pH scale Logarithmic scale –expressing H +1 concentration, [H +1 ] If pH changes by factor of 1, [H +1 ] changes by a factor of 10 pH = -log[H +1 ]pH = -log[H +1 ]

Molarity to pH To determine pH: Express [H +1 ] in scientific notation Remember, [ ] means concentration of whatever is inside brackets The log is the power of 10The log is the power of 10

Molarity to pH for Pure water There’s a tiny bit of ionization:  H 2 O  H +1 + OH -1 -7[H +1 ] = [OH -1 ] = 1 X M log[H +1 ]-7pH = -log[H +1 ] = -(-7) = 7

pH M or -3 1 X M 0.01 M or -2 1 X M 0.1 M or -1 1 X M 1 M or 0 1X10 0 M [H +1 ]

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) = 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 M Then pOH = -log(10 -3 ) = -(-3) = 3

pH + pOH = 14 pH = 3, pOH = pH = 7, pOH = pH = 10, pOH = pH = 9, pOH = pH = 1, pOH =

ACID or BASE? Acids: [H +1 ]  [OH -1 ] Bases: [OH -1 ]  [H +1 ]

pH range 0 to 14 –with 7 being neutral pH = 0, strongly acidic pH = 14, strongly basic

If the pH = 4If the pH = 4 The [H +1 ] = ? The pOH = ? The [OH -1 ] = ?10 Antilog(-pH) = [H +1 ] Antilog(-4) = Antilog(-10) = [OH -1 ] 1x = [OH -1 ]

If the [OH -1 ] = 1 X The pOH = ? The pH = ? The [H +1 ] = ? pOH = -log[OH -1 ] = -log(10 -3 ) = -(-3) = 3 14 – 3 = 11 Antilog(-11) = 1x M

I f the [H +1 ] = 1 x M The pH = ? The pOH = ? The [OH - ] = ?5 14 – 5 = 9 Antilog(-9) = 1x10 -9

6 1x x Acidic or Basic pOH[OH - ] (M) pH[H + ] (M) 11 1x x A 59 1x10 -9 B x A 8 1x x10 -6 B

How to safely test pH Instruments – use a pH meter Indicators – use a series of indicators See if the substance reacts with a metal other than Cu, Ag, or Au NEVER “taste”

Indicator substance that changes color over a narrow pH range Use several indicators to narrow down pH range of substance