缓冲溶液(Buffer solutions)

Removal of the hydroxide ions by reacting with hydrogen ions
Basic buffer solutions
Example:a mixture of ammonia and ammonium chloride solutions
contains these important things: lots of unreacted ammonia; lots of ammonium ions from the ammonium chloride; enough hydroxide ions to make the solution basic.
Using plasmatic(血浆) carbonates for example
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Buffers are widely used in our daily-life.For example, all injections and eye drops are buffered.Furthermore, buffers are also important in industry.
• A basic buffer solution has a pH greater than 7. Basic buffer solutions are commonly made from a weak base and one of its salts.
• Example: A weak base and its conjugate acid NH3 / NH4+ (pH=9.25) We also can change the pH of the buffer solution by changing the ratio of base to salt.
• Example : A weak acid and its conjugate base CH3CO2H / CH3CO2- (pH=4.76) We can change the pH of the buffer solution by changing the ratio of acid to salt.

Removal of the hydrogen ions by reacting with hydroxide ions
Adding a base to this buffer solution
Because the ammonia formed is a weak base, it can react with the water - and so the reaction is slightly reversible. That means most of the hydroxide ions are removed from the solution.
equal concentrations 4.760 0.025, 0.025 0.01 6.865 9.180
0.008695, 0.030430 7.413
equal concentrations 9.250
Example :

Preparation of a buffer solution with pH =
solution
KHC4H4O6 KHC8H4O4 CH3CO2HCH3CO2Na KH2PO4-Na2HPO4 KH2PO4-Na2HPO4 Na2B4O7-10H2O NH4+ -NH3
Concentration(mol/L) pH(25 ℃)
Saturation,25 ℃ 0.05 3.557 4.008
Acidic buffer solutions
Example:a mixture of acetic acid and sodium acetate
contains these important things: lots of un-ionised acetic acid; lots of acetate ions from the sodium acetate; enough hydrogen ions to make the solution acidic.
Adding an acid to this buffer solution

Removal by reacting with ammonia
Most, but not all, of the hydrogen ions will be removed. The ammonium ion is weakly acidic, and so some of the hydrogen ions will be released again.
Choose weak acid with pKa close to required pH. Substitute into Henderson–Hasselbalch equation. Solve for the ratio of [conjugate base]/[acid].
Common buffer solutions
[CH3CO2H] = (1 + 0.1) / 1.1 = 1.0 M and [CH3CO2-] = (0.3 - 0.1) / 1.1 = 0.18 M pH = 4.76 + log (0.18 / 1) = 4.02
Upon addition of 100 ml of NaOH 1 M :
Upon addition of 100 ml of NaOH 1 M : [OH-] = 1 / 1.1 = 0.091 M pOH = -log (0.091) = 1.04 pH = 14 - 1.04 = 12.96
Use the Henderson–Hasselbalch equation in reverse.
[CH3CO2H] = (1 - 0.1) / 1.1 = 0.82 M [CH3CO2-] = (0.3 + 0.1) / 1.1 = 0.36 M
pH = 4.76 + lg (0.36 / 0.82) = 4.40

One liter of neutral water
Upon addition of 100 ml of HCl 1 M : [H3O+] = 1 / 1.1 = 0.091 M pH = -log (0.091)= 1.04
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Uses of the buffer solution

The pH of human blood must stay in a relatively narrow range around pH = 7.4 (7.35-7.45), in order to ensure cellular survival. It can be maintained within these limits thanks to buffer systems .
pH = pKa + log ([Ac-] / [HAc]) 5.2 = 4.76 + log([Ac-] / [HAc]) 0.44 = log ([Ac-] / [HAc]) [Ac-] / [HAc]= 10-0.44 = 2.75
mix 2.75 moles of acetate and 1 mole of acetic and complete to 1 liter with distilled water.
5.2 pKa (CH3CO2H / CH3CO2-)=4.76，so that this mixture acetic acid (CH3CO2H) / sodium acetate (CH3CO2-) can be used.

According to the Henderson-Hasselbalch relationship:
Adding an acid to this buffer solution
The buffer solution must remove most of the new hydrogen ions, otherwise the pH would drop markedly.
Since most of the new hydrogen ions are removed, the pH won’t change very much ——but because of the equilibrium involved, it will fall a little bit.
Adding a base to this buffer solution

Removal by reacting with acetic acid
Because most of the new hydroxide ions are removed, the pH doesn't increase very much.


A buffer solution has to contain things which will remove any hydrogen ions or hydroxide ions that you might add to it otherwise the pH will change. Acidic and basic buffer solutions achieve this in different ways. Acidic buffer solutions Basic buffer solutions
A buffer solution is one which tend to resist changes in pH when small amounts of acid or base are added to it and its pH is not affected by dilution.
Buffer solutions
outline

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Definition Classification How buffers work Buffer calculations Preparation a buffer solution Uses of the buffers
Acidic buffer solutions
Basic buffer solutions
• An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt.
Henderson-Hasselbalch equation
Example :
Through calculation,please compare the additions of acid or base to a buffer solution and to pure water.
one liter of a buffer solution 1 M in acetic acid and 0.3 M in sodium acetate Upon addition of 100 ml of HCl 1 M :