HH OO33 ++ OHOH__++

KKeqeq==[products][products]

[reactants][reactants]== [H[H++ ] ][OH[OH-- ] ]

[H[H22O]O]22

[H[H22O] is O] is effectivelyeffectivelyconstant.constant.

Instead a different constant is used where the denominator is Instead a different constant is used where the denominator is ignoredignored

KKww= 1.0 X 10 = 1.0 X 10 -14-14

pH is a measure of the acidity and basicity of a solutionpH is a measure of the acidity and basicity of a solution

when [Hwhen [H++] = [OH] = [OH--] the solution is neutral and pH is 7] the solution is neutral and pH is 7when [Hwhen [H++] > [OH] > [OH--] the solution is acidic and pH is less than 7] the solution is acidic and pH is less than 7when [Hwhen [H++] < [OH] < [OH--] the solution is basic and pH is more than 7] the solution is basic and pH is more than 7

a change in 1 pH units is = a ten fold change in hydrogen ion a change in 1 pH units is = a ten fold change in hydrogen ion concentrationconcentration

KKww == [H ][H ]++[OH ][OH ]__

Therefore: Therefore:

And as measured in pure waterAnd as measured in pure water

[H ] = [H ] = ++ [OH ] = 1 X 10[OH ] = 1 X 10__ --77

Instead a different constant is used where the denominator is Instead a different constant is used where the denominator is ignoredignored

KKww= 1.0 X 10 = 1.0 X 10 -14-14

pH is a measure of the acidity and basicity of a solutionpH is a measure of the acidity and basicity of a solution

when [Hwhen [H++] = [OH] = [OH--] the solution is neutral and pH is 7] the solution is neutral and pH is 7when [Hwhen [H++] > [OH] > [OH--] the solution is acidic and pH is less than ] the solution is acidic and pH is less than

77when [Hwhen [H++] < [OH] < [OH--] the solution is basic and pH is more than 7] the solution is basic and pH is more than 7

a change in 1 pH units is = a ten fold change in hydrogen ion a change in 1 pH units is = a ten fold change in hydrogen ion concentrationconcentration

Instead a different constant is used where the denominator is Instead a different constant is used where the denominator is ignoredignored

KKww= 1.0 X 10 = 1.0 X 10 -14-14

pH is a measure of the acidity and basicity of a solutionpH is a measure of the acidity and basicity of a solution

pH = -log[HpH = -log[H++]]

when [Hwhen [H++] = [OH] = [OH--] the solution is neutral and pH is 7] the solution is neutral and pH is 7when [Hwhen [H++] > [OH] > [OH--] the solution is acidic and pH is less than 7] the solution is acidic and pH is less than 7when [Hwhen [H++] < [OH] < [OH--] the solution is basic and pH is more than 7] the solution is basic and pH is more than 7

a change in 1 pH units is = a ten fold change in hydrogen ion a change in 1 pH units is = a ten fold change in hydrogen ion concentrationconcentration

KKww == [H ][H ]++[OH ][OH ]__

Therefore: Therefore:

And as measured in pure waterAnd as measured in pure water

[H ] = [H ] = ++ [OH ] = 1 X 10[OH ] = 1 X 10__ --77

The extent of ionization of a weak acid The extent of ionization of a weak acid is a function of its acid dissociation is a function of its acid dissociation

constant pKaconstant pKaBronsted and Lowry acid and basesBronsted and Lowry acid and bases– acid donates protonsacid donates protons– bases accepts protonsbases accepts protons

Strong acids dissociate nearly fullyStrong acids dissociate nearly fully

Weak acids only partially dissociateWeak acids only partially dissociate

Acids with Ka < 1 are considered weak acidsAcids with Ka < 1 are considered weak acids Ka for acetic acid is 1.76 x 10Ka for acetic acid is 1.76 x 10-5-5 -> difficult to work -> difficult to work with so instead use log scalewith so instead use log scale::

pKpKaa = -log K = -log Kaa

So the pKa of acetic acid is =So the pKa of acetic acid is = -log 1.76 x 10 -log 1.76 x 10-5-5 = 4.75 = 4.75

