緩沖溶液(Buffersolutions)

1、Buffer solutions outlinel1 Definition l2 Classificationl3 How buffers work l4 Buffer calculationsl5 Preparation a buffer solution l6 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 comm

2、only made from a weak acid and one of its salts - often a sodium 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. A basic buffer solution has a pH greater than 7. Basic buffer solutions a

3、re 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.vA buffer solution has to contain things which will remove any hydrogen ions or hydroxide io

4、ns that you might add to it - otherwise the pH will change. Acidic and basic buffer solutions achieve this in different ways.lAcidic buffer solutions lBasic buffer solutionsAcidic buffer solutions Example:a mixture of acetic acid and sodium acetate contains these important things: vlots of un-ionise

5、d acetic acid;vlots of acetate ions from the sodium acetate;venough hydrogen ions to make the solution acidic.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, t

6、he pH wont change very much but because of the equilibrium involved, it will fall a little bit.Adding a base to this buffer solutionvRemoval by reacting with acetic acid Because most of the new hydroxide ions are removed, the pH doesnt increase very much.vRemoval of the hydroxide ions by reacting wi

7、th hydrogen ionsBasic buffer solutions Example:a mixture of ammonia and ammonium chloride solutions contains these important things:vlots of unreacted ammonia;vlots of ammonium ions from the ammonium chloride;venough hydroxide ions to make the solution basic.Adding an acid to this buffer solutionvRe

8、moval 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.vRemoval of the hydrogen ions by reacting with hydroxide ionsAdding a base to this buffer solution Because the ammonia fo

9、rmed 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.Henderson-Hasselbalch equation Example : Through calculation,please compare the additions of acid or base to a buffer solution and to pur

10、e 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 : 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 : CH3CO2H = (1 - 0.1) / 1.1 = 0.

11、82 M CH3CO2- = (0.3 + 0.1) / 1.1 = 0.36 M pH = 4.76 + lg (0.36 / 0.82) = 4.40lOne 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 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 = 12.9

12、6 Use the HendersonHasselbalch equation in reverse. Choose weak acid with pKa close to required pH. Substitute into HendersonHasselbalch equation. Solve for the ratio of conjugate base/acid.Common buffer solutionssolutionConcentration(mol/L) pH(25 )KHC4H4O6 Saturation,25 3.557KHC8H4O40.054.008CH3CO2

13、H-CH3CO2Na equal concentrations 4.760KH2PO4-Na2HPO40.025, 0.0256.865KH2PO4-Na2HPO40.008695, 0.0304307.413Na2B4O7-10H2O0.019.180NH4+ -NH3equal concentrations 9.250Example :nPreparation of a buffer solution with pH = 5.2 pKa (CH3CO2H / CH3CO2-)=4.76，so that this mixture acetic acid (CH3CO2H) / sodium acetate (CH3CO2-) can be used. nAccording to the Henderson-Hasselbalch relationship: pH