Lab 4. Volumetric Determination of Impure Sodium Carbonate (Na2CO3) Introduction: To determine the total amount of carbonate in unrefined sodium carbonate, soda ash, a titration is done using a standardized solution of HCl. Aqueous HCl is a strong acid and therefore almost completely disassociates into H+ and CL-. Therefore, when HCl is used in a titration, the H+ is the titrant. Carbonate in aqueous solution is able to accept a proton, i.e. it acts as a base. When carbonate accepts the H+ a bicarbonate ion is formed. Na2CO3(aq) + HCl(aq) NaHCO3(aq) + NaCl(aq)
This is not the complete reaction for the titration because bicarbonate is able to accept one more proton. This reaction produces carbonic acid which decomposes to sodium chloride, water and carbon dioxide. NaHCO3(aq) + HCl(aq) NaCl(aq) + H2O(l) + CO2(g) These two reactions equate to a 2 to 1 ratio of HCL to Na2CO3 which is expressed in the equation below. 2HCl + Na2CO3 2NaCl + H2O + CO2
The first two reactions are indicative of the two equivalence points of this reaction. Therefore, two indicators are needed to visualize the end-points. The first indicator is phenolphthalein which will turn from the pink ionic form while in a base, to a colorless form indicating the first end-point in this experiment. At this point exactly one mole of HCl has been added per mole of carbonate. This reaction of phenolphthalein occurs from pH 10 to 8.3 which is within 1 pH of the equivalence point for the carbonate to bicarbonate reaction. The second reaction has an equivalence point at ~ pH 3.7. This is where sodium bicarbonate reacts with a proton to produce an excess amount of CO2 very quickly. To visualize this, an indicator that changes color within 1 pH range of the equivalence point is used. For this experiment bromocresol green (BCG), which changes from blue to green when an end-point is reached within the pH range of 5.5 to 3.8. BCG will turn from green to yellow at pH’s below 3.8 which is well below the end-point of the bicarbonate to CO2 reaction. On a titration curve these two end-points would be where the greatest negative rate change in pH occurs on the graph. Because bicarbonate releases CO2 quickly when approaching the second end-point a premature end-point is visualized in BCG, requiring the mixture to be boiled to release this CO2 and allow for the actual end-point to be visualized. Hypothesis: I propose that by knowing the amount, in weight, of soda ash titrated and the amount, in volume, of standardized HCl used the percent of carbonate in unknown 253 can be determined. By increasing the amount of soda ash used the amount of Na2CO3 will increase requiring the standardized HCl to increase along with it. This dependent increase creates a ratio and subsequently the mass percent of carbonate. The independent variable is the mass of the unrefined soda ash and the dependent variable is the mass of the sodium carbonate. The controls used in the experiment are the indicators used, the temperature of the solution and the pressure. Experimental: HCl that was previously standardized to 0.12 M was obtained. Soda ash was dried appropriately two hours before that lab started. A clean and dry weighing bottle was used to carry the unknown soda ash. The total amount of carbonate in the unknown soda ash was determined by titration. A buret was rinsed with DI water and then 5 – 10 mL of HCl solution. The buret was filled with HCl. The weigh-by-difference method was used to obtain 0.2 to 0.25 g of soda ash and quantitatively transferred to a 250 mL Erlenmeyer flask. The soda ash was dissolved in 25 mL of DI water and three drops of phenolphthalein indicator was added. The mixture was titrated until the solution was colorless. Then, three drops of bromocresol green indicator was added to the solution. The titration was continued until the premature endpoint was reached. At this point the solution was boiled and then cooled to room temperature. The...
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