Ionisation Constants of Acids in Water Solution at 298 K

Formula

pKa

 

Formula

pKa

H2SO3

1.90

 

H2S

7.02

HSO4-

1.99

 

H2PO4-

7.20

H3PO4

2.13

 

HSO3-

7.20

CH2ClCOOH

2.86

 

HOCl

7.54

HNO2

3.14

 

N2H5+

7.94

HF

3.17

 

HCN

9.22

HCOOH

3.74

 

H3BO3

9.24

CH3CHOHCOOH

3.86

 

NH4+

9.24

C6H6COOH

4.20

 

C6H5OH

9.98

C6H5NH3+

4.61

 

HCO3-

10.33

CH3COOH

4.76

 

CH3CH2NH3+

10.67

CH3CH2 CH2COOH

4.82

 

HPO42-

12.36

CH3CH2COOH

4.87

 

HS-

12.89

NH3OH+

5.96

 

CH3CH2OH

>15.7

H2CO3

6.35

 

H2O

15.7

 

Common Acid-Base Indicators

Indicator Name

Colour Change

(low to high pH)

pKa

methyl yellow

red yellow

3.1

methyl orange

red yellow

3.7

methyl red

pink yellow

5.1

bromothymol blue

yellow blue

7.0

cresol red

yellow red

8.3

phenolphthalein

colourless red

9.6

alizarin yellow

yellow lilac

11.1