Weak Acids and weak Bases Flashcards
to determine if a drug is largely ionized or largely non-ionized you will need to know what 3 things?
- If the drug is a weak acid or weak base (must memorize)
- the pKa of the drug
- the pH of the target solution
weak acids become more ________ when pH decreases (acidic) and more _______ when pH increases (basic)
non-ionized
ionized
weak bases become more ______ when pH increases (basic) and more _____ when pH decreases (acidic)
non-ionized
ionized
the pKa is your reference what is it?
it’s when the pH of a solution is the same os the pKa of the drug the drug will be 50% non-ionized and 50% ionized (equilibrium)
why is the pH of the target solution inportant to know
when you know the pH of the solution the weak acid or weak base is being placed in, you use your reference pKa to determine if the weak acid or weak base in the question is more the 50% ionized or less than 50% non-ionized in the solution
which is the active part? ionized or non-ionized
non-iionized
what are examples of weak acids?
thiopental
Barbiturates
propofol
Weak Acids:
what do they do with the H+ ion
weak acids GIVE UP H+ ions (but do not dissociate completely)
Weak Acids:
whay are they called Weak Acids
acids give up hydrogen
called weak b/c they don’t dissociate completly
Weak Acids:
ex
Hbarb = weak acid; non-ionized; lipid soluble
Barb- = conjugate base; ionized; water soluble (has charge)
Weak Acids:
pH = what?
pH = pKa
pH of body for all problems unless otherwise stated
7.4
Weak Acids:
what happens to weak acids as the pH falls (H+ increases)
become more non-ionized
Weak Acids:
Acid + Acid =
nonionized
(acid in an acidic enviroment = non-ionized or more active free drug available)
Weak Acids:
the pKa of a Weak Acids is what?
it is the pH at which 50% of the Weak Acid is in the ionized form and 50% of the Weak Acid is in the non-ionized form
Weak Acids:
an ex of pKa with Weak Acids is as follows:
the pKa for thiopental is 7.6 so is the pH of the pt was 7.6 there would be 50% ionized and 50% non-ionized. what would happen it the pH was 7.4?
more than 50% would be non-ionized (in this case to be exact 68% would be non-ionized)
Weak Acids:
are the pKa’s different for different Weak Acids?
yes one Weak Acid may have a pKa of 2.3 while another may have a pKa of 11.2
a given Weak Acid may have any pKa
the pKa is constant for a given Weak Acid
Weak Acids:
The (ionized or nonionized) drug penetrates the BBB and the placental barrier. the more (ionized or non-ionized) the drug the greater transfer to the brain or fetus
Non-ionized
Non-ionized
Weak Acids:
strictly speaking the non-ionized form of the Weak Acid is called the ______, and the ionized form is called the _____ ____
acid
conjugate base
Weak Acids:
what are the steps in order to solve any Acid base question
- is drug weak acid or weak base
- what is the pKa of the drug
- what is the pH of the target solution
Weak Acids:
when drawing a graph if dealing with weak acids non-ionizd over the Acidic portion of the graph
(acid + acid = nonionized) and then ionized over the basic aspect of the graft.
also for all puproses get use to writting less ionized which is the same as more non-ionized and less un-ionized which is the same as more ionized
Weak Acids:
assume drug Y is a barb
state if the drug has more ionized or non-ionized
do steps 1, 2 , 3 in order
more NON-ionized
Weak Acids:
assume drug Y is a barb
state if the drug has more ionized or non-ionized
do steps 1, 2, 3
More IONIZED
Weak Acids:
the greater the pKa of a weak acid the greater amount of the drug in the _________ form at physiologic pH of 7.4
non-ionized
Weak Acids:
will induction with thiopental be faster is the pt is acidodic or alkolotic? explain.
thiopental (weak acid)
induction would be faster if ACIDODIC
when a pt is acidodic, more of the injected thiopental will be non-ionized (acid + acid = nonionized)
thiopental will diffuse in greater amounts across the BBB b/c more of the drug is in the non-ionied form
non-ionizd drug readily penetrates the BBB
Weak Acids:
should you mix thipoental with a low pH solution? explain
Thiopental = weak acid
no
when thiopenal is mixed with a solution of low pH (pH 3.0) most of the drug will become non-ionized.