The pH is a measure of acidity and the pKa is a measure of acid The pH is a measure of acidity and the pKa is a measure of acid strengthstrength

The extent of ionization of a weak acid The extent of ionization of a weak acid is a function of its acid dissociation is a function of its acid dissociation

constant pKaconstant pKaBronsted and Lowry acid and basesBronsted and Lowry acid and bases– acid donates protonsacid donates protons– bases accepts protonsbases accepts protons

Strong acids dissociate nearly fullyStrong acids dissociate nearly fully

Weak acids only partially dissociateWeak acids only partially dissociate

Acids with Ka < 1 are considered weak acidsAcids with Ka < 1 are considered weak acids Ka for acetic acid is 1.76 x 10Ka for acetic acid is 1.76 x 10-5-5 -> difficult to work -> difficult to work with so instead use log scalewith so instead use log scale::

pKpKaa = -log K = -log Kaa

So the pKa of acetic acid is =So the pKa of acetic acid is = -log 1.76 x 10 -log 1.76 x 10-5-5 = 4.75 = 4.75

The pH is a measure of acidity and the pKa is a measure of acid The pH is a measure of acidity and the pKa is a measure of acid strengthstrength

HA + H O H O + AHA + H O H O + A22 33++ __

acid base conjugate conjugate acid base conjugate conjugate acid baseacid base

The extent of ionization of a weak acid The extent of ionization of a weak acid is a function of its acid dissociation is a function of its acid dissociation

constant pKaconstant pKaBronsted and Lowry acid and basesBronsted and Lowry acid and bases– acid donates protonsacid donates protons– bases accepts protonsbases accepts protons

Strong acids dissociate nearly fullyStrong acids dissociate nearly fully[H[H++] = [ acid] ] = [ acid] and thusand thus pH = -log [acid] pH = -log [acid]

Weak acids only partially dissociateWeak acids only partially dissociate

Acids with Ka < 1 are considered weak acidsAcids with Ka < 1 are considered weak acids Ka for acetic acid is 1.76 x 10Ka for acetic acid is 1.76 x 10-5-5 -> difficult to work -> difficult to work with so instead use log scalewith so instead use log scale::

pKpKaa = -log K = -log Kaa

So the pKa of acetic acid is =So the pKa of acetic acid is = -log 1.76 x 10 -log 1.76 x 10-5-5 = 4.75 = 4.75

The pH is a measure of acidity and the pKa is a measure of acid The pH is a measure of acidity and the pKa is a measure of acid strengthstrength

HA + H O H O + AHA + H O H O + A22 33++ __

acid base conjugate conjugate acid base conjugate conjugate acid baseacid base

The extent of ionization of a weak acid The extent of ionization of a weak acid is a function of its acid dissociation is a function of its acid dissociation

constant pKaconstant pKaBronsted and Lowry acid and basesBronsted and Lowry acid and bases– acid donates protonsacid donates protons– bases accepts protonsbases accepts protons

Strong acids dissociate nearly fullyStrong acids dissociate nearly fully[H[H++] = [ acid] ] = [ acid] and thusand thus pH = -log [acid] pH = -log [acid]

Weak acids only partially dissociateWeak acids only partially dissociate

Acids with Ka < 1 are considered weak acidsAcids with Ka < 1 are considered weak acids Ka for acetic acid is 1.76 x 10Ka for acetic acid is 1.76 x 10-5-5 -> difficult to work -> difficult to work with so instead use log scalewith so instead use log scale::

pKpKaa = -log K = -log Kaa

So the pKa of acetic acid is =So the pKa of acetic acid is = -log 1.76 x 10 -log 1.76 x 10-5-5 = 4.75 = 4.75

The pH is a measure of acidity and the pKa is a measure of acid The pH is a measure of acidity and the pKa is a measure of acid strengthstrength

HA + H O H O + AHA + H O H O + A22 33++ __

acid base conjugate conjugate acid base conjugate conjugate acid baseacid base

KK == [H ][H ]++[A ][A ]

__

[HA][HA]aa

pKa pKa and Ka and Ka are are used used for for all all

acids.acids.

