Experiment

1. EXPERIMENT No.:-2
OBJECTIVE:
To determine the strength of Sodium hydroxide (NaOH) and Sodium
carbonate (Na2
CO3
) from given water sample. Given standard N/40
Sodium carbonate to standardize given HCl solution.

2. HCl
Na2
CO3
+ Methyl Orange

3. •Reagents Required:
• Sodium Carbonate (Na2
CO3
)
• Hydrochloric Acid (HCl)
• Distilled Water
• Water Sample containing NaOH and Na2
CO3
mixture.
•Indicators and there End points:
• Phenolphthalein (Pink to colourless)
• Methyl orange (Yellow to Pink)

4. Resonance Structure of Methyl Orange

5. H+
OH―
H+
OH―
Benzenoid Structure,
Colourless,
Exists predominantly in
acidic medium
Quinonoid Structure,
Pink in colour, ,
Exists predominantly in
Basic medium
STRUCTURE OF PHENOLPHTHALEIN

6. Theory
The titration is based upon the specific pH range for each indicator. The phenolphthalein
indicator gives pink colour in pH range 8.2 – 10.0 while methyl orange gives pink colour in
pH range 4.4 – 3.0. It has been observed that the presence of sodium hydroxide and sodium
carbonate always provide a pH higher than 8.2 to the solution. Initially when standard
hydrochloric acid is added, the OH-
ions are first converted to H2
O and then CO3
-2
ions are
converted to HCO3
-
. When first two steps [(i) and (ii)] completed the pH of solution becomes
less than 8.2 and the pink colour of phenolphthalein disappears. At this point, methyl orange
is added as indicator to find the complete neutralization. On addition of further acid, the
HCO3
-
ions are converted to CO2
and H2
O. It means step (iii) is completed. When all HCO3
-
ions are used up the pH of solution becomes less than 4.4 instantaneously. At this point
methyl orange gives pink colour.

7. Reactions Involved:
•Chemical Reactions:
HCl + NaOH NaCl + H2
O
HCl + Na2
CO3
NaHCO3
+ NaCl
NaHCO3
+ HCl NaCl + H2
O + CO2
Ionic Reactions:
OH¯ + H+
H2
O
CO3
2-
+ H+
HCO3
-
HCO3
-
+ H+
H2
O + CO2
Phenolphthalein
Methyl orange
Phenolp

8. Procedure
I. Standardization of given HCl solution:
(i) Take 10 mL of standard Na2
CO3
with the help of pipette into a conical flask. Add 1-2 drops of methyl orange.
(ii) Run the acid solution from the burette into conical flask drop wise with constant shaking of solution. Find the end point
when light yellow colour solution turns pink. Note volume of acid used. Repeat this step 4-5 times till you get at least two
concordant readings (Table 3.1).
II. Titration of given water sample with HCl solution:
(i) Pipette out 10 mL of given water sample into a conical flask. Add 1-2 drops of phenolphthalein indicator. The solution turns
pink.
(ii) Add HCl solution from burette into the conical flask with constant shaking till the pink colour disappears. Note the reading
on the burette (V2
).
(iii) After discolouration, add 1-2 drops of methyl orange into the solution.
(iv) Titrate now again with acid till yellow coloured solution turns pink again. Note the volume of acid used (V3
). Repeat the
steps to get concordant readings (table 3.2).

9. Observations
Observation Table 3.1:
S. No.
Volume of
Na2
CO3
taken
(mL)
Burette Readings
Volume of HCl
used (mL)
Concordant
reading (Vo
)
(mL)
Initial Final
1. 10 mL 0.0 7.7 7.7
7.7 mL
2. 10 mL 7.7 15.4 7.7
3. 10 mL 15.4 23.1 7.7
4. 10 mL

10. Observations
Observation Table 3.2:
S. No.
Volume of Water
Sample taken
(mL)
Burette Readings Volume of HCl used (mL)
Concordant
reading (Vo
)
(mL)
Initial
(V1
)
Final P
(V4
= V2
– V1
)
M
(V5
= V3
– V2
)
P
(V2
)
M
(V3
)
1. 10 mL 0.0 7.0 12.3 7.0 5.3 P = 7.0 mL
M = 5.3 mL
2. 10 mL 12.3 19.3 24.6 7.0 5.3
3. 10 mL 24.6 31.6 36.9 7.0 5.3
4. 10 mL

11. Calculations
1. To calculate Normality of given HCl solution, apply normality equation:
=> NHCl
X VHCL
= NNa2CO3
X VNa2CO3
=> NHCl
= NNa2CO3
X VNa2CO3
/ VHCl
=>NHCl
= NNa2CO3
X 10 / VHCl

12. Calculations
1. To calculate Normality of Sodium hydroxide (NaOH) and Sodium carbonate
(Na2
CO3
) from given water sample:
V4
= Volume of HCl when whole of NaOH and half of carbonate is neutralized
= [OH-] + ½[CO3
2-
]
V5
= Volume of HCl when rest half of carbonate is neutralized
= ½[CO3
2-
]
Hence, Volume of HCl required for the neutralization of NaOH = V4
-V5
Volume of HCl required for the neutralization of Na2
CO3
= 2V5

13. Calculations
1. To calculate Normality of Sodium hydroxide (NaOH) and Sodium carbonate
(Na2
CO3
) from given water sample:
• Normality of NaOH in given solution (NNaOH
) = (NHCl
× V4
-V5
)/10
• Normality of Na2
CO3
in given solution (NNa2CO3
) = (NHCl
× 2V5
)/10
•
• Strength of NaOH in given solution = NNaOH
× Equivalent wt. of NaOH g/L
• Strength of Na2
CO3
in given solution = NNa2CO3
× Equivalent wt. of Na2
CO3
g/L

14. Results
• Strength of NaOH in given solution = ………………. g/L
• Strength of Na2
CO3
in given solution = ……………….. g/L

15. Precautions:
(i) Before starting the experiment, the glass apparatus must be perfectly cleaned.
(ii) For each titration, the initial reading of burette should always be the same.
(iii) Always read lower meniscus of solution level in burette.
(iv) Near the end point, add acid solution dropwise and after addition of each drop, see
the colour against white background.
(v) Do not blow last drop of solution from pipette. Just tap the tip of pipette to the walls
of the flask.