power of hydrogen

http://www.lowimpactdevelopment.org/school/images/pH2.jpg

pH scalepH scale• Logarithmic scale

–expressing H+1 concentration, [H+1]• If pH changes by factor of 1, [H+1] changes by factor of 10• pH = -log[HpH = -log[H+1+1]]

http://www.stormcenter.com/envirocast/2003-04-01/images/article4-1_sm.gif

Molarity to pH

To determine pH:• express [H+1] in scientific notation• remember, [ ] means concentration of

whatever is inside brackets• log is the power of 10log is the power of 10

Molarity to pH for Pure water

tiny bit of ionization in pure water:H2O H+1 + OH-1

• [H+1] = [OH-1] = 1 X 10-7-7 M• pH = -log[Hlog[H+1+1]] = -(-7-7) = 7

pH

33221100pH

.001 Mor

1 X 10-3-3

M

0.01 Mor

1 X 10-2-2

M

0.1 Mor

1 X 10-1-1

M

1 Mor

1X1000 M

[H+1]

http://www.agen.ufl.edu/~chyn/age2062/lect/lect_02/2_14.gif

pH to Molarity

pH = -log[H+1], solve for [H+1]• -pH = log[H+1]• antilog(-pH) = [H+1]• say pH = 5, then –pH = -5• antilog(-5) = 10-5

• the –pH becomes the power of 10!the –pH becomes the power of 10!

pOH

• by analogy: pOH is defined as –log[OH-1]• express [OH-1] in scientific notation• if [OH-1] = 1.0 X 10-3 M • then pOH = -log(10-3) = -(-3) = 3

pOH = 14 10 7 4 0

pH range

• 0 to 14 (7 is neutral)• pH = 0, strongly acidic• pH = 14, strongly basic

ACID or BASE?

• acids: [H+1] [OH-1]

• bases: [OH-1] [H+1]

Relationships of: pH, pOH, [H+1] and [OH-1]

• product of [H+1] and [OH-1] is always 1x10-1M• add exponents when multiple in scientific notation

• sum of pH and pOH is always 14

pH + pOH = 14pH + pOH = 14

• pH = 3, pOH = • pH = 7, pOH = • pH = 10, pOH =• pH = 9, pOH =• pH = 1, pOH =

1111774455

1313

•if pH = 4if pH = 4

• [H+1] = ?

• pOH = ?

• [OH-1] = ?

1010

1 x 101 x 10-4 -4 MM

1x101x10-10-10 M M

• if the [OH-1] = 1 X 10-3

• pOH = ?

• pH = ?

• [H+1] = ?

pOH = -log[OHpOH = -log[OH-1-1]]= -log(10= -log(10-3-3) = -(-3) = 3) = -(-3) = 3

14 – 3 = 1114 – 3 = 11

1x101x10-11-11 M M

• if the [H+1] = 1 x 10-5M

• pH = ?

• pOH = ?

• [OH-] = ?

55

14 – 5 = 914 – 5 = 9

1x101x10-9-9MM

6

1x10-2

1x10-5

3

Acidic or Basic

pOH[OH-](M)(M)

pH[H+](M)(M)

11111x101x10-3-3 1x101x10-11-11 AA

55991x101x10-9-9

BB

22 12121x101x10-12-12 AA

881x101x10-8-8 1x101x10-6-6 BB

How to safely test pH

• instruments – use a pH meter• indicators – use a series of indicators• see if substance reacts with a metal other

than Cu, Ag, or Au• NEVER “taste”

pH meters

Indicator

• substance that changes color over narrow pH range

• use several indicators to narrow down pH range of substance

• See Table M