Acids and Bases, making an Acid-Base Indicator using Red Cabbage11/04/13

Can you give an example of a strong acid, what would its pH value be?

Do you think citrus acid found in fruits is a strong acid, what do you think its pH value is?

Do you know any strong bases what would its Ph would be?

Give an example of a weaker base and estimate its pH

What are Acids and Bases?

The Arrenhius definition of an acid and a base and further a strong acid/base and weak acid/base

The Bronsted- Lowry definition of an acid and base

The pigment found in plants and an example of an indicator that uses this pigment

Why this pigment can act as a natural indicator How to prepare a natural indicator using red

cabbage

What we should know after this class!

The Arrenhius Definition: An Acid is a substance that dissociates in water

to produce H+ ions. HCl→ H+ + Cl- HNO3→ 2H+ + NO3

-

A strong acid is one which dissociates almost completely in water to give H+ ions such as HCl.

A weak acid is one which dissociates only partially in water to give H+ i.e. ethanoic acid.

But What makes an Acid an Acid and a Base a Base!

A Base is a substance that dissociates in water to produce OH- ions.

NaOH → Na+ + OH-

Mg(OH)2→ Mg2+ + 2OH-

A strong base is one which dissociates almost completely in water to give OH- i.e. NaOH

A weak is one which only partially dissociate in water i.e. Mg(OH)2

Arrenhius Base

An Acid is a proton donor, i.e. donates a proton.

A Base is a proton acceptor, i.e. accepts a proton.

HCl + H2O→ H3O+ + Cl- The HCl has donated a proton to water,

therefore it is an Acid. Since the molecule of water accepts the proton, it is the Base.

Bronsted- Lowry Definition

NH3 + H2O→ NH4+ + OH-

What does this tell us about water?

Pick out the Acid and Base!

It is clear from the reaction of Water with HCl and NH3 that water can act as either an Acid or a Base.

A substance that can do this is said to be Amphoteric (the greek word for on both sides!)

The Answer!

Many indicators we use today such as Phenolpthalein indicator and Methyl orange indicator are derived from natural plant materials such as red cabbage, blackberries, blueberries and beet plants.

All these plants contain a pigment molecule called an Anthocyanin, the active ingredient used in indicators.

They are water soluble and acts as Indicators by changing colour in different Ph’s.

Making our own Indicator

Is food in the cell vacuole in plants but also in the leaves, stems and roots

The natural occurring colour of the pigment in most plants is a red-purple colour

In flowers bright reds and purples are adaptive for attracting pollinators

There are also know to protect the plant cells from high-light damage by absorbing ultraviolet light.

Anthocyanin

Contains the pigment molecule Anthocyanin and works as a natural indicator by changing colour when solutions of different pH’s are added.

pH: 2 , 4 , 6, 8 , 10, 12 Colour: Red,Purple,Violet,Blue,Blue-Green,

Greenish yellow

Red Cabbage Indicator

Place about 2 cups full of red cabbage in a blender, cover it with boiling water and blend!

The water used should be deionised water. The liquid should be at around pH 7 after

blending Pour about 50-100ml of the indicator into each

beaker. Add the 4 unlabelled solutions to the indicator

until a colour change is detected. Isolate the unlabelled solutions by the colour

changes that occur!

Making the Indicator

Repeat the procedure using blue grapes which also contains the Anthocyanin pigment.

Compare the results which indicator works best and is the most accurate.

Making the indicator continued!