Analysis of the Mixture of Carbonate and Bicarbonate

Geoff Simpson

Department of Chemistry and Biochemistry, McMurry University, Abilene, TX 79697

 

            The purpose of this experiment was to calculate the weight percents of carbonate and bicarbonate contained in the unknown sample.

 

Experimental Section

 

Apparatus and Materials:  The reagents used in this experiment were standard NaOH (Sigma-Aldrich, Inc., St. Louis, MO), standard 0.1038 M HCl (Pharmco Products, Inc., Brookfield, CT), 10% wt aqueous BaCl2, unknown solids, and CO2 free water. Na2CO3 (Mallinckrodt, Inc., Paris, KY) was used to standardize the HCl, and NaOH was standardized with potassium hydrogen phthalate (KHP) (Mallinckrodt, Inc., Paris, KY).  The indicators used in this experiment were bromocresol green and phenolphthalein.  A previously calibrated 50 mL buret was also used.

 

Procedure

            Between 2.0-2.5 grams of an unknown sample was weighed out in a 250 mL volumetric flask.  The unknown was diluted with deionized water to the mark and inverted.  Next, 25 mL of the unknown solution was pipeted into a 250 mL flask, and then titrated with 0.1 M standard HCl.  This was performed using bromocresol green indicator.  This procedure was repeated two more times using two more separate 25 mL portions.

            Next, 25 mL of unknown solution and 50 mL of standard 0.1 M NaOH was pipeted into a 250 mL flask and mixed.  Then 10 mL of 10% wt BaCl2 was added and mixed again.  Phenolphthalein indicator was added and titrated with standard HCl.  This procedure was repeated 2 more times using two more separate solutions. 

 

Results and Discussion

 

Table 1 illustrates the results of the standardization of hydrochloric acid solution.

 

Table 1. Standardization of HCl

Mass of Na2CO3 

 

Vol. of HCl Titrated (mL)

Molarity of HCl (mol/L)

0.1323 g

25.59

0.09647

0.1327 g

24.72

0.1013

0.1336 g

21.95

0.1149

0.1331 g

24.63

0.1020

 

The average HCl molarity was calculated to be 0.1037 M, while the standard deviation was 0.0079 and the relative standard deviation was 7.62%.  Sources of uncertainty in standardizing the HCl could be not weighing out the Na2CO3 accurately, as well as the volume pf HCl titrated.

 

Table 2 illustrates the results of the standardization of solium hydroxide (NaOH).

 

Table 2. Standardization of NaOH

Mass of KHP (g)

Vol. of NaOH titrated (mL)

Molarity of NaOH (mol/L)

0.5102

27.95

0.08938

0.5106

28.31

0.08832

0.5098

28.13

0.08876

 

The average NaOH molarity was 0.08882, while the standard deviation was calculated to be 5.325 E-4.  The RSD was found to be 0.60%. 

 

For this experiment, 2.3617 g of unknown solid was weighed out.  For the first set of titrations, the indicator bromocresol green was used.  Table 3 illustrates the results from these titrations.  These are the corrected values by factoring in the correction on the pipet.

 

Table 3. Titration Results using Bromocresol Green Indicator

Trial

Vol. of HCl used (mL)

Avg. HCl Volume (mL)

Standard Deviation

1

36.455

36.76

0.35

2

36.685

 

 

3

37.142

 

 

The percent relative standard deviation was calculated to be 0.95%.

 

Table 4 illustrates the results from the titrations from the addition of NaOH to the unknown solution, and adding phenolphthalein indicator.

 

Table 4. Titration Results of addition of NaOH using Phenolphthalein Indicator

Trial

Vol. HCl used (mL)

Avg. Vol. HCl (mL)

Standard Deviation

1

30.28

30.42

0.20

2

30.32

 

 

3

30.65

 

 

The percent RSD was calculated to be 0.68%.

 

To calculate the weight percent of HCO3 and CO3 2- , you must first find the total alkalinity.  The total alkalinity is calculated using the following equation:

 

Equation 1.    Total Alkalinity = [HCO3 ] + 2[CO3 2 ]

 

The total alkalinity was calculated to be 0.003812 mols of H+ for this experiment.  This can now be used along with the bicarbonate concentration to calculate the carbonate concentration.  The amount of HCl left over should be subtracted from the total amount of NaOH in solution, which in this experiment yields 0.001286 mol OH that are reacted, which is equal to the moles of HCO3 in the solution.  The HCO3 moles are then subtracted from the total alkalinity, and used to solve for x.  X is equal to the amount of CO3 2- present in the solution.  The CO3 2- and HCO3 moles are then converted into grams, and divided by the initial amount of unknown solid weighed out (2.3617 g).  This yields the weight percents.  The weight percent for NaHCO3 was calculated to be 39.12% (+/- 7.4%), while the weight percent for Na2CO3- was calculated to be 56.7% (+/- 17%).

 

BaCl2, when added to the solution, can precipitate some of the OH out of the solution, therefore yielding a higher amount of HCO2.  The carbonate amount present is calculated using the bicarbonate amount present, and if this occurs, than the amount of carbonate present is going to be lower due to the bicarbonate amount being higher.

 

With the standardization of the HCl, an error must have occurred and therefore yielded a high amount of uncertainty on the carbonate and bicarbonate amounts present.  This could have been because of not reading the endpoint correctly and/or not reading the color change correctly during the titration. 

 

Conclusion

 

In this experiment, the amounts of carbonate and bicarbonate present in an unknown solid were calculated.  The amount of carbonate present in the solution was 56.7% (+/- 17%) wt%, while the amount of bicarbonate present in the solution 39.12% (+/- 7.4%) wt%.  A significant error in this experiment caused uncertainties to be yielded.  This could have occurred during the procedure part of the experiment or by misreading the measurements taken in this experiment.