Acid/Bases & Buffers (pH) | PDF

pH, Acid / Bases and Buffers Asst. Prof. Tuğçe DEMİREL YALÇINER 6th floor No:619 Faculty of Medicine/ Department of Medical Biochemistry  Learning Objectives ✓ Define Chemical Bonding ✓ Understand the pH scale. ✓ Differentiate between...

pH, Acid / Bases and Buffers Asst. Prof. Tuğçe DEMİREL YALÇINER 6th floor No:619 Faculty of Medicine/ Department of Medical Biochemistry  Learning Objectives ✓ Define Chemical Bonding ✓ Understand the pH scale. ✓ Differentiate between acids and bases ✓ Function of Buffer.  Water Characteristics The covalent bonds within the water molecule, like the hydrogen bonds between the molecules, can also break spontaneously. This is known as water dissociation. H2O → H+ + OH- Water Hydrogen Ion Hydroxide Ion  Ionization of Water Water molecules have a very slight tendency to ionize to a hydrogen ion and a hydroxide ion H2O  H+ + OH-. Actually, free protons (H+) do not exist in solution, and instead, hydrogen ions formed in water are immediately hydrated to form hydronium ions (H3O+). Through proton hopping, the short hops of protons between a series of hydrogen-bonded water molecules results in a very rapid net movement of a proton over a long distance. Thus the movement of hydronium (and hydroxide) ions in an electric field is extremely fast compared to the movement of ions such as Na+ and Cl-.  Water Characteristics This doesn’t happen often, but enough. If you have a beaker of pure water, 1/10,000,000 molecules are hydrogen ions. H2O → H+ + OH- Water Hydrogen Ion Hydroxide Ion Another important characteristic of water… Water can form acids and bases. Dissociation of Water Hydronium ion + Hydroxyl ion - Neutral water has equal amounts of H+ and OH - Acids: Excess of H+ in aqueous solution Bases: Excess of OH- in aqueous solution Acids & bases neutralize each other.  pH If you want to mathematically express the concentration of the hydrogen ions found in a solution, you would refer to the solution’s “pH”. Power of H ion  Key Features of pH Scale Acid, Base, Neutral Range Acids are molecules that donate hydrogen ions to a solution. In a chemical formula, acids start with “H”. Ex. HCl, HF (hydrochloric acid, hydrofluoric acid) Bases are molecules that combine with hydrogen ions to remove them from a solution In a chemical formula, end with “OH”. Ex. NaOH, KOH (sodium hydroxide, potassium hydroxide) Scale is Logarithmic pH Scale Designates the H+ and OH- Concentrations Measurements of Acidity & Alkalinity (pH) Acidity of a solution > measured by concentration of hydrogen ions (H+). pH ranges: 0 (very acidic) to 14 (very basic). Change in just one unit of scale = tenfold change in H+ concentration. If concentration of H+ = OH - … neutral.  pH scale is logarithmic Change in just one unit of scale = tenfold change in H+ concentration.  More Examples of pH from Daily Life Images: pH scale, Wiki From the Virtual Cell Biology Classroom on ScienceProfOnline.com  There are several points related to the pH notation to be taken into account: 1. No direct relationship exists between the magnitudes of [H+] and pH. When the value of [H+] increases, the pH decreases and vice versa. 2.A pH of 7 indicates neutrality only at 25°C. As the [H+] changes with temperature, so does the value of pH. Pure water (neutral) has a pH 7.5 at 0°C and 6.1 at 100°C. At 37°C, the human body temperature, neutral pH is 6.8. 3.Usually, the pH of a solution varies between 0 and 14. Measurement of pH is one of the most important and frequently used procedures in biochemistry. The pH affects the structure and activity of biological macromolecules, so a small change in pH can cause a large change in the structure and function of a protein. There are two ways to determine the pH of an aqueous solution 1.pH strip 2.pH meter. Ways to measure pH l pH meter l Electrode measures H+ concentration l Must standardize (calibrate) before using. Ways to measure pH l Indicator dyes and test strips l Less precise l Each indicator is only good for a small pH range (1-2 pH units) l But may be good for field usage, or measuring small volumes, or dealing with noxious samples.  Why is pH important in biology? pH affects solubility of many substances. pH affects structure and function of most proteins - including enzymes. Many cells and organisms (esp. plants and aquatic animals) can only survive in a specific pH environment.  pH of blood and urine are commonly used in medical diagnoses - Acidosis: pH of the blood below the normal value of 7.4. For example, uncontrolled diabetes - Alkalosis: pH of the blood is higher than 7.4. May follow vomiting of acidic gastric contents. ✓ A solution is neutral when the hydrogen ion concentration is equal to the hydroxide ion concentration. ✓ Pure water is neutral because [H+] = [OH−]. ✓ At 25°C, pure water has 1 · 10−7 M [H+] and 1 · 10−7 M [OH−], but this markedly changes with temperature. ✓ For example, at 0°C [H+] or [OH−] = 3.4 · 10−8 and at 100°C, 8.8 · 10−7. When a molecule or anion accepts H+ (also known as a Brønsted–Lowry base), a “conjugated acid” of that molecule is formed. When an acid loses H+, its “conjugated base” is formed. base ACIDS AND BASES STRENGTH MONOPROTIC/POLYPROTIC ACIDS  Weak Acids and Bases Have Characteristic Acid Dissociation Constants Reaction proceeding in a single direction and reaching completion; STRONG ACID Reaction proceeding in both directions and reaching equilibrium; WEAK ACID ✓ Strong bases ionize completely in a solution. ✓ Similar to weak acids, the ionization constants of weak bases (Kb) reflect their degree of ionization. pKa:acid strength  Why is pH important to living organisms? Key molecules, known as enzymes, function at set pH values. All our cells function within a certain pH range. If the fluids bathing those cells is “off” these cells won’t function at max capacity. Other than just cells, our blood needs to be at a certain pH.  But… The chemical reactions of life constantly produce acids and bases within cells. These have a tendency to throw off the pH values. We need some sort of mechanism to minimize how much the pH is altered. Which type of solution is one with a pH of 8? a. acidic b. basic c. neutral A solution with a pH of 5.0 _____. a) is basic b) has a hydrogen-ion concentration of 5.0M c) is neutral d) has a hydroxide-ion concentration of 1 x 10-9 M –9 THANK YOU…