Acids and Bases :  pH

1909 - S. P. L. Sørenson - Danish biochemist who suggested the use of a logarithmic scale to express the concentration of the H3O+ ion

pH - The negative of the log of the H3O+ (hydronium) ion concentration:

pOH - The negative of the log of the OH- (hydroxide) concentration:

Water-dissociation equilibrium constant (Kw) - The product of the
equilibrium concentration of the H3O+ and OH- ions in an aqueous solution is
equal to 1.00 x 10-14 at 25C:

When the logs of both sides are taken:

NOTE: When the pH is doubled, the [H3O+ ] decreases by a factor of 100.
  When the pH is quadrupled, the [H3O+ ] decreases by a factor of 10,000

A substance is acidic if the pH is less than 7.

A substance is basic if the pH is greater than 7.

A substance is neutral if the pH is equal to 7.

In pure water, the concentration of the hydronium and hydroxide ions are equal.  At 25C:


 

Relationship Between[H3O+] and [OH-]
with pH and pOH of an Aqueous Solution
[H3O+] (M)   [OH-] (M)   pH   pOH
1.0 1.0 x 10-14
0
14
Acidic
1.0 x 10-1 1.0 x 10-13
1
13
1.0 x 10-2 1.0 x 10-12
2
12
1.0 x 10-3 1.0 x 10-11
3
11
1.0 x 10-4 1.0 x 10-10
4
10
1.0 x 10-5 1.0 x 10-9
5
9
1.0 x 10-6 1.0 x 10-8
6
8
1.0 x 10-7 1.0 x 10-7
7
7
Neutral
1.0 x 10-8 1.0 x 10-6
8
6
Basic
1.0 x 10-9 1.0 x 10-5
9
5
1.0 x 10-10 1.0 x 10-4
10
4
1.0 x 10-11 1.0 x 10-3
11
3
1.0 x 10-12 1.0 x 10-2
12
2
1.0 x 10-13 1.0 x 10-1
13
1
1.0 x 10-14 1.0
14
0

 


pH of 0.1 M Solutions of Common Acids and Bases

Compound Name
Formula
pH
hydrochloric acid HCl 1.1
sulfuric acid H2SO4 1.2
sodium bisulfate NaHSO4 1.4
sulfurous acid H2SO3 1.5
phosphoric acid H3PO4 1.5
hydrofluoric acid HF 2.1
acetic acid HC2H3O2 2.9
carbonic acid H2CO3 2.8
hydrogen sulfide H2S 4.1
sodium dihydrogen phosphate NaH2PO4 4.4
ammonium chloride NH4Cl 4.6
hydrocyanic acid HCN 5.1
Compound Name Formula pH
sodium sulfate NaSO4 6.1
sodium chloride NaCl 6.4
sodium acetate NaC2H3O2 8.4
sodium bicarbonate NaHCO3 8.4
sodium biphosphate Na2HPO4 9.3
sodium sulfite NaSO3 9.8
sodium cyanide NaCN 11.0
ammonia NH3 11.1
sodium carbonate NaCO3 11.6
sodium phosphate Na3PO4 12.0
sodium hydroxide NaOH 13.0

 



 

Acid-base indicator - A weak acid or weak base which changes color when it gains or loses an H+ ion.

Examples of Acid-Base Indicators
Indicator
 
pH Range
 
Color Change
Methyl violet
Thymol blue
Bromophenol blue
Methyl orange
Methyl red
Litmus
Bromocresol purple
Bromophenol red
Bromothymol blue
Cresol red
Thymol blue
Phenolphthalein
Alizarin yellow
0.0 - 1.6
1.2 - 2.8
3.0 - 4.6
3.2 - 4.4
4.4 - 6.2
5 - 8
5.2 - 6.8
5.2 - 6.8
6.2 - 7.6
7.2 - 8.8
8.0 - 9.6
8.0 - 10.0
10.0 - 12.0
yellow  blue-violet
red  yellow
yellow  blue-violet
red yellow-orange
red  yellow
pink  blue
yellow  purple
yellow  red
yellow  blue
yellow  red
yellow  blue
colorless  pink
yellow  red-violet

Next:  "Factors that Affect the Relative Strengths of Acids and Bases"