Tris has a of 8.06 at

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

Tris has a

pK_a

of 8.06 at

25\,^\circ\text{C}

, and its

pK_a

decreases by approximately 0.028 units for every

1\,^\circ\text{C}

increase in temperature. Which of the following best explains why Tris is usually a poor choice for maintaining pH 7.0 in reactions carried out at

37\,^\circ\text{C}

? (a) Its

pK_a

drops to roughly 7.30 at

37\,^\circ\text{C}

, leaving it unable to buffer effectively around pH 7. (b) Its

pK_a

falls to about 7.72 at

37\,^\circ\text{C}

, still more than 0.5 pH units above the desired value. (c) Its

pK_a

rises to approximately 8.40 at

37\,^\circ\text{C}

, placing it even farther above pH 7.0. (d) According to Henderson–Hasselbalch, the

[A^-]/[HA]

ratio for Tris at pH 7 would be nearly

1:10

, so it cannot neutralize added acid.