Good Luck on the Exam

National University –SUDA Faculty of Clinical and Industrial Pharmacy

Second Year (Batch-PA-13)-Semester Four

Professional Skills-2- Laboratory Skills-1

(PA-SKILL-221)

Course Coordinator: Us. Ghada Mustafa Eltayeb,

Phone Number (00249-912242444 / 00249-990997710).

Assistant Coordinator: Us. Maria Alamin Hamed,

Phone Number (00249-913714903 / 00966567560185)

Ghada

Volumetric analysis involving

acids and alkalis

By the end of this lesson the student is expected ……..

Lecture, No-5

1. To prepare standard solutions

2. To practice acid-alkali titrations

3. To Calculate the concentration of

volumetric analysis

4. To Write a laboratory report on volumetric

analysis experiment

Lecture Objectives

Standard solutions

A solution of accurately known molar

concentration or molarity is called a

standard solution.

Key point

A _s_t_a_n_d_a_rd solution is a solution of

accurately known molar concentration or

molarity.

Apparatus for preparing standard solutions

Table 19.1 Names and uses of the apparatus commonly used in

the preparation of standard solutions.

Name Use

1. Weighing bottle

(a) Glass weighing bottle

(b) Plastic weighing bottle

cap cap

(a) (b)

It is used to hold the

sample to be

weighed.

Name Use

2. Electronic balance

(a) (b)

It is used to weigh

the solid quickly and

accurately (accuracy

up to (a) 0.01 g or

(b) 0.0001 g).

Names and uses of the apparatus commonly used in the

preparation of standard solutions.

Name Use

3. Volumetric flask It is used to make up

a solution to a

specific volume (e.g.

250.0 cm3)

accurately.

* Before use, rinse

the volumetric flask

with distilled water.

Table 19.1 Names and uses of the apparatus commonly used in

the preparation of standard solutions.

Table 19.1 Names and uses of the apparatus commonly used in

the preparation of standard solutions.

Name Use

4. Pipette

bulb

It is used to deliver a

specific volume (e.g.

25.0 cm3) of a

solution accurately.

* Before use, rinse

the pipette with

distilled water, then

with the solution it

is going to deliver.

19.1 Standard solutions

P. 8 / 74

Table 19.1 Names and uses of the apparatus commonly used in

the preparation of standard solutions.

Name Use

5. Pipette filler

(a) The simple type

(b) The traditional type with valves

valves

It is used to suck up

a solution into a

pipette.

* As a safety

precaution, always

fill a pipette with a

pipette filler. Do NOT

use your mouth.

Name Use

6. Beaker It is used to hold

solutions.

Names and uses of the apparatus commonly used in the

preparation of standard solutions.

Preparing standard solutions

Primary standard

A primary standard is a substance which can

be used to prepare a standard solution

directly.

Learning tip

Only very few substances can be used as a

primary standard. Examples include

anhydrous sodium carbonate and oxalic acid

crystals.

A good primary standard should have the following properties:

readily available with high purity

chemically stable (should not react

with

oxygen, carbon dioxide and water vapour in

air)

high solubility in water

does not absorb water vapour from air.

does not lose water of crystallization to

the atmosphere on exposure to air if it is

a hydrated substance

involatile and non-toxic

Preparing a standard solution by dissolving a pure solid

1000

250.0 dm3 106.0 g mol–1 = 0.10 mol dm–3

= 2.65 g

Steps for preparing 250.0 cm3 of an

approximately0.10 M sodium carbonate solution

Stage A: Weighing the pure solid

1. Calculate the mass of anhydrous sodium

carbonate required.

Mass of Na2CO3 required

= number of moles of Na2CO3 molar mass of Na2CO3

2. Use an electronic balance to weigh out accurately the required mass of anhydrous sodium carbonate.

weighing

bottle anhydrous

sodium

carbonate

Steps for preparing a standard

solution by dissolving a pure

solid in distilled water.

