An Acid And Base Titration Biology Essay

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Most food or household products contain chemicals, and some of them are able to destroy things and harm peoples health. The aim for this experiment is calculate the concentration of the acid and base using titration. The end point of a reaction should be observed after chemical solutions were mixed in a conical flask.

An acid is a substance which can dissolve in water solution, and changes it is colour from colourless to red after litmus was added into it. Moreover, acid can react with metal and yield hydrogen gas, and react with bases to yield salts (Answers, 2010). An Acid can donating proton in chemical reaction, such as, HCl. A base can act as proton receivers that turns litmus blue, and yield salts after it react with acids. In addition, it can yield hydroxyl ions (OH-) when it dissolves in water (Answers, 2010).

Phenolphthalein indicator (molecular equation: C20H14O4, figure 1) is a weak acid which is a common indicator for titration. The pH scale which phenolphthalein can change colour is from 8.2-10.0, thus, phenolphthalein can only indicates alkaline as red or pink colour (Baike. Baidu, 2010). According to Lister et al (2000), the phenolphthalein is acidic and needs to produce H+ to change colour.

Figure 1 The Structure of Phenolphthalein (T4j, 2010)

Starch, is a natural nutrient carbohydrate, which molecular equation is (C6H10O5) n, composed of a great deal of glucose monosaccharide units, and occurs in most seeds (Answer, 2010). In this experiment, starch was an indicator for observed iodine, and produced blue-black colour.

Syntel Laboratory Ltd (2008) suggests that, Sodium thiosulphate (Na2S2O3) is used for neutralization of iodine solution, which the chemical reaction is

2Na2S2O3 (aq) + I2 (s) â†' Na2S4O6 (aq) + 2NaI (s)

Bleach, is a common houseware with the main component being calcium hypochlorite (Ca(ClO)2) and calcium chloride (CaCl2). Bleach is a kind of base.

Vinegar (figure 2) is a common seasoning in daily life. Lane (2010) stated that, contains ethanoic acid (also called acetic acid). The molecular formula of vinegar is C2H4O2.

Figure 2 The Structure of Vinegar (Chemistry, 2010)

According to Answers (2010), Sodium hydroxide, which chemical equation is NaOH, is a white crystalline substance absorbs carbon dioxide (CO2) and water easily from the air. The chemical equation of reacts with carbon dioxide is:

2NaOH (aq) + CO2 (l) â†' Na2CO3 (aq) + H2O (l)

The chemical equation of reacts with water is:

NaOH (aq) â†' Na+ (aq) + OH- (aq)

Sodium hydroxide is a strong base and able to destroy or dissolve other substance easily.

Method

Chemicals:

Vinegar, bleach, NaOH (0.1M), Phenolphthalein Indicator, sodium thiosulphate solution (0.1M), Potassium Iodine (1M), Sulphuric Acid (dilute) and Starch Indicator.

Equipment:

Test tube x 6, Funnel x 7, Beaker x 7, Burette x 4, conical flask x 4, pipette and pipette filler.

Part A

A little NaOH (0.1M) and a little vinegar was put into 2 test tubes used a dropping pipette. 1 drop of phenolphthalein indicator solution was dropped into each test tube, the solutions were observed.

A funnel was put in the top of a burette and NaOH (0.1M) was poured. The NaOH bottle was placed under the burette and the NaOH was poured back into the bottle until the burette level was 0.0cm3. The exact initial burette reading was written down.

25 cm3 of vinegar was poured into the small beaker. 2.5cm3 vinegar was put into the conical flask using a 5cm graduated pipette, the value was written down. 20cm3 of water was poured into the conical flask. 4 drops of phenolphthalein indicator solution was added.

The conical flask was put under the burette and some white paper put under the flask to improve visibility. The NaOH (0.1M) solution was added from the burette into the flask, which was shaken continuously to mix the solutions. 1 minute later, some colour was changed and was observation disappeared quickly as the end point was neared. NaOH (0.1M) was added slowly. 1 drop was added at a time when the end point was appeared. The final burette reading was written down. The steps were repeated three times

B.

