Page 1
Determination of the Components of a Base Mixture
Bryan Angelo Lazo, Camille Jane Villena, Jonniel Vince Cruz
Group 3
College of Science, Pamantasan ng Lungsod ng Maynila
Received: 10 March 2014
Abstract
This experiment aimed at identifying the components, and percentage compositions, of a known and an unknown base mixture using double indicator method. The data necessary to find the constituents present was obtained by performing a stepwise titration using phenolphthalein and methyl orange indicator. At the end of the experiment, it was confirmed that the known sample contained 0.0967(±0.0144) M Na2CO3 and 0.117(±0.017) M NaHCO3; and it was also found out that the unknown sample contained 0.145 (±0.003)M of Na2CO3 and 0.111 (±0.005)M of NaHCO3 with a percentage error of 49% and 9%, respectively .Thus, qualitative and quantitative determination of the constituents in a solution of compatible mixtures provided important examples as to how neutralization titrations can be applied to analyze mixtures. From the error of the experimental results, it is recommended to use precipitants such as barium carbonate in solutions to avoid the interference of previous species to the succeeding endpoint.
INTRODUCTION
Alkalinity roughly refers to the amount of bases in a solution that can be converted to uncharged species by a strong acid. Common ions that mainly contribute to the alkalinity of a water sample are OH-, HCO3-, and CO32-. Total alkalinity is formally defined as the equivalent sum of the bases that are titratable with strong acid. Total Alkalinity is measured by adding a strong acid until all the anions listed above are converted to uncharged species. It is determined by titrating with an acid standard solution to a fixed end point at a pH of 4.5 (Dacumos and Falcatan, 2013).
The total alkalinity is not affected by temperature, pressure, or pH, though the values of individual constituents are, mostly being conversions between HCO3- and CO3 2. (Wikipedia contributors, 2014)
The double indicator method is needed when OH-, HCO3-, and CO32- are the only sources of alkalinity. In a double indicator method, phenolphthalein and methyl orange or phenolphthalein and bromocresol green is usually used. The ions present can be determined using titration to a certain pH.
In this experiment, the concepts of the double indicator method were applied in identifying the components of the given and unknown base mixture. The percent composition of the base mixture is determined in the experiment.
METHODOLOGY
Ten milliliters aliquot of the unknown sample, twenty milliliters of boiled distilled water, and two drops of phenolphthalein indicator were transferred into an Erlenmeyer flask. The solution was then titrated with standard HCl solution to the colorless end point. When the end point was reached, the volume in the buret was recorded as the final volume of the acid at the phenolphthalein indicator and as the initial volume of acid at the methyl orange end point.
After addition of 5 drops methyl orange indicator, the titration was continued until the solution's color was near orange. The solution was then boiled for 3 minutes and then cooled to room temperature. No indication of overtitration was observed; thus, the titration was resumed until the solution turn light orange in color.
RESULTS AND DISCUSSION
Analysis of Sample
The experimental procedure was divided into two divisions: unknown and known analysis; using a compatible mixture of Na2CO3 and NaHCO3 in the determination of known sample. Known analysis comprised three trials, each using 10.00 (±0.02) mL aliquots of the mixture and diluting it to 30.00 (±uncertainty) mL marks with boiled distilled water. During titration, 0.0896 (±0.0006) M of standardized HCl was used to titrate the sample at all endpoints. Using a basic sample, phenolphthalein was used to signal the presence of a relatively strong base: NaOH or Na2CO3, as governed by its small pkb values; where pH values of each during equivalence point correspond to the transition range of a suitable indicator. After the first endpoint, volumes of NaOH are recorded as approximately uniform, ranging at volumes 2.10-2.20 mL. Volume for the endpoint was intended as the final reading for first; but initial reading of the second endpoint relatively differed due to some factors: removal of interferences in the buret (air, contaminants), and the leakage and draining of some titrant at the neck of buret.
Titration was continued using methyl orange indicator with a transition range 3.1-4.4, which was close to the pka of H2CO3: 6.35 where all of the HCO3- ions were already titrated starting from the base mixture. As the solution began to change from yellow to near orange, which is at pH= 4.4, titration was temporarily stopped and boiled for 3 minutes and cooled to room temperature.
In solution as well as in its solid state, hydroxides of sodium, potassium, and barium react rapidly with atmospheric carbon dioxide to produce the corresponding carbonate:
CO2 + 2OH- CO3- + H2O
In the titration of base mixture, an even sharper endpoint can be achieved by boiling the solution to eliminate carbonic acid and carbon dioxide. The sample is titrated to the first appearance of acidic color of indicator. Boiling effectively destroys the buffer solution from first endpoint with carbonic acid by evaporating it into carbon dioxide ion. Afterwards, a substantially larger decrease in pH occurs during final additions of acid, thus giving a more abrupt color change. (Skoog, et al.,2013)
As the titration was resumed, volumes of HCl used ranges from 4.60-4.80 milliliters, indicating that the sample contained uniform solutions of base mixture to titrate all the remaining HCO3- species left in the sample.