SBA note

Do not waste time trying to weigh out exactly 2.65

g of the solid. However, the mass weighed out

should be accurate (e.g. to be at least 0.01 g).

Stage B: Dissolving the pure solid

(3

)

anhydrous

sodium

carbonate

sodium

carbonate

solution

3. Dissolve the solid in about 100 cm3 of distilled

water in a beaker. Stir the solution with a glass

rod.

100 cm3 of

distilled

water

glass rod

beaker

Stage C: Making up the solution to 250.0

cm3 in a volumetric flask

(4)

4. Pour the solution to a 250.0 cm3 volumetric

flask through a filter funnel.

glass rod

sodium

carbonate

solution filter funnel

volumetric flask

(250.0 cm3

type)

5. Wash the beaker and the glass rod with distilled water several times. Pour all the washing to the volumetric flask.

plastic

wash

bottle distille

d

water

washin

g

sodium

carbonate

solution

distille

d

water

(5)

beake

r distille

d

water

6. Add distilled water to the volumetric flask but stop about 2 cm below the graduation mark.

7. Use a dropper to add distilled water until the

bottom of the meniscus reaches the graduation

mark. droppe

r

250.0 cm3

graduatio

n mark

distille

d

water

(6

)

(7

)

8. Stopper the flask and invert it several times to mix the contents well.

stoppe

r

meniscus

invert it

several

times

standard sodium

carbonate

solution

(8)

Example 19.2

Preparing a standard solution by diluting a

concentrated solution of known molarity

Steps for preparing 250.0 cm3 of 0.0120 M

sodium carbonate solution from

0.120 M sodium carbonate solution

1. Calculate the volume of the original solution

required for dilution. number of moles of Na2CO3 = number of moles of Na2CO3

before dilution after dilution

M1V1 = M2V2

∴ V1 = 25.0 cm3

0.120 V1

1000 = 0.0120 250.0

1000

2. Use a pipette to transfer 25.0 cm3 of the original solution to a 250.0 cm3 volumetric

flask. pipett

e

filler

0.120 M sodium

carbonate

solution

25.0

cm3

pipette

meniscu

s

25.0

cm3

pipette

0.120 M sodium

carbonate

solution

0.120 M

sodium

carbonat

e

solution

250.0 cm3

volumetric

flask

(1) and (2)

Steps for preparing a standard solution by diluting a

concentrated solution.

NOTE You should use the index finger instead

of the thumb to control the flow of solution in a pipette.

3. Add distilled water to the volumetric flask but stop about 2 cm below the graduation mark.

4. Use a dropper to add distilled water until the

bottom of the meniscus reaches the graduation

mark. beake

r distille

d

water

droppe

r distille

d

water 250.0 cm3

graduatio

n mark

(4

)

(3

)

invert it

several

times stopper

meniscus

0.0120 M sodium

carbonate

solution (5)

5. Stopper the flask and then invert it several times to mix

the contents well.

19.2 Acid-alkali titrations

Volumetric analysis and titration

Volumetric analysis

is a method to determine the amount (or

concentration) of a substance in a

sample.

requires the measurement of volumes of

the solutions involved.

is also known as titrimetric analysis as it

is done by titration.

Titration is an experimental technique used for

determining the amount (or concentration) of

a substance in a solution.

In a titration, a solution of known concentration

(i.e. a standard solution) is slowly added to

another solution of known volume, until the two

solutions have just reacted completely.

Apparatus for titration

Name Use

1. Burette

stopcock

It is used to deliver

variable volumes (ranging

from 0.05 to 50.00 cm3) of

a solution accurately.

* Before use, rinse the

burette with distilled

water and then with the

solution it is going to

deliver.

Names and uses of some apparatus used in acid-alkali titration.

Name Use

2. Conical flask It is used to hold a

solution to be titrated. Its

conical shape allows it to

be swirled gently without

spilling out the solution.

