Chemical equilibria Buffer solutions

Transcription

1 Chemical equilibria Buffer solutions

2 Definition The buffer solutions have the ability to resist changes in ph when smaller amounts of acid or base is added. Importance They are applied in the chemical practice (reactions performed at an optimal ph), in biochemistry and in physiology.

3 Composition of the buffer solutions They contain a weak acid and its salt (or its conjugate base) e.g. CH 3 COOH and CH 3 COONa. They can be prepared from a weak base and its salt (or conjugate acid) e.g. NH 3 solution and NH 4 Cl.

4 Acidic buffers I The buffer is composed of acetic acid (c acid ), it ionizes only slightly. CH 3 COOH(aq) CH 3 COO - + H + K = [ CH 3 [ CH COO 3 [ H COOH = (at 5 0 C) In order to prepare a buffer, sodium acetate is added to the acetic acid solution. The sodium acetate (c salt ), dissociates completely. The acetate concentration was increased, the undissociated acid concentration was also increased. [CH 3 COOH = c acid [CH 3 COO - =c salt

5 Acidic buffers III According to the law of mass action: K a = c salt [H c acid [H + = Kac c salt acid A general expression for the acidic buffers [H + = K a [ HA [ A log [H + = log K a + log ph = pk a + log [ A [ HA [ A [ HA The equation above is the Henderson-Hasselbalch equation describing the ph value of an acidic buffer system, it depends only on the ratio of the components.

6 Basic buffers E.g. it consists of NH 3 solution and NH 4 Cl NH 3 (aq) + H O(l) OH - + NH 4 + K b = [ NH 4 [ NH [ OH 3 [OH - = K b [ NH [ NH 3 4 poh = pk b + log [ HB [ B Henderson-Hasselbalch equation for basic buffers

7 The features of the buffer solutions I The ph value of a certain buffer is determined by salt/weak acid ratio (acidic buffer) or by the salt/weak base ratio (basic buffer). At the different buffers the ph depends on the pk a or pk b values of the acid or base. The buffer works efficiently if the salt/weak acid ratio is in the following range: 0,1< salt/weak acid <10 Based on this fact we can prepare a good buffer from a weak acid and a salt pair in the range: ph = pk s +/-1

8 The features of the buffer solutions II Buffer capacity Definition: The amount of strong acid (e.g. HCl) or strong base (e.g. NaOH) in moles that can be added to one litre of buffer in order to change the ph of the buffer by one unit. Acid capacity of a buffer- base capacity of a buffer

9 The features of the buffer solutions III The effect of the dilution on the ph of the buffer (ph is not influenced by dilution) and on the buffer capacity (the buffer capacity depends on the dilution).

10 Calculation The function of the buffers I 1. Calculate the ph value of the buffer made by mixing 0.5 L molar acetic acid and 0.5 L molar sodium acetate solutions. (K a =1.8x10-5 ) After mixing the solutions [CH 3 COOH = [CH 3 COO - = 1 mol/l ph = pk s + log [ A [ HA ph = log [1 [1 = 4.74

11 The function of the buffers II Calculation. Calculate the change in ph if 0.4 g NaOH (strong base) is dissolved in one litre of the buffer solution previously mentioned. The NaOH neutralizes a certain amount (mole) of acid producing the same amount (mole) of salt. ph = pk + lg c c salt acid x x = lg lg = = = 1.0 The change in ph is: 0.01 unit.

12 The function of the buffers III Calculation 3. By what factor does the ph of 1 L of distilled water change, if 0.4 g NaOH is dissolved in it. ph 1 = 7, 0.4 g NaOH = 0.01 mole [OH - = 0.01 M, poh =, ph = 1, ph = 1 The change in ph is: 5 units.

13 Changes in buffers on treatment with acids or bases

14 Physiological buffers I The buffer systems of the organism Hydrogen carbonate /carbonic acid buffer (blood) Phosphate buffer (having an intracellular role) The protons of the acids can originate from: carbonic acid (volatile acid) Bound acids: phosphoric acid and sulphuric acid Organic acids: lactic acid and acids of ketone bodies.

