Introduction to pHIntroduction to pH

BY

G. Ram KumarDepartment of Chemistry

Pydah College P.G. Courses

Gambheeram, Visakhapatnam

S.P.L Sorenson introduced the concept of pH in the year 1909 Definition

Negative Logarithm of the hydrogen ion concentration

pH = - log aH3O+ or pH = - log [H+]

Where [H+] is in molarity

pH < 7 then acidic

pH = 7 then neutral

pH > 7 then basic

Battery Acid

Normal Stomach Acid (1.0 - 3.0)

Normal Rainwater (5.6)

Milk

Pure Water, Blood

Seawater, Shampoo

Household Ammonia (10.5 - 11.9)

Oven Cleaner

H20 H+ + OH-

Ionic product of water (W) at 25 0C is 10-14 mol/l

[H+] = [OH-] = 10-7 mol/l

pH + pOH = 14

Acid-base equlibria in water

pH Indicators – using equilibria to detect changes in pHLitmus - is a weak acid. It has a seriously complicated molecule which we will simplify to HLit. The "H" is the proton which can be given away to something else. The "Lit" is the rest of the weak acid molecule.

The un-ionised litmus is red, whereas the ion is blue.

Adding hydroxide ions

Adding hydrogen ions

Methyl orange

Phenolphthalein

The half-way stage happens at pH 8.3-10 A mixture of pink and colourless is simply a paler pink,

The pH range of indicatorsThe importance of pKindThink about a general indicator, HInd - where "Ind" is all the rest of the indicator apart from the hydrogen ion which is given away:

Think of what happens half-way through the colour change.

indicator pKind

litmus 6.5

methyl orange 3.7

phenolphthalein

9.3

That means that the end point for the indicator depends entirely on what its pKind value is. For the indicators we've looked at above, these are:

There is a gradual smooth change from one colour to the other, taking place over a range of pH. As a rough "rule of thumb", the visible change takes place about 1 pH unit either side of the pKind value. indicator pKind

pH range

litmus 6.5 5 - 8

methyl orange 3.7 3.1 - 4.4

phenolphthalein 9.3 8.3 -

10.0

Choosing indicators for titrationsStrong acid vs strong base

HCl Vs NaOH

Strong acid vs weak baseHCl Vs NH4OH

Weak acid vs strong baseCH3COOH Vs NaOH

Weak acid vs weak baseCH3COOH Vs NH4OH

Na2CO3 Vs HCl

Buffer Using equilibria to stabilize pH

Definition - a solution which resists the change in its pH values on its dilution or addition of small amounts of acid or base to it

Acidic Buffer

CH3COOH + CH3COONa

Basic Buffer

NH4Cl + NH4OH

Hendreson equations

Acidic bufferspH = pKa + log (salt/acid)

Basic buffers pOH = pKb + log (salt/base)pH = 14 – pKb + log(base/salt)

pH electrodes GLASS ELECTRODE

Glass electrode is a potentiometric sensor made from glass of a specific composition.

silicate matrix based on molecular network of silicon dioxide (SiO2) with additions of other metal oxides, such as Na, K, Li, Al, B, Ca, etc.

1 - a sensing part of electrode, a bulb made from a specific glass2 - sometimes electrode contain small amount of AgCl precipitate inside the glass electrode3 - internal solution, usually 0.1M HCl for pH electrodes 4 - internal electrode, usually silver chloride electrode or calomel electrode5 - body of electrode, made from non-conductive glass6 - reference electrode, usually the same type as 47 - junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.

Importance of pH Biological process & Industrial process generally occur at specified pH values onlyCrops require soils of specific pH values for optimum growth and better yieldsMany chemical and analytical procedures require maintenance of specific pH

Blood has specific pH ( 7.35 ) and this must always be maintainedMany physiological processes influence the pH, but one of the largest contributors is the CO2 content of the blood. CO2 + H2O = HCO3

-1 + H+1

Thanks for your attention