pH and pOH,pH and pOH,Indicators and BuffersIndicators and Buffers

Chapter 19Chapter 19

What is pH?What is pH? pHpH is a logarithmic measure of hydrogen ion concentration, originally is a logarithmic measure of hydrogen ion concentration, originally

defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909. defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909. pH = -log[HpH = -log[H++] ] where where loglog is a base-10 logarithm and is a base-10 logarithm and [H+][H+] is the concentration of is the concentration of

hydrogen ions in moles per liter of solution. hydrogen ions in moles per liter of solution. According to the Compact Oxford English Dictionary, the "p" stands for the According to the Compact Oxford English Dictionary, the "p" stands for the

German word for "power", German word for "power", potenzpotenz, so pH is an abbreviation for "power of , so pH is an abbreviation for "power of hydrogen". hydrogen".

The pH scale was defined because the enormous range of hydrogen ion The pH scale was defined because the enormous range of hydrogen ion concentrations found in aqueous solutions make using Hconcentrations found in aqueous solutions make using H++ molarity molarity awkward. For example, in a typical acid-base titration, [H+] may vary from awkward. For example, in a typical acid-base titration, [H+] may vary from about 0.01 M to 0.0000000000001 M. It is easier to write "the pH varies about 0.01 M to 0.0000000000001 M. It is easier to write "the pH varies from 2 to 13". from 2 to 13".

The hydrogen ion concentration in pure water around room temperature is The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10about 1.0 × 10-7-7 MM. .

A pH of 7 is considered "neutral", because the concentration of hydrogen A pH of 7 is considered "neutral", because the concentration of hydrogen ions is exactly equal to the concentration of hydroxide (OH-) ions produced ions is exactly equal to the concentration of hydroxide (OH-) ions produced by dissociation of the water. by dissociation of the water.

Increasing the concentration of hydrogen ions above 1.0 × 10Increasing the concentration of hydrogen ions above 1.0 × 10-7-7 MM produces produces a solution with a pH of less than 7, and the solution is considered "acidic". a solution with a pH of less than 7, and the solution is considered "acidic".

Decreasing the concentration below 1.0 × 10Decreasing the concentration below 1.0 × 10-7-7 MM produces a solution with a produces a solution with a pH above 7, and the solution is considered "alkaline" or "basic". pH above 7, and the solution is considered "alkaline" or "basic".

Comparing solution aciditiesComparing solution acidities pH is often used to compare solution acidities. pH is often used to compare solution acidities. For example, a solution of pH 1 is said to be 10 For example, a solution of pH 1 is said to be 10

times as acidic as a solution of pH 2, because the times as acidic as a solution of pH 2, because the hydrogen ion concentration at pH 1 is ten times hydrogen ion concentration at pH 1 is ten times the hydrogen ion concentration at pH 2. the hydrogen ion concentration at pH 2.

This is correct as long as the solutions being This is correct as long as the solutions being compared both use the same solvent. compared both use the same solvent.

You can't use pH to compare the acidities in You can't use pH to compare the acidities in different solvents because the neutral pH is different solvents because the neutral pH is different for each solvent. different for each solvent.

For example, the concentration of hydrogen ions in For example, the concentration of hydrogen ions in pure ethanol is about 1.58 × 10pure ethanol is about 1.58 × 10-10-10 MM, so ethanol is , so ethanol is neutral at pH 9.8. A solution with a pH of 8 would neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water! be considered acidic in ethanol, but basic in water!

Negative pHNegative pH When you are given the molarity of an acid solution, and asked to When you are given the molarity of an acid solution, and asked to

determine the pH, a molarity greater than 1 produces a negative pH. determine the pH, a molarity greater than 1 produces a negative pH. Is this possible? Yes.Is this possible? Yes. If the molarity of If the molarity of hydrogen ionshydrogen ions is greater than 1, you'll have a negative is greater than 1, you'll have a negative

value of pH. value of pH. E.g., you might expect a 12 M HCl solution to have a pH of -log(12) = -E.g., you might expect a 12 M HCl solution to have a pH of -log(12) = -

1.08. 1.08. Why don't you hear more about negative pH? Why don't you hear more about negative pH? There are some complications in high molarity acid solutions that make There are some complications in high molarity acid solutions that make

pH calculations from acid molarity inaccurate and difficult to verify pH calculations from acid molarity inaccurate and difficult to verify experimentally: experimentally: – Even strong acids don't dissociate completely at high concentrations. Some of Even strong acids don't dissociate completely at high concentrations. Some of

the hydrogen remains bound to the chlorine, making the pH higher than you'd the hydrogen remains bound to the chlorine, making the pH higher than you'd expect from the acid molarity. expect from the acid molarity.

