Impure Sodium Carbonate
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
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