Changed sharply when only one drop of acid or base is added at the equivalent point, that is the end point.
In this experiment, methyl orange is used because:
Its end point matching the equivalent point of the acid-base titration. Its working range lies on the vertical part of the pH curve. The value pKin is near the pH of the solution at the equivalent point.
Although there are more than one indicator for acid-alkali titration, the other one, phenolphthalein cannot be used .There should be correct choice of indicators in different types of titrations.
1. For strong acid and strong alkali, any indicator will do. This is due to the equivalent point of the titration closes to the neutral point. The vertical part of the pH curve is very obvious and long.
2. For strong acid and weak alkali, only methyl orange can be used as an indicator because of the equivalent point within the acidic range. Since the methyl orange has the transition from 3.2 to 4.2, its pKin is 3.7 and be close to the pH of the solution at the equivalent point.
3. For weak acid and weak alkali, no indicators can be used as there is no vertical part where the equivalent point lies.
4. For weak acid and strong alkali, phenolphthalein should be used as the equivalent point lies on a alkaline range. Phenolphthalein has a high pkin value (9.1) with transition range from 8.2 to 10.
5.For strong acid and carbonate , methyl orange is a ideal indicator. As acid reacts with sodium carbonate to form acidic gas carbon dioxide, the resulting solution is slight acidic due to the dissolved acidic gas. Thus, the equivalent point reached in an acidic solution. Hence, Methyl orange, which has a low pKin value is used in this time.
Although the end point this time can not be determined by using indicator for all acid-alkali titration, we can determine it by alternative means like using pH meter, and by conductivity measurements.
Procedure:
1 Preparation, washing and cleaning of apparatus.
a . The pipette was washed by tap water first , followed by distilled water and finally
With the sulphuric acid.
b. The conical flask was washed by tap water first, and then distilled water .
c. The burette was washed with tap water, distilled water and finally rinsed with sodium carbonate solution.
d. Beaker, filter funnel, volumetric flask should be rinsed with distilled water .
* Beaker used to contain sulphuric acid, filter funnel for translating carbonate solution into burette should be dried completely lest the dilution of solution.
2 To make a standard solution of sodium carbonate
a. Weigh about 2.65g of sodium carbonate .A weighing bottle should be used to contain the weighed sodium carbonate. The sodium carbonate was weighed on an electronic balance.
b. Weigh the mass of weighing bottle first, then the weigh bottle with the sodium carbonate.
c. Distilled water was added in the weighing bottle to dissolve the sodium carbonate. A glass rod was used to stir for helping dissolve of sodium carbonate.
d. The solution was then transferred to the 250ml volumetric flask.
e. Distilled water was repeatedly poured into the bottle in order to dissolve all left particles.
f. When the graduated mark was nearly reached, a dropper should be used to add water into the volumetric flask to make an exact 250ml solution.
4 Preparation of 25 ml of sulphuric acid
a. Sulphuric acid was contained in the beaker.
b. The pipette filler was used to pull up the sulphuric acid through the pipette to the graduated mark.
c. The 25 ml of solution was released in the conical flask. ( when the solution stopped dripping out, touched the bottom of the titled conical flask with the tip of pipetter for a few seconds.)
d. Few drops (<5) of methyl orange is added in the conical flask .The solution will then become red.
5 Preparation of sodium carbonate solution in burette
a. The stopcock is opened to fill the part below the burette with solution.
b. After the stopcock is closed, solution is continued to fill in the burette and the initial reading is taken to 2 d.p.
6 +
7 Performing the titration
a. One trial and at least 3 consistent results should be obtained as a succeed experiment.
b. Operate the stopcock with the left hand and swirl the conical flask with the right hand.
c. For trial, open the stopcock until an obvious color change from red to yellow was observed.
d. After the color change, the level of solution was recorded and the added volume was found.
e. Based on the trial result, the volume of sodium carbonate solution added was estimated and use it in the first, second and third time.
f. Repeat step 4,5,6.
g. Starting the first time, the sodium carbonate solution should be added to the predicted level and the stopcock is then stopped. The solution should be orange in color now.
h. The solution is then added drop by drop now. After adding each drop of solution, the level should be recorded until the color turns to yellow.
i. The resultant volume should be recorded just before the color change of yellow.
j. The steps f to I should be repeated for at least 3 times to obtain a consistent result.
Discussion:
The final result is deviated from the exact answer. This is due to several errors. For instance, the reading is the end point reading, it is not the equivalent point. In addition, the end point may be difficult to observe as changing from red to orange is not particularly clear. To tackle these problems, a pH meter can be used to find out the point instead of using indicators.
Besides, there may be some impurities in the sodium carbonate. Then, the incomplete washing and cleaning of apparatus may cause errors. For example, the beaker, pipette has not dried and contains some water inside. This will lead to the dilution of the acid. So, we should make sure that these apparatus are completely dried.
The volumetric flask may not wash and clean thoroughly with a brush or the brush is dirty itself, this will cause some water in flask stick on the glass. Consequently, the flask will not contain 250ml solution exactly. We should use some dishwashing liquid and a clean brush to clean the volumetric flask.