Known sample
TRIAL
1
2
3
Volume of sample(mL)
10.00(±0.02)
10.00(±0.02)
10.00(±0.02)
Volume of NaOH
(mL)
Phenolphthalein
Endpoint
Initial
0.00(±0.03)
15.10(±0.03)
21.60(±0.03)
Final
2.10(±0.03)
21.60(±0.03)
31.10(±0.03)
Actual
2.10(±0.04)
6.50(±0.04)
9.50(±0.04)
Methyl orange
Endpoint
Initial
2.35(±0.03)
9.50(±0.03)
16.50(±0.03)
Final
7.10(±0.03)
14.30(±0.03)
21.20(±0.03)
Actual
4.75(±0.04)
4.80(±0.04)
4.60 (±0.04)
COMPONENTS
CONCENTRATION (M)
Na2CO3
0.0952 (±0.0082)
0.0998(±0.0086)
0.0952(±0.0082)
x= 0.0967(±0.0144)
Na2HO3
0.120(±0.010)
0.118(±0.010)
0.113(±0.010)
x= 0.117(±0.017)
Table 1. Data in titration of known base mixture In the unknown analysis, analogous results were obtained: with volumes 3.20-3.25 mL for Phenolphthalein and 5.60-5.80 mL for methyl orange endpoint, showing it contained the same components as the known sample: Na2CO3 and NaHCO3 since Vph < ½Vmo for separate sample; Vph
Table 1. Data in titration of known base mixture
instruments used, the known sample contained 0.0967(±0.0144) M Na2CO3 and 0.1177 (±0.017) M NaHCO3, similar to the quantities of the unknown sample: 0.145(± 0.003) for Na2CO3 and 0.111 (±0.005) M for NaHCO3.
The experiment used compatible mixtures such that the species will not interfere or react with each other, thereby avoiding false results as the analyte would change its identity before it was titrated. Assuming only NaOH and NaHCO3 mixture was available, titration was not feasible as sodium hydroxide is a strong base, and sodium bicarbonate can act as an acid, causing a reaction to occur. Both are said to be "incompatible" as no mixture of the two can exist in stability without reacting to form sodium carbonate and water, depending on the relative concentrations of the starting reagents.
Double indicator method can also be applied to determine the concentrations of Na3PO4 and NaH2PO4 provided that systematic treatment of equilibrium is considered since the concentrations are dependent on the relative concentrations of the two to form the intermediate specie: Na2HPO4.
Thus, same molarities of each must be present such that same equivalents of product will be produced; removing all side reactions provided that all potential interferents were masked in the sample. Given that the primary condition was satisfied, equilibrium constants of components must then be analyzed such that the chosen indicator transition ranges will fit at equivalence point to the pH of sample. This would also affect the completeness of the reaction as some reagents are not "titratable" due to some errors involved in the instruments or modes of analysis used.
Unknown sample (No. 3)
TRIAL
1
2
3
Volume of sample(mL)
10.00(±0.02)
10.00(±0.02)
10.00(±0.02)
Volume of NaOH
(mL)
Phenolphthalein
Endpoint
Initial
0.00(±0.03)
3.30(±0.03)
0.00(±0.03)
Final
3.25(±0.03)
6.55(±0.03)
3.20(±0.03)
Actual
3.25(±0.04)
3.25(±0.04)
3.20(±0.04)
Methyl orange
Endpoint
Initial
9.58(±0.03)
15.60(±0.03)
3.30(±0.03)
Final
15.60(±0.03)
21.30(±0.03)
8.90(±0.03)
Actual
5.80(±0.04)
5.70(±0.04)
5.60 (±0.04)
CONCENTRATION
Mean Molarity
True Value
% Error
Na2CO3
0.146
(±0.002)
0.146
(±0.002)
0.143
(±0.002)
0.145
(±0.0144)
0.285
49.12
Na2HO3
0.114
(±0.003)
0.110
(±0.003)
0.110
(±0.003)
0.111
(±0.017)
0.122
9.02
Table 2. Data in titration of unknown base mixture Most species require a pka difference of 3.0 such that the buffer region at pka ±1.0 will not coincide with the endpoint. When this happened, no abrupt change of pH will be observed as it is included in 10-90 %
Table 2. Data in titration of unknown base mixture
range for an effective constant pH. (Harvey, 2000) Also, the relative concentrations of titrant and analyte do not differ logarithmically as each proton will not simultaneously be independent at all equilibria present; and less sharp endpoint will be achieved.
CONCLUSION
In the experiment, it was found out that mixtures of unknown molarity from the initial solution showed how neutralization titrations can be employed to analyze mixtures using double indicator method. When known values were used, it had two components: Na2CO3, and NaHCO3-with a mean molarity of 0.0967 (±0.0144) M and 0.117 (±0.017) M respectively. Unknown values yielded the same identity of compound; with a mean molarity of 0.145 (±003) M for Na2CO3 and 0.111 (±0.005) M for NaHCO3.
REFERENCES
1. "Alkalinity." Wikipedia. Wikimedia Foundation, 22 Feb. 2014. Web. 09 Mar. 2014.
2. Dacumos, Nemia T., and Anilyn M. Falcatan. "Determination of the Components of a Base Mixture." Analytical Chemistry II Laboratory Manual. p.12.
3. Harvey, David. "Titrimetric Methods of Analysis." Modern Analytical Chemistry. Boston: McGraw-Hill, 2000. N. pag. Print.
4. Skoog, Douglas A., Donald M. West, and F. James. Holler. "Principles of Neutralization Titrations." Fundamentals of Analytical Chemistry. Fort Worth: Saunders College Pub., 2013. N. pag. Print.