* Before use, rinse

the conical flask with

distilled water.

Table19.2 Names and uses of some apparatus used in acid-alkali

titration.

Name Use

3. Stand and clamp They are used to support

the burette.

Name Use

4. White tile It is used to allow clear

observation of the colour

change of the indicator.

Procedure for acid-alkali titration

A standard solution of an acid (or an alkali) is

slowly added to a known volume of a solution of

an alkali (or an acid), until the acid and alkali

have completely reacted with each other.

The stage at which the acid and the alkali have

just reacted completely during titration is called

the equivalence point.

Learning tip

The procedure for an acid-carbonate (e.g. HCl

and Na2CO3) titration is more or less the same

as that for the acid-alkali titration.

Key point

The equivalence point of an acid-alkali titration

is the point at which an acid and an alkali have

just

_c_o_m_p_l_e_t_e_ly_ reacted with each other.

The equivalence point of an acid-alkali titration

can be estimated by using a pH meter or a

data- logger connected with a pH sensor.

It is not possible to detect the equivalence

point using an acid-base indicator.

During the acid-alkali titration, a suitable acid-

base indicator is usually added.

The colour of the indicator changes sharply at

or very near the equivalence point, then the

end point of titration is reached.

Key point

_En_d_p_o_i_n_t_ is the point at which the

indicator changes colour sharply.

Detection of end point by using an acid-base indicator

alkal

i

conical

flask alkali

alkal

i

1. Fill the 25.0 cm3 pipette (using a pipette filler)

with the alkali until the graduation mark is

reached. Transfer 25.0 cm3 of the alkali to a

conical flask.

25.0

cm3

pipette

(1

)

Procedure for performing an acid-alkali titration.

2. Fill the burette with the acid. Make sure that the jet is filled completely (no air bubbles inside the jet).

Learning tip

The solution in the burette is often known as

titrant.

white

tile

clam

p

stan

d aci

d

burett

e

(2

)

3. Add a few drops of an acid-base indicator to the alkali. Note the colour

of the solution. 4. Take the initial burette reading, with your eyes

on the same level as the meniscus.

indicato

r

alkal

i (3

)

meniscu

s

eye at

prope

r

level

read as 0.6 cm3

5. Slowly add the acid to the alkali until the mixture just turns to another persistent colour. This is the end point of titration.

burett

e

acid

solution mixture + indicator

(5)

6. Take the final burette reading. Calculate the volume of the acid added to reach the end

point. 7. Repeat the titration (steps 1 to 6) at least three

times, until we get three sets of data for

calculating the average volume of the acid

used.

Learning tip

The volume of titrant used to react completely

with the solution in the conical flask is called

titre.

methyl

orange

pH = 2 pH =12

phenolphthalei

n

Choosing a suitable acid-base indicator

Methyl orange and phenolphthalein can give a sharp colour change at the end point of titration. Litmus is not used because it does not give a sharp colour change.

(a) pH = 2 pH = 12 (b

)

Figure The sharp colour changes of (a) methyl orange and

(b) phenolphthalein in solutions of pH = 2 and pH = 12.

(There is often a marked change in pH at the end point of

titration. The change from pH = 2 to pH = 12 is used as an

example here.)

Titration type Example Suitable

indicator(s)

Strong acid – strong alkali

HCl(aq) vs. NaOH(aq)

methyl orange;

phenolphthalein

Strong acid – weak alkali

(Strong acid – carbonate)

HNO3(aq) vs. NH3(aq)

HCl(aq) vs. Na2CO3(aq)

methyl orange

Weak acid – strong alkali

CH3COOH(aq) vs. KOH(aq)

phenolphthalein

Weak acid – weak alkali

CH3COOH(aq) vs. NH3(aq)

(no indicator

would be suitable)

The correct choice of indicators for different types of acid-alkali

titrations.

19.2 Acid-alkali titrations