LEC1: pH and Buffers

Question 1

where can a buffer be useful?

Answer 1

+/- 1 pH unit around the pKa

Question 2

formula for dissociation constant of an acid?

Answer 2

Question 3

how do weak acids/abses act as buffers?

Answer 3

weak acids & bases only partially dissociate into charged species
buffers provide a reservoir of acid that consumes added base, reservoir of base that consumes added acid
therefore dampen pH changes

Question 4

what do acids and bases tend to do, what is each’s effect on pH?

Answer 4

acids donate protons, cause a decrease in pH
bases accept protons, cause an increase in pH by removing protons from H2O

Question 5

what do acids/bases each do re: proton exchange?

Answer 5

acids donate protons
bases accept protons

Question 6

what is pH for pure water under normal conditions?

Answer 6

pH = 7

Question 7

what is the definition of pH?

Answer 7

pH = -log[H⁺]

pH is the measure of the H⁺ion concentration

Question 8

what is the effect of buffers on pH?

Answer 8

buffers dampen pH changes

Question 9

what is the K_a?

Answer 9

dissociation constant of an acid

Question 10

what does this curve show?

Answer 10

concentration curve for buffer

shows relationship between acid added/base added (x axis) & pH (y axis)

see small changes to pH when add base (to the R) or acid (to the L) because this is where buffer is useful

see if add a LOT of base or acid, the conjugate acid/base is exhausted, buffer is no longer useful as a buffer

Question 11

what is Henderson-Hasselbach equation?

Answer 11

Question 12

what is the pKa of a weak acid?

Answer 12

the pH at which 1/2 of the molecules are dissociated into ions

Question 13

what is the maximum buffering capacity of a weak acid?

Answer 13

1 pH unit above/below the pKa

here, both acid and conjugate base are present, can consume added ions

Question 14

how is an acid and a base when each is above its pKa?

Answer 14

acid above its pka: unprotonated, charged

base above its pKa: unprotonated, neutral

Question 15

what buffers human blood?

Answer 15

bicarbonate buffering system

Question 16

what is the Henderson-Hasselbach equation for the formation of bicarbonate?

Answer 16

Question 17

what does this graph represent?

Answer 17

titration curve of carbonic acid/bicarbonate system

pH = 6.1 is known neutral state; see that between pH=5.1-7.1, small changes in pH

Question 18

what is the pH of blood?

Answer 18

7.4

Question 19

at physiologic pH of blood, what is ratio of bicarbonate:carbonic acid? explain

Answer 19

[H₂CO₃] to [HCO₃^-] is 1:20

physiological mechanisms can regulate this concentration

1) [H₂CO₃] modulates with breathing rate, affecting CO₂levels in the blood - rxn becomes more acidic
2) [HCO₃^-] modulates by changing the rate of its elimination by the **kidney **- rxn becomes more basic

Question 20

when does a drug get absorbed?

Answer 20

if a drug is a weak acid, aka **protonated & uncharged, **it can be absorbed into the stomach

Question 21

describe how aspirin works, the form its found in in the stomach

Answer 21

pH of the stomach = 1.5

aspirin is a weak acid, pH= 3.5

therefore at pH = 1.5, aspirin will be protonated and uncharged, and therefore can cross membrane > stomach & intestines

protonated form is 100x more abundant than charged form

Question 22

what is charge of each part of an amino acid? & what is amino acid’s charged at pH=7?

Answer 22

weakly basic amino group, pKa = 9-10

weakly acidic carboxyl group, pKa = 2

overall, at pH=7, charge of an amino acid=0

Question 23

structure of an amino acid?

Answer 23

Question 24

what does it mean that amino acids are optical isomers?

Answer 24

their stereoisomers are nonsuperimposable mirror images

Question 25

what is the simplest amino acid? how is it found in nature?

Answer 25

glycine; R group = H

overall charge is positive, both groups are protonated, if in highly acidic environment

overall charge is **neutral **in physiologic form (normal!)

overall charge is negative, both groups are unprotonated, if in highly basic environment

Question 26

what is this? explain what is happening

Answer 26

represents the acidic/basic propteries of an amino acid w/ an uncharged R-group, aka glycine

see 2 regions where it behaves as a buffer:

1) pK1, the pKa of carboxyl group
2) pK2, the pKa of the amino group

Question 27

what is the isoelectric point?

Answer 27

the pH in which the amino acid is electrically neutral - charges balance

Question 28

what is the pI of amino acids w/ R groups that cannot be ionized?

Answer 28

6

Question 29

how are amino acids re: protonation, charge, at physiological pH?

Answer 29

carboxyl group is unprotonated, negative charged

amino group is protonated, positively charged

Question 30

how does the body control the level of CO₂ in blood?

Answer 30

[H₂CO₃] modulates with breathing rate, affecting CO₂ levels in the blood

Henderson-Hasselbalch rxn becomes more acidic

Question 31

how does the body control bicarbonate concentration?

Answer 31

[HCO₃^-] modulates by changing the rate of its elimination by the kidney

H-H rxn becomes more basic

Question 32

how does a buffer reduce the effect of added acid or base on the pH of a solution?

Answer 32

buffers dampen pH changes by providing a reservoir of acid that consumes added base / a reservoir of base that consumes added acid

at +/- 1 pH unit above or below a weak acid’s pKa, added ions are consumed by buffer

Question 33

why is the maximum buffering capacity of a weak acid at +/- 1 pH unit of its pKa?

Answer 33

because in this pH range both the acid and its conjugate base are present and can consume the added ions

Question 34

is a weak acid charged or uncharged at 2 pH units above its pKa?

Answer 34

charged

Question 35

is a weak base charged or uncharged at 2 pH units above its pKa?

Answer 35

neutral

Question 36

why is the carbonic acid/bicarbonate system an effective buffer of blood even though its combined pKa is more than 1 more pH unit below that of blood pH?

Answer 36

the combined pKa of carbonic acid/bicarbonate = 6.1

blood pH=7.4

works b/c components can be regulated by physiological mechanisms