volumetric2(acid base titration)

21st February 2011 1 DrSabiha/CHM421/Dec 10-Apr11

TITRIMETRIC ANALYSIS Topic 4

ACID-BASE TITRATION

CHOOSING A TITRANT

In theory, any strong acid or strong base can be used as titrant.

The reason for this is that most reaction involving a strong acid or a strong base is QUANTITATIVE.

Examples of strong acid titrants : 1. Hydrochloric acid (HCl), 2. Nitric acid (HNO3), 3. Perchloric acid (HClO4), 4. Phosphoric acid (H3PO4) 5. Sulphuric acid (H2SO4)

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Weak acid titrant 1. Acetic acid ,(CH3COOH) 2. Ammonium ion, (NH4-) 3. Hydrogen flouride, (HF) 4. Carbonic acid, (H2CO3) 5. Nitrous acid, (HNO2) 6. Hydrogen sulphide, (H2S) 7. Hydrogen cynide, (HCN)


Strong base titrant Weak base titrant Sodium hydroxide, NaOH Potassium hydroxide, KOH Magnesium hydroxide, Mg(OH)2 Barium hydroxide , Ba(OH)2

Ammonium hydroxide, NH4OH Amine acetate. Carbonate, CO32- Flouride ion, F- Sodium carbonate, NaCO3


You can see that neither indicator changes colour at the equivalence point.

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Strong acid vs strong base

21st February 2011 DrSabiha/CHM421/Dec 10-Apr11

However, the graph is so steep at that point that there will be virtually no difference in the volume of acid added whichever indicator you choose. However, it would make sense to titrate to the best possible colour with each indicator.

If you use phenolphthalein, you would titrate until it just becomes colourless (at pH 8.3) because that is as close as you can get to the equivalence point.

On the other hand, using methyl orange, you would titrate until there is the very first trace of orange in the solution. If the solution becomes red, you are

getting further from the equivalence point.


Strong acid vs weak base

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This time it is obvious that phenolphthalein would be completely useless. However, methyl orange starts to change from red to orange-yellow very close to the equivalence point. You have to choose an indicator which changes colour on the steep bit of the curve.


Weak acid vs strong base

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This time, the methyl orange is hopeless! However, the phenolphthalein changes colour exactly where you want it to.


Weak acid vs weak base

The curve is for a case where the acid and base are both equally weak - for example, ethanoic acid and ammonia solution. In other cases, the equivalence point will be at some other pH.


You can see that neither indicator is any use. Phenolphthalein will have finished changing well before the equivalence point, and methyl orange falls off the graph altogether.

It may be possible to find an indicator which starts to change or finishes changing at the equivalence point, but because the pH of the equivalence point will be different from case to case, you can't generalise.

On the whole, you would never titrate a weak acid and a weak base in the presence of an

indicator. 10 21st February 2011 DrSabiha/CHM421/Dec 10-Apr11

CALCULATION Mol = mass (g) molar mass (g/mol) Molarity (M) = Mol Volume (L)


Mass (g ) = Liter (L) x Molarity [mol/L] x Molar Mass [g/mol]

aA + bB cC + dD

MAVA = a

MBVB b


CALCULATION ACID-BASE Example1 : A sample weighing 0.8365 g is a diprotic acid was dissolved in 75.0 mL distilled water. Indicator was added and the solution was titrated with sodium hydroxide solution. 35.35 mL was needed to reach the end point. Molar mass of acid is 150 gmol-1. 1. State whether the equivalence point is greater or

less than 7. 2. Name a suitable indicator that can be used in the

above titration. 3. Calculate the molarity of the base.


First write the reaction equation: Equation: H2A + 2OH- = 2H2O + A2-

Mole acid = = 5.5767 x 10-3 mole

150

0.835


Molarity of acid = = 0.0744 M =

= 0.3157 M Mb= 0.3157 M

L 0.075

10 x 5.5767 -3 mol

VM

VM

bb

aa

2

1

35.35

75 x 0.0744 x 2


Example 2: Calculate the concentration of vinegar in

%w/v for a titration of 25 ml of vinegar which was

primarily diluted 10 times (25mL into 250mL), reacted

with 22.4 ml of 0.1 M NaOH standard solution.

16 DrSabiha/CHM421/Dec 10-Apr11 21st February 2011

Mass of vinegar = (Litersvinegar)(Mvinegar)(Fwgtvinegar) = (0.25)( )(60) = 1.34g In %w/v, concentration of vinegar = (1.34/25) X 100 = 5.4%

25

0.1 x 22.4

17 DrSabiha/CHM421/Dec 10-Apr11 21st February 2011

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1) Write and balance the equation

CH3COOH + NaOH CH3COONa + H2O

MaVa = a = Ma x 25 = 1 MbVb b 0.1 x 22.4 1

Ma = 0.0896 M of acid

DrSabiha/CHM421/Dec 10-Apr11 21st February 2011

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2) Calculate mole of acid , n = Molarity (M) x Volume (Liter) = 0.0896 (mol/L) x 0.250 L = 0.0224 mol 3) Mass of acid (g) = molar mass x mol = 60 g/mol x 0.0224 mol = 1.344g 4) % w/v = (g/mL) x 100 = (1.344g / 25 mL ) x 100 = 5.4%

DrSabiha/CHM421/Dec 10-Apr11 21st February 2011

volumetric2(acid base titration)

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