# Atkins/Jones - 12.01-12.07 Aqueous Equilibrium

## 32 terms · 12.1 Buffer Action 12.2 Designing a Buffer 12.3 Buffer Capacity 12.4 Strong Acid-Strong Base 12.5 Strong Acid-Weak Base and Weak Acid-Strong Base Titrations 12.6 Acid-Base Indicators 12.7 Stoichiometry of Polyprotic Acid Titrations

### Qualitative Analysis

The identification of substances and ions present in a aqueous solution sample.

### "Mixed" Solutions

Solutions that contain a weak acid or a weak base and one of its salts. A means of stabilizing the pH of aqueous solutions such as blood plasma, seawater, etc. (i.e. buffers).

### Buffer

A solution in which the pH resists changes when small amounts of strong acids or bases are added.

### 12.1 Buffer Action Summary

A buffer is a mixture of a weak conjugate acid-base pair that stabilizes the pH of a solution by providing both a source and a sink for protons.

### Acid Buffer

A buffer made of a weak acid and its salt, pH>7.

### Basic Buffer

A buffer made of a weak base and its salt, pH<7.

### Calculating the pH of a buffer solution (acid example)

Identify the weak acid and its conjugate base. Write the proton transfer equilibrium between them and rearrange the expression for Ka to give [H₃O⁺]. Calculate the pH.
(Use tabled values and approximate conjugate acid/base concentrations by their initial values).

### Calculating the pH change of a buffered solution (acid example)

Calculate the new concentration of acid (initial concentration minus how much reacts [based on rxn stoichiometry]). Do the same for the conjugate base. Calculate the pH from such.

### Buffer composition

Buffers are often made with equal amounts of conjugate acids/bases. This means that, since the acids/bases are so weak, and dissociate so little, in calculating Ka, the concentrations can be set equal to their initial concentrations so that they cancel out.
tl;dr when [HA]ini=[A⁻]ini, pH=pKa

### Henderson-Hasselbalch Equation

pH = pKa + log [base]ini/[acid]ini

### 12.2 Designing a Buffer Summary

The pH of a buffer solution is close to the pKa of the weak acid component when the acid and base have similar concentrations.

### Buffer Capacity

The maximum amount of acid or base that can be added before the buffer loses its ability to resist large changes in pH.

### Buffer capacity relies on...

A more concentrated buffer is more resistant to pH changes. The relative concentrations of weak acid/base also matter; when the weak base is at least 10% of the weak acid, the buffer better resists the addition of acid, and when the weak acid is at least 10% of the weak base, the buffer better resists the addition of base.

### Buffer effectiveness

When the acid is 10× as abundant as the base ([acid] = 10[base], the pH is given by
pH = pKa + log [base]/10[base] = pKa + log 1/10 = pKa - 1
When the base is 10× more abundant than the acid, then pH = pKa + log 10[acid]/[acid] = pKa + 1
Buffers act most effectively when the pH is within a range of ±1 unit of pKa.

### 12.3 Buffer Capacity Summary

The capacity of a buffer is determined by its concentration and pH. A more concentrated buffer can react with more added acid or base than can a less concentrated one. A buffer solution is generall most effective when the pH is in the range pKa ± 1.

### Analyte

The unknown sample in a titration.

### Titrant

The solution of known concentration in a titration.

### Stoichiometric p\Point

When the amount of OH⁻ or H₃O⁺ added as titrant is equal to the amount of H₃O⁺ or OH⁻ initially present in the analyte.

### pH Curve

A plot of the pH of the analyte solution against the volume of titrant added during a titration.

### Physiological Buffers (Blood)

Biological systems are highly reliant on pH buffers, ex. in the blood. Blood uses HCO₃⁻/H₂CO₃ in a ration of ~20:1, with most of the carbonic acid in the form of CO₂.

### Alkalosis/Acidosis

Alkalosis: when the pH of blood rises above normal levels.
Acidosis: when the pH of blood falls below normal levels.
Reliant on the ratio of HCO₃⁻ to H₂CO₃ present.

### How blood pH is maintained

H₂CO₃: exhalation as CO₂
HCO₃⁻: excretion in urine

### 12.4 Strong Acid-Strong Base Summary

in the titration of a strong acid with a strong base or vice versa, the pH changes slowly initially, changes rapidly through pH=7 at the stoichiometric point, and then changes slowly again.

### At the halfway point of an acid-base titration, since pH = 7...

[HA] = [A⁻] and thus pH = pKa

### 12.5 Strong Acid-Weak Base and Weak Acid-Strong Base Titrations Summary

Halfway to the stoichiometric point, the pH is equal to the pKa of the acid. The pH is greater than 7 at the stoichiometric point of the titration of a weak acid and strong base. The pH is less than 7 at the stoichiometric point of the titration of a weak base and strong acid.

### pH Meter

A device that uses a special electrode to measure H₃O⁺ concentration.

### Acid-Base Indicator

A water-soluble organic dye with color that depends to pH.

### How indicators work

An indicator changes color with pH because it is a weak acid that has one color in its acidic form and another color in its conjugate base form. Because it is a weak acid, it takes place in a proton transfer equilibrium and has its own Ka/pKa.

### End Point of an Indicator

When the concentrations of its acid and base forms are equal, i.e. [acid]=[base]; the color change occurs when pH=pKin (in=indicator).

### 12.6 Acid-Base Indicators Summary

Acid-base indicators are weak acids that change color close to pH = pKin; an indicator should be chosen so that its end point is close to the stoichiometric point of the titration.

### Polyprotic Acids

Acids with more than one donatable H⁺.

### 12.7 Stoichiometry of Polyprotic Acid Titrations Summary

The titration of a polyprotic acid has a stoichiometric point corresponding to the removal of each acidic hydrogen atom. The pH of a solution of a polyprotic acid undergoing a titration is estimated by considering the primary species in solution and the proton transfer equilibrium that determines the pH.