Objective: The purpose of this experiment is to determine the composition of a mixture of solutions of NaOH and Na2CO3 by double indicator method.

Experiment 4: Acid-base Titration using Method of Double Indicators

Date: 2010/10/7

I. Objective: The purpose of this experiment is to determine the composition of a mixture of solutions of NaOH and Na2CO3 by double indicator method.

II. Introduction:

Consider a mixture of NaOH(aq) and Na2CO3(aq). Reaction between HCl(aq) and

Na2CO3(aq) takes place in two stages:

HCl(aq) + Na2CO3(aq) → NaHCO3(aq) + H2O(l) -------------------------(1)

HCl(aq) + NaHCO3(aq) → NaCl(aq) + CO2(g) + H2O(l) -----------------(2)

While that between HCl(aq) and NaOH(aq) completes in only one step:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) -----------------------------(3)

The hydrochloric acid is placed in the in the burette and is added, slowly and with constant swirling, to the mixture of NaOH(aq) and Na2CO3(aq).

Solution mixture of reaction (1) at the equivalence point is alkaline, while that of reaction (2) is acidic and that of reaction (3) is neutral. Therefore the whole titration should have three breaks in the pH curve, corresponding to the above three stages. Reactions (1) and (3) can be indicated by phenolphthalein and that of reaction (2) can be indicated by methyl orange.

III. Procedure:

1. All apparatus should be rinsed properly with corresponding solution

2. 25cm3 of the mixture of NaOH(aq) and Na2CO3(aq) was pipetted into conical flask.

3. Burette was filled with standardized 0.1M hydrochloric acid

4. 2 drops of phenolphthalein were added to the mixture.

5. Initial reading was recorded and the burette was run until colour of the solution changes from pink to colourless.

6. Final reading was recorded and 3 drops of methyl orange were added.

7. Burette was run again until colour of the solution changes from yellow to red.

8. Final reading was recorded and composition ...

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III. Procedure:

1. All apparatus should be rinsed properly with corresponding solution

2. 25cm3 of the mixture of NaOH(aq) and Na2CO3(aq) was pipetted into conical flask.

3. Burette was filled with standardized 0.1M hydrochloric acid

4. 2 drops of phenolphthalein were added to the mixture.

5. Initial reading was recorded and the burette was run until colour of the solution changes from pink to colourless.

6. Final reading was recorded and 3 drops of methyl orange were added.

7. Burette was run again until colour of the solution changes from yellow to red.

8. Final reading was recorded and composition of the mixture of NaOH(aq) and Na2CO3(aq) could be calculated.

IV. Data, Calculation and Result

Phenophthalein indicator:

Mean titre = (25.00+24.95+24.95+25.00)/4

= 24.98 cm3

Methyl orange indicator:

Mean titre = (9.85+9.95+9.90+9.90)/4

= 9.90 cm3

Number of mole of HCl(Methyl orange indicator)= 9.90/1000X0.1

= 9.9X10-4mol

Number of mole of NaHCO3= 9.9X10-4mol

Number of mole of Na2CO3= 9.9X10-4mol

Mass of Na2CO3= 9.9X10-4 X(23X2+12+16X3) = 0.105g

Number of mole of HCl(Phenophthalein indicator)=24.98/1000X0.1

=2.498X10-3mol

Number of mole of the mixture of NaOH and Na2CO3=2.498X10-3 mol

Number of mole of NaOH=2.498X10-3 - 9.9X10-4

=1.508 X10-3 mol

Mass of NaOH=1.508 X10-3X(23+16+1)=0.0603g

Mass ratio of Na2CO3 to NaOH=0.105:0.0603 =7:4

Therefore, the mixture contains approximately 64% Na2CO3 and 36% NaOH

V. Discussion

The mean titre with the use of phenophthalein indicator is 24.98 cm3 while the mean titre with the use of methyl orange indicator is 9.90 cm3 and the mixture contains approximately 64% Na2CO3 and 36% NaOH.

There are some sources of errors:
(1) Inconsistent reading of the burettes

(2) General errors associated with titration (overshooting, arithmetic errors)

(3) Methyl orange was added when the solution didn’t become totally colourless.

Improvement:

(1) Great care is needed during the addition of methyl orange.

(2) Great care is needed during titration to avoid using too much titre as the end point is not very sharp.

VI. Questions

1. HCl is not a primary standard and need to be standardized. Suggest a primary standard for the present experiment and explain briefly how to perform the standardization.

Ans: Anhydrous sodium carbonate is a suitable chemical for preparing a standard solution (as a primary standard). The molarity of the given hydrochloric acid can be found by titrating it against the standard sodium carbonate solution prepared. The end-point is marked by using methyl orange as indicator.

2. Write down the equation for the reaction involved before titration in the present experiment.

Ans: 2NaOH +CO2 →Na2CO3

3. Sketch a pH vs volume of HCl curve, indicate the stages of colour change of the indicators.

4. Define the equivalence point and the end point of a titration. Why does one choose an indicator so that the two points coincide?

Ans: Equivalence point is the point at which titrant has been added to react exactly with the substance being determined while end point is the point at which the indicator changes colour. Choosing an indicator that the two points coincide helps to minimize titration error.

5. Suggest another way to detect the equivalence point without the use of indicator.

Ans: pH meter.

VII. Conclusion

The mixture is composed of approximately 64% Na2CO3 and 36% NaOH