pH and Indicators (chpt 18)

Self-ionisation of water

Pure water can conduct tiny electric current, as water dissociates slightly to form ions (endothermic as it favours higher temps)

Represented as H₂O ⇌ H⁺ + OH⁻

Equilibrium constant

Kc = [H⁺][OH⁻]/[H₂O]

Ionic product of water (Kw)

Kw = Kc[H₂O] = [H⁺][OH⁻]

Is temperature dependent

Neutral solution of water

[H⁺] = 10⁻⁷ (at 25° C, where 7 = pH)

Acidic solution of water

[H⁺] > 10⁷⁻ ie. [H⁺] > [OH⁻], eg. (10⁻⁵) > (10⁻⁹)

Basic solution of water

[H⁺] < 10⁻⁷ ie [H⁺] < [OH⁻], eg. (10⁻⁸) < (10⁻⁶)

pH

= -log₁₀[H⁺], where square brackets indicate concentration in moles per litre

Negative logarithm to the base 10 of hydrogen ion concentration measured in moles per litre

Strong acid

Good proton donor

Examples of acidic substances

Stomach acid (HCl) - pH 1, grapefruit - pH 4, milk - pH 6.2

Weak acid

Poor proton donor

Acid dissociation constant Ka

Ka = [H⁺][A⁻]/[HA]

The larger the value of Ka, the higher the concentration of H₃O⁺ and the stronger the acid

Strong base

Good proton acceptor

Examples of basic substances

Toothpaste - pH 9, Soap - pH 11, Limewater - pH 12.1

Weak base

Poor proton acceptor

Conjugate bases for acids

• Strong acid: weak conjugate base

• Weak acid: strong conjugate base

Finding pH when given pOH and visa versa

pH + pOH = 14

pOH

pOH = -log₁₀[OH⁻]

Limitations to pH scale

• pH scale is limited to 0 - 14 range, although possible to have pH values outside in theory

• Does not work w very concentrated solutions, ie. concentration above 1M as complete dissociation doesn’t always occur, and can’t predict no. H⁺ or OH⁻ ions

• Limited to aqueous solutions

Calculating pH of weak acids/bases

To find, must know extent which acid/base dissociates, depends on Ka/Kb (dissociation constants)

Acids: [H⁺] = -log√Ka x Ma(cid)

Bases: [OH⁻] = -log√Kb x Mb(ase)

Where Ma is concentration of acid in moles/L and Mb is concentration of base in moles/L

After finding extent of dissociation, then can find pH using its formula and H⁺/OH⁻ found

Acid-base indicator

Substance that changes colour according to pH of solution in which it is placed (most are weak acids, very little used so negligible contribution of H⁺/OH⁻ ions)

Litmus paper indicator example

HIn (red) ⇌ H⁺ + In⁻ (blue). In keeping w Le Chatilier’s

• HIn is acid form of indicator and In⁻ s conjugate base

• If H⁺ ions from acid added to blue litmus, litmus turns red to keep in accordance w Le Chatelier’s Principle (equ. shifts to left to absorb added H⁺ ions)

Weak base indicators

Dissociate slightly as follows: XOH ⇌ X⁺ + OH⁻

• Addiction of acid removes some of OH⁻ ions from right, therefore colour of X⁺ predominates, and vie versa with OH⁻

Range of an indicator (Methyl orange, litmus, phenolphthalein)

pH interval over which there is a clear change of colour for that indicator

• Methyl Orange → range: 3-5, acid colour: red, base colour: yellow (definite red pH 3 and lower, definite yellow pH 5 and higher)

• Litmus → range: 5-8, acid colour: red, base colour: blue (definite red pH 5 and lower, definite blue pH 8 and higher)

• Phenolphthalein → range: 8-10, acid colour: colourless, base colour: pink (definite no colour pH 8 and lower, definite pink colour pH 10 and higher)

pH titration

• Base placed in burette and acid in beaker. Done to allow us to start at low pH value and work up

• Acid kept stirred by placing magnetic stirring bar in beaker and stirred by magnetic stirrer

• pH sensor placed in acid solution, measures pH as base added. Connected to computer and pH recorded by datalogging software

• Beaker used instead of conical flask as beaker more suitable for magnetic stirring bar and pH sensor

• Graph of pH vs time plotted for solution showing change of pH related to volume of base added: pH titration curve

• No indicator used as not necessary with pH sensor

Types of acid-base titrations

• Strong acid against strong base

  • 0.1M NaOH to 0.1M HCl

  • Any indicator as all three fit in vertical part of curve (litmus not as reliable as other two)

• Strong acid against weak base

  • 0.1M NH₃ to 0.1M HCl

  • Methyl orange (vertical between 7 and 3)

• Weak acid against strong base

  • 0.1M NaOH to 0.1M CH₃COOH

  • Phenolphthalein (vertical between 10 and 7)

• Weak acid against weak base

  • 0.1 NH₃ to 0.1M CH₃COOH

  • Not possible to detect end point using indicator, so weak acid titrated against strong base and weak base against strong acid