pH and Buffers

Acids and Bases Acids: H + donors  HCl  H + + Cl -  CH 3 COOH  CH 3 COO - + H + Bases: H + acceptors  NaOH + H +  Na + + H 2 O  Na 2 CO H +  2 Na + + H 2 CO 3 Amphoteric (both H + donor and acceptor)  H 2 O + HCl  H 3 O + + Cl -  H 2 O + NH 3  OH - + NH 4 +

Conjugate acid-base pair Examples HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) NH 3(g) + H 2 O (l)  NH 4 + (aq) + OH - (aq) acid baseConjugate acid Conjugate base acid base Conjugate acid Conjugate base

pH Scale pH = - log [ H + ]  Acids: pH < 7  Bases: pH > 7 E.g., [H + ] = 1 x mol/L pH = - log (1 x ) = 3 (acid) E.g., [H + ] = 1 x mol/L pH = -log (1 x ) = 12 (base) E.g., [H + ] = 1 x mol/L pH = -log (1 x ) = 7 (neutral)

Strong and Weak Acids HCl  H + + Cl - CH 3 COOH  CH 3 COO- + H+

Buffers Buffers – solutions that tend to resist changes in pH as acid or base is added Composed of a weak acid AND its conjugate base E.g., HA  H + + A - (HA is the weak acid and A - is its conjugate base)  The buffer would be composed of HA + A -  Adding extra H + (acid) would result in H + + A -  HA  Adding extra OH - (or any other base) would result in OH - + HA  A - + H 2 O

stry/essentialchemistry/flash/buffer12. swfhttp:// stry/essentialchemistry/flash/buffer12. swf Maintenance of pH is vital to all cells Cellular processes depend on enzyme activity

Phosphate System The phosphate system buffers the intracellular fluid of cells at physiological pH range (6.9 and 7.4) Made up of H 2 PO 4 - and HPO 4 2-  Adding extra H + : HPO H +  H 2 PO 4 -  Adding extra OH - (or another base): H 2 PO OH -  HPO H 2 O

Bicarbonate System Important buffer system of blood plasma H 2 CO 3 and HCO 3 - This system works well since critical concentration of H 2 CO 3 (dissolved CO 2 ) is maintained relatively constant through dissolved CO 2 produced in the tissues and available as gaseous CO 2 in the lungs

 Carbonic anhydrase add H 2 CO 3 (acid) by dissolving CO 2 (produced in tissues) CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 -  When excess H + are added, the reaction shifts to the left CO 2 + H 2 O ← H 2 CO 3 ← H + + HCO 3 -  When H + are removed, the reaction shifts to the right CO 2 + H 2 O → H 2 CO 3 → H + + HCO 3 -

pH control in the kidneys pH of blood is kept constant  H + into kidneys  HCO 3 - reabsorption in blood  If blood is too basic, less H + is excreted Phosphate buffer is the only in the urine  If too much acid in urine then, H 2 PO 4 - is formed

pH of Urine Normal range for urine is between 6.5 and 7.25  Diet determines the pH level  Diet rich in meats, pH tends to be acidic  Diet rich in fruits and vegetables, pH tends to be alkaline