EXPERIMENT NO. 7
Quantitative Analysis of Soda Ash by Double-Indicator Titration ABSTRACT
This experiment aims to analyze the percent composition of a substance mixture by using double indicator titration. The analyte used is the soda ash which is titrated with an HCl titrant, standardized by 1 o Na2CO3. The indicators used are phenolphthalein for basicity and methyl orange for acidity. The two volumes of the titrant are then used to calculate percent composition of soda ash analyte. Statistical parameters such as relative standard deviation and confidence limits are also calculated. At the end of the experiment, the calculate percent /calculated average percent by mass of Na 2CO3 is 27.6% with a relative standard deviation of 64.1 ppt and confidence intervals of 43.5% - 11.7% and the average percent by mass of NaHCO 3 is 5.23% with a relative standard deviation of 128 ppt and confidence intervals of 11.3% - -0.8%. With these results and the fulfilled objectives, the experiment was a success
Introduction
This experiment focuses in the analysis of soda ash, a widespread industrially used substance in petroleum refining, glass making, detergent manufacture, water treatment etc. It can be a pure compound of sodium carbonate, sodium bicarbonate or sodium hydroxide or the mixture of the three with compatible percent compositions. These compositions compositions are determined by acid-base titrations, a volumetric process in determining the concentration of a substance in a certain solution which uses a base or acid as as the titrant. The reaction between sodium carbonate and hydrochloric acid occurs in 2 stages, with the formation of bicarbonate ion as the intermediate product. (1)
(2) Equation 1 shows the production of the bicarbonate ion in the first titration and equation 2 shows that the bicarbonate is consumed in the second titration.
Graph 1. Endpoints of HCl with Na 2CO3 and NaHCO3 Graph 1 shows that sodium carbonate has two endpoints, the phenolphthalein endpoint which indicates neutralization of the carbonate ion and the methyl orange endpoint which indicates neutralization of the bicarbonate ion. Some terms concerning titration are: analyte, titrant, standard, equivalence point, end point, indicators. Analytes are the unknown substances to be analyzed. Standards are the substances with known concentrations and they provide a reference to determine unknown concentrations or to calibrate analytical instruments. Primary standards, reagents that are extremely pure, stable, have no waters of 1
hydration, and have high molecular weights, are used to determine the accurate concentration of the standard solution. Titrant is the reagent inside the burette that would be dropped in the analyte. The volume of the titrant that is used to completely neutralize the base or acid component of the analyte will be used to calculate for the concentration of the unknown substance in the analyte. The complete neutralization is called the equivalence point, the point in which stoichiometrically equivalent quantities are brought together. End point is the next drop right after the equivalence point has been reached. These points are qualitatively observed through the use of indicators. Indicators are substances that changes color through the presence of an acid or base in a solution. In this experiment, purity of soda ash would be determined by double indicator titration using HCl as the titrant, Na 2CO3 as the primary standard, and methyl orange and phenolphthalein as the indicators. Methodology After the proper PPE’s have been worn, 500ml
distilled water was boiled and the titration set-up and the needed solutions were prepared. 1000 ml of 0.050M HCl titrant was prepared by the monitor. Each of the groups would perform one of the three trials of standardization. 100.3 mg, 146.2 mg and 111.3 mg of 1 o standard Na2CO3 with 99.9% purity was dissolved in 25 ml boiled distilled water. 3 drops of phenolphthalein indicator was then added. The standard was titrated till phenolphthalein endpoint before adding 3 drops of methyl orange indicator. The solution was then titrated till the orange color was observed then boiled and cooled to room temperature and titrated till the first hint of red color. For the sample analysis, two trials with two replicate solutions were prepared by dissolving 114.3mg of soda ash sample for trial 1 and 115.1 mg of soda ash sample in trial 2 in two separate 250 ml Erlenmeyer flasks containing 75 ml boiled distilled water. 25 ml of the solution was then transferred to 2 separate 250 ml Erlenmeyer flasks then 3 drops of phenolphthalein was added to each. Each was then titrated to phenolphthalein endpoint. 3 drops of methyl orange was added and the solutions are titrated till orange color was observed. The solution is boiled then cooled to room temperature before titrated till methyl orange endpoint (red color). If the solution was already red when boiled, the process should be repeated with the remaining solution. The
initial and final volumes with the corresponding uncertainties of the HCl titrant were recorded. All titrated solutions and drained into the sink with copious amounts of water. Results and Discussion
The data obtained in the first part was used to determine the average molarity of the standard HCl titrant by using the equation 1 in the appendix. First, the moles of Na2CO3 are calculated. Since the mole ratio HCl: Na2CO3 is 2:1, moles of Na 2CO3 is multiplied by 2. Then the molarity of the HCl is determined by dividing its moles by the net volume of HCl used. In the sample analysis, two volumes are obtained. The data shows that the soda ash is composed of Na2CO3 and NaHCO3. This is because V ph
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Table 1.Colors of Indicators at Different pH conditions Indicator pH range Color Phenolphthalein pH<3.1 Red 3.1
