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Foundations of Biomedical Science > CBS - pH and Buffers > Flashcards

CBS - pH and Buffers Flashcards

1
Q

What is pH?

A

pH is a measure of hydrogen ion concentration. It is a measure of the acidity or alkalinity of a solution.

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2
Q

Which type of hydrogen ions does acidity depend on?

A

Acidity depends only on free hydrogen ions

not those still bound to anions

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3
Q

What is the range of blood pH?

A

It is kept within a very narrow range, 7.35 to 7.45.

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4
Q

What is the living range for pH?

A

7.0 - 7.8

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5
Q

Where do the acids in our body come from?

A

Some acids enter through foods (chillies, lemons, vinegar)

However, most of them are generated by:

  • breakdown of proteins
  • incomplete oxidation of fats or glucose,
  • loading and transport of carbon dioxide in the blood
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6
Q

Why is the urine pH much higher than blood pH?

A

The reason why the range of urine is much higher than blood is because the kidneys are one of the organs that regulate the amount of hydrogen ions in our body (by filtering them out through the urine).

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7
Q

What is the acid-base balance regulated by in the body?

A
  • the lungs (long term)
  • the kidneys (long term)
  • systems in the blood known as chemical buffers (immediate term)
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8
Q

How do buffers resist abrupt and large swings in pH of body fluids?

A

They do it by:
- releasing H+ (acting as acids) when the pH begins to rise

  • binding H+ (acting as bases) when the pH drops
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9
Q

What is the molarity of pure water?

A

Pure water is a 55.6M (55.5 recurring) solution.

This means that a litre of water contains 55.5555555555 moles of water.

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10
Q

What is the concentration of [H+] ions at neutrality?

A

At neutrality, [H+] =[OH-] = 10^-7M

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11
Q

What is the equation to calculation pH from H+ concentration?

A

pH = - log [H+]

so, for example, when [H+] = 10 -2 M
pH is 2

If we know either the hydroxyl ion concentration or the hydrogen ion concentration of a solution, then we can figure out the pH

Since we know the concentration of H+ and OH- must be 10^14 altogether.
when [H+] is 10^-2, then [OH-] is 10^-12
when [H+] is 10^-4, then [OH-] is 10^-10

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12
Q

What is the hydrogen ion concentration in the blood?

A

blood pH is 7.4
7.4 = - log [H+]
[H+] = 3.98 x 10&-8 M

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13
Q

Explain what weak acid dissociation means.

A

What weak acid dissociation means is that not all of the acid actually dissociates (rather than it dissociating further to 2H+ and CO3 2-).

If we were to decrease the pH of the solution, we would get more carbonic acid undissociated. If we were to increase the pH, we would get less carbonic acid, and more bicarbonate ions.

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14
Q

What is the pKa?

A

At some point, the acid is half dissociated. The pH at which it is half dissociated is known as pKa.

pKa = –log Ka
Ka is the dissociation constant.

The pKa gives us an idea of at which pH this acid will buffer best. The lower the pKa, the stronger the acid.

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15
Q

What is the Henderson-Hasselbalch equation?

A

pH = pKa + log ([conjugatebase]/[acid])

It is a convenient way to relate the pH of a solution, the pKa of a weak acid and the relative amounts of dissociated and non-dissociated (unprotonated and protonated) forms of the acid.

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16
Q

What is the relation between pKa and buffering?

A

Buffering is the ability of a solution to resist a change in pH when acid or alkali is added.

At the pKa there are equal amount of dissociated and non dissociated forms of the acid (conjugate base and acid)

Thus, at the pKa, buffering is best.

17
Q

What are physiologically important buffers?

A

Carbonic Acid:
H2CO3 → HCO3-
pKa 6.1

(A form of) Phosphoric Acid:
H2PO4- → HPO42-
pKa 6.8

Proteins can also act as buffers sometimes.

18
Q

How can we use the Henderson-Hasselbalch equation with knowledge of the the carbonic acid buffer to distinguish the cause of a person’s acidosis?

A

H2CO3→HCO3- pKa 6.1

[H2CO3], carbonic acid is the acid, and [HCO3-], bicarbonate is the conjugate base.

We can measure bicarbonate directly, but we can’t measure the level of carbonic acid in the blood.

However, we know that H2CO3 is proportional to the pCO2.

So, we can measure the partial pressure of carbon dioxide in the blood, and from that , determine the reason for their acidosis.

