Buffer Systems Flashcards
pH and pKa
- Acid
Base
pH is a logarithmic measure of the hydrogen ion concentration of an aqueous solution it is affected by temperature
pKa
* dissociation constant of a buffering solution in equilibrium.
* negative log of the dissociation constant.
* pH= pKa
* [salt] = [acid]
- Acid-when dissolved in water, an acid donates a hydrogen ion (H+)
- Base: accepts a hydrogen ion (H+)
- pH = – log [H+]
- decrease in 1 pH unit represents a 10-fold INCREASE in [H+]
What is a buffer and what are its applications
-. When the pH doesnt change upon adding small amount of acid or base
* Weak acid and it’s corresponding salt = acidic buffer
OR
* Weak base and it’s corresponding salt= basic buffer
Applications of a buffer
* Maintain a constant pH in a reaction in the lab (e.g., clinical
tests such as enzyme tests)
* Maintaining the pH in microbiological media, tissue cultures
* Maintain pH of blood in the human body
* pH range 7.35-7.45
General Action of a Buffer
- free ions that act to change the pH when and H+ or OH- is added
- In a buffered solution:
- Buffer component and the free ions combine to form molecule
that stays undissociated in solution - Removes the excess free ions and results in only a slight change in pH
- Buffer generally effective within ± 1 of the pKa
pH Balance maintained by
- main organs of excretion are lungs (volatile) and kidneys (nonvolatile)
- regulation of [H+]
- body fluids are supplied with buffer systems and act quickly
Henderson-Hasselbalch equation
note the acid and base equation
pH and pKa for buffers
Direct relationship, pH of a buffer and its pKa
Max buffer capacity when [salt] and [acid] are equal concentrations
-pKa is a constant value, the pH will vary of a buffer because of [salt] and
[acid]
When [salt] > [acid], pH > pKa
When [salt] = [acid], pH = pKa
When [salt] < [acid], pH < pKa
Best buffering range
- A buffer can function well when salt : acid ratio is 1:10 or salt : acid ratio is10:1
Look at slide for equation
- Therefore, pH = pKa ± 1.0
- If acetic acid/sodium acetate buffer, pKa = 4.8
- Best buffering range, pH = 4.8 ± 1.0 = 3.8-5.8
BUFFER SYSTEMS IN THE HUMAN BODY
Primary EC buffer system
Phosphate, protein, and bicarb
Primary IC buffer systems
phosphate, protein , and hemoglobin
“open” buffer systems: phosphate, bicarbonate
“closed” buffer systems: protein, hemoglobin
Buffer systems: Bicarbonate
HCO3-/H2CO3-most important buffer pair in plasma
pKa of 6.1 cannot buffer at pH 7.4
* Chemically speaking bicarbonate is not a buffer in action
HCO3- - regulated by kidneys
PCO2 - regulated by lungs
open buffer system
Buffer systems: Phosphate
important intracellularly as organic phosphate (2,3-DPG in red cells)
* excretion of acids in the urine
* action similar to bicarbonate buffer system
* Open buffer system
Buffer systems: Protein
- Most plentiful non-bicarbonate buffer of the body
-most powerful - presence of both free acidic and basic radicals
- can accept H or donate H as metabolism requires
- Each albumin molecule contains 16 histidines
- imidazole groups of histidines (pK 7.3)
- H+ ion sequestered
- Closed buffer system
- protein buffers react much more slowly
- concentration in mmol is lower than bicarbonate
Buffer systems: Hemoglobiin
- primary intracellular buffer
*2 buffer pairs: De/oxygenated - buffering of H+ and CO2 depends on Hb concentration of blood
- CO2 is an acid
- Closed buffer system
DO WE NEED TO KNOW THE SLIDE
The isohydric principle
- the buffer systems all work together
- H+ is common to all the systems
- if [H+] changes, the balance of all
systems change at the same time (the isohydric principle) - in other words, the buffer systems actually buffer each other
- all four buffer systems act in concert
Transport of oxygen
Oxygen-hemoglobin dissociation curve
- oxygen combines loosely and reversibly with Hb
- basis for oxygen transport from lungs to tissues
- increased PO2 causes oxygen to bind to Hb (lungs)
- decreased PO2 causes oxygen to be released from Hb (tissues)
- PO2 = partial pressure of oxygen
- relationship seen on oxygen-hemoglobin dissociation curve
-the log going up is the reduced blood returning from tissues
and the plateau is the oxygenated blood leaving the lungs
*P50 affinity of Hb for oxygen
- partial pressure of oxygen (PO2) at which Hb is 50% saturated
- P50 is increased when its more
difficult for Hb to bind O2 - curve shifts to the right
- P50 is decreased when its easier for Hb to bind O2
- curve shifts to the left
Shift of dissociation curve
What is a shift to right?
- increased [H+] and decreased pH leads to decreased affinity of Hb for O2
- increased PCO2 leads to decreased affinity of Hb for O2
- increased temperature leads to decreased affinity
therefore
* increased 2,3-diphosphoglycerate (2,3-DPG) leads to decreased affinity
* promotes oxygen release to tissues in hypoxia
