Determine the percentage purity of commercial sodium carbonate, you are provided with 0.05 N HCL.
Report the following:
i. detailed procedure.
ii. Method to prepare 0.05 N HCL solution.
iii. Detailed calculations with supposed/hypothetical values.
iv. Discuss results
Answers
Answer:
Last updatedJun 30, 2020
Analytical Chemistry Labs
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Contributed by Oliver Seely
Professor Emeritus (Chemistry) at California State University Dominguez Hills
Crude sodium carbonate, Na2CO3, is commonly called soda ash. It is frequently used as a commercial neutralizing agent. Besides the carbonate small amounts of sodium hydroxide, NaOH, and sodium hydrogen carbonate, NaHCO3, may also be present. Titrating with standard acid, usually HCl, makes it possible to determine the total alkalinity of the soda ash. It is common practice to report the total alkalinity as percent sodium carbonate or sodium oxide, Na2O. Since samples are frequently non-homogeneous, the method of aliquot portions is usually employed. Instead of weighing out three separate samples of soda ash, one accurately weighs out a larger amount. This is then transferred into a volumetric flask, dissolved in water and then diluted to an accurately known volume. From this solution are then taken samples or aliquots on which the titration is carried out.
DISCUSSION
The titration involved in the determination of the carbonate content is an example of a weak base being titrated with a strong acid. The two weak bases involved are CO2−3 with Kb1=2.1×104 and HCO−3 with Kb2=2.4×10−8 . The reactions involved are:
CO2−3+H3O+⇌HCO−3+H2O(1)
and
HCO2−3+H3O+⇌H2CO3+H2O(2)
The equivalence point pH for reaction 1 occurs at a pH of about 8.3, hence a suitable and commonly used indicator is phenolphthalein. The equivalence point for reaction 2 occurs at a pH of roughly 4.0. Indicators that have been used are methyl red, methyl orange, methyl purple and bromocresol green. Bromocresol green will be used in the analysis that you will perform. At the beginning of the titration, CO32- exists to the practical exclusion of the other carbonate species. When one equivalent of acid has been added, almost all of the CO32- has been changed into HCO3-. Addition of a further equivalent of acid changes practically all hydrogen carbonate into carbonic acid, H2CO3. The latter is in equilibrium with water and CO2. The steep portions of the titration curve near the two equivalence points are not so steep and do not extend over so large a pH range as is required for a titration accuracy of 0.1 relative percent.
Near the HCO3- equivalence point pH of 8.3 the change in pH caused by adding 1.0 mL of acid is only about 0.3 units and 10 mL are needed for a pH change of 1 unit. The situation near the second equivalence point at pH 4.0 is somewhat more favorable. About 4 mL of acid are needed for a pH change of 1 unit. The accuracy of the titration can be improved considerably by removal of the CO2 just before the second equivalence point has been reached. An accuracy of better than 0.1 relative percent may be obtained by the experimental procedure outlined below.