Objecctives: To demonstrate acid base titration using different indicators.

To learn to calculate molarity based on titrations.

To sketch the pH curve when strong acid react with strong alkali.

Introduction: Titration is the quantitative measurement of an analyte in solution by reacting it

completely with a reagent solution (titrant). The point at which all of the analyte is

consumed by titrant is called the equivalence point. For titration of a strong acid with a

strong base, the equivalence point occurs at a pH of 7. Titration is also a procedure

used in chemistry in order to determine the molarity of an acid or a base. A chemical

reaction is set up between a known volume of a solution with an unknown concentration

and a known volume of a solution with a known concentration. A solution of known

concentration used in a titration is called a standard solution. From the known quantity

and molarity of the standard solution and the measured volume of unknown solution

used, the unknown concentration can be calculated by using the formula:

Ma = molarity of acid

Va = volume of acid

Mb = molarity of base

Vb = volume of base

ab = Ratio of no. of mole of acid to no. of mole of base

Indicators are weak organic acids or bases that are different colors in their

dissociated and undissociated states. Indicators do not change colour sharply at one

particular pH. Instead, they change over a narrow range of pH. As they are used in low

concentrations, indicators do not appreciably alter the equivalence point of a titration.

The indicator changes color when the end-point has been reached which is at the point

of neutralization (complete reaction). For a properly performed titration, the volume

difference between the end point and the equivalence point is small. The difference is

called titration error and will always be ignored if a suitable indicator is used.

MaV aMbV b = ab

In this experiment, an unknown concentration of HCl acts as a titrant while 25mL

0.1M of NaOH acts as an analyte. An indicator is added into the analyte before titration.

The HCl solution is slowly added from the burette until the mixture in the Erlenmeyer

flask changes colour. The 3 indicators used in this experiment are phenolphthalein,

methyl red and bromocresol green. The volume of HCl is measured and the molarity of

HCl is calculated. The titration curve is drawn as well.

Apparatus & materials:Retort stand and clamp, measuring cylinder, beakers, distilled water, unknown

concentration hydrochloric acid solution, 0.1M NaOH, burette, Erlenmeyer flask,

Indicators: Methyl red, phenolphthalein and bromocresol green.

Procedures:

1) The apparatus were rinsed with distilled water before experiment and then set up as

diagram above.

2) 0.1M NaOH solution was added to a 250mL Erlenmeyer flask.

3) 25.00mL of a hydrochloric acid solution of unknown concentration might be added to

the burette, which was set up over the Erlenmeyer flask containing the 0.1M NaOH

solution.

4) 3 drops phenolphthalein, acid base indicator, was added to the 0.1M NaOH solution

in the Erlenmeyer flask. The colour change of the solution was observed and

recorded. The color that supposed to be changed was shown in table 1.

Table 1: Acid, base amd endpoint color different pH indicator

Indicator Color pH Rangeacidic endpoint basic

bromocresol green yellow Green blue 4.0-5.6

methyl red red Yellow yellow 4.4-6.2

phenolpthalein colorless light pink red 8.0-10

5) Hydrochloric solution was slowly added from the burette until the mixture in the

Erlenmeyer flask changes colour. The change was observed and recorded.

6) Steps 1-5 was repeated with other two indicators bromocresol green and methyl red.

Results:

Volume of

hydrochloric used

(mL)

Indicator

Phenolpthalein Methyl red Bromocresol green

Initial reading 0 0 8.6

Final reading 8.6 8.6 17.3

Total volume used 8.6 8.6 8.6

Volume of NaOH used 25 25 25

Colour change Pink

Colourless

Yellow

Dark orange

Light blue

Dirty green

Calculation:

1) MaV aMbV b =

ab

From the chemical equation in question 2, the ratio of number of mole of acid to

number of mole of base is 11 . Thus,

MaVa= MbVb

Molarity of HCl, Ma = (MbVb)/ Va

= (0.1 x 25)/ 8.6

= 0.2907 mol

2) Balance equation of the neutralization process of acid base titration:

HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

3) Graph of pH vs titrant:

4) The reasons for using different indicators in this experiment is because the end point

of acid base titration will have different pH values. For instance, strong acid reacts with

strong base will have a pH of about 7, strong acid reacts with weak base will have a low

pH which is acidic, weak acid reacts with strong base will have a high pH which is

alkaline. An indicator is considered suitable only if it undergoes a change of colour at

the pH near the end point.

Discussion: From the result, it shows that the volume of HCl required to neutralize the NaOH

is constant that is 8.6mL. Hence, the result is considered accurate as the volume of HCl

required is not affected by the different indicators used. The only inaccuracy is the

colour change of the indicators. The colour of indicators at the end point from the result

is different from the actual endpoint colour shown in table 1 due to some errors.

Discussing on the pH vs titrant graph, when a solution of HCl was added to the

solution of NaOH, the pH progressively decreases. This is because OH- ions from the

8.6 mL

Phenolphthalein

Methyl red

Bromocresol green

base will react with H+ ions of the acid to form water. This decreases the concentration

of OH- ions and therefore pH decreases. The pH of the solution decreases only slightly

in the beginning until just before the equivalence point, there is a sudden fall through pH

7 as the concentration of H+ ions increases sharply. The pH continues to decrease after

the stoichiometric point but then levels off because of the presence of excess strong

acid in the solution. At the equivalence point the pH is 7, but has decreased sharply

from 11 to 3.5 just before this point. Any indicator, which has effective range between

pH 3.5 to 11 may be used to detect the equivalence point. Hence, phenolphthalein,

methyl red and bromocresol green are good indicators in this range and this is the

reason that all of these indicators show their colour change.

Experimental errors:In this experiment, there are some mistakes like use more than acid volume to titrate

solution of NaOH. The volume had passed the end point, so the volume uses is much

more than needed. So, the colour of phenolphthalein had changed to colourless instead

of light pink and the colour of methyl red had changed to dark orange instead of yellow

when it reached the end point.

Another error is the using of beaker that containing HCl previously to fill NaOH. This will

affect the pH of the mixture.

Precaution steps:Titrate the HCl slowly and shake the Erlenmeyer flask for about 30 seconds when the

solution show changing in colour.

Make sure all the apparatus are clean by rinsing them with distilled water before use.

Ensure that there is no bubbles trap at the tip of the burette during the filling of HCl.

Avoid from make a parallax error when taking the reading of the burette.

Conclusion:In conclusion, 25mL of 0.1M of NaOH requires 8.6mL of 0.2907M of HCl to reach its

endpoint where all of the indicators (phenolphthalein, methyl red, bromocresol green)

show colour change.

References: Anne Marie Helmenstine, Ph.D. , n.d.,Titration basics.[Online]. Available from: <

http://chemistry.about.com/od/acidsbases/a/aa082304a.htm> Accessed on 2

June 2013.

Lawrence Kok, 2011, Acid Base Titration, pH Titration curves and Indicators.

[Online] Available from: < http://lawrencekok.blogspot.com/2011/11/ib-chemistry-

on-acid-base-titration-ph.html> Accessed on 2 June 2013.

n.a. , n.d.,Acid base titration.[Online]. Available from: < http://chem-

guide.blogspot.com/2010/04/acid-base-titration.html> Accessed on 2 June 2013.