Submitted: February 27, 2013
Department of Chemical Engineering, Faculty of Engineering
University of Santo Tomas
A standard acid solution like HCl can be used as titrant for the analysis of both soda ash and a carbonate-bicarbonate mixture. In the analysis of soda ash, the volume needed to neutralize the soda ash is used to compute for its alkalinity, in this experiment we obtained a 17.6 % alkalinity with an error of 15.14% In the analysis of a carbonate-bicarbonate mixture two indicators (phenolphthalein and methyl orange) were used. The first endpoint determines the half-neutralization of the carbonate and the second determines that of the bicarbonate. The experiment results to an analysis of 4.92% carbonate and 5.07% bicarbonate content in the unknown sample.
Soda ash or sodium carbonate is a sodium salt of carbonic acid. It is commonly known for its everyday use as water softener. It can be naturally extracted from plants of synthetically produced from large amounts of sodium. It may contain small to moderate amounts of chlorides and hydroxides as impurities. The hydroxide present in sodium carbonate reacts with an acid titrant like HCl and its total alkaline strength is increased. Titration of soda ash with a standard acid solution with methyl orange as indicator, neutralizes its carbonate ions. The usual endpoint of this titration is at pH 4.
A standard HCl can also be used to neutralize a carbonate-bicarbonate mixture through the use of the double indicator method. A bicarbonate is an immediate form of deprotonation of carbonic acid while carbonates as mentioned above is a salt from carbonic acid. In the analysis of a carbonate- mixture, there are two endpoints, first is the phenolphthalein endpoint which is the volume needed to half-neutralize the carbonate content. The second endpoint uses another indicator, the methyl orange, when the solution turns orange in color, this endpoint determines the volume at which the bicarbonate was neutralized. The usual endpoint of the phenolphthalein is around a pH of 8.3 while the methyl orange endpoint is usually at a pH of 4.
Chemicals and Reagents
All chemical reagents used in this experiment (Na2CO3, standardized NaOH solution, standardized HCl solution, unknown carbonate-bicarbonate solid, Phenolphthaelin and methyl orange indicators) were provided inside the laboratory. The buret and other apparatuses used in the titration process were also provided inside the lab. A pH meter was also made available inside the balance room for the analysis of the carbonate-bicarbonate mixture.
Preparation and Standardization of HCl Solution
Approximately 500 mL of distilled water was placed in a beaker and 4.2 mL of concentrated HCl was allowed to flow slowly along the wall of the beaker. The solution was then kept in a labelled storage bottle. Two burets were prepared, the first buret was used to transfer 20 ml of the standard NaOH solution from experiment 3 into a dry beaker, the other buret was filled with the prepared acid solution and was set-up for titration. 3 drops of methyl orange was added to the beaker containing the NaOH solution. The base was titrated with the HCl solution until the formation of an orange-colored solution. The molarity of the acid was computed and resulted to an average molarity of 0.0727 M HCl. The standardized HCl solution was used as titrant for both the analysis of the soda ash and the analysis of the carbonate-bicarbonate mixture.
Determination of Alkalinity of the Soda Ash
The unknown soda ash sample was weighed from 0.3000-0.6000 g. The sample was then dissolved in 150 mL distilled water and was transferred into a 250-mL volumetric flask. Distilled water was added quantitatively until the solution reached the blue mark of the flask. The contents were mixed thoroughly by inverting the flask...