Dissociation constants and Dissociation constants and pKa values pKa values

C hemi cal Ka (M ) pKa

Formic acid 1.77 x 10-4 3.8

C arbonic acid 4.30 x 10-7 6.4

Bicarbonate 5.61 x 10-11 10.2

Am m onium 5.62 x 10-10 9.2

Note Note relationsrelations

hip hip between between pKA, Ka pKA, Ka and acid and acid strength.strength.

MoreMoreAcidicAcidic

Titrations and pKaTitrations and pKapKa of a weak acid is pKa of a weak acid is determined determined experimentally by experimentally by titration. titration.

pKa is when the pKa is when the concentration of acid concentration of acid and base is equal in and base is equal in a titration (ask a titration (ask yourself if that then yourself if that then equal to pH 7).equal to pH 7).– There is a point There is a point in a titration of in a titration of a weak acid where a weak acid where the change in pH the change in pH is very little. is very little. This is the buffer This is the buffer action of the action of the acid.acid.

Many acids have more than one ionizable group (polyprotic)Many acids have more than one ionizable group (polyprotic)

The relationship between pH and pKa FOR A The relationship between pH and pKa FOR A WEAK ACID is described by the Henderson-WEAK ACID is described by the Henderson-

Hasselbalch equationHasselbalch equationHA HHA H++ + A + A--

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

Multiply by -1Multiply by -1

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

[A[A--] ]

[HA][HA]-Log[H-Log[H++] = -Log K - ] = -Log K - Log Log

Multiply by -1Multiply by -1

aa

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

[A[A--] ]

[HA][HA]-Log[H-Log[H++] = -Log K - ] = -Log K - Log Log

Multiply by -1Multiply by -1

SubstituteSubstitutepH = -Log[HpH = -Log[H++]]pKa = Log KpKa = Log Kaa

aa

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

[A[A--] ]

[HA][HA]-Log[H-Log[H++] = -Log K - ] = -Log K - Log Log

Multiply by -1Multiply by -1

SubstituteSubstitutepH = -Log[HpH = -Log[H++]]pKa = Log KpKa = Log Kaa

aa

[A[A--] ]

[HA][HA]pH = pKa - Log pH = pKa - Log

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

[A[A--] ]

[HA][HA]-Log[H-Log[H++] = -Log K - ] = -Log K - Log Log

Multiply by -1Multiply by -1

SubstituteSubstitutepH = -Log[HpH = -Log[H++]]pKa = Log KpKa = Log Kaa

aa

[A[A--] ]

[HA][HA]pH = pKa - Log pH = pKa - Log

Remove (-) and Remove (-) and invert last invert last

termterm

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

[A[A--] ]

[HA][HA]-Log[H-Log[H++] = -Log K - ] = -Log K - Log Log

Multiply by -1Multiply by -1

SubstituteSubstitutepH = -Log[HpH = -Log[H++]]pKa = Log KpKa = Log Kaa

aa

[A[A--] ]

[HA][HA]pH = pKa - Log pH = pKa - Log

Remove (-) and Remove (-) and invert last invert last

termterm[A[A--] ]

[HA][HA]pH = pKa + Log pH = pKa + Log

The relationship between pH and pKa is The relationship between pH and pKa is described by the Henderson-Hasselbalch described by the Henderson-Hasselbalch

equationequation

KK == [[HH++] [A] [A--] ]

[[HAHA]]aa

Arrange to getArrange to get

Multiply each Multiply each side by logside by log

Start with a weak acidStart with a weak acidHA HHA H++ + A + A--

[H[H++] ] KK

==[A[A--] ]

