Acids, bases & buffers

• At r.t.p water is predominantly found in the H2O form
• A few molecules may dissociate into H+ and OH- ions
• Another few may be found as a hydroxonium ion H3O+
• The hydroxonium ion is formed via the reaction between two water molecules:

H2O + H2O --> H3O+  + OH-

• Acids can be defined as proton donors
• Strong acids completely dissociate in water
• Weaks acids only partially dissociate in water
• Weak acids have a pKa value
• pKa is constant which denotes the point at which 50% of the protons have been donated into solution 
• When pKa=pH the equilibrium will be central
• Bases can be defined as proton acceptors
• A measure of basicity is the ability of the molecule to remove a hydrogen ion from water 

• A buffer is a solution that can resist minor changes in pH by 'mopping up' excess ions
• A buffer solution is comprised of 50% conjugate acid and 50% conjugate base
• The buffering region is usually +/- 1 pH unit
• The buffering capaity is greatest when pH=pKa 

• Physiological pH= 7.4
• Outside cells the buffer is carbonic acid (pKa = 6.1):

H₂CO₃ + H₂O ↔ HCO₃- + H₃O+

• Inside cells the buffer is hydrogen phosphate (pKa = 7.2):

H₂PO₄^- + H₂O ↔ HPO₄^2- + H₃O+

• The exception to this is the lysosomes, which operate at about pH 4.5
• Stomach acid has a pH of 1.3, but gastric juice has a pH of 1.5-3
• This is because the epithelial cells secrete bicarbonate, which neutralises stomach acid, raising the pH

• Universal indicator: good for all pH ranges
• Phenolphtalein: turns pink when the solution becomes alkali
• Methyl orange: becomes yellow when the solution becomes alkali
• Poinsettia plants contain anthrocyanin and so make good indicators above pH 3
• Red cabbage paper is also good for all pH ranges