Non-ionized drugs, b/c it is poorly water soluble, tends to precipitate.
weak acids should not be mixed with a low pH solution bc a precipitate might form
RULE: mixing Weak Acids with a low pH solution may lead to form a precipitate
Weak Acids:
why is thiopenal mixed and stored in a solution with a pH of 10-11
thiopental = Weak Acid
most of the drug is in the ionized form
ionized drugs are highly water soluble
a precipitate will not form when thiopental is stored with a solution witf a high pH
Weak Bases:
weak bases do what with H+
weak bases accept a H+, but never do so completely
Weak Bases:
picking up a H+ (proton) means it becomes pronated or ___-
charged
Weak Bases:
label what is non-ionized
LA = Lipid soluble, NON-ionized; weak base
H+ = base
LAH+ = Ionized; water soluble; conjugate acid
what is teh general rule for all acid problems
like + like = non-ionized
Weak Bases:
Weak Bases become more _____ as pH increases (H+ decrease)
non-ionized
Weak Bases:
Base + base = ?
Base + Base = non-ionized
Weak Bases:
the pKa of a weak base is what?
the pH at which 50% of the Weak Base is in the ionized and 50% of the base is in the non-ionized form
Ex- Lidocaine has pKa of 7.7 is the pH is 7.7, 50% of the lidocain is non-ionized and 50% of the lidocaine is ionized
but is the lidocaine is injected into a pH of 7.4 less than 50% (24% in this case) will be in the non-ionized form
Weak Bases:
is the pKa different for different Weak Bases?
yes
one Weak Base may have a pKa of 2.6 while another may have a pKa of 9.8.
a given Weak Base may have any pKa, however the pKa is constant for a given Weak Base
Weak Bases:
what drugs are weak bases? and how to remember
think of LA and you use them for a BLOK
Benzos
LA
Opioids
Ketamine
what drugs are weak acids again?
Thiopental
Propofol
Barbs
Weak Bases:
how do you set up the line to determine if the drug will be more ionied or unionized
draw your line just as you did for the weak acids
but remember like + like = non-ionized
so for the acid we placed non-ionized on the acidic side… this time with Weak Bases we put the non-ionized on the basic side
Weak Bases:
say you gave drug V and its a benzo. with a pKa of 9.1 state if the drug will be more ionized or non-ionized
do it in order 1,2,3
more ionized
Weak Bases:
say you gave drug V and its a benzo. with a pKa of 4.5 state if the drug will be more ionized or non-ionized
do it in order 1,2,3
more non-ionized
Weak Bases:
the pKas of LA range from 7.6 to 9.1, which LA will be the most NON-IONIZED at a pH of 7.4, the one w/ a pKa of 7.6 or the one with a pKa of 9.1?
the one with a pKa of 7.6
for all weak acid and weak base questions what is the equation name that is utilized to calculate the answer
Henderson-Hasselbach equation
Trapping of LA in Fetal Circulation:
why does this occur?
the pH of the fetus is lower than the pH of the mom.
The NON-ionized form of circulating LA crosses the placenal barrier
once in the fetus, equilibrium b/t ionized and non-ionized drug is re-established
B/c fetl pH is lower than mom’s, however a greater amount of the drug is ionized
the ionize form of the LA cannont cross the placental barrier, hence the LA is trapped in the fetus
Trapping of LA in Fetal Circulation:
picture to illustrate
Trapping of LA in Fetal Circulation:
the (higher or lower) the fetal pH, the greater the amount of LA in the ionizied form in teh fetus (aka greater the ion trapping)
Lower
Trapping of LA in Fetal Circulation:
what combination will most facilitate trapping of LA by the fetus?
- Maternal ALKalosis and Fetal ALKalosis
- Maternal ALKalosis and Fetal ACIDosis
- Maternal ACIDosis and Fetal ALKalosis
- Maternal ACIDosis and Fetal ACIDosis
2) Maternal ALKalosis and Fetal ACIDosis
Trapping of LA in Fetal Circulation:
What condition most prevents passage of LA from the fetus to the mom
- Maternal ALKalosis
- Maternal ACIDosis
- Fetal ALKalosis
- Fetal ACIDosis
4) Fetal ACIDosis
Weak acid and Weak Base problems:
Sauvage sulfate has a pKa of 4.5. will this drug be more than 50% of less than 50% non-ionized at physiological pH (7.4)?
more than 50% will be non-ionized
Weak acid and Weak Base problems:
Sodium Schaus has a pKa of 3.8. wll this drug be more than 50% or less than 50% IONIZED at pH 7.4
more than 50% will be IONIZED
Weak acid and Weak Base problems:
Propofol has pKa of 11. is it a weak acid or weak base?
weak acid
Weak acid and Weak Base problems:
if you see one of the following salts: Sodium drugs, Ca++ drugs, Magnesium drug are the weak acid or bases?
Weak acids
(think of Sodium thiopental)
Weak acid and Weak Base problems:
if you see one of the folloowing salts: chloride, sulfate is it weak acid or base
Weak base
think of morphine sulfate