[HA][HA]aa

Log[HLog[H++] = Log K + ] = Log K + Log Log [A[A--] ]

[HA][HA]aa

[A[A--] ]

[HA][HA]-Log[H-Log[H++] = -Log K - ] = -Log K - Log Log

Multiply by -1Multiply by -1

SubstituteSubstitutepH = -Log[HpH = -Log[H++]]pKa = Log KpKa = Log Kaa

aa

[A[A--] ]

[HA][HA]pH = pKa - Log pH = pKa - Log

Remove (-) and Remove (-) and invert last invert last

termterm[A[A--] ]

[HA][HA]pH = pKa + Log pH = pKa + Log

Henderson -HasselbalchHenderson -Hasselbalch

What is the H-H Equation Used For?What is the H-H Equation Used For? This is used to determine the concentration of This is used to determine the concentration of

acid and base at a given pH. It is Also used acid and base at a given pH. It is Also used to determine the pH of a known solution. These to determine the pH of a known solution. These concepts are used to calculate buffer strength concepts are used to calculate buffer strength and understand the pH of a biological solution.and understand the pH of a biological solution.

Remember that buffers are mixtures of weak acids Remember that buffers are mixtures of weak acids and their conjugate bases that resist pH by and their conjugate bases that resist pH by shifting the equilibrium between the acid and shifting the equilibrium between the acid and base in response to the pH of a solution.base in response to the pH of a solution.

Case 1) when the Case 1) when the concentration of base equals concentration of base equals

the acid.the acid.

When pH = pKa 50% of the acid is dissociatedWhen pH = pKa 50% of the acid is dissociated

pH = pKa + LogpH = pKa + Log[A ][A ]

[HA][HA]

__

= pKa + Log= pKa + Log[ x ][ x ]

[ x ][ x ]

pH = pKapH = pKa

Case 2) when the pH is Case 2) when the pH is above or below 1 pH unit of above or below 1 pH unit of

the pKathe pKa

Then 90% of the buffer is in the conjugate base Then 90% of the buffer is in the conjugate base forfor

If pH is 2 units different then 99% of buffer is If pH is 2 units different then 99% of buffer is in the conjugate base formin the conjugate base form

What does this mean about the buffering ability What does this mean about the buffering ability if more acid or base is added?if more acid or base is added?

5.00 = 4.00 + Log5.00 = 4.00 + Log[A ][A ]

[HA][HA]

__

1.00 = Log1.00 = Log

10.0 = 10.0 =

[A ][A ]

[HA][HA]

[A ][A ]

[HA][HA]

__

__

Calculate the pH of a Calculate the pH of a mixture of 250 mM acetic mixture of 250 mM acetic acid and 100 mM Na acetate.acid and 100 mM Na acetate. The pKa of acetic acid is The pKa of acetic acid is 4.75.4.75.

Calculate the pH of a Calculate the pH of a mixture of 250 mM acetic mixture of 250 mM acetic acid and 100 mM Na acetate.acid and 100 mM Na acetate. The pKa of acetic acid is The pKa of acetic acid is 4.75.4.75.Start with the HH equationStart with the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Calculate the pH of a Calculate the pH of a mixture of 250 mM acetic mixture of 250 mM acetic acid and 100 mM Na acetate.acid and 100 mM Na acetate. The pKa of acetic acid is The pKa of acetic acid is 4.75.4.75.Start with the HH equationStart with the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Recognize that a compound that ends with -ate is Recognize that a compound that ends with -ate is the base form of a weak acid and plug in the the base form of a weak acid and plug in the

known termsknown terms

Calculate the pH of a Calculate the pH of a mixture of 250 mM acetic mixture of 250 mM acetic acid and 100 mM Na acetate.acid and 100 mM Na acetate. The pKa of acetic acid is The pKa of acetic acid is 4.75.4.75.Start with the HH equationStart with the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Recognize that a compound that ends with -ate is Recognize that a compound that ends with -ate is the base form of a weak acid and plug in the the base form of a weak acid and plug in the

known termsknown terms

pH = 4.75 + Log (0.1/0.25)pH = 4.75 + Log (0.1/0.25)

