Thursday, February 9, 2012

pH Value, indicators, charge relations

pH Value, indicators, charge relations

pH Value:

The pH value is the negative logarithm to the base 10 of the hydrogen ion concentration.

Derivation:

In pure water, the concentration of hydrogen ions [H3O+] is equal to the concentration of hydroxyl ions [OH], viz

[H3O+] = [OH-] = 10-7 mol/L

At this equilibrium, the water has a neutral reaction. In practice, the minus sign is dropped, 10-7 = pH 7.

According to the above equation, if the hydrogen ion concentration in an aqueous system is increased, the hydroxyl ion concentration becomes less, and the system has an acid reaction.

If the hydroxyl ion concentration is increased, the hydrogen ion concentration decreases accordingly, and the solution has an alkaline reaction.

pH = 1 ←

pH = 7

→ pH = 14

acid

neutral

alkaline

Determination of pH value:

1. Potentiometric (electrometric)

With a hydrogen electrode or glass electrode (measuring range pH 0 – pH 14)

2. Colorimetric

With indicators or indicator papers.

Common indicators

Indicator

Effective pH range

Color change

Concentration

of solution

Thymol Blue

1.2 – 2.8

red – yellow

0.1% in 20%

alcohol

Dimethyl Yellow

2.9 – 4.0

red – yellow

0.1% in 90%

alcohol

Bromophenol Blue

3.0 – 4.6

yellow – blue

0.1% in 20%

alcohol

Congo Red

3.0 – 5.0

blue – red

1% in water

Methyl Orange

3.2 – 4.4

red – yellow

0.1% in water

Bromocresol Green

3.8 – 5.4

yellow – blue

0.1% in 50%

alcohol

Methyl Red

4.8 – 6.0

red – yellow

0.2 % in 90%

alcohol

Litmus

5.0 – 8.0

red – blue

0.3 % in 90%

alcohol

Bromocresol Purple

5.2 – 6.8

yellow – purple red

0.04 % in 90%

alcohol

Bromothymol Blue

6.0 – 7.6

yellow – blue

0.1% in 20%

alcohol

Neutral Red

6.8 – 8.0

red –

yellowish orange

0.1 in 70% alcohol

Phenol Red

6.6 – 8.0

yellow – red

0.02 % in 90%

alcohol

o-Cresol Red

7.0 – 8.8

yellow – purple red

0.1% in 20%

alcohol

Tropaeoline 000

7.6 – 8.9

yellowish green –

pink

1% in water

Phenolphthalein

8.2 –10.0

colorless – pink

1.0 % in 50%

alcohol

Thymol Blue

8.0 – 9.6

yellow – blue

0.1% in 20%

alcohol

Thymolphthalein

9.4 –10.6

colorless – blue

0.1% in 90%

alcohol

Alizarin Yellow R

10.1 –12.0

yellow – red

0.1% in 50%

alcohol

Tropaeoline O

11.3 –13.0

yellow – orange red

0.1% in water


Example: pH colour range of bromocresol green

Color: Yellow = pH 3.5 and lower

Yellow-green = pH 4.0

Green = pH 4.5

Blue-green = pH 5.0

Blue = pH 5.5 and higher

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