Acid Base Indicators

Experiment #6

What are acids and bases?There are many different definitions for classifying a substance as an acid or a base.

Definitions are based on different theories

Lewis’s theory of acids and bases

Lowry-Bronsted theory of acids and bases

Arrhenius theory of acids and bases

Arrhenius theory of acids and bases

Proposed in 1884 by Svante Arrhenius

An acid is a substance that contains hydrogen and produces H+ in aqueous Solution. A base is a solution that contains the OH group and produces Hydroxide ions OH-, in aqueous solution

Neutralization is defined as the combination of H+ ions with OH- ions to form H2O molecules.

H+ (aq) + OH-(aq) H2O(l)

Very Limited in Scope!!

Lowry-Bronsted theory

Proposed in 1923 by N. Bronsted and T.M.Lowry

An acid is defined as a proton donor, H+, and a base is Defined as a proton acceptor

The complete ionization of hydrogen chloride, HCl in wateris an acid-base reaction

Step 2: H+ (aq) + H2O(l) H3O+

An acid-base reaction is the transfer of a proton from an acid to a base

Overall: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) (LB)

Step 1: HCl(aq) H+(aq) + Cl-(aq) (Arrehnius)

Overall: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) (LB)

acid conjugateacidbase conjugate

base

Overall: NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) (LB)

base conjugateacidacid conjugate

base

Whether water can act as an acid or base depends on the other species present

H2O (l) + H2O(l) H3O+ (aq) + OH-(aq)

Autoionization of water

Simplified notation

H2O(l) H+ (aq) + OH-(aq)

Water is said to be amphiprotic

Strength of acidsAcids and bases are classified as strong or weakdepending on the extent to which they can ionize.

Higher ionization ≡ Strong

Strong acid does not necessarily mean that it is the most corrosive acid. HF is a weak acid but it is one of the most corrosive acids.

Strength of some acids:HClO4 > HI > HBr > HCl > HNO3

H2SO4 > HNO3

Strong acids and strong bases are completely ionized

Weak acids and weak bases are partially ionized

Lewis theory

An acid is any species that can accept a share in an electron pair. A base is any species that can make available or donate a share in an electron pair.

A more broader definition. There is no need for the molecule to have hydrogens.

Proposed in 1923 by G. N. Lewis

What happens when salts are dissolved in water?

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(aq)

Acid Base Salt Water

NaCl(s) + H2O NaOH(aq) + HCl(aq)

Salt Water Base Acid

Depending on the strength of the acid and base that are formed in solution, the medium will be either acidic or basic

How do I know if the resulting solution is acidic or basic?

Measure pH of the solution

What is pH?

It is the measure of hydrogen ions in solution

OHLogpH 3

What is pH?

pH=1.0

pH=14.0

pH=7.0

Acidic

Neutral

Basic

OHLogpH 3

HLogpH

What are some of the ways to measure pH?

1. Using an acid-base indicator

2. Using pH paper

3. Using pH meter

4. ……………..

What are acid-base indicators? How do they work?

An organic dye, whose color depends on the concentration of H3O+ or pH

HIn(aq) + H2O(l) H3O+(aq) + In-(aq)

acid conjugateacidbase conjugate

baseWhen the indicator is added to the solution of interest, depending on the pH of the solution, the indicator stays in either its acid form or its conjugate form.

Indicator "Acidic" Color "Basic" color(MO)methyl orange Red below pH 4.0 Yellow above pH 4.4(BG)bromcresol green Yellow below pH 4.5 Blue above pH 4.8(BB)bromthymol blue Yellow below pH 6.9 Blue above pH 7.3(TB)thymol blue Yellow below pH 8.0 Blue above pH 8.4(P)Phenolphthalein Colorless below pH 9.3 Pink above pH 9.7(AY)alizarin yellow Yellow below pH 10.8 Red above pH 11.2

If we are to pick a suitable indicator for following the pH change in the reaction,We have to pick one that is suitable over the pH range of the reaction.

Indicators cannot give a particular numerical value for pH. They can only give a range for the pH of the solution.

Ionization of acids

All acids undergo ionization. Strong acids ionize more than the weak ones. Strong acids are almost100% ionized. Since the weak acids are partially ionized, the measure of the extent of ionization is given by an expression called acid ionizationconstant, Ka.

In our experiment today, we would like to measurethe Ka for a weak monoprotic acid.

What is a monoprotic acid?

Usually acids have one or more hydrogen atoms (H)which they end up donating as protons (H+).

# of H atoms Name Example

1 monoprotic acid (HA) HCl – hydrochloric acid CH3COOH - acetic acid

2 diprotic acid (H2A) H2SO4 – sulfuric acid

3 triprotic acid (H3A) H3PO4 – Phosphoric acid

Ionization constant of an acidFor a monoprotic weak acid (HA) dissolved in water,

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

acid conjugateacidbase conjugate

base

eqbeqb

eqbeqb

eq OHHA

AOHK

2

3

cOHthatassumewillweOHOHSinceeqbinitialeqb

222 ,

cHA

AOHK

eqb

eqbeqb

eq

3

Ionization constant of an acid

cHA

AOHK

eqb

eqbeqb

eq

3

eqb

eqbeqb

eqa HA

AOHcKK

3

eqb

eqbeqb

a HA

AOHK

3

Ionization constant of the acid

eqb

eqbeqb

a HA

AOHK

3

Ionization constant of an acid

Taking log of the equation on both sides,

eqb

eqbeqb

a HA

AOHLogLogK

3

][][

3 HAA

LogOHLogLogKa

cb

LogaLogaswrittenbecancab

LogSince )()(

Ionization constant of an acid

][][

3 HAA

LogOHLogLogKa

Multiplying both sides of the equation by -1

][][

3 HAA

LogOHLogLogKa

pHOHLogandpKLogKbut aa 3,

][][

HAA

LogpHpKa

Ionization constant of an acid

][][

HAA

LogpHpKa

][][

HAA

LogpKpH a

Henderson-Hasselbach equation

In the above equation, if we can make the [A-] = [HA]

][][

HAHA

LogpKpH a

Ionization constant of an acid

][][

HAHA

LogpKpH a

)1(LogpKpH a

0 apKpH

apKpH

250mL

Vinitial

0.2 MNaOH

250mL

Vfinal

100mL

1.0 g of HAIn 100mL ofwater

50 mL of HA+ 2 drops phenolphthalein

Vfinal- Vinitial =Vused

Add Vused amount ofwater to the solution in the volumetric flask

Mix the solution involumetric flask to theSolution in the erlenmeyerflask

Measure the pH of thefinal solution with indicators and pH paper