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I.
Introduction
In the acid-base titration, the equivalence point is known with the help of indicator which changes its color at the end point. In the titration of the polyacidic base or polybasic acid there are more than one endpoint. One indicator is not able to give color change at every endpoint. So, to find out each end point, more than one indicator must be used (Class 11 Chemistry Notes Stoichiometry - Double Indicator Acid Base Titration, 2012). Sodium Carbonate is a Bronsted Base that is used as a primary standard for the standardization of strong acids. It hydrolyzes in two steps:
Sodium Carbonate can be titrated to give end points corresponding to the stepwise additions of protons to form HCO3- and CO2 (Christian, 2004). When phenolphthalein is used in the above titration, it changes its color at the first end point when HCO3- is formed and with it, the second end point is not known. Similarly, with methyl orange, it changes color at second end point only and the first end point cannot is not known. It is because all indicator changes color on the basis of pH in the medium (Class 11 Chemistry Notes Stoichiometry - Double Indicator Acid Base Titration, 2012). Phenolphthalein changes color at pH 8.3-10:
While, methyl orange changes color at pH 3.1-4.4:
Calculations in double indicator titration involves determination of the volume of the volume of acid used when using phenolphthalein and methyl orange indicators, determining the concentration of salt in a given mixture and also determining the percentage composition of a given salt mixture. This experiment should be able to guide the student on how to determine qualitatively and quantitatively the components of carbonate mixture.
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II.
Methodology
This experiment is composed of three parts. The first procedure is to prepare a dilution of 1:25 from the original stock of an unknown sample using a 100 mL volumetric flask. Dilution is the process of reducing the concentration of solute usually simply by adding more solvent while, dilution factor is the ratio of the final volume /aliquot volume, where final volume = aliquot + diluent . Thus, 4 mL of the original unknown sample is needed to prepare 100 mL dilution of 1:25. The second part of the experiment is the qualitative analysis of components in which methyl orange and phenolphthalein were used separately on the titration of the 10.0 mL of diluted unknown sample with the 0.05 N standard HCl solution.Volume of HCl used for each indicator had to be compared to be able to identify the components present: if Vph>1/2 Vmo, then NaOH & Na2CO3 are the components present; if Vph<1/2Vmo then, NaHCO3 & Na2CO3 are present. The third part of the experiment involves quantitative analysis of the components found to be present in the sample. The procedure for this part is almost the same as the second part except that in the titration using methyl orange, the sample solution was boiled with a few mL less HCl than the expected end point, then allowed to cool before the titration was completed at peach end point. Boiling removes the CO2 from the buffer system of HCO3-/CO2, leaving only HCO3- in the solution. In the titration using phenolphthalein, 10mL of 10%BaCl2 was added allowing the formation of BaCO3 precipitate so that the only carbonate that would react with HCl is HCO3-. If NaOH & Na2CO3 are present, the volume of HCl used to titrate will determine the NaOH present. If NaHCO3 & Na2CO3 are the components found to be present in the sample, 25.00 mL of standard 0.05N NaOH had to be added. Once completed, a blank titration had to be performed to determine the amount of HCl which reacted with HCO3- alone. Blank titration is an analysis on the added reagents which is absent from the solution involved in blank titration. It is standard practice to run such blanks and subtract the results from those for the sample. This is one way to minimize titration error. For each part, two trials were required to carry out however during the quantitative analysis, there was an error incurred in trial 2 during the titration with methyl orange hence, unable to continue the rest of the procedure.
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III.
