A volumetric analysis is a method used to find out the amount of solute in a solution. It involves preparation of a primary standard, which is a solution of known molarity, and titration. When choosing a good primary standard, the requirements are that it should be cheap, stable in atmosphere and should not be hygroscopic to prevent the loss of solution. Also, it should have a high purity and high molar mass to prevent serious weighing error. In this experiment, sodium carbonate is chosen to be the primary standard, because it suits all the aforementioned criteria.
Methyl orange should be used as the indicator, because in this experiment, a strong acid Sulphuric acid and a weak alkali sodium carbonate were involved. The pH of the resulting solution would be below 7, i.e. acidic. As a result, methyl orange, having a pH transition range of 3.1 to 4.4, should be used instead of the others.
All the apparatus should be rinsed or washed before titration, since some dirt may be left on them in the previous experiments.
They should be cleaned with tap water first. Then distilled water is used because it contains no ions, and thus won’t react with the solutions and lead to errors in the calculated molarity. Also, the burette and pipette should be rinsed by the solution to be delivered. However, the conical flask doesn’t need to be washed like that because the number of moles of sodium carbonate in it would still remain unchanged, even a great deal of water is added into it.
A labeled diagram for this titration is as followed:
4) Chemicals:
2.654g Na2CO3, 200cm3 Sulphuric acid, 16 drops of methyl orange
5) Apparatus:
250cm3 conical flask
X 2
250cm3 beaker
X 2
50cm3 burette
X 1
25cm3 pipette
X 1
Pipette filler
X 1
Weighing bottle
X 1
Stand
X 1
Clamp
X 1
Filter funnel
X 1
Glass rod
X 1
Dropper
X 1
White tile
X 1
Plastic wash bottle
X 1
6) Procedures:
i. Using an electronic balance, 2.654g of sodium carbonate was weighed in the weighing bottle.
ii. The beakers and volumetric flask were cleaned with tap water and distilled water. The weighed sodium carbonate was then transferred into a beaker and dissolved with distilled water. A glass rod was used to stir the solution until the solid had all dissolved. After that, the resulting solution was transferred into the volumetric flask through a filter funnel. The beaker, the filter funnel and the glass rod were rinsed by distilled water and the washing was poured into the volumetric flask. Distilled water was added to the graduation mark of the volumetric flask.
iii. The volumetric flask was stopped and inverted for several times.
iv. The pipette was rinsed using tap water followed by distilled water and sodium carbonate. The conical flasks were also rinsed by distilled water. Next, 25cm3 of the sodium carbonate solution was pipetted into a conical flask.
v. The burette was rinsed using distilled water and Sulphuric acid. Sulphuric acid was then added to it through a filter funnel. Next, it is clamped vertically in a stand.
vi. The initial reading of the burette was taken.
vii. Four drops of methyl orange indicator solution were added to the conical flask. The acid was run into the flask while the flask was swirled continuously. When an orange colour was seen, the acid was added drop by drop, until a yellow colour appeared. At this time, no more acid was added. The final burette reading was taken.
viii. Other than washing the burette, procedures (ii) to (vii) were repeated for three times to obtain three consistent results.
7) Results:
Trial
1
2
3
Final burette reading (cm3)
39.20
28.40
29.00
27.70
Initial burette reading (cm3)
10.25
1.70
2.70
1.15
Volume of Na2CO3 used (cm3)
28.05
26.70
26.30
26.55
Mean volume of Na2CO3 solution used to react with Sulphuric acid =
(26.70+26.30+26.55)/3
=26.52 cm3
8) Calculation/interpretation:
Equation of the reaction:
Na2CO3 (aq) + H2SO4 (aq) � Na2SO4 (aq) + H2O (l) +CO2 (g)
No. of moles of Na2CO3 dissolved: mass of Na2CO3 / molar mass of Na2CO3
= 2.654/ (23*2 + 12 + 16*3)
= 2.654/106
= 0.025mol
No. of moles of Na2CO3 used: 0.025 * (26.52/250)
= 0.00266mol (3 sig. fig.)
Ratio of no. of moles of Na2CO3 to H2SO4 = 1:1
No. of moles of H2SO4 = 0.00266mol
Molarity of H2SO4 = No. of moles of H2SO4/ volume of H2SO4 (dm3)
= 0.00266/ 0.02652
= 0.100M (3 sig. fig.)
9) Discussion:
If the above titration is repeated with the sodium carbonate in the conical flask and 25ml Sulphuric acid in the burette, the calculated molarity of Sulphuric acid will still be the same. This is because there will be still the same number of moles of sodium carbonate and Sulphuric acid used to react with each other.
In the experiment, immediately after the end point was reached, the burette reading was marked down. In fact, the reading should be read at the time the equivalent point is reached. The equivalent point is the point at which all the sodium carbonate in the conical flask has reacted with Sulphuric acid, i.e. neutralization has just completed. However, it is not possible to take burette reading at this point because there is no colour change to methyl orange at this instant (the transition range of methyl orange is 3.1-4.4). Only when the reaction reaches the end point will methyl orange change from red to yellow.
The end point could not be determined by using indicator for all acid-alkali titration. This is due to the resulting pH of the solution; the product sodium sulphate is acidic. Different indicators have different pH transition range, which is the pH range for it to change its colour. Among litmus, methyl orange and phenolphthalein, methyl orange is the only one having a pH transition range below pH 7. In this case, only methyl orange should be used as the indicator.
On the other hand, improvement should be made to run the experiment more smoothly. For instance, in the first trial, it was not necessary to run the Sulphuric acid that slowly, since the trial reading isn’t counted in calculating the average volume of Sulphuric acid used. It should be made clear that the trial is used to roughly indicate the amount of Sulphuric acid required to completely neutralize sodium carbonate. So it should not take too long in trial in order to save time.
Besides, after one titration, the conical flask used should be left behind as a colour sample for the next titration, because it was always difficult to distinguish between the orange and yellow colour of methyl orange at the end point. Having the previous colour sample in a conical flask could prevent confusion of whether the colour is orange or yellow.
Regarding the possible errors, first of all, when doing titration, there may be some Sulphuric acid left on the inner wall of the conical flask. As a consequence, the recorded reading of Sulphuric acid used would be larger than the actual amount. Thus, the calculated molarity of Sulphuric acid would be larger. In a bid to eliminate this error, the inner wall of the conical flask should be washed by distilled water occasionally during the titration process to leash the Sulphuric acid back into the solution.
Secondly, it was rather difficult to drip exactly one drop of Sulphuric acid from the burette into the conical flask at the final stage. Due to human error, usually more than one drop of Sulphuric acid was added. As a result, more practice for turning the stopcock of the burette from the horizontal position to slightly incline is essential to obtain a more accurate result.
10) Conclusion:
The concentration of the Sulphuric acid was found to be 0.100M.
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