Buffer Solutions and Henderson-Hasselbalch
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Buffer Solutions and Henderson-Hasselbalch

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Questions and Answers

The pKa of NaHCO3 is ______.

10.25

In the Henderson-Hasselbalch equation, [TRIS]/[TRIS-H+] = ______ at pH 8.0.

0.83

The Henderson-Hasselbalch equation relates pH, pKa and acid/base ______.

concentrations

A buffer is more effective at ______ concentration.

<p>higher</p> Signup and view all the answers

Buffer solutions can consist of a weak acid and its salt (conjugate ______).

<p>base</p> Signup and view all the answers

In buffering, addition of H3O+ is neutralized by ______ from the buffer.

<p>AcO-</p> Signup and view all the answers

If the pKa of benzoic acid is 4.19, what is the ratio of [PhCO2H]/[PhCO2−] necessary for pH ______?

<p>3.5</p> Signup and view all the answers

The weak acid AcOH has a pKa of ______.

<p>4.76</p> Signup and view all the answers

The addition of acid to 0.1 M TRIS/TRIS-H+ results in [TRIS] = ______.

<p>0.040 M</p> Signup and view all the answers

When 0.005 M H+ is added, [TRIS-H+] becomes ______ M after addition.

<p>0.060</p> Signup and view all the answers

The pH of a buffer solution can only be reduced slightly with the addition of a very strong ______.

<p>acid</p> Signup and view all the answers

Common buffers include NaH2PO4 / Na2HPO4 in the pH range of ______.

<p>6-8</p> Signup and view all the answers

Before adjustment, the concentration of [TRIS] when 0.02 M TRIS/TRIS-H+ is used is ______ M.

<p>0.009</p> Signup and view all the answers

HNO3 is a very strong acid, fully dissociated into ______ and NO3-.

<p>H+</p> Signup and view all the answers

The value of pH is calculated using the formula pH = pKa − log10(/).

<p>A, AH+</p> Signup and view all the answers

The ratio of TRIS to TRIS-H+ at a pH of 8.0 involves the use of pKa which is ______.

<p>8.08</p> Signup and view all the answers

The behavior of ions in solution is affected by the degree of ______.

<p>ionisation</p> Signup and view all the answers

In pharmaceutical solutions, real behavior is crucial to consider rather than ideal ______.

<p>behavior</p> Signup and view all the answers

Buffer solutions consisting of KH2PO4 / K2HPO4 have a desired pH range of ______.

<p>6-8</p> Signup and view all the answers

The buffering effect can be diminished if the addition of acid reaches equal or greater ______ than the buffer concentration.

<p>concentrations</p> Signup and view all the answers

The activity of component i is represented as ______.

<p>a_i</p> Signup and view all the answers

The equation 𝜇𝑖 = 𝜇𝑖° + 𝑅𝑇 ln ______ quantifies deviations from ideality.

<p>i</p> Signup and view all the answers

The Debye Huckel Limiting Law uses a constant A, which is equal to ______ for water at 25 °C.

<p>0.509</p> Signup and view all the answers

Ionic strength, I, is calculated using the formula I = 1/2 ______.

<p>Σ z_i^2</p> Signup and view all the answers

In the example of Na3PO4, the ionic strength I was calculated to be ______ M.

<p>0.60</p> Signup and view all the answers

The mean activity coefficient for Na3PO4 was found to be ______.

<p>0.066</p> Signup and view all the answers

In the expression pKa + pKb = ______, pKw represents the ion product of water.

<p>pKw</p> Signup and view all the answers

To calculate mean activity a±, you first need to determine the mean ionic concentration ______.

<p>[±]</p> Signup and view all the answers

For the solution of MgCl2, the assumption is that it is fully ______ in solution.

<p>dissociated</p> Signup and view all the answers

The term 𝜇𝑖 corresponds to the ______ potential of component i in solution.

<p>chemical</p> Signup and view all the answers

Study Notes

Henderson-Hasselbalch Equation

  • Relates pH, pKa, and acid/base concentrations
  • For bases, use the pKa of the conjugate acid

Buffer Solutions

  • Solution of weak acid and its salt (conjugate base) or a weak base and its salt (conjugate acid)
  • E.g., acetic acid/sodium acetate
  • Acetic acid (AcOH) is weakly dissociated (pKa 4.76); sodium acetate (AcONa) is fully dissociated

Buffering Effect

  • Addition of hydroxide (OH-) is neutralized by acetic acid (AcOH)
  • Addition of hydronium (H3O+) is neutralized by acetate (AcO-)
  • Example: 0.045 mol/L solution of AcOH and 0.045 mol/L solution of AcONa

