ACID-BASE TITRATION

by Debbra Marcel

WHAT IS ACID-BASE TITRATION? Acid-base titration is a technique to determine the concentration of an acid or base solutions by neutralizing the unknown concentration of a solution (acid/base) with the known concentration of base/acid.

The neutralization reaction is determined by using an indicator where the indicator will change its colour at the end point of the titration. This method allows the quantitative analysis for the unknown acid/alkali concentration.

What is Neutralisation??? • Acids and alkalis are like ‘chemical

opposite’. If we add just the right amount of acid to an alkali, they ‘cancel each other out’, and we get a neutral solution. A chemical reaction takes place. The acid and alkali react together. In any chemical reaction a new substances are made. In this case a salt and water are formed. The reaction is called neutralisation. We can show the reaction like this:

Acid + Base Salt + Water Acid + Alkali Salt + Water

Did you know?

Table salt is often called ‘common salt’. Its chemical name is sodium chloride. There are many other salts that can be made by adding different acids and alkalis together.

• During the neutralisation, the actual reaction that occurred is between one hydrogen ion H+ from the acid and one hydroxide ion, OH- from the alkali to form one molecule of water, H2O.

• The ionic equation between an acid an alkali can be constructed and written as shown below.

Chemical Equation: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) H+Cl-(aq) + Na+OH-

(aq) Na+Cl-(aq) + H2O(l) Ionic Equation: H+

(aq) + OH-(aq) H2O(l)

EQUATION

OVERVIEW

• We fill a burette with the solution of reagent of known concentration.

• The burette is provided with the scales of volume expressing cubic centimeters and 10 of cubic centimeters.

• Now we add a known volume of the solution being investigated to a conical flask followed by a few drops of an indicator.

• The reagent of known concentration is added in small portion until the indicator just changes colour (end point of the titration-achieved when all the OH- ions combine with all the H+ ions in the solution to form water which is neutral)

• The volume of the liquid added from the burette is calculated because the titration technique is based on measurement of volume (volumetric analysis)

THREE COMMON TYPES OF INDICATORS AT DIFFERENT PH VALUES.

Indicator Colour in acids Colour in neutral

solution Colour in alkalis

Methyl orange Red Orange Yellow

Phenolphthalein Colourless Colourless Light Pink

Litmus Red Orange Blue

PROCEDURE OF ACID-BASE TITRATION 1) Transfer 25.0 cm3 of sodium hydroxide, NaOH, into a conical flask

by using pipette. 2) Put a few drops of phenolphthalein into the sodium hydroxide,

NaOH solution. 3) Clamp the burette vertically on the retort stand and fill in with

0.1 mol dm-3 hydrochloric acid, HCl. 4) Place the conical flask containing sodium hydroxide solution on

top of the white tile at the base of the retort stand. 5) Record the initial volume of the hydrochloric acid in the burette. 6) Add the hydrochloric acid into the conical flaks slowly until the

pink solution changes to colourless while continuously shaking the conical flask.

7) Record the final volume of the hydrochloric acid on the burette. 8) Repeat the titration process three times to obtain more accurate

volume of hydrochloric acid at the end point.

PROCEDURE OF ACID-BASE TITRATION

(Practical steps)

Answer: To determine the concentration of an acid or base solutions by neutralizing the unknown concentration of a solution (acid/base) with the known concentration of base/acid.

Pre-step 1

The pipette rinsed with distilled water followed by sodium hydroxide (NaOH) solution

Step 1

25.0 cm3 of sodium hydroxide, NaOH, transferred into a conical flask by using pipette.

Step 2

A few drops of phenolphthalein added into the sodium hydroxide, NaOH solution.

Pre-step 3

The burette is rinsed with distilled water and a little of hydrocloric acid (HCl)

Step 3

The burette clamped vertically on the retort stand and filled in with 0.10 mol dm-3 hydrochloric acid, HCl

Step 4

The conical flask containing sodium hydroxide solution placed on top of the white tile at the base of the retort stand.

Step 5

The initial volume of the hydrochloric acid in the burette is recorded.(must be at 0.0 cm³)

Step 6

The hydrochloric acid is added into the conical flaks slowly until the pink solution changes to colourless while continuously shaking the conical flask.

Step 7

The final volume of the hydrochloric acid on the burette is recorded.

22.0 cm3

Step 8

The titration process repeated three times to obtain more accurate volume of hydrochloric acid at the end point.

GRAPH CHANGES DURING TITRATION

Green button= before end point Red button = end point

BASIC CONCEPT OF THE CALCULATION.

a Acid + b Alkali Salt + Water

Molarity of acid = MA Molarity of alkali = MB

Volume of acid = VA Volume of alkali = VB

a

b

= constant

(fixed value)

MAVA/1000 = a

MBVB/1000 b

MAVA = a

MBVB b

SOLUTION:

1. Write a balanced equation for the reaction. Deduced the mole ratio of NaOH to HCl

2. Calculate the number of moles of HCl.

3. Calculate the number of moles of NaOH.

4. Equate the mole ratio from step 3 with the mole ratio from step 1 and 2. Solve the value of x.

Answer:- 1. NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l)

2. Moles of HCl.

= 0.01 mol dm-3 x 0.022 dm3

= 0.0022 mol

0022.0

0250.0

1

1 x

HClofmolesofNumber

NaOHofmolesofNumber

0250.0

0022.0x 088.0

3. Moles of sodium hydroxide, NaOH = x mol dm-3 x 0.0250 dm3 = 0.0250x mol

4. Moles equation:-

-The molarity of sodium hydroxide, NaOH solution = 0.088 mol dm-3.

Answer:- The molarity of sodium hydroxide, NaOH solution = 0.088 mol dm-3