increase the molarity of the HCl. The dissolved carbon dioxide can also be a source of error because it can interfere with the methyl orange color thus increasing the molarity of the titrant. The students highly recommend the use of well calibrated instrument and glassware for measurements. Experimenters should make sure that the glassware used in solution preparation was clean and free of chemical contamination. Careful and correct burette reading of the volume of the titrant used should also be observed. The observation for the color change which signifies the endpoint of the titration using different indicator should be close to the exact time and this can be confirmed through several trials on each set-up. Conclusion
Based on the results, it was confirmed that the experiment is quite a success even if the calculation are quite small. It is successful with respect to the methodology and the logic behind it and that the students understood and fulfilled the objectives. At the end of the experiment, the calculated average percent by mass of Na2CO3 is 27.6% with a relative standard deviation of 64.1 ppt and confidence intervals of 43.5%-11.7% and the average percent by mass of NaHCO3 is 5.23% with a relative standard deviation of 128 ppt and confidence intervals of 11.3% - -0.8%. This implies that the errors discussed earlier have quite a large effect on the data, decreasing its accuracy. But because the objectives are fulfilled, the experiment is still deemed a success. Application
Soda ash is a versatile substance in the industry because of its uses. It is used in glass-making, petroleum refining etc. Therefore, industries would need to determine the soda ash composition to know and control their yields and they usually use double-indicator titration. References
Text book references: [1]Skoog, Douglas A., West, Donald M., Holler, F. James. ANALYTICAL CHEMISTRY an 3
Introduction. 6th edition. Saunders College Publishing. USA 1993 [2] Brown, Theodore L., Lemay, Eugene H. Jr., Bursten, Bruce E. Chemistry the Central Science. 8 th edition. Prentice Hall Inc. reprinted by Pearson Education Asia Pte. Ltd. 2002
b.Relative Standard Deviation c. Confidence Limits
Internet references: [1]KWOK The Chem Teacher http://kwokthechemteacher.blogspot.com/2011/ 02/double-indicator-titration.html [2]Engineering Tool Box (for pH values) http://www.engineeringtoolbox.com/bases-phd_402.html http://www.engineeringtoolbox.com/acids-phd_401.html
Sample Calculations
For Trial 1 Replicate 1: Molarity of HCl Standard
Appendix Working Equation
1. Molarity of Standard HCl
Composition of Soda Ash a.
2. Composition of Soda Ash
a. b.
b. 3. Error propagation uncertainties a. Addition and Subtraction b.Multiplication and Division
4. Statistical Parameters a. Standard Deviation
Error propagation uncertainties a. Addition and Subtraction (For the uncertainty of net volume in trial 1 rep. 1)
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1.) water? b.Multiplication and Division (For the uncertainty of the molarity of standard HCl titrant)
Why is there a need to use boiled distilled
- Boiling the water would remove carbon dioxide gas which may produce carbonic acid when dissolved in water. Carbonic acid might react with the base components of soda ash, and would cause lower percentages of the components 2.) Why is a mixture of NaOH and NaHCO3 incompatible? - OH- from NaOH and HCO3- from NaHCO3 would react with each other as showed in equation 1: (1)
Statistical Parameters Standard Deviation (average% Na2CO3 )
This reaction decreases both OH- and HCO3concentration and production of carbonate ions which can lead to altered volumes of the HCl standard. 3.) Why is there a need to boil the solution before reaching the methyl orange endpoint? - The titration to obtain V mo is aiming to neutralize all the HCO3-. But this leads to formation of carbonic acid which alters the color of the solution with the methyl orange indicator.
Relative Standard Deviation
Confidence Limits (intervals of %Na2CO3 at 95% confidence level)
Confidence intervals = 43.5% - 11.7%
4.) What are the basic components of the unknown soda ash sample based on the volume relationship at the phenolphthalein and methyl orange endpoints? - If the data obtained is analyzed, one might say that the soda ash components are NaHCO3 and Na2CO3. The two volumes V mo and V ph are equal theoretically in the titration of Na2CO3. This is because V ph of HCl is used to neutralize the carbonate ion into a certain number of moles of bicarbonate ion and the V mo of HCl is used to neutralize the same number of moles of the bicarbonate ion. So if the bicarbonate and carbonate ions are equal theoretically, V mo and V ph should also be. In the experiment, the V mo obtained is greater than V ph, which means there are more bicarbonate ions in the solution than carbonate ion. This implies that NaHCO3, a base that can supply bicarbonate ions, is also present in the soda ash
Answers to Questions 5
5.) What are the possible sources of errors and their effect on calculated parameters(increase, decrease, no effect or indeterminate) - Errors may come from various sources such as the uncertainties in the measurements of the volumes and weights of the components of the experiment. The uncertainties of the mass of the samples can also contribute to the errors in the solution. The uncertainties in measurements of the solutions can cause increase/decrease in the needed volume of the solutions and can change the results in the analysis of soda ash. Errors in reading the volume of the titrant used can cause errors in the calculated HCl concentrations; lower molarity if higher volume was read and a lower molarity if the volume read is lower. Another possible source of error is the determination of the endpoint of the titration since it was based on the color change of the solution and the students could have decided that the endpoint was reached earlier than the exact time and some could have seen the “phantom” pink color. This can increase the molarity of the HCl. The dissolved carbon dioxide can also be a source of error because it can interfere with the methyl orange color thus increasing the molarity of the titrant.
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