Results and Discussion
Table 5-1: Preparation of the Sample Trial 1(Canceran) A 1:25 4.0 90.0 100.00
Trial 2(Sanchez) A 1:25 4.0 90.0 100.00
Table 5-2: Qualitative Analysis of Components of the Mixture 5-2a: Methyl Orange as Indicator Trial 1(Canceran) Volume of sample titrated, mL 10.0 Concentration of HCl, N 0.05 Initial volume of HCl, mL 0 Final Volume of HCl, mL 17.70 Volume of HCl used for titration, mL (Vmo) 17.70
Trial 2(Sanchez) 10.0 0.05 0 19.85 19.85
5-2b: Phenolphthalein as indicator volume of sample titrated, mL Concentration of HCl, N Initial volume of HCl, mL Final Volume of HCl, mL Volume of HCl used for titration, mL (Vph)
Trial 1(Canceran) 10.0 0.05 38.90 44.90 6.00
Trial 2(Sanchez) 10 0.05 0.01 6.15 6.14
5-2c: Determination of Components Volume relationship of standard HCl used to reach end points
Trial 1(Canceran)
Trial 2(Sanchez)
Vph < 1/2 Vmo NaHCO3 & Na2CO3
Vph < 1/2 Vmo NaHCO3 & Na2CO3
Unknown sample dilution factor Volume of the original stock, mL Volume of water added, mL Final volume of the solution, mL
components present in the sample
Table 5-3: Quantitative Analysis of NaHCO3 & Na2CO3 5-3a: Methyl Orange as Indicator Trial 1(Canceran) Volume of sample titrated, mL 10.0 Concentration of HCl, N 0.05 Initial volume of HCl, mL 6.1 Final Volume of HCl, mL 23.65 Volume of HCl used for titration, mL (Vmo) 5-3b: Phenolphthalein as indicator volume of sample titrated, mL volume of standard 0.05 N NaOH added to the sample volume of 10% BaCl2 added to the sample Concentration of HCl, N
Trial 2(Sanchez)
17.55 Trial 1(Canceran) 10.0 25.00 10.0 0.05
Trial 2(Sanchez)
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Initial volume of HCl, mL Final Volume of HCl, mL
0 22.40
Volume of HCl used for titration, mL (Vph)
22.40
5-3c: Blank Preparation volume of water volume of 10% BaCl volume of standard 0.05 N NaOH Concentration of HCl, N Initial volume of HCl, mL Final Volume of HCl, mL Volume of HCl used for blank titration, mL difference in volume of HCl used for blank & sample titration 5-3d: Calculation of the percent components of carbonate mixture
Trial 1(Canceran) 10.0 10.0 25.00 0.05 0 29.45 29.45
Trial 2(Sanchez)
7.05 Trial 1(Canceran)
amount of NaHCO3 present in the original sample, g
0.02961 g
% NaHCO3 (w/v)
0.2961%
Average of %NaHCO3 Average deviation
n/a n/a
amount of Na2CO3 present in the original sample, g
0.02782 g
% Na2CO3 (w/v)
0.2782%
Average of %Na2CO3 Average deviation
Trial 2(Sanchez)
n/a n/a
The group analyzed unknown sample “A”. Phenolphthalein is used to detect the first end point, an approximation that H+ from HCl is equivalent to CO3-2 to produce HCO3-. Methyl orange is used to detect the second end point which is an approximation that H+ from HCl is equivalent to HCO3- to produce H2CO3. Based from the qualitative analysis done, Vph<1/2 Vmo which means that the components present are NaHCO3 and Na2CO3. This is because there is more HCO3needed to be protonated since it was coming from 2 sources, than CO3-2 to be converted HCO3which was coming from Na2CO3 only. On the quantitative analysis, it was calculated that the sample solution contains 0.02961 g NaHCO3 / 10.0 mL of solution giving 0.2961% w/v NaHCO3 and 0.02782 g Na2CO3 /10.0 mL of solution giving 0.2782% w/v Na2CO3. Determination of the amount of NaHCO3 and Na2CO3 came from the equation: Total meq = meq NaHCO3 + meq Na2CO3 = Vmo x NHCl
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Where, meq NaHCO3 = (Vph blank – Vph sample) x NHCl Determination of meq of NaHCO3 using this equation came from the titration of both blank and sample solution in which both solutions contains BaCl2 which would remove CO32- so that all that would participate on the titration are HCO3- and NaOH. Since NaOH was also present in the blank sample, the difference in the volume of HCl used determined the NaHCO3. This method which is called blank titration gives result with less titration error. Another technique that was carried out during this experiment is the boiling of the sample a few mL less than the expected end point to remove CO2, leaving only HCO3- in the solution. If the sample solution was not boiled the presence of CO2 would interfere to the measured value of HCO3-. Likewise, boiling the sample solution too much would make some of the molecules of the solution to evaporate resulting to loss. It also important to use boiled distilled water to make sure that the sample is free from unwanted substances which may react with the titrant and interfere with the measurement of the analyte. The group was able to complete trial 1 only because of some errors incurred while doing the 2nd trial thus no other data to compare trial 1 from.
IV.
Summary and conclusion
Diprotic acids and other diprotic bases can be titrated stepwise just as sodium carbonate does. During titration up to the first equivalence point, an HA-/H2A (for acids) or HB-/B-2(for bases) buffer region is established. At the first equivalence point, a solution of HA- (acids) or HB(bases) exists. Beyond this point, A-2/HA-(for acids) or H2B/HB-(for bases) exists. In performing quantitative and qualitative analysis of an unknown sample of diprotic acid or base by means of titration, it is important to choose an indicator which changes color as close as possible to that equivalence point to minimize titration error. Other way of minimizing titration error is comparing the titration results to that of a blank sample.
V.
References
Class 11 Chemistry Notes Stoichiometry - Double Indicator Acid Base Titration. (2012, October 11). Retrieved July 2013, from Aglaem Schools: http://schools.aglasem.com/?p=4685 CHM130L Laboratory Manual. (2013).
Page 6 of 6 Christian, G. D. (2004). Analytical Chemistry. Danvers, MA: John Wiley & Sons, Inc. Clark, J. (2006, December). Retrieved July 2013, from http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html Hage, D., & Carr, J. (2011). Analytical Chemistry and Quantitative Analysis. New Jersey: Pearson Prentice
