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Page 1 of 4 Titrimetric Determination of Sodium Carbonate.pdf Titrimetric Determination of Sodium Carbonate Introduction: This experiment involves the titration of sodium carbonate by a strong acid, HCl. This type of analysis is important in determining the buffering capacity of natural waters through a measurement of the carbonate, CO 3 2- and bicarbonate, HCO 3 - concentrations. The equilibria reactions are: H 2 CO 3 (aq) + H 2 O ( l ) H 3 O + (aq) + HCO 3 - (aq) pK a1 = 6.4 HCO 3 - (aq) + H 2 O ( l ) H 3 O + (aq) + CO 3 2- (aq) pK a2 = 10.3 CO 2 (g) + H 2 O ( l ) H 2 CO 3 (aq) pK g = 1.48 In this experiment a solution of hydrochloric acid will be standardized against pure sodium carbonate and then used to determine the percentage of sodium carbonate in an unknown sample. Reagent grade anhydrous sodium carbonate is suitable for use as a primary standard for titrations of strong acids. In this experiment, since the standard is the same compound as the substance being analyzed, determinate errors in the detection of the end-point will be minimized. The analytical reaction that will happen during your titration is H 3 O + (aq) + CO 3 2- (aq) H 2 CO 3 (aq) H 2 O ( l ) + CO 2 (g) (Cl - and Na + are spectators.) Note that each mole of carbonate requires two moles of acid for complete titration. Titration to the bromocresol green end-point ensures that all of the carbonate and bicarbonate have been converted to H 2 CO 3 and then to H 2 O and CO 2 . Before you come to class: Record the molar mass of sodium carbonate, Na 2 CO 3 . Write each of the three equilibria reactions (see above). Write the analytical reaction (see below) in your lab notebook. Record the acid-base indicator that will be used in this experiment as well as the color change that occurs. List all of the reagents (formulas and names) in your notebook. . Also include the NFPA ratings for any compounds with a non-zero rating. Familiarity with the reagents will help you avoid costly mistakes during the experiment. Reagents: sodium carbonate Unknown soda ash sample 0.1 M HCl bromocresol green indicator Safety Considerations: You will be drying your unknown in the oven this week, and you will be heating your titration solutions to boiling. Do not touch hot class with your hands or with nitrile gloves. Use your silicone mitt, your crucible tongs, and/or your beaker tongs to carry hot glassware back to your station. Check all glassware for cracks and/or stars before using it. If your glassware is damaged, throw it in the broken glass and ask your instructor for another one. When heated, glassware with cracks and/or stars will shatter.

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Page 1: 2-Titrimetric Determination of Sodium Carbonatecoryw.people.cofc.edu/resources/2-Titrimetric-Determination-of...Titrimetric Determination of Sodium Carbonate.pdf ... point ensures

Page 1 of 4 Titrimetric Determination of Sodium Carbonate.pdf

Titrimetric Determination of Sodium Carbonate

Introduction: This experiment involves the titration of sodium carbonate by a strong acid, HCl. This type of analysis is important in determining the buffering capacity of natural waters through a measurement of the carbonate, CO3

2- and bicarbonate, HCO3- concentrations. The

equilibria reactions are:

H2CO3 (aq) + H2O (l) ⇄   H3O+ (aq) + HCO3

- (aq) pKa1 = 6.4

HCO3- (aq) + H2O (l) ⇄   H3O+

(aq) + CO32-

(aq) pKa2 = 10.3 CO2 (g) + H2O(l) ⇄ H2CO3 (aq) pKg = 1.48

In this experiment a solution of hydrochloric acid will be standardized against pure sodium carbonate and then used to determine the percentage of sodium carbonate in an unknown sample. Reagent grade anhydrous sodium carbonate is suitable for use as a primary standard for titrations of strong acids. In this experiment, since the standard is the same compound as the substance being analyzed, determinate errors in the detection of the end-point will be minimized. The analytical reaction that will happen during your titration is

H3O+

(aq) + CO32-

(aq) ⇄ H2CO3 (aq) ⇄ H2O (l) + CO2 (g) (Cl- and Na+ are spectators.)

