INTRODUCTION
Determining the concentration of a solution requires comparison with a standard of known
properties. In acid–base chemistry, this is achieved through titration, a technique in which a
solution of known concentration is gradually added to another solution until a neutralization
reaction is complete. At the equivalence point, the number of moles of hydrogen ions equals the
number of moles of hydroxide ions, indicating complete reaction. Since this point is not directly
visible, an indicator such as phenolphthalein is used to signal the endpoint by a colour change.
In this experiment, a sodium hydroxide (NaOH) solution was prepared and standardized using
potassium hydrogen phthalate (KHP) as a primary standard. KHP is suitable due to its high
purity, stability, and solid form, which allows accurate weighing. The neutralization reaction
between NaOH and KHP occurs in a 1:1 molar ratio, enabling the moles of NaOH to be
determined directly from the known moles of KHP. Using stoichiometric relationships and
titration data, the concentration of the NaOH solution can therefore be calculated
PROCEDURE
Equipment And Chemicals
Sodium hydroxide
25 ml burette
Phenolphthalein indicator
100 ml Erlenmeyer flask
KHP
50 ml graduated cylinder
100 ml volumetric flask
150 ml beaker
Procedure
1. Preparation of NaOH
i.
Two pellets of NaOH were weighed carefully into a 100ml beaker
ii.
20ml of distilled water was added to dissolve the pellet and it was carefully
swirled.
iii.
The solution was carefully transferred into a 100ml volumetric flask. The beaker
was rinsed with subsequent volumes of distilled water to ensure the contents are
transferred into the volumetric flask.
iv.
The volume of the solution was then made up to the mark on the volumetric flask.
2. Preparation of Burette
v.
A 50ml burette was cleaned and rinsed in distilled water.
vi.
It was then rinsed with 1 5ml portion of the prepared NaOH solution (conditioning
the burette). The solution was drained through the tip of the burette and disposed
of.
vii.
The burette was then filled with the NaOH solution. Caution was taken to ensure
that the tip was also filled and there were no air bubbles in the tip.
viii.
Some of the NaOH was drained until the burette read 0.0ml.
3. Preparation of KHP Standard
ix.
0.15g of dried KHP was weighed to the milligram on a quarter A4 paper. The
crystals are then transferred into a clean 100ml Erlenmeyer flask.
x.
20ml of distilled water was added to the flask. The flask was shaken carefully to
dissolve the crystals.
xi.
Three drops of phenolphthalein indicator were added
4. Standardization of NaOH
xii.
A white background was secured underneath the flask containing the KHP
solution.
xiii.
With constant swirling, the NaOH was added drop-wise to the KHP solution.
xiv.
The NaOH was continuously added (with constant swirling) to the KHP solution
until the KHP solution turns a faint shade of pink that persists. This is taken as the
endpoint.
xv.
The volume of NaOH was recorded in the data table.
xvi.
The steps ix-xv were repeated up to 5 times.
DATA COLLECTION
Preparation of the primary KHP solution
Run 1
Run 2
Run 3
Mass of KHP
0.15
0.15
0.15
Formula mass of KHP ( 204.2 g/moles)
204.2
204.2
204.2
Moles of KHP (mass of KHP/formula mass KHP)
7.35 × 10−4
7.35 × 10−4
7.35 × 10−4
Final Burette reading in ml
11.5
33
45
Initial burette reading in ml
0
22
33
Volume of NaOH added (final – initial) in ml
11.5
11.0
12.0
Volume of NaOH added in litres (ml / 1000)
1.15 × 10−2
1.10 × 10−2
1.20 × 10−2
Titration of KHP
Average volume of NaOH added in litres
1.15 × 10−2
DATA ANALYSIS
Calculations
1. Average volume of NaOH
Mean
µ =
𝑥1 + 𝑥2 + 𝑥3 + ⋯ + 𝑥𝑖
𝑁
µ =
(1.15 × 10−2 ) + (1.10 × 10−2 ) + (1.20 × 10−2 )
