Weak Acids and Weak Bases Flashcards
When does a drug usually precipitate?
FYI: nonionized drugs are usually lipid soluble and penetrate BBB and placental barrier; ionized drugs are typically water soluble;
So, if you have a drug dissolved in water and its mostly in the nonionized form, it is likely that the drug will precipitate
Weak acids become more ________ when pH decreases and more _______ when pH increases.
nonionized, ionized
ex) pKa of thiopental = 7.6–> if put into pH of 7.4, since thiopental is a weak acid and it was placed into a slightly more acidic environment than it’s pKa…. more of the injected thiopental will be in the nonionized form
Weak bases become more ________ when pH decreases and more _______ when pH increases.
ionized, nonionized
What is pKa?
when the pH of the aqueous solution is the same as the pKa of the drug, the drug will be 50% nonionized and 50% ionized.–> PKa is the pH at which the drug reaches 50%/50% equilibration, not whether or not it is an acid or a base.
What are classic weak acids found on board exam?
thiopental and other barbiturates; propofol
Weak acids give up ______, but not completely.
H+
Acid + Acid = ________
nonionized; weak acids become more nonionized as pH falls (H+ increases)
Strictly speaking, the nonionized form of the weak acid is the _____, and the ionized form is called the ______.
acid, conjugate base
The _____ the pKa of a weak acid, the ______ the amount of drug in nonionized form at physiological pH (7.4).
greater; greater
Should thiopental be mixed with a low pH solution?
No. b\c it is a weak acid and when placed in an acidic solution more of the drug becomes non-ionized–> nonionized will form precipitate
Why is thiopental stored and placed in a solution with pH 10-11?
when it is stored in a solution with a pH of 10-11, most of the drug will be in ionized form—> ionized drug is highly water soluble–> precipitate will not form when thiopental is stored in a solution with a high pH
Name 4 of the primary weak bases seen on the board exam.
1) local anesthetics
2) ketamine
3) opioids
4) benzo’s
Weak bases accept _______, but never do so completely.
hydrogen ions (H+)
Weak bases become more ________ as pH increases.
non-ionized as H+ decreases
Base + Base = ________
nonionized
EX) pKa of lidocaine is 7.7; if pH is 7.7, 50% of the lidocaine would be in the ionized form and 50% in the nonionized form–> when lidocaine is injected into the body at pH=7.4, less than 50% of the drug will be nonionized (base + acid=ionized)
Strictly speaking, the non-ionized form of a weak base is the _____, and the ionized form is called the _______.
base; conjugate acid
What does protonated mean?
a substance has accepted a hydrogen ion
If placed in a patient with pH of 7.4, would a LA with a pKa of 9.1 or 7.6 be more nonionized?
- 6
* when looking at the number graph, weak bases… nonionized is everything to right of pKa and with weak acids nonionized is everything on left
The ____ the fetal pH, the ______ the amount of local anesthetic is in ionized form in the fetus. What is this called?
lower; greater
ion trapping
What combination will most facilitate trapping of local anesthetic by the fetus?
maternal alkalosis and fetal acidosis
What condition most prevents passage of local anesthetic from fetus to mom?
fetal acidosis
KNOW THE FOLLOWING INFO
Weak acids unit with positively charged ions such as Na+, Mg+, or Ca++–> If you see one of the following salts–> sodium drug, calcium drug, magnesium drug–> you would recognize as the salt form of a weak acid.
Ex) sodium pentobarbital
KNOW THE FOLLOWING INFO
Weak bases unite with negatively charged ions such as Cl- or SO4—> if you see one of the following salts–> drug chloride, drug sulfate–> you would recognize the drug as the salt form of a weak base.
Ex) lidocaine hydrochloride and morphine sulfate
