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titration acid-base - EXPERIMENT NO 5 Determination of...

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EXPERIMENT NO. 5 Determination of Carbonate in Soda Ash by Titration with Hydrochloric Acid Nuyda, J.B. Chemistry Department, College of Science, Adamson University, Ermita, Manila 1000 Philippines Keywords: acid-base titration, back titration, acid/base ratio Results and Discussion Acid-base titrations are widely used methods of quantitative analysis in Chemistry as well as in many other fields such as biology, medical technology and pharmacy. These methods are accurate and convenient due to the fact that acid- base reactions have stoichiometric relationships and go to 99.9% completion (Schenk et al., 1981). Experiment 5, the “Determination of Carbonate in Soda Ash by Titration with Hydrochloric Acid” is an example of composition analysis that makes use of the principles of acid-base titration. Soda ash is a white crystalline solid primarily used for the manufacture of glass, paper and detergents, among others. It is also used to regulate the pH of some chemical systems (Mineral Information Institute, 2012). Sodium carbonate, Na 2 CO 3 , acts as a base and can therefore be titrated with an strong acid such as hydrochloric acid (HCl). The stepwise neutralization of Na 2 CO 3 with HCl is given by the following reactions: H 3 O + (aq) + CO 3 2- (aq) HCO 3 - (aq) + H 2 O (l) (1) H 3 O + (aq) + HCO3 - (aq) H 2 CO 3(aq) + H 2 O (l) (2) Reaction (1) has an equilibrium constant value (K b ) of 2.1x10 -4 while (2) has a K b of 2.3x10 -8 (de Levie, 1997). A standardized solution of HCl was used as titrant and a few drops of bromocresol green as indicator. Bromocresol green is blue in alkaline conditions and yellow in acidic conditions. It has a pH range of 3.8-5.4 (de Levie, 1997). Standardization was performed using Na 2 CO 3 as primary standard. A second titrant, sodium hydroxide (NaOH), was used for back- tritration, a technique that brings back a titrated solution to its end point in case an excess of the original titrant was added (Waser, 1964). Instead of standardizing the NaOH solution, which takes considerable time, its volume ratio with the standardized acid was taken. To determine the acid/base ratio, the standardized HCl solution was titrated with NaOH, using phenolphthalein as indicator. Acid/base ratio is used to determine how much acid (in the case of the experiment) has been added in excess without the need of knowing the precise concentration of the base solution. This ratio is helpful in case overtitration—addition of excess titrant— is incurred. In the experiment, three replicate samples of soda ash of unknown
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Na 2 CO 3 concentration, labeled Trials 1, 2 and 3 and weighing 0.1679g, 0.1656g and 0.1518g, respectively, were titrated with a standardized HCl solution. The HCl solution was standardized by titrating three solutions of known Na 2 CO 3 concentration. Using the equation below, M HCl = (3) where M HCl = molarity of HCl (mol/Liter) mol Na = mol Na 2 CO 3 m Na = mass of Na 2 CO 3 (grams) MW Na = molecular weight of Na 2 CO 3 (grams/mole)
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