Quantitative Analysis of Soda Ash by Double Indicator Titration Kelvin L. Labarez Department of Materials, Metallurgical and Mining Engineering College Engineering University of the Philippines, Diliman Quezon City, Philippines January 22, 2013 Prof. Gillian Macusi
INTRODUCTION The purpose of this experiment is to familiarize students with double indicator titration. Polyprotic acids and base ba sess rele re leas as e or ab sorb so rb more mo re than th an on e mole of H+ or OH- per mole of analyte. This means that a simple titration is not enough to determine the analyte composition. Using the relationship betw be tw een ee n the th e ma in co mpon mp onen entt (Sod (S od ium iu m Carbonate) with its dissociated components, the composition of Soda Ash can be determined. METHODOLOGY 500ml of 1.0M stoch HCl was prepared, and from there 500ml of 0.050M HCl was prepared. Into three Erlenmeyer Flasks, 0.1g of Na2CO3 was added to each. Each flask was filled with 50ml of boiled distilled water and mixed. These were then added with 3 drops of methyl orange then titrated. At the point when the solution turns from yellow to orange, titration is temporarily stopped and the solutions are boiled. If the solution turns red, it means it has been overtitrated. Since Na2CO3 reacts slowly, there may be a chance that the solution has already been added with too much HCl but the reaction has not yet manifested. There may also be CO2 gas developing within the reaction, and this + leaves excess H . To deal with this problem, the titration is stopped when the equivalence point is near, and the solution is boiled to catalize and complete the reaction. This ensures that the appropriate amount of titrant
was added. After boiling, the solution is [1] further titrated to completion. For the sample, 0.1g of soda ash is used and 50ml of boiled distilled water, all of these are added to Erlenmeyer flasks for three trials. Phenolphthalein is added then the solutions are titrated. After the titration of phenolphthalein is complete, Methyl Orange is used and titrated. It is stopped midway similar to what was done earlier. RESULTS AND DISCUSSION A. Standardization of HCl
The exact concentration of stored titrant may not be completely accurate. It is necessary to know the exact concentration of the titrant to know the exact concentration of the analyte. A primary standard of Na2CO3 of known 99.9% purity is used. The reaction is as follows. Na2CO3 + 2HCl <-> 2NaCl + H2CO3 A solution with Methyl Orange indicator 2changes color when all CO3 has been converted to H2CO3. In the reaction, every two moles of HCl corresponds to a mole of Na2CO3. Using the known mass of the primary standard, it formula mass, the stoichiometric ratio of HCl and Na 2CO3 and the volume of the titrant. The concentration of the titrant can be determined. B. Analysis of Soda Ash
The main component in this experiment is Na2CO3. There are two equations involved in this scenario. 2-
-
-
CO3 + H2O <-> HCO3 + OH HCO3 + H2O <-> H2CO3 + OH 2-
When the solution is titrated with HCl, CO3 is used up and transformed into HCO3 , when all that remains is HCO 3 , the pOH is given by√ . Its pH given by 14-pOH is 8.33 which is approximately the point when a solution with drops of phenolphthalein indicator changes from colorless to pink and vice versa. This indicates that all that any strong bases in 2[2] addition to CO3 has been “used up”. At this point only HCO3 remains. Methyl orange changes the solution color also at the point when all the HCO3 is used up. Therefore, the amount of HCl used indicates the amount of HCO3 to react with. The HCl has a 1:1 mole ratio with OH- and 2CO3 (with respect to the first reaction only).Therefore, the amount of HCl used 2indicates the amount of CO3 to react with. If the solution contains just Na2CO3, the 2moles of CO3 is the same as the total amount of HCO3 that can be generated as the solution is titrated. If the amount of HCl used in the first titration (amount to reach the phenolphthalein indicated pH) is greater than the second titration (amount to reach the methyl orange indicated pH), it means that the solution contains strong bases in addition to Na2CO3. If the HCl used in the first is less than the second, it means that there is more HCO3 2than CO3 ; and the solution contains Na2CO3 and amounts of NaHCO3. If they are equal, then only Na2CO3 is present. If the solution is immediately colorless at the addition of phenolphthalein, there is no Na2CO3 present and it contains only
NaHCO3. If after the first titration, the solution is already orange, that means that there was no Na2CO3 and it only contains a [3] strong base. Soda Ash is an impure solution of Na2CO3, it may contain small amounts of Na2CO3 or NaOH. The results of the titrations are shown in table 1. Table 1: Titrant used in each trial Volume of HCl used 1st Titration 2nd Titration Trial 1 1.7 3 Trial 2 2.9 4.3 Trial 3 2.5 3.6 Average 2.366666667 3.633333333 From this we can conclude that the solution contains Na2CO3 and NaHCO3. C. %Composition of Soda Ash
Since Na2CO3 reacts first, the volume of the first titration can be related to the + concentration of Na2CO3. The moles of of H 2from HCl is equal the moles of CO3 . [HCl] 1st titration = [Na2CO3]total The volume of the second titration can be related to NaHCO3. [HCl] 2nd titration = [NaHCO3]total However, some of the NaHCO3 comes from the Na2CO3. The additional NaHCO3 is an impurity. [NaHCO3]total - [Na2CO3]total = [NaHCO3]from impurities
Therefore g Na2CO3 total g NaHCO3 total
x100% =
% g Na2CO3
The following summarizes important data: Table 2: Summary of Significant Data %Na2CO3 trial 1 8.47% trial 2 15.64% trail 3 13.41% mean 12.51% range 7.71% standard deviation 3.67% RSD (ppt) 293.6ppt confidence limit from 4.25% (95%) to 20.76% CONCLUSION While gravimetric methods can be used to measure moisture content, it is not precise. Many external factors come into play as the sample is constantly being exposed to the air. It absorbs moisture as it is moved in and out of the dessicator. Magnetic weight balances while accurate, are prone to mistaken calibration and positioning. REFERENCES
[1] Kolthoff I. M. Textbook of st Quantitative Inorganic Analysis, 1 Ed., Minneapolis. 1947 [2] Skoog D.A.; West D.M.; Holler F.J.; Crouch S.R.; Show C.C.; Introduction to Analytical Chemistry, 8th ed.; Cengage Learning Asia Pte Ltd, Pasig 2012 pp 124-186 [3] Howard Triebold, Quantitative analysis with applications to agricultural and food products.