3
µ = 1.15 × 10−2 𝐿
2. Number of Moles of KHP used
Trial 1
𝑚
𝑛 =
𝑀𝑟
𝑛 =
0.15𝑔
204.2𝑔/𝑚𝑜𝑙
𝑛 = 7.35 × 10−4 𝑚𝑜𝑙
Trial 2
𝑚
𝑛 =
𝑀𝑟
𝑛 =
0.15𝑔
204.2𝑔/𝑚𝑜𝑙
𝑛 = 7.35 × 10−4 𝑚𝑜𝑙
Trial 3
𝑚
𝑛 =
𝑀𝑟
𝑛 =
0.15𝑔
204.2𝑔/𝑚𝑜𝑙
𝑛 = 7.35 × 10−4 𝑚𝑜𝑙
3. Average number of moles of KHP from the chosen trials.
Mean
𝑥1 + 𝑥2 + 𝑥3 + ⋯ + 𝑥𝑖
𝑁
(7.35 × 10−4 ) + (7.35 × 10−4 ) + (7.35 × 10−4 )
µ =
3
µ =
µ = 7.35 × 10−4 𝑚𝑜𝑙
4. 𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐾𝐻𝐶8 𝐻4 𝑂4(𝑎𝑞) → 𝑁𝑎𝐾𝐶8 𝐻4 𝑂4(𝑎𝑞) + 𝐻2 𝑂(𝑙)
𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐾𝐻𝐶8 𝐻4 𝑂4(𝑎𝑞) → 𝑁𝑎𝐾𝐶8 𝐻4 𝑂4(𝑎𝑞) + 𝐻2 𝑂(𝑙)
(Self-Balanced)
NaOH : KHP
1:1
x : 7.35 × 10−4
The average number of moles of NaOH used in the reaction is 7.35 × 10−4 𝑚𝑜𝑙𝑒𝑠, since
all reactions take place in a 1:1 ratio.
5. Molarity of NaOH
𝑛
𝑀 =
𝑉
7.35 × 10−4 𝑚𝑜𝑙
𝑀 =
1.15 × 10−2 𝐿
𝑀 = 6.39 × 10−2 𝑀
DISCUSSION
The objective of this experiment was to determine the concentration of sodium hydroxide
(NaOH) by standardizing it against potassium hydrogen phthalate (KHP), a primary standard.
KHP is suitable for this purpose due to its high purity, stability, and monoprotic nature, which
ensures a straightforward 1:1 stoichiometric reaction with NaOH.
The calculated average volume of NaOH used in the titration was 1.15 × 10−2 L, with individual
titres of 11.5 mL, 11.0 mL, and 12.0 mL. These values are reasonably close, showing moderate
precision, although the spread of 1.0 mL suggests some experimental variability. Ideally, titres
should agree within ±0.1–0.2 mL, so this indicates minor inconsistencies in technique.
The calculated molarity of NaOH was 6.39 × 10⁻² M (0.0639 M). This is lower than what might
be expected if the NaOH solution had been prepared at a higher concentration, suggesting
possible dilution effects or preparation inaccuracies.
Several factors could explain the variation and slight inaccuracy:
•
Endpoint detection error: Phenolphthalein changes colour over a pH range, so judging
the “faint pink” endpoint is subjective and may lead to overshooting.
•
Burette reading errors: Parallax error when reading the meniscus could affect recorded
volumes.
•
NaOH instability: NaOH absorbs carbon dioxide from the air, forming sodium carbonate,
which reduces the effective concentration of hydroxide ions.
•
Transfer losses: During preparation, some NaOH solution may not have been fully
transferred from the beaker to the volumetric flask despite rinsing.
•
Inconsistent swirling: Uneven mixing during titration can delay the endpoint and
increase titre volume.
Despite these limitations, the results are internally consistent enough to support the calculated
concentration as a reasonable estimate.
CONCLUSION
The concentration of the sodium hydroxide solution was successfully determined by titration
with KHP. The average molarity of NaOH was found to be:
0.0639 M
The experiment demonstrated the principle of acid-base titration and the use of a primary
standard for solution standardization. While the results were reasonably consistent, minor
experimental errors affected precision. Improving endpoint detection and measurement
techniques would increase accuracy in future trials.