analytical chemistry lab 4 report - Lab 4 Volumetric...

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Lab 4: Volumetric Determination of Impure Sodium Carbonate (Na 2 CO 3 ) Introduction A titration involves a reagent solution, also known as a titrant, being added to an analyte until the reaction is complete. Titration is one technique used in volumetric analysis to determine the concentrations of acids and bases. When sodium carbonate (Na 2 CO 3 ) is titrated it with a strong acid, such as hydrochloric acid (HCl) the titration will have 2 equivalence points. The equivalence points “correspond to the adding of each successive proton to the base, and occur at pH 8.31 and pH 3.69.” (1) The equivalence points are observed using two separate indicators, phenolphthalein and bromocresol green. Phenolphthalein will have a color change in the pH range 8.0-9.6 and bromocresol green’s transition range is between pH 3.8-5.4. Phenolphthalein is added first and at the end point sodium carbonate becomes sodium bicarbonate (NaHCO 3 ). The solution is then heated to a boil and cooled so that the carbon dioxide in the solution is released. Bromocresol green is the second indicator used to detect the second equivalence point; at the end point all the carbonate is converted to H 2 CO 3 . The independent variables for this experiment are the indicators: phenolphthalein and bromocresol green. The volume of HCl added to the analyte is the dependent variable. Heating the solution to remove CO 2 from the sample is also a dependent variable. The control for this experiment would be the standard percentage of total sodium carbonate in soda ash. Titrating an acid will allow us to determine the unknown amount of base in a sample and vice versa. A

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