Note that each mole of carbonate requires two moles of acid for complete titration. Titration to the bromocresol green end-point ensures that all of the carbonate and bicarbonate have been converted to H2CO3 and then to H2O and CO2. Before you come to class:

• Record the molar mass of sodium carbonate, Na2CO3. • Write each of the three equilibria reactions (see above). • Write the analytical reaction (see below) in your lab notebook. • Record the acid-base indicator that will be used in this experiment as well as the color

change that occurs. • List all of the reagents (formulas and names) in your notebook. . Also include the NFPA

ratings for any compounds with a non-zero rating. Familiarity with the reagents will help you avoid costly mistakes during the experiment.

Reagents: sodium carbonate Unknown soda ash sample 0.1 M HCl bromocresol green indicator Safety Considerations:

• You will be drying your unknown in the oven this week, and you will be heating your titration solutions to boiling. Do not touch hot class with your hands or with nitrile gloves. Use your silicone mitt, your crucible tongs, and/or your beaker tongs to carry hot glassware back to your station.

• Check all glassware for cracks and/or stars before using it. If your glassware is damaged, throw it in the broken glass and ask your instructor for another one. When heated, glassware with cracks and/or stars will shatter.

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Safety Considerations continued: Waste Disposal: All solutions in today’s lab can be disposed of down the sink with running water. Labeling solutions: You will be using your 1 L bottles (plastic and brown glass) to hold different solutions over the course of the semester. Whenever you put a new solution in the bottle, you must label the bottle correctly – contents, NFPA diamond, your initials, the date. Obtain a label from the roll in the lab and look up the NFPA diamond information from the MSDS for the solution. Be sure to look up the MSDS for the correct concentration – the NFPA diamond for 12 M HCl is different from that for 1 M HCl. If you can’t find the MSDS for your exact concentration, use the closest concentration that is higher than yours (better to be too safe than not safe enough). When you are finished, your labeled bottle should look similar to the picture to the left. You’ll be labeling solutions like this for many experiments this semester. Proper labeling is an important part of working in a safe laboratory environment, so be sure you and your neighboring students are always working with solutions that are properly labeled.

Summary Procedure: A stock solution of 0.1 M HCl will be provided. You will need about 500 mL of this solution to complete this experiment. The HCl should be standardized against reagent grade (pure) sodium carbonate (a primary standard). By doing this you will determine the exact molarity (to 4 significant figures) of the HCl solution and use this number for all calculations, but you can label your solution as 0.1 M HCl.

Preparing unknown for pilot titration and drying

1. Do this before putting your unknown in the oven. Weigh one 0.45 gram sample of your unknown for titration along with the pure sodium carbonate. The results of this titration will be used to estimate the sample size of the unknown to be used. You’ll come back to this sample in step #13.

2. Transfer remainder of unknown to weighing bottle. You may label your weighing bottle with your initials and its contents using a Sharpie (this can be removed later using methanol or acetone). Put the bottle and its lid (apart, do not put the top on the weighing bottle at this time) into a 250 mL beaker and place a small ribbed watch glass on top. Using a Sharpie, label the beaker with your initials and place it in the 110+ °C oven for one hour.

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3. When you take your sample out of the oven, you’ll need to put it in the dessicator. Make sure you know how to use the dessicator before you attempt to remove the lid. Ask your instructor if you don’t know how. One of the most commonly broke pieces of glassware in the quant lab is a dessicator lid, and they cost hundreds of dollars to replace. Put the weighing bottle and lid into a dessicator immediately after removing from oven, in order to keep it dry as it cools. Once it is cool you may put the lid on the weighing bottle.

Weighing of Primary Standard Na2CO3:

4. Note the % Na2CO3 (assay) on the reagent bottle. Record this in your notebook. Use this % purity in your calculations.

If the primary standard sodium carbonate was already dried for you, go on to step 7.

5. Dry 1.5 to 2.0 g of pure Na2CO3 in a glass weighing bottle in the oven for at least 1 hour. Drying not only removes water but also can reduce bicarbonate impurities.

