Acids, bases, pH and buffers

A hydrogen atom is one proton and one electron. Remove the electron and a lone proton with a +1 charge is left: H+.

• A hydrogen atom with an extra electron:
• Correct: A hydrogen atom that has lost its electron, leaving a lone proton:
• A molecule of hydrogen gas:

Concentration is amount divided by volume. The same amount in half the volume is twice as concentrated.

• Correct: It doubles:
• It halves:
• It stays the same:

Denaturation breaks the weak bonds, such as hydrogen bonds and ionic bonds, that hold a protein's fold. The chain stays whole, but its shape and function are lost.

• Its peptide bonds break and it falls apart into amino acids:
• It gains extra amino acids:
• Correct: It loses its folded shape and stops working, while its sequence stays the same:

The pH values differ by 2 units, and each unit is a factor of ten: 10 × 10 = 100. The lower pH is the more acidic fluid, so the urine at pH 5 holds 100 times more hydrogen ions.

• The pH 7 fluid has 2 times more hydrogen ions: Two errors: the scale is not linear, and a higher pH means fewer hydrogen ions, not more.
• The pH 5 fluid has 2 times more hydrogen ions: The direction is right, but the pH scale counts in powers of ten. Two units is 100-fold, not 2-fold.
• The pH 7 fluid has 100 times more hydrogen ions: The size is right, but the direction is reversed. Lower pH means more hydrogen ions.
• Correct: The pH 5 fluid has 100 times more hydrogen ions: Correct. Two units of pH is a hundredfold difference, and the lower pH has more H+.

pH fell by 0.30 units. Because log 2 is about 0.3, a fall of 0.3 doubles [H+]: from about 40 nmol/L at pH 7.40 to about 80 nmol/L at 7.10.

• Correct: It has roughly doubled: Correct. A 0.3 fall in pH is a doubling of hydrogen ions, from about 40 to about 80 nmol/L.
• It has risen by about 4%: About 4% is the change in the pH number itself (0.3 out of 7.4). The pH scale is logarithmic, so the hydrogen ion change is far larger.
• It has roughly halved: Halving would go with a rise in pH of 0.3. His pH fell, so hydrogen ions went up.
• It has risen about 30-fold: A 30-fold change reads the 0.30 as if each tenth were tenfold. Only a whole unit is tenfold; 0.3 units is twofold.

Acidemia is blood pH below 7.35. At 7.29 her blood holds more hydrogen ions than normal. The process causing it is an acidosis.

• Alkalemia: Alkalemia is blood pH above 7.45. Hers is below the normal range.
• Normal blood pH: Normal is 7.35 to 7.45. A value of 7.29 is below that band.
• Neutral: Neutral is pH 7.00. Her blood is still slightly basic in chemical terms, but it is too acidic for blood.
• Correct: Acidemia: Correct. Below 7.35 is acidemia.

Many amino acid side chains gain or lose a hydrogen ion as [H+] changes, which changes their charge. That disturbs the ionic bonds holding each protein's fold. Shape sets function, and proteins do almost every job in a cell, so many functions change together.

• Hydrogen ions break the peptide bonds of every protein: Peptide bonds are covalent and are not broken by these pH changes. The weak bonds that hold the fold are what change.
• Extra hydrogen ions dissolve the fats around each cell: Lipids are nonpolar and are not dissolved by hydrogen ions. The effect runs through protein shape.
• Correct: Hydrogen ions change side-chain charges, altering the shape of proteins: Correct. Changed charges disturb protein folding, and changed shape means changed function.
• Acidic blood cannot carry water to the cells: Water is the solvent of blood at any pH. The pH effect acts on protein shape, not on water.

The weak base bicarbonate takes up added H+ first, forming the weak acid carbonic acid, which keeps free H+ low. Carbonic acid then splits into carbon dioxide and water, and exhaling the carbon dioxide removes the acid from the body.

• Correct order: 1. Extra hydrogen ions enter the blood 2. Bicarbonate takes up the hydrogen ions 3. Carbonic acid forms and holds most of those hydrogen ions 4. Carbonic acid splits into carbon dioxide and water 5. The lungs breathe the carbon dioxide out

A buffer holds acid rather than removing it, and every hydrogen ion bicarbonate takes up converts one bicarbonate into carbonic acid. While bicarbonate is plentiful, pH barely moves. Once the store runs low, new acid has little to take it up, so pH falls sharply.

• His buffer was working in reverse and releasing acid: A buffer does not create acid. His falling bicarbonate shows it was taking up hydrogen ions, as a buffer should.
• Correct: Each buffered H+ used up a bicarbonate; few were left: Correct. Buffers have a limited capacity, set by how much weak base is left.
• Bicarbonate turned into a strong acid once its level fell: Bicarbonate is a weak base and stays one at any level. It becomes carbonic acid, a weak acid, only by taking up H+.
• The pH scale becomes linear below 7.35: The pH scale is logarithmic at every value. The sudden drop comes from losing buffer, not from a change in the scale.

In pure water, 0.001 mol of freed H+ in a liter gives [H+] = 10⁻³ mol/L, pH 3. In blood, bicarbonate takes up nearly all of it and breathing removes the carbon dioxide formed, so the pH falls by only a few hundredths.

• Both fall to about pH 3: Blood is buffered. Bicarbonate takes up most of the added H+, so blood does not approach pH 3.
• Correct: The water falls to about pH 3; the blood falls by only a few hundredths: Correct. Unbuffered water swings by four whole pH units; buffered blood barely moves.
• The blood falls further than the water, because blood already holds acid: The acid already in blood is mostly held as weak carbonic acid alongside plenty of bicarbonate. That pair is what resists the change.
• Neither changes, because 1 mL is too little to matter: In pure water, 1 mL of strong acid raises [H+] ten-thousandfold, from 10⁻⁷ to 10⁻³ mol/L.