BUFFERS 

 

 

DEFINITION - Solutions which are able to resist pH change when a small amount of acid or base is added.

Mixture of weak acids and their salts
Mixture of weak bases and their salts

Buffers come in buffer pairs
   Acetic acid/sodium acetate
   Ammonium Hydroxide/Ammonium Chloride

pH is neg log of H+ concentration
pKa = neg log of Ka

A relationship exists between pH and pKa 

pH = pKa + log  [salt]
                          [acid]

Henderson-Hasselbalch Equation
pKa's can be gotten from tables

pKa
            acetic acid                                4.74
            lactic acid                                 3.86
            formic acid                               3.77
            trichloroacetic (TCA)                0.70 

Which of these is strongest? Rember it is - log of K 

Also can have pKa's for bases.  [ammonium]+ = pKa = 9.26 

If we had the same concentration of salt and acid the pH would be the same as pKa.

   conc of H+ = 1 M/liter
               conc of salt = 1 M/liter 

    pH = pKa + log  [salt]      
                                           [acid]

             pH = pKa + log  1   
                                        1
                                           log 1 is 0
             pH = pKa + 0
            pH = pKa 

I have 1000 ml of a solution that is 0.1 N acetic acid and 0.1 M sodium acetate. pKa of acetic acid is 4.74. What is the pH? 

pH = pKa + log  [salt] 
                                      [acid] 

pH = 4.74 + .1 
                              1000 
                                 1 
                             1000

            pH = 4.74 + log 1
            pH = 4.74 + 0
            pH = 4.74

         O
1. What is the pH of a .1M formic acid (HC      ) solution that contains 0.2 moles
                                                                    OH
of sodium formate per liter?       The pKa of formic acid is 3.75

2. Make a buffer of pH 5.00 using sodium acetate and acetic acid. pKa = 4.76.
   What ratio of molar concentrations of sodium acetate, acetic acid will be                             

 

    Physiological Buffers 

Purpose: maintain acid-base balance in living system
Buffer system of human body.
*****bicarbonate          -                     carbonic acid
               HCO3-                                       H2CO3 

organic phosphates
            proteins
            lactic acid

Blood pH   7.4  (7.35 - 7.45)
                         <7.00 > 7.8 dead 

Regulatory organs for pH maintenance
            1. lungs (respiratory)
            2. kidney (metabolic)

Lets look at *****                     CO2 + H2O ----> H2CO3
                                                                                                    carbonic acid 

Where do we get CO2? Metabolism of sugars, fats proteins for energy.  There are CO2 receptors in blood vessels that are sensi­tive to CO2 concentration.   If the level of CO2 goes up the person breathes deeper and/or faster. 

After generating CO2 the CO2 can
            1. be dissolved in blood as a gas
            2. react with H2O to form carbonic acid H2CO3
             3. react with hemoglobin Hb
     *** 4. be converted to a bicarbonate HCO3-

    CO2 + H2O <----->  H2CO3 <---->  H+ + HCO3- 

carbonic acid     bicarbonate

 Without an enzyme (carbonic anhydrase) the following reaction would be very slow

    H2CO3 -----> H2O + CO2

 Big Picture

 Muscles generate CO2 which goes to HCO3-

 HCO3- goes to the lungs and gets a H+ from Hb    hemoglobin

HCO3- + H+ -----> H2CO3 ---------> CO2 + H2O
                            carbonic acid                  exhale

acidosis <7.4   build up of carbon dioxide
hypoventilation - respiratory - lung alteration of CO2 

alkalosis >7.4   low levels of carbon dioxide
hyperventialtion - respiratory - lung alteration of CO2

 

Condition                                 CO2                   pH        Causes

Respiratory                                                                       shallow breathing Hypoventilation
acidosis                                                                           (drugs)  lung damage 

Respiratory                                                                       Hyperventilation (anxiety)
alkalosis                                                                           High Temperature 

Metabolic
Kidney Damage
Vomiting
Diarrhea
Ingestion of large amounts of H+ or (OH)- 

The major buffering system is the carbonic acid H2CO3  /  bicarbonate HCO3-
The pKa of carbonic acid is 6.37
The blood pH is  7.3 to 7.4, so the bicarbonate HCO3- to carbonic acid H2CO3 is 10 to 1

Use the Henderson-Hasselbach equation to show this.

 

Normal concentrations in the blood are 0.025 M bicarbonate to 0.0025 M carbonic acid

The second major buffer is hydrogen phosphate ion (HPO4)- 2 and  dihydrogen phosphate (H2PO4)-

The ratio of H(PO4)-2  to H2(PO4)- is 1.6 to 1

Wednesday, March 21, 2007 01:28:03 PM