EXPT. 4: Quantitative Analysis of Soda Ash by Double-Indicator Titration

I. Soda Ash common name for sodium carbonate (Na2CO3) primarily used in glass-making

II. Volumetric Titration reaction between a standard solution and an unknown solution (analyte) to determine stoichiometric or equivalence point can be used to calculate for the amount of the analyte based on the volume of standard solution used

A. Standard solution strong acids or basesdue to pH changes of weak acids or bases near the equivalence ptcommonly used HCl or NaOH standardized using a primary standard- high purity, stability towards air, absence of hydrate water, ready availability, reasonable solubility, reasonably large molar mass

B. Acid-Base Titration based on neutralization reactions also called acidimetric or alkalimetric titration H3O+ + OH- 2H2O (1)

III. For the Experiment :) acid-base double indicator titration-Phenolphthalein and methyl orange-phenolphthalein changes color after reactions 1 and 2-methyl orange after reaction 3 two component complex system-CO32- and HCO3- primary standard Na2CO3 of known reaction with the standard acid titrant is shown belowCO32- + H3O+ HCO3- + H2O (2) Kb = 2.1x104 HCO3- + H3O+ H2CO3 + H2O CO2 + H2O (3)Kb = 2.4x10-8 unknown sample may contain sodium carbonate, sodium bicarbonate, sodium hydroxide or a mixture of the 3

METHODOLOGY

I. Standardization of the HCl SolutionTitration of dissolved Na2CO3 in boiled distilled water with methyl orange indicatorStop titration: yellow to near orangeSolution was boiled and allowed to cool to room temperature (yellow solution)Titration was continued until end point (orange solution)

II. Analysis of Soda Ash SampleTitration of dissolved soda ash sample in boiled distilled water with phenolphthalein indicator --end point: pink to clear solution Methyl orange indicator was added and titration was continuedStop titration: yellow to near orangeSolution was boiled and allowed to cool to room temperature (yellow solution)Titration was continued until end point (orange solution)

RESULTS AND DISCUSSION

Table 1: Volume of Acid Relationships in the Analysis of Mixtures Containing Na2CO3, NaHCO3 and NaOH

Constituent(s) Volume Relationship

NaOHV2 = 0

Na2CO3V1 = V2

NaHCO3V1 = 0

Na2CO3and NaOHV1 > V2

Na2CO3 and NaHCO3V1 < V2

NaHCO3 and NaOH , incompatible:(mixture results in the formation of Na2CO3 until one of them, or even both, will be used up

Unknown Soda Ash Sample: V1 < V2basic components: Na2CO3 and NaHCO3

boiled distilled water was used to prevent Carbonate Error

Boiling of solution before methyl orange end point (near orange):- to remove the buffer created at that point due to the large amount of dissolved CO2and small amounts of H2CO3 and unreacted HCO3--to achieve a sharper end point Carbonate ErrorCO2 + 2OH- CO32- + H2O

NaOH as primary standard? contains carbonate from reaction with atmospheric CO2 contains adsorbed water; making it difficult to weigh exactly

basic solutions: stored in polyethylene bottles rather than volumetric glassware silicon - used in glass wares because of the reaction forming sodium silicates concentration of base will decrease slowly

Possible Sources of Errors:difficulty of determining the exact end point of the methyl orange indicator >color change was gradual>rate of change of pH at equivalence point was not as rapid as with an HCl-NaOH titration >over titrationfailure to cool solution to room temperature before completing titration>changes in temperature during titration affect the rate at which the reaction will take place

Increase, Decrease, or No Effect?

1. In the standardization of HCl solution, Marionne failed to observe the actual Methyl Orange end point and over titrated the solution. What will happen to the calculated % Na2CO3?

2. If Abi forgot to boil the solution right after she observed the phenolphthalein endpoint, what would be the effect on the calculated %Na2CO3?

3. Marionne titrated the soda ash sample without removing the bubbles under the buret. What was the effect on the calculated % Na2CO3?on % NaHCO3 ?

4. What would be the effect on the calculated amount of sodium carbonate if Abi forgot to use boiled distilled water when she prepared the samples?

5. If someone unknowingly exhaled on the sample being titrated after the phenolphthalein endpoint, what would be its effect on the calculated parameters?

Answers?1. V , M , % Na2CO3 2. V --- % Na2CO3 3. Vphth, % Na2CO3, % NaHCO34. VMO, %Na2CO35. color will revert back, Vphth, %NaHCO3

Prepared by: Mar & Abi :D