6. Allow the sodium carbonate to cool in a desiccator. 7. Weigh by difference (to the nearest 0.1 mg) three or four 0.20-0.25 gram portions of the

dry material into clean 500 mL Erlenmeyer flasks.

Titration of Standard Sodium Carbonate: perform 3 trials.

8. Add about 100 mL of distilled water to each sample of pure sodium carbonate in the 500 mL flask and swirl gently to dissolve the salt.

9. Add enough drops of bromocresol green indicator to one of the flasks so that a strong blue color is observed. Be sure to use the same bottle of indicator each time and add the same number of drops. Titrate with the HCl solution to an intermediate green color.

10. At this point stop the titration and boil the solution gently on one of the hotplates in the hood. (Make sure your buret isn’t leaking and don’t start using it for another sample – you still need to finish this titration to get to the yellow endpoint, and you’ll use the entire titrated volume for your calculations. So this buret needs to stay as-is until this sample is completely titrated.) Bring the solution to a boil for 2 to 3 minutes, taking care that no solution is lost in the process. Cool to room temperature – ask your instructor for some ice if it’s not already available. Bromocresol green is an example of an indicator whose color strongly depends upon temperature.

11. After allowing the flask to cool, When it’s cool, wash the inside walls of the flask with distilled water from a wash bottle and then finish the titration to the yellow endpoint. Titrate to the nearest half-drop. Perform at least three additional replicates. Complete one titration before going on to the next. The boiling step is necessary to yield a sharper endpoint by removing dissolved CO2 from the solution and forcing the equilibrium

H3O+

(aq) + HCO3- (aq)  ⇄ H2CO3 (aq) ⇄ CO2 (g) + H2O(l) to be driven to the right.

12. With your titration data, calculate the concentration of the HCl solution. The concentration of your HCl should be very close to 0.1000 M.

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"Pilot Titration" of Unknown Sodium Carbonate:

13. Add about 100 mL of distilled water to the pilot unknown sample in the 500 mL flask and swirl gently to dissolve the salt.

14. Add the bromocresol green indicator to the flask and titrate with the HCl solution to the intermediate green color.

15. At this point stop the titration and boil the sample and continue the procedure as was done for the primary standard.

16. Using the number of mL of HCl required in the pilot titration, calculate by ratio the number of grams to be taken for each unknown analysis that would require approximately 35 mL of the 0.100 M HCl.

mL used in pilot titration = 35 mL 0.45 g g to use for titrations

17. When you calculate the number of grams that would require approximately 35 mL, if you determine that you don’t have enough unknown to do three trials, do the following:

1. Weigh out one sample with the amount you have calculated for 35 mL. 2. Take the remainder and divide it into two – get the exact weight for each. By

doing this, you have 3 samples so you can do 3 trials.

Titration of Unknown Sodium Carbonate: perform 3 trials.

18. Add about 100 mL of distilled water to each sample of the unknown sodium carbonate in the 500 mL flask and swirl gently to dissolve the salt.

19. Add the bromocresol green indicator to one of the flasks, and titrate with the HCl solution to an intermediate green color.

20. At this point stop the titration and boil the solution gently using a Bunsen burner and tripod stand. Bring the solution to a boil for 2 to 3 minutes, taking care that no solution is lost in the process. Cool to room temperature and wash the inside walls of the flask with distilled water from a wash bottle.

21. After allowing the flask to cool, titrate to the yellow endpoint. Titrate to the nearest half-drop.

22. Do your calculations of % Na2CO3 as you go. Your result for each trial should be within 1% of each other.

When finished, be sure to rinse out your buret several times with water from the sink and store it in the community buret holder, upside down with the stopcock open (always.) Also rinse out all Erlenmeyer flasks and any other equipment from your drawers. Be sure that common glassware gets put away in the common drawers, while student glassware gets stored in the L01, L02 or L03 drawer (always.)

Report the result in terms of % Na2CO3 in your unknown. Your final result should be between 15% to 55%. Last revision: 7/7/13