Doc Brown's Chemistry  Theoretical Chemistry - Equilibria - Chemical Equilibrium 6.5

6.5 Case studies of uses of buffers in aqueous media

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses) KS4 Science GCSE/IGCSE Chemistry reversible reactions-equilibrium * KS4 Science GCSE/IGCSE notes acids and bases * KS4 Science GCSE/IGCSE notes acid-base theory

Equilibria Part 6 sub-index: 6.1 Salt hydrolysis * 6.2 Acid-base indicator theory, pH curves and titrations * 6.3 Buffers - definition, formulation and action * 6.4 Buffer calculations * 6.5 Case studies of buffer function * EMAIL query?comment

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle-rules * Part 2. K_c and K_p equilibrium expressions and calculations * Part 3. Equilibria and industrial processes * Part 4. Partition, solubility product and ion-exchange * Part 5. pH, weak-strong acid-base theory and calculations * Part 6. Salt hydrolysis, Acid-base titrations-indicators, pH curves and buffers * Part 7. Redox equilibria, half-cell electrode potentials, electrolysis and electrochemical series * Part 8 Phase equilibria-vapour pressure, boiling point and intermolecular forces

6.5 Case studies of the function and uses of buffers in aqueous media

• Case study 6.5.1 Other common buffer solutions and their use in the laboratory.

  • Potassium hydrogen benzene-1,2-dicarboxylate is an 'all in one' buffer solution of pH 4.0

    • (i) H⁺_(aq) + ^-OOC-C₆H₄-COOH_(aq) HOOC-C₆H₄-COOH_(aq)

    • ^{(ii) -}OOC-C₆H₄-COOH_(aq) + OH^-_(aq) ^-OOC-C₆H₄-COO^-_(aq) + H₂O_(l)

    • (i) removes hydrogen ions and (ii) removes hydroxide ions.

    • Buffers can be made by mixing the salt with the original benzene-1,2-dicarboxylic acid to give buffers in the range 2.2-3.8

  • A mixture of salts of a polybasic/polyprotic acid e.g. the salts KH₂PO₄, Na₂HPO₄ and Na₃PO₄ from phosphoric(V) acid (a tribasic/triprotic acid) can give buffer solutions in the range pH 6-12 e.g.

    • from Na₂HPO₄: HPO₄^2-_(aq) + H⁺_(aq) H₂PO₄^-_(aq)  (removes hydrogen ions)

    • from Na₃PO₄: PO₄^3-_(aq) + H⁺_(aq) HPO₄^2-_(aq) + H₂O_(l) (removes hydrogen ions)

    • from KH₂PO₄: H₂PO₄^-_(aq) + OH^-_(aq) HPO₄^2-_(aq) + H₂O_(l) (removes hydroxide ions)

    • from Na₂HPO₄: HPO₄^2-_(aq) + OH^-_(aq) PO₄^3-_(aq) + H₂O_(l) (removes hydroxide ions)

    • The HPO₄^2- ion is amphoteric, acting both as a proton donor and acceptor and phosphate(V) ions are important in the buffering of intracellular fluids in living organisms (see Case study 6.5.2 below).

  • Buffer solutions are used to accurately calibrate pH meters.

• Case study 6.5.2 The importance of buffering in biological systems

  • Case study 6.5.2a The importance of a stable pH cannot be over emphasised for the efficient function of enzyme-proteins which control most of the chemistry of life. Most enzymes can only operate at their maximum catalytic capacity over a narrow range of pH and three examples are shown on the left. 6.5.2b and 6.5.2c are examples of conjugate acid-base pairs that fulfil this important buffering operation are described below.

  • 6.5.2a Optimum pH conditions enzyme function.

    • Pepsin operates best in very acid conditions around pH 1.5 to 2.5

      • Pepsin is a vertebrate endopeptidase enzyme secreted as the zymogen pepsinogen by chief cells  of gastric pits and spontaneously formed from it below pH 5 (by an intramolecular process) when it is active. This type of acid protease enzyme hydrolyses peptides to their constituent amino acids.

    • Carbonic anhydrase operates most efficiently when its close to neutral ~pH 6.5 to 7.5

      • Carbonic anhydrase is a zinc-containing metalloenzyme of e.g. vertebrate red blood cells and catalyses reactions such as

        • H₂O + CO₂ H⁺ + HCO₃^-

        • a reversible reaction under different pH conditions and it speeds up carbon dioxide transport.

        • It is also involved in blood pH regulation of the kidney.

    • Trypsin is most efficient in slightly alkaline conditions pH 7.5 to 8.5

      • Trypsin is a protease enzyme secreted in an inactive form (trypsinogen) in the pancreas and is converted into an active form by enterokinase. The active form converts proteins into peptides in the digestion systems and the buffering medium is sodium hydrogen carbonate (see section 6.5.2c), and the reaction is essentially a partial hydrolysis breakdown of proteins.

    • For more on this See section 2b.6 in Enzyme structure, function and denaturing

  • 6.5.2b Inside cells hydrogenphosphate(V) ions act as the major intracellular buffer system, with contributions from organic phosphates such as glucose-6-phosphate and ATP.

    • HPO₄^2-_(aq) + H⁺_(aq) H₂PO₄^-_(aq) or H₂PO₄^-_(aq) + OH^-_(aq) HPO₄^2-_(aq)

  • 6.5.2c The major extracellular buffer is the 'carbonic acid'-'bicarbonate' or hydrogencarbonate system which enables e.g. blood, to function as an extraordinary effective buffer operating at about pH 7.

    • (i) CO_{2(g, lungs)} + aq (ii) CO_2(aq*) + H₂O_(l) (iii) H₂CO_3(aq) (iv) HCO₃^-_(aq) + H⁺_(aq)

    • (v) H₂CO_3(aq) + OH^-_(aq) (vi) HCO₃^-_(aq) + H₂O_{(l)  *(aq) = intra/extracellular fluids}

      • Note the negative anions must be counter balanced by positive ions such as sodium, Na⁺ otherwise an unwanted potential would be set up.

    • (iv) to (iii) removes hydrogen ions and (v) to (vi) removes hydroxide ions.

    • The effectiveness of the system depends on the reservoir of dissolved carbon dioxide in the blood plasma and the gas in the lungs.

      • If hydroxide ions are removed via reaction (v) to (iv), the depleted H₂CO₃ is readily replaced via the reaction sequence (i) to (ii) and (ii) to (iii).

      • If hydrogen ions are removed via (iv) to (iii) the reverse sequence of 1. can restore the system to the original pH.

    • The ability of mammals to maintain a fairly constant [HCO₃^-]/[H₂CO₃] ratio in blood plasma is reflected in the rate of CO₂ production in the cell oxidation reactions of respiration and the rate of CO₂ loss by expiration.

    • The blood plasma of man is about 7.4 and any deviation below 7.0, or above 7.8, as can happen in disease, can cause irreparable damage.

    • Intracellular and extracellular systems are very pH sensitive and small changes in pH can produce ill-effects in living organisms, hence, e.g. the bodies irritation by all except the very weakest of acids and alkalis in contact with the skin.

• Case study 6.5.3 Shampoos

  • to complete

• Case study 6.5.4: *

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