Calculate the pH of a Calculate the pH of a mixture of 250 mM acetic mixture of 250 mM acetic acid and 100 mM Na acetate.acid and 100 mM Na acetate. The pKa of acetic acid is The pKa of acetic acid is 4.75.4.75.Start with the HH equationStart with the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Recognize that a compound that ends with -ate is Recognize that a compound that ends with -ate is the base form of a weak acid and plug in the the base form of a weak acid and plug in the

known termsknown terms

pH = 4.75 + Log (0.1/0.25)pH = 4.75 + Log (0.1/0.25)

Remember [X] indicates molar concentrationRemember [X] indicates molar concentration

Calculate the pH of a Calculate the pH of a mixture of 250 mM acetic mixture of 250 mM acetic acid and 100 mM Na acetate.acid and 100 mM Na acetate. The pKa of acetic acid is The pKa of acetic acid is 4.75.4.75.Start with the HH equationStart with the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Recognize that a compound that ends with -ate is Recognize that a compound that ends with -ate is the base form of a weak acid and plug in the the base form of a weak acid and plug in the

known termsknown terms

pH = 4.75 + Log (0.1/0.25)pH = 4.75 + Log (0.1/0.25)

Remember [X] indicates molar concentrationRemember [X] indicates molar concentration

pH =4.35 pH =4.35

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

ArrangeArrange

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

ArrangeArrange

1.14 = Log [A1.14 = Log [A--]/[HA]]/[HA]

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

ArrangeArrange

1.14 = Log [A1.14 = Log [A--]/[HA]]/[HA]

Inv Log on both sidesInv Log on both sides

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start Again we recognize this is a weak acid and start

with the HH equationwith the HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

ArrangeArrange

1.14 = Log [A1.14 = Log [A--]/[HA]]/[HA]

Inv Log on both sidesInv Log on both sides

Ratio = 13.80Ratio = 13.80

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start with Again we recognize this is a weak acid and start with

the HH equationthe HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

ArrangeArrange

1.14 = Log [A1.14 = Log [A--]/[HA]]/[HA]

Inv Log on both sidesInv Log on both sides

Ratio = 13.80Ratio = 13.80

That means that for each mole of acid there are 13.8 That means that for each mole of acid there are 13.8 moles of basemoles of base

What is the ratio of lactic What is the ratio of lactic acid to lactate in a buffer acid to lactate in a buffer at pH of 5.00.at pH of 5.00. The pKa of The pKa of lactic acid is 3.86?lactic acid is 3.86?Again we recognize this is a weak acid and start with Again we recognize this is a weak acid and start with

the HH equationthe HH equation

pH = pKa + Log [ApH = pKa + Log [A--]/[HA]]/[HA]

Plug in the known termsPlug in the known terms

5.00 = 3.86 + Log [A5.00 = 3.86 + Log [A--]/[HA]]/[HA]

ArrangeArrange

1.14 = Log [A1.14 = Log [A--]/[HA]]/[HA]

Inv Log on both sidesInv Log on both sides

Ratio = 13.80Ratio = 13.80

That means that for each mole of acid there are 13.8 That means that for each mole of acid there are 13.8 moles of basemoles of base

What is the concentration What is the concentration of base and acid you need of base and acid you need to add to make a 50 mM to add to make a 50 mM solution of lactate buffer solution of lactate buffer at pH 4.0? at pH 4.0? The MW of The MW of Lactic acid is 91 amu and Lactic acid is 91 amu and sodium lactate is 102 amu.sodium lactate is 102 amu.

This is for you to take This is for you to take home try and we will home try and we will calculate an answer with Dr calculate an answer with Dr Provost - Provost - Also start to do Also start to do the pH Homework!the pH Homework!