ph and titration methods -...

pH and Titration MethodsEdward A. Mottel

Department of ChemistryRose-Hulman Institute of Technology

Autoionization of WaterWater is a very weak electrolyte

H2O(l) H+(aq) + OH–(aq)

2 H2O(l) H3O+(aq) + OH–(aq)

Dissociation constantKw = [H+] [OH– ] = 1.0 x 10–14 (@ 25 °C)

Note: H+(aq) ≡ H3O+ (aq)

pH Base 10 logarithmic scale of hydrogen ion

concentration• pH = -log [H+ ]

If pH = -log [H+ ], what does [H+ ] equal?

[H+ ] = 10–pH

Be carefuluse log and not ln

pH

pure waterat 25 °C Kw = [H+] [OH–] = 1.0 x 10–14

1.0 x 10–7 M 1.0 x 10–7 M

pH = -log(1.0 x 10–7) = 7.0

What’s the pH of pure water?

pH

If the pH of an aqueous solution is 3.50 at 25 °C,what is the hydroxide ion concentration?

pH

6

[H+ ] = 10–pH

[H+ ] = 10–3.50 = 3.2 x 10–4 M

Kw = [H+] [OH–] = 1.0 x 10–14 M2

[OH–] = Kw / [H+]

[OH–] = (1.0 x 10–14 M2) / 3.2 x 10–4 M

= 3.2 x 10–11 M

pH

7

0

14

acids

basesneutral @ 25 °C

1 M HCl

1 M KOH

pH values can be outside 0-14

Titration

analyte

titrant

an analytical technique to find the concentration of a solution

buretRemember to:•Rinse the buret•Remove air from tip•Added distilled water to the mixture does not affect calculations

Thermometric Titration

T (°C)

Vacid

acid-base reactionsare usually exothermic

Equivalence Pointvolume

Moles of acid = Moles of base

Titration

HF + NaOH NaF + H2O

Balanced equation

reactant 2

reactant 1

an analytical techniqueto determinethe concentrationof a solution

Titration


nacid · Macid · Vacid = nbase · Mbase · Vbase

Most acids have one acidic proton (HCl, CH3COOH),some have two (H2SO4, H2CO3),

some have three (H3PO4, citric acid)

the purpose of a titrationis to determine theequivalence point

Titration


nacid · Macid · Vacid = nbase · Mbase · Vbase

Most bases accept one proton (NaOH, NH3),some accept two (Ca(OH)2), some accept three (Al(OH)3)

Equivalence Point Detection Physical methods

• Thermometric • Potentiometric• Visual indicator• Conductometric

Potentiometric Titration 20.0 mL of 0.400 M HCl with Aqueous Ammonia

0 4 8 12 16 20 24 280

2

10

8

6

4

pH

Volume NH3 (mL)


What is the concentration of the ammonia?

Equivalence Point Detection Visual indicator methods

• Color change • Endpoint versus equivalence point

Endpoint: when the indicator changes colorEquivalence point: moles of acid = moles of base

• Factors to consider when selecting an indicator

SO3

C CH3H3C

Br

Br

OH

OBrBr

O

SO3

C CH3H3C

Br

Br

O

BrBr

Bromocresol Green

Common Acid-Base Visual IndicatorsIndicator Acidic Color Basic Color pH Range

thymol blue red yellow 1.2-2.8methyl orange orange yellow 2.9-4.4

bromocresol green yellow blue 3.8-5.4methyl red red yellow 4.4-6.0

bromothymol blue yellow blue 6.0-7.6cresol red yellow red 7.2-8.8

phenolphthalein colorless red 8.3-10.0thymolphthalein colorless blue 9.3-11.0alizarin yellow yellow red 10.0-12.1


0 4 8 12 16 20 24 280

2

10

8

6

4

pH

Volume NH3 (mL)

phenolphthalein

bromocresol green

equivalencepoint

endpoint

endpoint

For this titration,which indicatorwould be better?

Conductometric Titration

Λcorr

Vacid


Depending on the acid-base system studied,the graph may have many different shapes.


Why does the line change slope?

Λcorr

Vacid


HCladded K+ OH– H+ Cl– Total

Ions0 50 50 0 0 100

20 50 30 0 20 10040 50 10 0 40 10060 50 0 10 60 120

80 50 0 30 80 160100 50 0 50 100 200

H2O(l) + KCl(aq)KOH(aq) + HCl(aq)


If the total number of ions isn’t changing before theequivalence point, why is the conductivity changing?

Λcorr

Vacid



0 4 8 12 16 20 24 280

2

10

8

6

4

pH

Volume NH3 (mL)


What is [OH-] at the start of the reaction?


0 4 8 12 16 20 24 280

2

10

8

6

4

pH

Volume NH3 (mL)


Calculate the pH of 0.400 M HCl.


0 4 8 12 16 20 24 280

2

10

8

6

4

pH

Volume NH3 (mL)


Calculate the pH after 10.0 mL of ammonia have been added.


0 4 8 12 16 20 24 280

2

10

8

6

4

pH

Volume NH3 (mL)


Calculate the pH after 24.0 mL of ammonia have been added.

ph and titration methods -...

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