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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
Determination of Carbonate and Bicarbonate Mixtures
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.
Determination of Carbonate and Bicarbonate Mixtures
There are 5 possible combinations:
1. NaOH 2. Na2CO3 3. NaHCO3 4. Na2CO3 + NaOH 5. Na2CO3 + NaHCO3
Standardization of HCl
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:
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:
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?
Question
Do we really need to cool down the solution after
boiling?
Question What's the reason behind boiling the solution after the first equivalence point?
Question
What will happen if we do not use CO2- free distilled water?
Question
Is the ice bath important?
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.
Fin