Chem 27.1 Experiment 3 Determination of Mixed Alkali(1)

Experiment 3

Titration of Mixed Alkali

Objectives

Prepare a standard hydrochloric acid solution.

Determine the carbonate

and bicarbonate content in a given sample.

Introduction

Alkalimetry

Specialized analytic use of

acid-base titration to

determine the concentration

of a base

Determination of Carbonate and Bicarbonate Mixtures

oCO3

2-, HCO3- and OH- can be

titrated with a strong acid in many combinations with conditions:

1. HCO3

- and OH- cannot coexist in solution since they react

HCO3- + OH- -> CO3

2- + H2O


2. No more than two of these

three constituents can exist in any solution because one of the present species will react with another, resulting in CO3

2- and whatever is in excess in the solution.


There are 5 possible combinations:

1. NaOH 2. Na2CO3 3. NaHCO3 4. Na2CO3 + NaOH 5. Na2CO3 + NaHCO3

Standardization of HCl

Roan

Sticky Note

2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2CO3(aq) H2CO3(aq) → CO2(aq) + H2O(aq)

Table 1. Weight of Na2CO3 and volume of HCl used for the standardization of the HCl

Trial Weight of Na2CO3

(g) Volume of HCl

(L) Molarity of HCl

1 0.2586 0.0469 0.1040

2 0.3431 0.0593 0.1092

3 0.2001 0.0375 0.1007

4 0.2020 0.0352 0.1083

Mean - - 0.1056

Computation:

Analysis of Unknown

Table 2. Volume of HCl used to reach phenolphthalein end point

Trial

Volume of HCl (L)

Phenolphthalein end pt.

1 0.0190

2 0.0177

3 0.0222

Mean 0.0196

Computation:

Roan

Sticky Note

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

Table 3. Volume of HCl used to reach bromocresol green end point

Trial

Volume of HCl (L)

Bromocresol Green end pt.

1 0.0229

2 0.0218

3 0.0237

Mean 0.0228

Computation:

Roan

Sticky Note

HCl(aq) + NaHCO3(aq) → NaCl(aq) + CO2(aq) + H2O(l)

Table 3. Volume of HCl used to reach bromocresol green end point

Trial

Volume of HCl (L)

Bromocresol Green end pt.

1 0.0229

2 0.0218

3 0.0237

Mean 0.0228

Computation:

Question

Why did we use phenolphthalein and bromocresol green as

indicators?

Roan

Sticky Note

The indicator transition range fall within the equivalence point (pH break) Sodium carbonate can be titrated to HCO3- using phenolphthalein or to carbonic acid using methyl orange, methyl red or bromocresol green

Question

Do we really need to cool down the solution after

boiling?

Roan

Sticky Note

Δ temperature → Δ equilibrium constant →Δ equivalence point

Question What's the reason behind boiling the solution after the first equivalence point?

Roan

Sticky Note

To enhance the sharpness of pH break

Roan

Sticky Note

Marked set by Roan

Question

What will happen if we do not use CO2- free distilled water?

Roan

Sticky Note

CO2 dissolved in water undergoes a reaction that produces carbonate. Additional CO2 would give a positive error to the final amount of CO32- obtained.

Question

Is the ice bath important?

Roan

Sticky Note

CO2 is soluble in lower temperature and is part of the carbonate system. Loss of CO2 will decrease the amount of carbonates → negative error

Discussion

Carbonate, bicarbonate, and hydroxide constitute a carbonate mixture but a maximum of two can only exist at the same time as the other one will be consumed completely

Constituen(s) in sample Volumes of Titrant

NaOH V0->phth = V0->bcg > 0

Na2CO3 V0->phth = Vphth->bcg

NaHCO3 V0->phth = 0 ; Vphth->bcg > 0

NaOH + Na2CO3 V0->phth > Vphth->bcg

Na2CO3 + NaHCO3 V0->phth < Vphth->bcg

Table 4. Five possible combinations of sample

Figure 1. Titration curve when sample is only NaOH

Figure 2. Titration curve when sample is only Na2CO3

Figure 3. Titration curve when sample is only NaHCO3

Figure 4. Titration curve when sample is NaOH and Na2CO3

Figure 5. Titration curve when sample is Na2CO3 and NaHCO3

Conclusion

Alkalimetry is the determination of the quantity of real alkaline in alkali solutions or salts, either through gravimetric or volumetric analysis.

Observe proper laboratory procedures and techniques

Brown, T., Lemay, E. & Bursten, B. (2003). Chemistry: The central science 9th ed. Singapore: Pearson Education, Inc.

Chang, R. (2005). Chemistry 8th ed. Singapore:

McGraw Hill Inc. Hargis, L. G. (1988). Analytical chemistry: principles

and techniques. NJ, USA: Prentice Hall. Skoog, D., West, D., Holler, J. & Crouch, S. (2010).

Fundamentals of analytical chemistry 8th ed. Philippines: Cengage Learning Asia Pte Ltd.

Chem 27.1 Experiment 3 Determination of Mixed Alkali(1)

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