DW Lecture 1: pH/Buffers Flashcards
What is the equation for pH?
pH = -log[H+]
What do acids do to the pH?
they donate protons and cause pH to decrease
What do bases do to the pH?
they accept protons and cause increase in pH (remove protons from H2O)
Why are weak acids/bases used as buffers?
because they only partially dissociate into charged species
What do buffers do?
dampen pH changes by providing a reservoir of acid that consumes added base and a reservoir of base that consumes added acid
How do you find the dissociation constant of an acid?
Ka = ( [H+] [A-] ) / ( [HA] )
What is equation for pKa?
pKa = -log(Ka)
What is the Henderson-Hasselbalch equation for a weak acid?
pH = pKa + log ( [A-] / [HA] )
What is the Henderson-Hasselbalch equation for a weak base?
pH = pKa + log ( [B] / [BH+] )
What is the general form of the Henderson-Hasselbalch equation?
pH = pKa + log ( [conjugate base] / [conjugate acid] )
Define what pKa means for a weak acid.
the pH at which one half of the molecules are dissociated into ions (i.e. [A-] = [HA])
measures the strength of the acid
What does a low pKa indicate?
stronger acid
Where is the maximum buffering capacity of a weak acid?
1 pH unit above or below its pKa (because in this pH range both the acid and its conjugate base are present and can consume the added ions (H+ or OH-)
What states are acids and bases in when they are above their pKa?
acid- unprotonated and charged
base- unprotonated and neutral
How is human blood buffered?
the bicarbonate system
H2O + CO2 < – > H2CO3 < – > H+ + HCO3-
What is the most appropriate form of the Henderson-Hasselbalch equation for the bicarbonate buffering system?
pH = 6.1 + log ( [HCO3-] / 0.03 [PaCO2] )
*where PaCO2 is the partial pressure of CO2 in mmHg
How can the bicarbonate buffer system be regulated by physiological mechanisms?
by changing heart rate/breathing (lung regulated) you can change the level of CO2 in the blood and modulate H2CO3 concentrations
by changing the rate of elimination of bicarbonate (HCO3) by the kidney you can change its concentration
How can you predict the effect of pH on the absorption of an ionizable drug?
Henderson-Hasselback equation can compute the ratio between the charged and uncharged forms of the drug ( [A-]/[HA] = 10^(pH-pKa))
then we know that 10^(pH-pKa) = (x / 100-x) so we solve for x to get the %[A-]
What are the acid/base features of amino acids?
amino group: weakly basic (pKa= 9-10)
carboxyl group: weakly acidic (pKa= 2)
R group: changes per aa
What is the pI?
isoelectric point
the pH at which a molecule’s net charge is zero
How can the isoelectric point be calculated?
pI = (pK1 + pK2)/2
