BUFFER NEW PPT (2).pptx

Buffer Learning Goal Describe the role of buffers in maintaining the pH of a solution; calculate the pH of a buffer. A buffer solution maintains the pH by neutralizing small amounts of added acid or base. An acid must be present to react with any OH− added, an...

Buffer Learning Goal Describe the role of buffers in maintaining the pH of a solution; calculate the pH of a buffer. A buffer solution maintains the pH by neutralizing small amounts of added acid or base. An acid must be present to react with any OH− added, and a base must be present to react with any H O + added. 3 General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer A buffer solution is a water solvent-based solution which consists of a mixture containing a weak acid and the conjugate base of the weak acid or a weak base and the conjugate acid of the weak base. They resist a change in pH upon dilution or upon the addition of small amounts of acid/alkali to them. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer What Is a Buffer Solution? The buffer solution is a solution able to maintain its hydrogen ion concentration (pH) with only minor changes in the dilution or addition of a small amount of either acid or base. Buffer solutions are used in fermentation, food preservatives, drug delivery, electroplating, printing and the activity of enzymes, and the blood oxygen-carrying capacity needs specific hydrogen ion concentration (pH). General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer Solutions of a weak acid and its conjugate base or weak base and its conjugate acid are able to maintain pH and are buffer solutions. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer Types of Buffer Solutions The two primary types into which buffer solutions are broadly classified are acidic and alkaline buffers. Acidic Buffers As the name suggests, these solutions are used to maintain acidic environments. Acid buffer has acidic pH and is prepared by mixing a weak acid and its salt with a strong base. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer Acidic Buffers An aqueous solution of an equal concentration of acetic acid and sodium acetate has a pH of 4.74. The pH of these solutions is below seven. These solutions consist of a weak acid and a salt of a weak acid. An example of an acidic buffer solution is a mixture of sodium acetate and acetic acid (pH = 4.75). General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer Alkaline Buffers These buffer solutions are used to maintain basic conditions. A basic buffer has a basic pH and is prepared by mixing a weak base and its salt with strong acid. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer Alkaline Buffers The aqueous solution of an equal concentration of ammonium hydroxide and ammonium chloride has a pH of 9.25. The pH of these solutions is above seven. They contain a weak base and a salt of the weak base. An example of an alkaline buffer solution is a mixture of ammonium hydroxide and ammonium chloride (pH = 9.25). General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer What is pH? The pH value of a system shows its acidity or alkalinity. What is pKa? The pH value at which a chemical species will take or donate a proton is known as the pKa. The negative base-10 logarithm of a solution’s acid dissociation constant (Ka) is pKa. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer What is pH? The pH value of a system shows its acidity or alkalinity. What is pKa? The pH value at which a chemical species will take or donate a proton is known as the pKa. The negative base-10 logarithm of a solution’s acid dissociation constant (Ka) is pKa. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer pKa Definition The pH value of a system shows its acidity or alkalinity. The acid dissociation constant is denoted by the symbol Ka. Some acids are powerful, while others are weak. In aqueous solutions, strong acids totally break down into their ions. Weak acids, on the other hand, partially dissociate, resulting in a balance between the acid and its conjugate base. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer We can determine whether an acid is a strong acid or a weak acid by looking at its pKa value. The acid is weak if the pKa value is high. Because a greater pKa number suggests that Ka is low, this is the case. The value of [A–][H+] should be lower than the value of [HA] in order for Ka to be low. This indicates that the acid has been partially dissociated. However, if the value of [A–][H+] is more than [HA], the Ka will be large and the pKa will be low. This shows that the acid is potent. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer What is the Henderson-Hasselbalch Equation? The Henderson-Hasselbalch equation provides a relationship between the pH of acids (in aqueous solutions) and their pKa (acid dissociation constant). The pH of a buffer solution can be estimated with the help of this equation when the concentration of the acid and its conjugate base, or the base and the corresponding conjugate acid, are known. General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  Buffer Handerson-Hasselbalch Equation pH of acid buffer = pKa + log ([base]/[acid]) pKa = -log[Ka] General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  PRACTICE 2 What is the pH of a buffer solution containing equal volumes of 0.3M Hac and 0.3M NaAc? The ka of Hac is 1.8x10-5 General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake  PRACTICE 3 General, Organic, and Biological Chemistry: Structures of Life, 5/e © 2016 Pearson Education, Inc. Karen C. Timberlake