Formula

pK_a

Formula

pK_a

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

HPO₄^2-

12.36

CH₃CH₂COOH

4.87

HS^-

12.89

NH₃OH⁺

5.96

CH₃CH₂OH

>15.7

H₂CO₃

6.35

H₂O

15.7

Indicator Name

(low to high pH)

pK_a

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