Water, Buffers, and pH - PDF

Water, pH and buffers SAFA YOUSIF  Importance of water Water is essential for life (the medium for life). It covers about 73% of the earth's surface and every living thing is dependent upon it. The human body is comprised of over 70% water, and it is a major component of...

Water, pH and buffers SAFA YOUSIF  Importance of water Water is essential for life (the medium for life). It covers about 73% of the earth's surface and every living thing is dependent upon it. The human body is comprised of over 70% water, and it is a major component of many bodily fluids including blood, urine, and saliva. Water plays a central role in the chemistry of all.life(chemical reaction occur in aqueous environment)  dissolve nutrients and easy to transport. Proteins, polysaccharides, nucleic acids and membranes all assume their characteristic shapes in response to water. The chemical properties of water are related to the functions of biomolecules,.entire cells and organisms :Water has two very important properties 1. physical properties..Chemical properties.2 Physical properties: 1. Water is a polar molecule: A water molecule is formed when two atoms of  hydrogen bond covalently with an atom of oxygen. This gives water an asymmetrical distribution of  charge. Molecules that have ends with partial negative and  positive charges are known as polar molecules. It is this polar property that allows water to  separate polar solute molecules and explains why.water can dissolve so many substances Uneven Charge Distribution: Dipolar: (partial negative and positive charges). Hydrogen Bonds: gives water unusual properties. Between water molecules, and between water and polar solutes. Four H-bonds per water molecule gives water its: high.boiling point, high melting point, large surface tension 2. Water is excellent solvent its ability to solvate a wide range of organic and inorganic molecules. Hydrophilic & Hydrophobic molecules: hydrophilic (Greek, “water-loving”): Compounds that dissolve easily in water. hydrophobic—non polar molecules: compound that poor dissolve in water such as lipids.and waxes 3.Osmosis Osmosis is the passage of water from a region of high water concentration through a semi concentration through a semi permeable membrane to a region of low water.concentration  Isotonic, hypotonic, hypertonic Isotonic: same osmotic pressure as blood.(same concentration of solutes) Hypotonic: lower osmotic pressure than blood ( less concentration of solutes). Hypertonic: higher osmotic pressure than blood ( more concentration of solutes)  : Chemical Properties.2 Ionizes to H and OH The Ionization of Water Water is slightly ionized into a hydrogen ion (H+).and a hydroxide ion (OH–) Concentrations of H+ and OH– At 25°C, water dissociates to the extent that [H+] and [OH–] are equal= 1x10-7M..The concentration of water is a constant (55.5 M)  PH pH as a shorthand way to know how much H+ is present in a solution. To avoid the use of large negative exponents, the pH scale was devised. pH is defined as the negative logarithm of the hydrogen ion concentration. pH = – log[H+] pOH = -Log[OH-]  pH Scale The pH of a solution indicates its acidity or basicity (alkalinity). The pH scale is an inverse logarithm that ranges from 0 to 14: anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is basic (or alkaline ). Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH in cells (6.8) and the blood (7.4) are both very close to neutral, whereas the environment in the stomach is highly acidic, with a pH of 1 to 2. Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH, and low concentrations a high pH.  Acid and base Acid An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, usually by dissociating one of its hydrogen atoms. An acid is a proton donor. ⁻HA → H⁺ + A Strong Acids: Strong acids completely ionize in water.  Strong acids go 100% to the right.  This is an irreversible reaction.  ⁻HCl → H⁺ + Cl  Weak Acids Weak acids do not ionize completely.  HF is a weak acid.  this is a reversible reaction.  ⁻HF ← → H⁺ + F   Base A base provides either hydroxide ions (OH– ) or other negatively-charged ions that react with hydrogen ions in solution, thereby reducing the concentration of H+ and raising the pH. A base is a proton accepter or a hydroxyl donor. ⁺B + H⁺ → BH Strong Bases  A strong base completely dissociates in solution into  the ions that make it up. This is an irreversible reaction.  NaOH → Na⁺ + OH⁻  Weak Bases Weak bases do not fully ionize in water.  This is a reversible reaction.  ⁺NH3 + H⁺ ← → NH4   Ionizatioan constants of Acids HA → H⁺ + A⁻ Ka = [H+][A-] / [HA] Ka= Measure of ability of an acid to dissociate. Strong acids: high Ka, ionized 100% into in water..Weak acids: low Ka, ionized partially (~10%) in water  The Physiological pH It is important in the human body that pH remains nearly constant † ~ 7 inside cells † 7.4 ± 0.05 in blood This can be achieved by the use of.BUFFERS  Buffers How can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Are solutions of a weak acid (HA) and its conjugate base, a weak base and its conjugate acid. Buffers resist the change in pH because they have acid to neutralize bases and bases to neutralize acids. Buffers usually consist of a weak acid and its conjugate base; this enables them to readily absorb excess H+ or OH–, keeping the system’s pH within a narrow range. Two common ways to produce this effect: 1. Weak acid + its e.g. CH3COOH/CH3COO- 2. Weak base + its conjugate acid + e.g. NH3/NH4 Note that buffers require an acidic component and a basic component  The buffer systems of the body (a) Proteins are the most important buffers in the body. They are mainly intracellular and include hemoglobin. (b) Phosphate buffer (H2PO4- : HPO42-) is mainly intracellular. (c) H2CO3 is a buffer found in the blood (extracellular). H2CO3 ↔ H⁺ + HCO3⁻ Carbonic acid bicarbonate ion Maintaining a constant blood pH is critical to a person’s well-being. The buffer that maintains the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, excess carbonic acid can be converted into carbon dioxide gas and exhaled through the lungs; this prevents too many free hydrogen ions from building up in the blood and dangerously reducing its pH; likewise, if too much OH– is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. #Without this buffer system, the body’s pH.would fluctuate enough to jeopardize survival  Thank You