The burette was washed and filled with sodium thiosulphate solution used a funnel. The reading was recorded. The exact reading was written down. 2cm3 pipette of bleach solution was filled by pipette, and 1cm3 of bleach was transferred. The exact value transferred was written down. 10cm3 of the 1M potassium iodine solution and 10cm3 of dilute sulphuric acid was added to the conical flask, iodine was produced. Sodium thiosulphate solution from the burette was added to the flask, and was swirled continuously. A few drops of starch indicator solution were added just before the end point, the colour of the solution was monitored. The thiosulphate solution was added dropwise, with thorough swirling. Finally, the end point of the titration was detected when the blue-black colour changed to colourless. The final burette reading was written down. The steps were repeated once.

Results

Titrate of Vinegar

At the beginning of this experiment, after NaOH (0.1M) was added into the flask, a violet colour appeared, lasting about 2 seconds, and then disappeared. After 10 cm 3 of NaOH (0.1M) added into flask, the colour last longer, about 6 seconds, and then disappeared. Finally, when the burette reading became 15.8 cm 3, the solution inside flask became violet.

Trials

Volume/cm 3

Trial 1

Trial 2

Trial 3

Initial burette reading

0

6.0

4.0

Final burette reading

15.8

18.55

9.93

Volume of NaOH (aq) for neutralisation

15.8

12.55

5.93

Table 2 The data in Titration of Vinegar

Titration of bleach

At the beginning of this experiment, after sulphuric acid was added into the flask, no change was observed. Then, KI (0.1M) was added into the flask, the solution turned to brown-yellow colour. Starch indicator was added into flask, the solution turned to blue-black colour from brown-yellow colour. Finally, as the thiosulphate solution was mixed with the solution, the solution faded to pale yellow. After the end point was occurred, the solution was faded to colourless.

Trials

Volume/ cm 3

Trial 1

Trial 2

Initial burette reading

0

4

Final burette reading

3.2

7.7

Volume of NaS2O3

3.2

3.7

Table 4 Titration of Bleach

Discussion

A.

The calculation of the concentration of ethanoic acid:

mole of NaOH = mole of CH3COOH

c (NaOH) x v (18.55 - 6) cm3

0.1mole/l x 12.55 x 10-3dm3

c (CH3COOH) = n (CH3COOH) / v (CH3COOH)

= 1.255 x 10-3 / 0.25 x 10-3

= 5.02 mol dm-3

(c means concentration, v means volume)

According to the data from result, the volume of NaOH (0.1M) for neutralisation in trial 1 is 15.8 cm3, the data is trial 2 is 12.55 cm3, in trial 3 is 5.93; the data in these trials was declines gradually. The reason may be because there are some chemical solutions remained in the flask, and influenced the data.

B.

The sodium hypochlorite:

mole of S2O3-

3.4 cm3 x 0.1 mol/l x 10-3

= 3.4 x 10-4 mol

∴ mole of I2 = ClO-

3.4 x 10-4/2 = 1.7 x 10-4 mol

c = n /v

=1.7 x 10-4/1 x 10-3

=0.17 mol dm3

According to the process of trial 2, the concentration of trial 1 was calculated 0.16 mol dm3, and the concentration in trial was calculated by the previous calculation, which equally to 0.17 mol dm3. However, the normal concentration should be 0.5-0.8 mol dm3.

In addition, because of the wrong units transfer in these two experiments and the unclear conical flasks, these two factors caused the result was different from the standard value, especially in experiment B, the error had a large difference of the normal value. In next experiment, all units problems should be avoid and preview it before the experiment; however, the equipments which used in experiment should be clean carefully and by the running tap, which can avoid chemicals remain in the equipments.

Conclusion

In conclusion, these experiments show clearly that the states and the colour changes during these two experiment after indicator was added into the conical flask. Moreover, the concentration of these two experiments were calculated by c = n/v.

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