Buffers and

Acid/Base Titration

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Buffered Solutions

A solution that resists a change in pH when either hydroxide ions orprotons are added.

Buffered solutions contain either:

A weak acid and its salt

A weak base and its salt

Acid/Salt Buffering Pairs

Weak AcidFormula

of the acidExample of a salt of the

weak acidHydrofluoric HF KF – Potassium fluoride

Formic HCOOH KHCOO – Potassium formate

Benzoic C6H5COOH NaC6H5COO – Sodium benzoate

Acetic CH3COOH NaH3COO – Sodium acetate

Carbonic H2CO3 NaHCO3 - Sodium bicarbonate

Propanoic HC3H5O2 NaC3H5O2 - Sodium propanoate

Hydrocyanic HCN KCN - potassium cyanide

The salt will contain the anion of the acid, and the cation of a strong base (NaOH, KOH)

Base/Salt Buffering PairsThe salt will contain the cation of the base, and the anion of a strong acid (HCl, HNO3)

BaseFormula of the base

Example of a salt of the weak acid

Ammonia NH3 NH4Cl - ammonium chloride

Methylamine CH3NH2 CH3NH2Cl – methylammonium chloride

Ethylamine C2H5NH2 C2H5NH3NO3 - ethylammonium nitrate

Aniline C6H5NH2 C6H5NH3Cl – aniline hydrochloride

Pyridine C5H5N C5H5NHCl – pyridine hydrochloride

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00milliliters NaOH (0.10 M)

pH

Titration of an Unbuffered Solution

A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

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milliliters NaOH (0.10 M)

pH

Titration of a Buffered Solution

A solution that is 0.10 M CH3COOH and 0.10 M NaCH3COO is titrated with 0.10 M NaOH

Comparing Results

Graph

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mL 0.10 M NaOH

pH

Buffered

Unbuffered

Comparing Results

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milliliters NaOH (0.10 M)

pH

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00milliliters NaOH (0.10 M)

pH

Unbuffered Buffered

In what ways are the graphs different?In what ways are the graphs similar?

Henderson-Hasselbalch Equation

][

][log

][

][log

acid

basepK

HA

ApKpH aa

][

][log

][

][log

base

acidpK

B

BHpKpOH bb

This is an exceptionally powerful tool, and it’s use will be emphasized in our problem solving.

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0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00milliliters NaOH (0.10 M)

pH

Weak Acid/Strong Base Titration

A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

Endpoint is above pH 7

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milliliters NaOH (0.10 M)

pH

Strong Acid/Strong Base Titration

A solution that is 0.10 M HCl is titrated with 0.10 M NaOH

Endpoint is at pH 7

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milliliters HCl (0.10 M)

pH

Strong Acid/Strong Base Titration

A solution that is 0.10 M NaOH is titrated with 0.10 M HCl

Endpoint is at pH 7 It is important to

recognize that titration curves are not always increasing from left to right.

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milliliters NH3 (0.10 M)

pH

Strong Acid/Weak Base Titration

A solution that is 0.10 M HCl is titrated with 0.10 M NH3

Endpoint is below pH 7

Selection of Indicators

Indicator TransitionsIndicator Low pH color Transition pH range High pH colorGentian violet (Methyl violet 10B) yellow 0.0–2.0 blue-violet

Leucomalachite green (first transition) yellow 0.0–2.0 green

Leucomalachite green (second transition) green 11.6–14 colorless

Thymol blue (first transition) red 1.2–2.8 yellow

Thymol blue (second transition) yellow 8.0–9.6 blue

Methyl yellow red 2.9–4.0 yellow

Bromophenol blue yellow 3.0–4.6 purple

Congo red blue-violet 3.0–5.0 red

Methyl orange red 3.1–4.4 orange

Bromocresol green yellow 3.8–5.4 blue

Methyl red red 4.4–6.2 yellow

Methyl red red 4.5–5.2 green

Azolitmin red 4.5–8.3 blue

Bromocresol purple yellow 5.2–6.8 purple

Bromothymol blue yellow 6.0–7.6 blue

Phenol red yellow 6.8–8.4 red

Neutral red red 6.8–8.0 yellow

Naphtholphthalein colorless to reddish 7.3–8.7 greenish to blue

Cresol Red yellow 7.2–8.8 reddish-purple

Phenolphthalein colorless 8.3–10.0 fuchsia

Thymolphthalein colorless 9.3–10.5 blue

Alizarine Yellow R yellow 10.2–12.0 red

Litmus red 4.5-8.3 blue

Source: Wikipedia