Buffering Effect Example

  • E.g., 2.25 x 10^-3 mol AcOH and AcONa
  • Add 1.0 x 10^-4 mol HNO3
  • AcO- + H+ → AcOH
  • [AcO-] = (2.25 x 10^-3 - 1.0 x 10^-4) = 2.15 x 10^-3 mol
  • [AcOH] = (2.25 x 10^-3 + 1.0 x 10^-4) = 2.35 x 10^-3 mol

Effective Buffering

  • pH of the solution is reduced by the addition of acid, but not greatly reduced (from 4.76 to 4.72)
  • Addition of equal or greater concentrations of added acid would break down the buffering effect
  • Common buffers include NaH2PO4/Na2HPO4 (pH range 6-8), KH2PO4/K2HPO4 (pH range 6-8), TRIS (pH range 7-9), and HEPES (pH range 6.8 – 8.2)

TRIS/TRIS-H+ Buffer Worked Example

  • pKa of TRIS-H+ is 8.08
  • What is the TRIS/TRIS-H+ ratio at pH 8.0?
    • pH = pKa + log10([TRIS]/[TRIS-H+])
    • -0.08 = log10([TRIS]/[TRIS-H+])
    • [TRIS]/[TRIS-H+] = 0.83

Addition of Acid to TRIS/TRIS-H+ Buffer

  • Starting with 0.1 M TRIS/TRIS-H+ buffer:
    • [TRIS] + [TRIS-H+] = 0.1 M
    • [TRIS]/[TRIS-H+] = 0.83
    • [TRIS] = 0.83[TRIS-H+]
    • [TRIS-H+] = 0.055 M
    • [TRIS] = 0.045 M
  • Adding 0.005 M H+:
    • [TRIS] = 0.040 M
    • [TRIS-H+] = 0.060 M
    • pH = pKa - log10([TRIS-H+]/[TRIS]) = 7.9

Addition of Acid to 0.02 M TRIS/TRIS-H+ Buffer

  • Starting with 0.02 M TRIS/TRIS-H+ buffer:
    • [TRIS] + [TRIS-H+] = 0.02 M
    • [TRIS]/[TRIS-H+] = 0.83
    • [TRIS] = 0.009 M
    • [TRIS-H+] = 0.011 M
  • Adding 0.005 M H+:
    • [TRIS] = 0.004 M
    • [TRIS-H+] = 0.016 M
    • pH = pKa - log10([TRIS-H+]/[TRIS]) = 7.48
  • Buffers are more effective at higher concentrations

Ions in Solution

  • Behavior of ions in solutions is affected by:
    • Degree of ionization
    • Extent of solvation
    • Ion-ion vs. ion-solvent interactions
    • External fields
    • Other phenomena
  • In pharmaceutical solutions, consider the effect of dissolved ions
  • Consider real (rather than ideal) behavior: activity rather than concentration

Activity of Ions in Solution

  • Activity (ai) quantifies deviations from ideality
  • μi = μi° + RT ln ai
  • ai = γi[i], where:
    • γi is the activity coefficient of component i
  • μi = μi° + RT ln [i] + RT ln γi

Activity Coefficient

  • log10 γ± = -Az+z-√I
    • Debye-Hückel Limiting Law
    • A is a constant (0.509 for water at 25 °C)
    • z+ and z- are the charges of the positive and negative ions
    • '±' implies mean value for both ions
    • I is the Ionic Strength

Ionic Strength (I)

  • Depends on the number of cations and anions in solutions
  • Quantifies the ionic field generated by a system of ions in solution
  • I = ½ Σi zi²[i]

Ionic Strength and Mean Activity Coefficient Worked Example: 0.1 M Na3PO4

  • Na3PO4 fully dissociates in water to 3Na+ and PO43-
  • zNa = 1; [Na] = 3 x 0.1 M
  • zPO4 = 3; [PO4] = 0.1 M
  • I = ½{[(0.3M)12] + [(0.1M)32]} = 0.60 M
  • log10ϒ± = −(0.509)(1)(3)√(0.6) = − 1.183
  • ϒ± = 0.066

Mean Activity of 0.1 M Na3PO4

  • a± = ϒ±[±] ([±] = mean ionic concentration of salt)
  • [±] = n√{([+])n+([-])n-} n = n+ + n-
  • For Na3PO4, [±] = 4√{(0.3)3(0.1)} = 0.228 M
  • a± (Na3PO4) = (0.066)(0.228 M) = 0.015 M
  • There is a significant difference between ionic activity and concentration
  • Activities determine equilibria for ions in solution

Ka, Kb and Kw

  • Note: AH and A- correspond to the acid and conjugate base (or corresponding base and conjugate acid)
  • pKa + pKb = pKw

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Henderson-Hasselbalch Equations and Buffer Solutions PDF

Description

This quiz covers the concepts of buffer solutions and the Henderson-Hasselbalch equation, exploring how pH, pKa, and concentrations of acids and bases relate. It also examines the buffering effect, including examples using acetic acid and sodium acetate. Test your understanding of these key concepts in chemistry!

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