Buffers and Titrations South Kingstown High Shool：緩沖器和滴定南金斯敦高中

1、buffersa buffer in actiondefinition of a bufferna buffer solution is one which resists changes in ph when small quantities of an acid or a base are added to it.vthe most important way that the ph of the blood is kept relatively constant is by buffers dissolved in the blood. h2co3 + h2o hco3- + oh-hc

2、o3- + h+ h2co3hco3- + 3oh h2o + h2co3vwhen a substance acts as both an acid and a base such as hco3- does, it is called amphoteric.how do buffer solutions work?na buffer solution contains a weak acid and base, which removes any hydrogen ions or hydroxide ions that are add keeping the ph constant. (n

3、ot neutral)nthe bicarbonate buffer:weak acid weak base h2co3 + h2o hco3- + oh-another buffer solutionnammonia is a weak base, and the position of this equilibrium will be well to the left:nany added hydrogen ions will react with the ammonia to make more of the conjugate acid, maintaining the ph.anot

4、her buffer solutionsnany added oh- ions will react with the weak acid to make the conjugate base and again maintain a constant ph.determination of the ph of a buffer solutionnexample: what is the ph of a solution of 0.11 m nac2h3o2 and 0.090 m hc2h3o2?(ka = 1.8 x 10-5)hc2h3o2 c2h3o2- + h+initial0.09

5、0.110change-x+x+xequil.09-x0.11 +xxka = x (0.11) /.09-x x = 1.47 x 10-5, ph = 4.83n(buffered solution will have a very small dissociation so + or x can be ignored)preparing buffer solutionsnthe henderson-hasselbalch equation gives the ratio of weak acid to base needed to maintain a constant ph.nph =

6、 pka + log a-/ha(the ph can be approximated: ph = pka +1preparing buffers1.example. what is the mole ratio of acetic acid to acetate ion needed to prepare a buffer solution at a ph = 5.00? ka= 1.8 x 10-5ka = h+ a- = h+ mol c2h3o2- ha mol hc2h3oph = 5 so, h+ = 1 x 10-5 mol c2h3o2- = h+ = 1x 10-5 = .5

7、6mol hc2h3o ka 1.8 x 10-5preparing buffers2.what is the ph of a solution containing 0.11 m of sodium acetate and 0.090 m acetic acid? ka = 1.8 x 10-5nph = pka + log a-/haph = 4.74 + log (0.11mol a-) (0.090 mol ha)ph = 4.83ntitration of a strong acid by a strong basethe equivalence point occurs at 25

8、.00 ml added base with a ph of 7.0.titrationsntitration of a weak acid by a strong baseqthis can be divided into four regions1)before the titration begins: this is simple a solution of weak acid2)during the titration, but before the equivalence point: the solution is a buffer3)at the equivalence poi

9、nt: the solution contains a salt of the weak acid, and hydrolysis can occur4)past the equivalence point: the excess added oh- is used to determine the ph of the solutiontitrations the titration curve for the titration of 25.00 ml of 0.200 m acetic acid with 0.200 m sodium hydroxide. due to hydrolysi

10、s, the ph at the equivalence point higher than 7.00.buffered regionntitration of a weak base by a strong acidqthis is similar to the titration of a weak acid by strong baseqagain dividing into four regions1)before the titration begins: this is a solution of a weak base in water2)during the titration

11、, but before the equivalence point: the solution is a buffer3)at the equivalence point: the solution contains the salt of the weak base, and hydrolysis can occur4)past the equivalence point: excess added h+ determines the ph of the solutiontitrations titration curve for the titration of 25.00 ml of

12、0.200 m nh3 with 0.200 m hcl. the ph at the equivalence point is below 7.00 because of the hydrolysis of nh4+.buffered regionntitration curves for diprotic acidsqthe features are similar to those for monoprotic acids, but two equivalence points are reachedthe titration of the diprotic acid h2a by a

13、strong base. as each equivalence point is reached, the ph rises sharply.titrationsna few general comments about indicators can be madeqmost dyes that are acid-base indicators are weak acids, which can be represented as hinqthe color change can be represented as:color)(another color) (oneform base form acid )()()(hininhkaqinaqhaqhinhintitrationsqthe color change will “appear” to the human eye near the equivalence point of the indicatorqat the equivalence point, the concentration of the acid and base form are equal,