Post Lab 4 .docx - 1 Section 2 Student Name Sydney Saunders...

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1 Section: 2 Student Name: Sydney Saunders TA: Sinduri Vuppala Experiment: Volumetric Titration of Impure Sodium Carbonate (Na 2 CO 3 ) Introduction Titrations within chemistry are characterized by slow addition of a solution to a titrant of a known concentration (“Titration”). Titrations can be used to determine the concentration of the solution of interest. The titration is continued until the reaction becomes neutralized. The point of neutralization is visualized by a color change, from the addition of an indicator. Whilst titrating, the titrant used must be a primary or secondary standard. Neutralization reactions are described as a reaction in which an acid and base react within an aqueous solution; thus producing a salt and water (“Acid-Base Neutralization”). Hydrochloric acid (HCl) is used to reduce the alkalinity of a solution until the pH is near 7(Deziel). The reaction proceeds through two stages, in which HCl reacts with sodium carbonate (Na 2 CO 3 ), to produce sodium bicarbonate (NaHCO 3 ). This reaction is modeled by Na 2 CO 3(aq) + HCl (aq) NaHCO 3(aq) + NaCl (aq). Within the second reaction, the sodium hydrogen reacts with HCl to produce CO 2 . This reaction is modeled by NaHCO 3(aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2(g) . The overall reaction is modeled by 2HCl +Na 2 CO 3 2NaCl + H 2 O + CO 2 . Within the titration there are 2 equivalence points, at a pH of 8.3 and a pH of 3.7. To identify how much HCl is needed to neutralize the base, the endpoints within the reaction are visualized by the change of color after the addition of two different indicators. The indicator phenolphthalein used is colorless within basic environments in the pH ranges less than 8.3. Phenolphthalein may be used as an indicator for pH values within the range of 8.3-10. The addition of the indicator bromocresol green, causes a color change within the lower pH range of 3.8-5.5, thus identifying the final step of the
reaction. The color change of bromocresol green is characterized by a color change from blue to green; thereby signaling the pre-mature endpoint. The solution is boiled and cooled to remove CO 2 , and titrated to the final endpoint.

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