1. pH and pOH, Indicators and Buffers Chapter 19

2. What is pH? pH is a logarithmic measure of hydrogen ion concentration, originally defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909. pH is a logarithmic measure of hydrogen ion concentration, originally defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909. pH = -log[H + ] pH = -log[H + ] where log is a base-10 logarithm and [H+] is the concentration of hydrogen ions in moles per liter of solution. where log is a base-10 logarithm and [H+] is the concentration of hydrogen ions in moles per liter of solution. According to the Compact Oxford English Dictionary, the "p" stands for the German word for "power", potenz, so pH is an abbreviation for "power of hydrogen". According to the Compact Oxford English Dictionary, the "p" stands for the German word for "power", potenz, so pH is an abbreviation for "power of hydrogen". The pH scale was defined because the enormous range of hydrogen ion concentrations found in aqueous solutions make using H + molarity 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 from 2 to 13". The pH scale was defined because the enormous range of hydrogen ion concentrations found in aqueous solutions make using H + molarity 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 from 2 to 13". The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10 -7 M. The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10 -7 M. A pH of 7 is considered "neutral", because the concentration of hydrogen ions is exactly equal to the concentration of hydroxide (OH-) ions produced by dissociation of the water. A pH of 7 is considered "neutral", because the concentration of hydrogen ions is exactly equal to the concentration of hydroxide (OH-) ions produced by dissociation of the water. Increasing the concentration of hydrogen ions above 1.0 × 10 -7 M produces a solution with a pH of less than 7, and the solution is considered "acidic". Increasing the concentration of hydrogen ions above 1.0 × 10 -7 M produces a solution with a pH of less than 7, and the solution is considered "acidic". Decreasing the concentration below 1.0 × 10 -7 M produces a solution with a pH above 7, and the solution is considered "alkaline" or "basic". Decreasing the concentration below 1.0 × 10 -7 M produces a solution with a pH above 7, and the solution is considered "alkaline" or "basic".

3. Comparing 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 times as acidic as a solution of pH 2, because the hydrogen ion concentration at pH 1 is ten times the hydrogen ion concentration at pH 2. For example, a solution of pH 1 is said to be 10 times as acidic as a solution of pH 2, because the hydrogen ion concentration at pH 1 is ten times the hydrogen ion concentration at pH 2. This is correct as long as the solutions being compared both use the same solvent. This is correct as long as the solutions being compared both use the same solvent. You can't use pH to compare the acidities in different solvents because the neutral pH is different for each solvent. You can't use pH to compare the acidities in different solvents because the neutral pH is different for each solvent. For example, the concentration of hydrogen ions in pure ethanol is about 1.58 × 10 -10 M, so ethanol is neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water! For example, the concentration of hydrogen ions in pure ethanol is about 1.58 × 10 -10 M, so ethanol is neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water!

4. Negative pH 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. 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. Is this possible? Yes. Is this possible? Yes. If the molarity of hydrogen ions is greater than 1, you'll have a negative value of pH. If the molarity of hydrogen ions is greater than 1, you'll have a negative value of pH. E.g., you might expect a 12 M HCl solution to have a pH of -log(12) = -1.08. E.g., you might expect a 12 M HCl solution to have a pH of -log(12) = -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 pH calculations from acid molarity inaccurate and difficult to verify experimentally: There are some complications in high molarity acid solutions that make pH calculations from acid molarity inaccurate and difficult to verify experimentally: –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 expect from the acid molarity. –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 concentration of hydrogen ions (or the activity) is much higher than the actual 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 = - 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 molarity. –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 well-known defect in glass pH electrode measurements is called the "acid error"; it is sensitive to experimental conditions and difficult to correct for.

5. How is [H + ] found from pH? How is [H + ] found from pH? A simple, working definition of pH is A simple, working definition of pH is –pH = - log [H + ] –To obtain the hydrogen ion molarity from the pH, remember that a logarithm of a number is really just the exponent when that number is written as a power of ten: –y = 10 log x –so the definition of pH solved for hydrogen ion molarity is –[H+] = 10 -pH –For example, the molarity of hydrogen ions in a pH 5 solution is 10 -5 M. –N.B. pH is only approximately equal to minus the log of the hydrogen ion molarity. Calculating pH

6. Calculating pOH How is [OH - ] found from pOH? How is [OH - ] found from pOH? A simple, working definition of pOH is A simple, working definition of pOH is –pOH = - log [OH - ] –To obtain the hydroxide ion molarity from the pOH, remember that a logarithm of a number is really just the exponent when that number is written as a power of ten: –y = 10 log x –so the definition of pH solved for hydroxide ion molarity is –[OH - ] = 10 -pOH –For example, the molarity of hydroxide ions in a pOH 10 solution is 10 -10 M. –N.B. pOH is only approximately equal to minus the log of the hydroxide ion molarity.

7. K a from pH The pH of a 0.115 M weak acid is 1.92. What is the K a ? The pH of a 0.115 M weak acid is 1.92. What is the K a ? HA + H 2 O  ¾ H 3 O + + A - [H 3 O + ] = 10 -1.92 = 0.012 M initial 0.115 change -0.012 0.012 0.012 equil 0.103 0.012 0.012 Ka = [H 3 O + ][A - ]/[HA] Ka = (0.012) 2 /0.103 Ka = 1.4 x 10 -3

8. Indicators A chemical which changes color according to the pH of its environment is considered an indicator. A chemical which changes color according to the pH of its environment is considered an indicator.

9. Buffers A solution that can maintain its pH value with little change when acids or bases are added to it is called a buffer. A solution that can maintain its pH value with little change when acids or bases are added to it is called a buffer. Buffer solutions are usually prepared as mixtures of a weak acid with its own salt (i.e., the conjugate base). Buffer solutions are usually prepared as mixtures of a weak acid with its own salt (i.e., the conjugate base). E.g., a 50:50 mixture of 1 M acetic acid and 1 M sodium acetate buffers pH around 4.7. E.g., a 50:50 mixture of 1 M acetic acid and 1 M sodium acetate buffers pH around 4.7.