Chance Boudreaux
DETERMINATION OF Na₂CO₃ BY MEANS OF BACK TITRATION Chance Boudreaux, Kennesaw State University Quantitative Analytical Chemistry Lab 2800 Instructor: Dr. H. Z. Msimanga INTRODUCTION: The objective of the experiment is to determine the % mass of sodium carbonate (Na₂CO₃) in the unknown sample. This is done by means of a back titration in order to ensure that the titration will be successful due to the possibility that the carbonate ion (CO₃²¯) can form two different reactions with hydronium (H₃O⁺). Back titration is a titration done in reverse. Instead of titrating the original sample, a known excess of standard reagent, in this case HCl, is added to the solution, and the excess is titrated. Using a back titration is helpful if the endpoint of the reverse titration is easier to identify than the endpoint of the normal titration. Back titrations are also helpful if the reaction between the analyte and the titrant is very slow, or when the analyte is in a non-soluble solid. Either of these two reactions can occur:
(1)
CO3
2
(2)
CO3
2
HCO H 3O HCO3
HCO 2 H 3O HCO3
H 2O
H 2O
These reactions cause a problem due to eq1 needing the indicator phenolphthalein and eq2 using indicator methyl orange.
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Chance Boudreaux
In order to perform a back titration of sodium carbonate, a standardized strong acid and base will be required. The data, in this report, was extracted ex tracted by the use of 0.09731M HCl and 0.09948M 0.0994 8M NaOH (this means phenolphthalein will be the indicator). The experiment will also require a 50mL buret, 10mL pipet, a graduated cylinder, 250mL volumetric flask, a balance, and a hot plate for each time you run it. Table 1: (preparation of each flask for titration)
Table 1
Trial 1
Trial 2
Trial 3
Trial 4
(g)
0.3395
0.3579
0.3049
0.3049
HCl (mL)
50.00
50.00
50.00
50.00
H₂O (mL)
25.00
25.00
25.00
25.00
Na₂CO₃
After the solutions are prepared they should be heated for 3-4 minutes to allow the reaction (eq3) to go until completion, as well as, to allow the CO₂ (g) that is formed to dissipate. This is so that the CO₂ does not adversely react with the phenolphthalein creating an unwanted precipitate.
(3)
2 NaCl CO2 H 2O Na2CO3 2 HCl HCl
After the solutions cool to room temperature begin titrating. The following equations (eq4, eq5, & eq6) are used to obtain grams of Na₂CO₃ from the sample after titrations are complete.
(4)
(5)
M NaOH L NaOH 1 mol HCl
1 mol NaOH M HCl
L HCl HCl
L HCl HCl excess
L HCl reacted
2
L HCl excess
Chance Boudreaux
(6)
M HCl L HCl reacted 1 mol Na2CO3 MW Na2CO3
2 mol HCl
g Na2CO3
(MW Na₂CO₃= 105.9885 g/mol)
(7)
(8)
g Na2CO3 g Total Na2CO3 S x _
*100 % mass % CO3
*100% % RSD RSD
x From here find g Na₂CO₃, than convert to mass % Na₂CO₃ as shown in table 2. Table 2: (Experimental results of Trials 1-4, % mean and %RSD)
Table 2
Trial 1
Trial 2
Trial 3
Trial 4
NaOH (mL)
24.52
24.42
24.50
24.38
Na₂CO₃ (g)
0.1308
0.1340
0.1148
0.1146
mass Na₂CO₃ (%)
38.53
37.44
37.65
37.59
mean Na₂CO₃ (%)
37.80
%RSD (%)
±1
(Unknown code #249 for stock sample) DISCUSSION: This experiment is created to quantify CO₃ in an unknown purity of Na₂CO₃. This is done by a means of a back titration technique, due to two possible reactions between H₃O⁺ and CO₃²¯. This back titration is done by adding a known excess amount a mount of standardized HCl to a weighted 3
Chance Boudreaux
sample of Na₂CO₃. The excess HCl was titrated with standardized NaOH, to quantify the amount of HCl that reacted with the sample. Using stoichiometry, the g of analyte (CO₃) can now be found and then converted to a % mass by dividing the total mass of the sample. The % CO₃ along with the %RSD, approximates the amount of CO₃ in a given sample from the stock bottle. The % relative standard deviation (%RSD) shows the tolerance (±) of a sample's mean. Due to the low %RSD of this data, it proves the data's mean to be a valid approximation. The %RSD of this data also shows that most of the deviation is from the imperfect blend of the unknown sample and the tolerance error of the instruments used, due to a relatively small amount of deviation in the mean. The other calculations supporting the precision of the data are done by a ttest. A t-test uses a normal distribution curve to see if your data fits within set parameters. The data acquired through this experiment test valid for the 50th confidence level, meaning that the data fits within the ±25% of the mean on the normal distribution curve. CONCLUSION: The results of the back titration concluded that we have a mean of 37.80% Na₂CO₃ with a ±1%RSD for the unknown Na₂CO₃ stock provided in the laboratory. The tcalc = 0.7407 for the 50% confidence level and the tcrit = 0.741 meaning that the data is appropriate for the 50th confidence level. REFERENCE: Harris, D. C. (2010). Quantitative chemical analysis. (Eighth Edition ed.). New York: W.H. Freeman and Company.
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Chance Boudreaux
ACKNOWLEGEMENTS: H. Z. Msimanga APPENDIX: The below example calculations are taken from trial 1:
(4)
0.09948 M NaOH 0.02452 L NaOH 1 mol HCl
1 mol NaOH 0.09731 M HCl
(5)
0.05000 L HCl HCl
0.02507 L HCl HCl excess
0.02507 L HCl excess
0.02493 L HCl reacted
(6)
0.09731 M HCl 0.02493 L HCl reacted 1 mol Na2CO3 105.9885 MW Na2CO3 2 mol HCl
(7)
0.1308 g Na2CO3 0.3395 g Total Na Na 2 CO3
0.4930
*100% 1 % RSD RSD
37.80
5
0.1308 g Na2CO3
*100 % 38.53 mass % Na2CO3
(eq8 uses data from every trial; standard deviation over the mean of % mass Na₂CO₃)
(8)