15 Hydrogencarbonate/carbondioxide buffer I Its role It maintains the ph of the blood plasma at a constant value of Deviations At ph>7.6 it is not possible to transfer the CO from the cells to the blood. ph < 7.3 enhances the exchange of gases in the lungs. Acidosis (at ph<7 coma) Alcalosis

16 Hydrogencarbonate/carbondioxide buffer II The equilibria in the buffer system of the blood Dissolution of carbondioxide CO (g) + H O(l) CO (aq) K 3 = Reaction of the dissolved CO with CO (aq) + H O(l) H CO 3 (aq) The first dissociation step of the carbonic acid H CO 3 (aq) + H O(l) H 3 O + + HCO 3 - K = [ CO [ CO [ H [ CO ( aq) ( g) CO 3 ( aq) K 1 = [ H [ H [ HCO CO 3 3

17 Hydrogencarbonate/carbondioxide buffer III The [H CO 3 was substituted into the K 1 expression from K. K 1 = [ H K [ HCO [ CO 3 ( aq) Into the new equation the [CO (aq) - obtained form the K 3 equation- was introduced. K 1 xk xk 3 = [ H [ HCO [ CO ( g) 3 Applying a new constant for the three constants and using Henry s law for the solubility of gases (it depends on the partial pressure and the absorption coefficient): K = [ H [ HCO3 CO P CO

18 Hydrogencarbonate/carbondioxide buffer IV The [H + and the ph is expressed. [H + = K CO P CO [ HCO3 ph = pk + log [ HCO3 CO P CO The concrete values are substituted into the equation (pk=6.1, p CO =53 mbar, CO =0.06 mmol/mbar at 37 ºC and [HCO 3- =4 mmol/dm 3 ). This equation gives the physiological arterial ph value. ph = log = log 0 = 7.4

19 Hydrogencarbonate/carbondioxide buffer IV

20 Phosphate buffer The second dissociation step is involved here. H PO 4 - H + + HPO 4 - K a = weak acid conjugate base [H + = K a [ H PO [ HPO 4 4 The ph is expressed. ph = pk a + log [ HPO [ H 4 4 If =1, PO [ HPO [ H PO 4 4 ph = pk a = 7.1 ph = log [ HPO [ H PO 4 4 Physiological importance of ph: the enzymes function within a quite small ph range optimally. It influences the distribution of the ions, too.

21 Acid base indicators I Definition They are dyes applied to distinguish between acidic and basic solutions by means of colour change. HInd + H O H 3 O + + Ind - acid colour alkaline colour The law of mass action is applied. K ind = [ Ind [ H [ HInd

22 Acid base indicators II The colour of the indicator is influenced by the [Ind- /[Hind ratio. [H + = K ind [ HInd [ Ind If the [Ind - /[Hind ratio is equal to = 1, ph = -logk ind = pk ind, pk ind or pi = indicator exponent, it characterizes the indicator. Its colour-change interval is pk ind +/- 1 ph unit.

23 Acid base indicators III The reason for the difference in colours The two colours can be seen in the presence of each other, if the minor one is minimum 10% present. Both of them can be observed, if 0,1 < < 10 Ind - HInd

24 Acid base indicators IV The colour-change interval and colour of some indicators

25 Acid base indicators V The structure of some indicators COOH CH 3 N N N COOH - H + + H + yellow CH 3 H N N + N CH 3 CH 3 red Methyl red colour-change interval:

26 Acid base indicators VI HO HO OH O - H + O + H + O - colourless O Phenolphthalein O violet colour-change interval: The importance of the acid-base indicators: 1. Determination of ph. They are used in titrations to indicate the end point.

27 Determination of ph I 1. Using indicator dyes (accuracy 1 ph unit) Or mixture of indicator dyes (universal indicator)

28 Determination of ph II. Using potentiometric method (more accurate) measuring the electrode potential (ph dependency) (see electrochemistry)

29 Titrations Acid base titration Definition It is a quantitative analytical method used for the determination of the concentration of an acid or base. A standard solution of known concentration is given to the unknown solution. The end point or equivalence point of the titration should be indicated (by visual indicators or potentiometric method).

30 Titration curves II Titration of a strong acid with a strong base The equivalence point is at ph = 7. Application of an indicator: the colour-change interval has to be at about ph = 7, e.g. between ph = 4 10.

31 Titration curves III Titration of a weak acid with a strong base CH 3 COOH(aq) + Na + + OH - CH 3 COO - + Na + + H O(l) The end-point is in the alkaline range (above ph 7) At the beginning of the curve there is a buffer region, where the inflection point corresponds to the pka value. [CH 3 COOH = [CH 3 COO - ph = pk a = 4.74

32 Titration curves IV