buffers and neutralisation Flashcards
what is a buffer
- minimises pH changes
- when small amounts of acid/base are added
forming a basic buffer solution
- weak base and its salt
- made from reacting weak base + strong acid
- E.g NH3 + NH4+Cl-
forming acidic buffer solutions
- weak acid and its salt
- Made from weak acid and strong base
- or mixing weak acid solution with a solution of its salt
forming buffer from a partial neutralisation of the weak acid
- excess WA is partially neutralised by the alkali
- NaOH
- forms a conjugate base
adding acid to a buffer solution
- [H+] inc
- H+ reacts with conjugate base
- H+ + CB = HA
- eq shift to left
- counteracts change
- restores original [H+]
adding alkali to buffer solution
- [OH-] inc
- OH - reacts with H+
- OH- + H+ = H2O
- HA dissociates
- eq shifts to right
- restores [H+]
when [HA] = [A-]
- pH = pKa
- Ka = [H+]
calc pH of a buffer solution
[H+] = Ka x [HA] / [A-] pH = -log[H+]
pH = pKa + log [A-] / [HA]
pH of blood plasma
7.35-7.45
healthy blood = 7.40
controlled by carbonic acid / hydrogen carbonate (H2CO3/HCO3-) buffer system
pH lower/higher than 7.35/7.45
- lower = acidosis = fatigue, shortness of breath
- higher = alkalosis = muscle spasms, nausea
addition of H+/OH- on blood buffer system
Acid:
- [H+] inc
- H+ react with CB HCO3-
- eq left
- decreasing [H+]
Alkali:
- [OH-] inc
- [H+] reacts with [OH-]
- H+ + OH- = H2O
- H2CO3 dissociates
- eq right
- [H+] restored
- H2CO3 = H+ + HCO3-
what is the equivalence point
- volume of one sol that exactly reacts with volume of another solution
- centre of the vertical section of pH curve
what is the end point
- indicator contains equal conc of HA and A-
indicator colour changes (basic)
- E.g methyl orange
- indicators are weak acids
basic soltuion:
- OH- + H+ = H2O
- HA dissociates
- eq to right
- colour change to orange at end point
- then yellow when eq to right
methyl orange :
acid = red
alkali = yellow
end point = orange
phenolphthalein colours
acid = colourless alkali = pink
