Quantitative Analysis of Soda Ash by Double-Indicator Method Mark Steven R. Santiago and Kristiene B. Sadiwa Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101 Philippines Date/s Performed: July 13, 2012; Date Submitted: July 19, 2012 Results and Discussions A mixture of carbonate (CO32-), bicarbonate, (HCO32-) and hydroxide (OH-) ions can be analysed and determined by titration with strong standard acid solution. Volumetric titrimetry can be employed to compute percent compositions of sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and sodium hydroxide (NaOH) in a soda ash sample through the application of neutralization concepts and titrimetric analyses. Volumetric titrimetry has been utilized in such analysis because of its rapid, convenient, accurate and readily automated results. Some knowledge in acid-base equilibria, however, is needed to fully apply such method in determining composition of a mixture of substances of basically similar properties. In a soda ash, no more than two of the three components described previously can exist in substantial amount in any solution because reaction will remove one of them. In particular, a mixture of NaOH and NaHCO3 is incompatible as explained by the reaction posited below. Two titrations were needed for the analysis of such mixtures, one with the use of an alkaline range indicator and the other with an acid rang indicator. The composition of the solution can then be inferred from the relative volumes of the standard acid needed to titrate equal volumes of the sample. Once the composition of the solution has been established, the volume data can be used to determine the concentration of each component in the sample. Table 1 below shows the volume relationships in the analysis of mixtures containing hydroxide, carbonate, and bicarbonate ions. If the volume of the standard acid needed for a methyl orange end point is equal to zero, the sample only contains sodium hydroxide. If the volume of the standard acid needed for a phenolphthalein end point is equal to the volume of the standard acid needed for the methyl orange end point, the component in the sample is only sodium carbonate. If the volume of the standard acid needed for a phenolphthalein end point is equal to zero, the sample only contains sodium bicarbonate. If the volume of standard acid needed for a phenolphthalein endpoint is greater than the volume of the standard acid needed for a methyl orange end point, the components in the sample are sodium hydroxide and sodium carbonate. But if the volume of the standard acid needed for a phenolphthalein end point is lesser than the volume of the standard acid needed for a methyl orange end point, the components in the sample are sodium carbonate and sodium bicarbonate.
As seen, the mixture of hydroxide and bicarbonate ions will just produce a carbonate ion. Essentially, a soda ash containing the mixture of NaOH and NaHCO3 is simply a soda ash containing Na2CO3.
Table 1. Volume Relationships in the Analysis of Soda Ash of Unknown Composition Component(s) of the Sample NaOH Na2CO3 NaHCO3 NaOH and Na2CO3 Na2CO3 and NaHCO3 Relationship between VHPH and VHMO in the titration of an Equal Volume of Sample VHMO = 0 VHPH = VHMO VHPH = 0 VHPH > VHMO VHPH < VHMO 1
In the table, VHPH is the amount of the standard acid needed to reach the phenolphthalein endpoint; similarly, VHMO is the amount of the standard acid needed to reach the methyl orange endpoint. Before titration can be successfully employed, standardization of the acid to be used as the titrant is accomplished, thus calling the titrant as a standard acid after it has been standardized. The 0.05 M HCl has been standardized by titrating 0.1000 grams Na2CO3. The carbonate salt acted as the primary standard in the standardization procedure. A primary standard is a highly purified compound that functions as a reference material in all volumetric titrimetric methods. Some...
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