Water-acid and Base Buffer

Water- Acid and Base -Buffer Water is the most abundant substance in living systems, making up 70% (or more) of the most of the organisms weight Chemical properties of Water • • 1- ionization : Water is dissociated to H + and OH – • 2-Solvent of Polar molecules : It can disolves the polar compounds • 3- Hydrogen bond: It Has potential to make Hydrogen bonds with itself and other molecules (organic molecules) and dissolves them • Because of this reason, it has a higher melting point, boiling point, and heat of vaporization than most other common solvents • Water has three forms liquid, solid and vapor • How is the hydrogen bond made? • In a each H2o molecule each hydrogen bears a partial positive charge (+δ) • and the oxygen atom bears a partial negative charge (- δ)equal to the sum of the two partial positives. • As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen atom of another (called a hydrogen bond..) Amphipathic compounds in water makes micells • Amphipathic compounds contain two regions that are polar (or charged) and regions that are nonpolar • What is an Acid? • An acid is a substance which, when dissolved in water, and releases protons. • The extent of dissociation, that is, the amount of protons released compared to the total amount of compound, is a measure of the strength of the acid. • For example, HCl (hydrochloric acid) is a strong acid, because it dissociates completely in water, generating free [H+] and [Cl-]. • Acidity can be measured on a scale called pH “the negative logarithm of the hydrogen ion concentration”. • pH= - log [H+] • How Can You actually determine the pH of a Solution? • Use a pH meter—read the number. • Use pH paper (color patterns indicate pH). Weak Acids and Bases Weak Acids and Bases Have Characteristic Dissociation Constants Acids may be defined as proton donors and bases as proton acceptors. A proton donor and its corresponding proton acceptor make up a conjugate acid-base • Acetic acid (CH3COOH) as a proton donor, and • the acetate anion (CH3COO-),the corresponding proton acceptor , constitute a conjugate acid base pair, related by the reversible reaction : The tendency of any acid (HA) to lose a proton and form its conjugate base (A) is defined by the equilibrium constant (Keq) for the reversible reaction Buffers • Buffers Are Mixtures of Weak Acids and Their Conjugate Bases • Buffers are aqueous systems that tend to resist against changes in pH when small amounts of acid (H) or base (OH) are added. • A buffer system consists of a weak acid (the proton donor) and its conjugate base (the proton acceptor). • As an example, a mixture of equal concentrations of acetic acid and acetate ion Regulation of H+ concentration (PH) of the intracellular and extracellular fluids The intracellular and extracellular fluids of multicellular organisms have a characteristic and nearly constant pH. Concentration of hydrogen ions is regulated sequentially by: 1-Chemical buffer systems , which act within seconds • H2CO3/HCO3-, H2PO4-/HPO2- and Proteins , Nucleic acids 2- The respiratory center in the brain stem , which acts within 1-3 min 3-Renal mechanisms , which require hours to days to effect pH changes Henderson- Hasselbalch equation Determination of PKa Pka is a PH at that point of PH,which [HA] =[A] Log [HA]/[A] = log 1 PH= Pka + Log 1 = Pka+0 PH=PKa • Normal PH= 7.4 • Acceptable PH range= 6.8< 7.4<7.8 • Acidosis : Occuer when blood PH< 6.8 • Alkalosis : Occure when blood PH> 7.8 Kinds of Buffers 1- Bicarbonate buffer H2CO3/HCO3- = acid/ base H2CO3 CO2+ H2O H2CO3 is a weak and unstable acid thereforein h2o medium it will dissociat to CO2+ H2O CO2 is a gas therefore for it’s concentration we have to use the pressure sign P instated of concentration sign [ ] a PCO2 equal with concentration of H2CO3 a= solubility coefficient of CO2 in water According toHenderson- Hasselbalch equation: PH=Pka + log HCO3-/ H2CO3=Pka + log HCO3-/aP CO2 2- Phosphate Buffer H2PO4H+ + HPO4 2Acid Base PH= Pka+ Log HPO4 2- / H2PO4- • Protein , nucleic acids buffer system • Consists of Plasma Proteins (albumin, hemoglobin) • Proteins are just chains of AA • The exposed amine group of the AA (NH2) accepts H+ ions when conditions are acidic • The exposed carboxyl group of AA can release H+ ions when conditions are basic Proteins can act as Acids or Bases 1.Lung and kidneys response to changes oh blood PH in two ways: 1- The lungs assist in maintaining a constant blood pH by removing (excreting) the CO2 2. while the kidney excretes acid in the form of H2PO4- and NH4+ and reabsorbs the alkali in the form of HCO3-.