– Because there are so few waters per acid formula unit, the influence of Because there are so few waters per acid formula unit, the influence of hydrogen ions in the solution is enhanced. We say that the effective hydrogen ions in the solution is enhanced. We say that the effective concentration of hydrogen ions (or the concentration of hydrogen ions (or the activityactivity) is much higher than the actual ) is much higher than the actual concentration. The usual general chemistry text definition of pH as -log [H+] concentration. The usual general chemistry text definition of pH as -log [H+] (negative the logarithm of the hydrogen ion molarity) is better written as pH = (negative the logarithm of the hydrogen ion molarity) is better written as pH = - log aH+ (negative the logarithm of the hydrogen ion activity). This effect is - log aH+ (negative the logarithm of the hydrogen ion activity). This effect is very strong, and makes the pH much lower than you'd expect from the acid very strong, and makes the pH much lower than you'd expect from the acid molarity. molarity.

– If you were to dip a glass pH electrode into the 12 M HCl solution to actually If you were to dip a glass pH electrode into the 12 M HCl solution to actually measure the pH, you would get a pH that was higher than the true pH. This measure the pH, you would get a pH that was higher than the true pH. This well-known defect in glass pH electrode measurements is called the "acid well-known defect in glass pH electrode measurements is called the "acid error"; it is sensitive to experimental conditions and difficult to correct for. error"; it is sensitive to experimental conditions and difficult to correct for.

How is [HHow is [H++] found from pH? ] found from pH? A simple, working definition of pH isA simple, working definition of pH is

– pH = - log [HpH = - log [H++] ] – To obtain the hydrogen ion molarity from the pH, To obtain the hydrogen ion molarity from the pH,

remember that a logarithm of a number is really just the remember that a logarithm of a number is really just the exponent when that number is written as a power of ten: exponent when that number is written as a power of ten:

– y = 10 log x y = 10 log x – so the definition of pH solved for hydrogen ion molarity so the definition of pH solved for hydrogen ion molarity

is is – [H+] = 10[H+] = 10-pH-pH – For example, the molarity of hydrogen ions in a pH 5 For example, the molarity of hydrogen ions in a pH 5

solution is 10solution is 10-5-5 M. M. – N.B. pH is only N.B. pH is only approximatelyapproximately equal to minus the log of equal to minus the log of

the hydrogen ion molarity. the hydrogen ion molarity.

Calculating pHCalculating pH

Calculating pOHCalculating pOH How is [OHHow is [OH--] found from pOH? ] found from pOH? A simple, working definition of pOH isA simple, working definition of pOH is

– pOH = - log [OHpOH = - log [OH--] ] – To obtain the hydroxide ion molarity from the pOH, To obtain the hydroxide ion molarity from the pOH,

remember that a logarithm of a number is really just the remember that a logarithm of a number is really just the exponent when that number is written as a power of ten: exponent when that number is written as a power of ten:

– y = 10 log x y = 10 log x – so the definition of pH solved for hydroxide ion molarity so the definition of pH solved for hydroxide ion molarity

is is – [OH[OH--] = 10] = 10-pOH-pOH – For example, the molarity of hydroxide ions in a pOH 10 For example, the molarity of hydroxide ions in a pOH 10

solution is 10solution is 10-10-10 M. M. – N.B. pOH is only N.B. pOH is only approximatelyapproximately equal to minus the log of equal to minus the log of

the hydroxide ion molarity. the hydroxide ion molarity.

KKaa from pH from pH

The pH of a 0.115 M weak acid is The pH of a 0.115 M weak acid is 1.92. What is the K1.92. What is the Kaa? ?

HA + H2O ¾ H3O+ + A-

[H3O+] = 10-1.92 = 0.012 Minitial 0.115change -0.012 0.012 0.012equil 0.103 0.012 0.012Ka = [H3O+][A-]/[HA]Ka = (0.012)2/0.103Ka = 1.4 x 10-3

IndicatorsIndicators A chemical which changes color according to the A chemical which changes color according to the

pH of its environment is considered an pH of its environment is considered an indicatorindicator..

BuffersBuffers

A solution that can maintain its pH A solution that can maintain its pH value with little change when acids or value with little change when acids or bases are added to it is called a bases are added to it is called a bufferbuffer. .

Buffer solutions are usually prepared Buffer solutions are usually prepared as mixtures of a weak acid with its own as mixtures of a weak acid with its own salt (i.e., the conjugate base). salt (i.e., the conjugate base).

E.g., a 50:50 mixture of 1 M acetic acid E.g., a 50:50 mixture of 1 M acetic acid and 1 M sodium acetate buffers pH and 1 M sodium acetate buffers pH around 4.7. around 4.7.