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Learn about the pH scale, the self-ionization of water, the ion-product constant for water, and how to measure pH using indicators.
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Section 2 - Hydrogen Ions and Acidity Introduction • To test a diagnosis of diabetic coma, a doctor orders several tests, including the acidity of the patient’s blood. • Results from this test will be expressed in units of pH. • You will learn how the pH scale is used to indicate the acidity of a solution and why the pH scale is used.
5. Hydrogen Ions from Water • As you already know, water molecules are highly polar and are in constant motion. • Occasionally, the collisions between water molecules are energetic enough to transfer a hydrogen ion from one water molecule to another. • The reaction in which water molecules produce ions is called the self-ionization of water.
Hydrogen Ions from Water (cont.) • Hydrogen ions (H+) are always joined to water molecules as hydronium ions (H3O+) • Sometimes called protons • The self-ionization of water occurs to a very small extent. • The concentrations of H+ and OH- are equal (1 x 10-7M) in pure water, which is a neutral solution.
6. Ion Product Constant for Water • When [H+] increases, [OH-] decreases, and vice versa. • If additional ions are added to a solution, the equilibrium shifts. • The concentration of the other type of ion decreases. • More water molecules are formed in the process.
Ion Product Constant for Water (cont.) • For aqueous solutions, the product of the hydrogen ion concentration and the hydroxide ion concentration equals 1.0 10-14. • The product of the concentrations of the hydrogen ions and hydroxide ions in water is called the ion-product constant for water (Kw).
Ion Product Constant for Water (cont.) • Not all solutions are neutral, when some substances dissolve in water they release hydrogen ions • An acidic solutionis one in which [H+] is greater than [OH-]. • A basic solutionis one in which [H+] is less than [OH]. • Basic solutions are also known as alkaline solutions. Unrefined hydrochloric acid, commonly called muriatic acid, is used to clean stone buildings and swimming pools. Sodium hydroxide, or lye, is commonly used as a drain cleaner.
Example 1: • Step 1: Ion Product Constant for Water (cont.)
Step 2: • Step 3: Ion Product Constant for Water (cont.)
7. The pH Concept • On the pH scale, which ranges from 0 to 14, neutral solutions have a pH of 7. • A pH of 0 is strongly acidic, a solution with a pH of 14 is strongly basic. • Calculating pH • The pH of a solution is the negative logarithm of the hydrogen-ion concentration. • To calculate the pH of a neutral solution:
The pH Concept (cont.) • A solution in which [H+] is greater than 1 10–7M has a pH less than 7.0 and is acidic. • The pH of pure water or a neutral aqueous solution is 7.0. • A solution with a pH greater than 7 is basic and has a [H+] of less than 1 10–7M.
The pH Concept (cont.) • Calculating pOH • The pOH of a solution equals the negative logarithm of the hydroxide-ion concentration. • A neutral solution has a pOH of 7, a solution with a pOH less than 7 is basic, a solution with a pOH more than 7 is acidic.
The pH Concept (cont.) • pH and Significant Figures • For pH calculations, you should express the hydrogen-ion concentration in scientific notation • Most pH values are not whole numbers. • When the pH is not a whole number use the yx function on your calculator. • When its coefficient is not 1, you will need to use the log function on your calculator.
Example 2: • Step 1: The pH Concept (cont.)
Step 2: • Step 3: The pH Concept (cont.)
Example 3: • Step 1: The pH Concept (cont.)
Step 2: • Step 3: The pH Concept (cont.)
8. Measuring pH • For small-volume samples, an indicator is used. • If you know the [OH-] of a solution, you can find its pH. • The ion-product for water defines the relationship between [H+] and [OH-]. • Therefore, you can determine [H+] by dividing Kw by the known [OH-]. Phenolphthalein changes from colorless to pink at pH 7–9.
Example 4: • Step 1: Measuring pH (cont.)
Step 2: • Step 3: Measuring pH (cont.)
Measuring pH (cont.) • Acid-Base Indicators • An indicator (HIn) is an acid or a base that undergoes dissociation in a known pH range. • An indicator is a valuable tool for measuring pH because its acid form and base form have different colors in solution. • The acid form dominates the dissociation equation at low pH, and the base form dominates the equilibrium at high pH.
For each indicator, the change from dominating acid form to dominating base form occurs in a narrow range of approximately 2 pH units. • Within this range, the color of the solution is a mixture of the colors is a mixture of the colors of the acid and base forms. • Knowing the pH range over which this color change occurs can give you a rough estimate of the pH of a solution. • At all pH values above this range, you would only seethe color of the acid form, at all values below you would only see the color of the base form. Measuring pH (cont.)
Measuring pH (cont.) • Indicators have certain characteristics that limit their usefulness. • At temperatures other than 25⁰ C an indicator may change color at a different pH. • If the solution being tested is not colorless, the color of the indicator may be distorted • Dissolved salts in a solution may also affect the indicator’s dissociation. • Indicator strips can help overcome these problems. Universal Indicators
Measuring pH (cont.) • pH Meters • Make rapid, accurate pH measurements. • Easier to use than liquid indicators or indicator strips. • Measurements are typically more accurate to within 0.01 pH unit of the true pH. • The color and cloudiness of the unknown solution do not affect the accuracy of the pH value obtained.