If the bicarbonate is low, we call it metabolic acidosis (probably a diabetic). If the carbonic acid is high, we call it respiratory acidosis.

You would have to treat the patients in a different way, depending on what the reason for their alkalosis is, even though their pH could be the same.

19
Q

What is it in proteins that allows them to act as buffers?

A

Amino acids.

20
Q

Why is a zwitterion not a good buffer?

A

The zwitterion is not a good buffer as that requires a molecule to have an acid and its own conjugate base, not an acid and any conjugate base.

21
Q

Why is glycine not a good physiological buffer?

A

Looking at the pKas (2.34 and 9.66), we can see that glycine will not be a good physiological buffer.

Additionally, in a protein it will not be free, rather it is in a peptide bond. Even if an amino acid is in good physiological range, if it is in a peptide bond, it can not dissociate.

Thus, we can conclude that it is not the alpha carboxyl and alpha amino groups, but rather the R groups that are involved in physiological buffering.

22
Q

As a recap, list the non-polar amino acids, and will the side chain dissociate?

A
  • Glycine (Gly)
  • Alanine (Ala)
  • Valine (Val)
  • Leucine (Leu)
  • Isoleucine (Ile)
  • Methionine Met)
  • Phenylalanine (Phe)
  • Tryptophan (Trp)
  • Proline (Pro)

The side chains are not going to dissociate.

23
Q

List the polar amino acids that will not dissociate.

A
  • Serine (Ser)
  • Threonine (Thr)
  • Cysteine (Cys)
  • Tyrosine (Tyr)
  • Aspargine (Asp)
  • Glutamine (Gln)

These are polar, but they do not dissociate.

Cysteine can, but we usually find it in a disulphide bond in a protein, so it cannot dissociate.

24
Q

List the polar amino acids that will dissociate.

A
  • Aspartic Acid (Asp)
  • Glutamic Acid (Glu)
  • Lysine (Lys)
  • Arginine (Arg)
  • Histidine (His)

These amino acids dissociate.

There is a carboxyl group in aspartic and glutamic acid (so very low pKa, thus low buffering range).
Lysine and arginine can also dissociate, but at a high pH.

We are then left with histidine, which has a pKa of 6.

25
Q

What makes haemoglobin a good blood buffer, and how can we explain its buffering range discrepancy?

A

The presence of a large number of histidine residues.

We can bring up the fact that the range of histidine is 6, and the pH of the blood is 7.3, so they are a bit far away. However, it is important to note that the histidine is not free.
The surroundings of an acid group influence the pKa.

Therefore, the pKa of histidine in haemoglobin is not 6, but rather it is slightly higher due to its environment.

26
Q

(from workshop)

What are the two pKas for glycine?

A

pKa1: 2.4 (alpha carboxyl group)

pKa2: 9.6 (alpha amino group)

27
Q

(from workshop)

What are the three pKas for aspartic acid?

A

pKa1: ~2.5 (alpha carboxyl group)

pKa2: ~3.5 (R-carboxyl group)

pKa3: ~9.6 (alpha amino group)

28
Q

(from workshop)

What are the three pKas for histidine?

A

pKa1: ~2.0 (alpha carboxyl group)

pKa2: ~6.0 (R-amino group)

pKa3: ~9.6 (alpha amino group)

29
Q

(from workshop)

What are the three pKas for arginine?

A

pKa1: ~2.5 (alpha carboxyl group)

pKa2: ~9.6 (alpha amino group)

pKa3: ~12.5 (R-amino group)

30
Q

(from workshop)

What is the equivalence point?

A

It is when an equivalent number of moles of base has been added to the weak acid.

It is the vertical part of the curve where no buffering occurs, as opposed to the horizontal part of the curve where buffering occurs.

31
Q

(from tutorial)

The concentration of carbonic acid, H2CO3, in human plasma is approximately 0.00125 M.

Assuming that this is the true concentration (of the undissociated weak acid) use the Henderson Hasselbalch equation to calculate the concentration of HCO3- in the
plasma, when the pH is 7.3.

You may assume that H2CO3 in blood has a pKa 1 value of 6.1.

A

antilog (1.2) = [HCO3-]/[H2CO3] = 15.85

Concentration of HCO3- is 15.85 x 0.00125 M = 0.02 M

Foundations of Biomedical Science (42 decks)

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