pH

pH

1. Acids

   • An acid releases protons
   • Some acids can release more than one proton

2. Bases

   • Proton acceptor
   • When in solution they accept a proton to form water

3. Conjugate pairing

   • Conjugate pairs are species that are linked by the transfer of a proton - theyre always on the opposite side of an equation
   • The species that has lost the proton is the conjugate base
   • The species that has gained a proton is the conjugate acid
   • Water reacts with acids to form a conjugate acid (H3O+) and with bases to form a conjugate base (OH-)

4. pH

   • pH is a measure of hydrogen concentration
   • pH = -log[H+]
   • [H+] = 10^-pH
   • Monobasic acids are acids that produce one mol of H+ from 1 mol of acid
   • For monobasic acids: [H+] = [Acid]
   • Kw is the equilibrium constant for water
   • Kw = [H+] [OH-]
     • Changing the concentration of [H+] or [OH-] has no effect on Kw as the equilibrium will shift changing the value for the other [] keeping Kw the same
     • Changing the temperature of the solution changes Kw as it will favor one side of the reaction

5. Acid dissociation constant Ka

   • Weak acids dont fully dissociate in solution so [H+] doesnt = [Acid]
   • Ka = ([H+] [A-]) / [HA]
   • pKA
     • pKa = -log(Ka)
     • The smaller the pKa the stronger the acid

6. Buffers

   • Buffers resist change in pH
   • A buffer solution minimises changes in pH when small amounts of acid or base are added
   • Acid buffers (pH less than 7) are made by setting up an equilibrium between a weak acid and its conjugate base
   • The equilibrium solution will contain:
     • Lots of undissociated acid (HA)
     • Lots of the acids conjugate base (A-)
     • Enough H+ ions to make the solution acidic
   • The conjugate pair work together to resist changes to pH
     • The conjugate base reacts with any excess of H+
     • The conjugate acid releases H+ when theres too much base
   • Our bloods pH is controlled by a buffer of carbonic acid and hydrogen carbonate

7. pH curves and titrations

   • You can measure pH using a pH meter
   • Depending on the strength of the acids and bases the curve will look different
   • The initial pH depends on the strength of the acid
   • To start with addition of small amounts of base have little impact on the pH of the solution
   • The vertical part is called the equivalence point and at this point [H+] = [OH-]
   • You need to add more weak base than a strong base to a strong acid to cause a pH change and the change is less pronounced - but you need to add less strong base to a weak acid to see a large change in pH
   • The final pH depends on the strength of the base - the stronger the base the higher the final pH
   • Indicators
     • pH curves can tell you what indicator to use
     • You need the indicator to change colour at the equivalence point of the reaction
     • Strong acid/strong base
       • Both indicators suitable as there is a rapid pH change over range of both indicators
     • Strong acid/weak base
       • Methyl orange range
     • Weak acid/strong base
       • Phenolphthalein range
     • Weak acid/weak base
       • Neither as there is no sharp pH change

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