Quantitative Analysis of Soda Ash by Double Indicator Titration

Kelvin L. LabarezDepartment of Materials, Metallurgical and Mining EngineeringCollege EngineeringUniversity of the Philippines, Diliman Quezon City, PhilippinesJanuary 22, 2013Prof. Gillian Macusi

INTRODUCTION The purpose of this experiment is to familiarize students with double indicator titration. Polyprotic acids and bases release or absorb more than one 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 between the main component (Sodium Carbonate) with its dissociated components, the composition of Soda Ash can be determined.

METHODOLOGY500ml 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 further titrated to completion.[1]

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 DISCUSSIONA. 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 changes color when all CO3 2- 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 Na2CO3 and the volume of the titrant. The concentration of the titrant can be determined.

B. Analysis of Soda AshThe main component in this experiment is Na2CO3. There are two equations involved in this scenario.

CO3 2- + H2O <-> HCO3- + OH-

HCO3- + H2O <-> H2CO3 + OH-

When the solution is titrated with HCl, CO3 2-

is used up and transformed into HCO3- , when

all that remains is HCO3-, the pOH is given

by√ pKb1+ pKb2. 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 addition to CO3 2- has been “used up”. [2]

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 CO3 2-(with respect to the first reaction only).Therefore, the amount of HCl used indicates the amount of CO3

2- to react with.If the solution contains just Na2CO3, the moles of CO3 2- 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

-

than CO3 2- ; 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 strong base.[3]

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 trialVolume of HCl used

 1st Titration

2nd Titration

Trial 1 1.7 3Trial 2 2.9 4.3Trial 3 2.5 3.6Average

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+

from HCl is equal the moles of CO32- .

[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

Thereforeg Na2CO3 total x100% = % g Na2CO3g NaHCO3 total

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 (95%)

from

4.25%

to20.76%

CONCLUSIONWhile 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 Quantitative Inorganic Analysis, 1st 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.