CHEM 2000
ANALYSIS OF BAKING POWDER

Introduction 

The soft, springy texture of breads, cakes and other baked goods is due to the presence of bubbles in the cooked dough. These bubbles are the result of adding a leavening agent to the batter. The leavening agent produces a gas, CO₂, which causes the batter to rise. A common leavening agent is baking soda or sodium bicarbonate, NaHCO₃. It reacts with acid to produce carbon dioxide.

NaHCO₃(aq) + H₃O⁺(aq) ® Na⁺(aq) + 2H₂O(l) + CO₂(g)

Leavening requires the presence of both baking soda and an acid. Acidic substances used in recipes for baked goods include sour milk, buttermilk, lemon juice and cream of tartar. Single acting baking powder is a mixture of baking soda and a dry acid such as cream of tartar, KHC₄H₄O₆. The gas producing reaction does not occur until water is added. The reaction is

NaHCO₃(aq) + KHC₄O₄O₆(aq) ® H₂O(l) + CO₂(g) + NaKC₄O₄O₆(aq) 


Baking soda and single acting baking powder have one drawback, and that is that the carbon dioxide is released into the batter all at once as soon as it is mixed. If the batter is not placed into the oven within a short time the gas will be lost and the batter will not rise during baking. 

This problem is avoided by the use of double acting baking powder. Double acting baking powder contains baking soda and two dry acids. Again, no reaction occurs between the acids and the baking soda until water is added. “Double acting” refers to the fact that carbon dioxide is produced in two stages. Some carbon dioxide is produced at room temperature when water is first added to the batter. This results from the reaction of one of the two acids with some of the baking soda. More carbon dioxide is produced when the batter is heated in the cooking process. This results from the reaction of the second acid with the remaining baking soda. The dry acids commonly used in double acting baking powder are calcium hydrogen phosphate, CaHPO₄, which reacts at room temperature and sodium aluminum sulfate, NaAl(SO₄)₄ which reacts at high temperature. The reactions are:
 

The purpose of today’s experiment is to make some observations of the behavior of baking soda and baking powder, to determine the percentage sodium bicarbonate in a commercial double acting baking powder, and to compare the relative amounts of carbon dioxide gas produced in the two stages of gas production.

 The analysis of baking powder for its sodium bicarbonate content will be accomplished with the apparatus shown below. Two experiments  are required to measure the amount of CO₂ available at room temperature and the amount produced at high temperature. In one experiment,  a weighed amount of baking powder, and a vial containing water are placed in the flask. The flask is tightly stoppered. The tubing from the flask is run into a calibrated container which is filled with water and placed in an inverted position in a tub of water. The flask is swirled, causing the water in the vial to mix with the baking powder. Reaction (1) occurs, and the carbon dioxide gas displaces some of the water in the calibrated volume. At this point, the baking powder sample still contains unreacted NaHCO₃ because the baking powder sample does not contain enough CaHPO₄ to react with all of the sodium bicarbonate, and reaction (2), which normally occurs at high temperature, does not occur under these conditions. To determine the total amount of CO₂ gas available from the baking powder, a second experiment  is conducted using the same mass of baking powder as before, and placing vinegar, an aqueous solution of acetic acid, CH₃CO₂H,  in the vial instead of water. The vial contains more than enough acid to react with all of the sodium bicarbonate in the sample, and all of the available CO₂ is released.  The reaction is:

In summary, we have:
Volume of CO₂ obtained with water in the vial = Volume of CO₂ available at room temperature

Volume of CO₂ obtained with vinegar in the vial = Total volume of CO₂ available

Figure 1. Apparatus 

The volume of CO₂ measured with vinegar in the vial is directly proportional to the total amount of sodium bicarbonate in the baking powder sample. The relationship between the mass of NaHCO₃ and the volume of CO₂ is established by means of a calibration graph. A sample calibration graph is shown in Figure 2, and consists of a plot of volume of CO₂ versus mass of NaHCO₃. The calculation of the percentage NaHCO₃ in a baking powder sample is illustrated in Example 1, below. 

Figure 2

Example 1: A 1.75 g sample of baking powder produced 70. mL of CO₂ gas when vinegar was placed in the vial in the apparatus in Figure 1. What is the percentage NaHCO₃ in the sample?
Solution: With vinegar in the vial, all of the NaHCO₃  reacts to produce CO₂.   From the calibration graph (Figure 2), we see that 70 mL of gas is produced by 1.4 g of NaHCO₃. Therefore, the sample contains 1.4 g of NaHCO₃. The percentage NaHCO₃ is given by:

Stockroom

Things for each group to borrow and return on the same day.

Procedure

Part 1,Observations

Observe, and record in your notebook the result of each activity below:
1. Add about 1g solid sodium bicarbonate to 50 mL of water. And stir.
2. Dissolve about 0.5 g of cream of tartar in 50 mL of water, add 1g of sodium bicarbonate and stir.
3. Add 1 g of commercial baking powder to 50 mL of water and stir.
4. Heat the solution prepared in 3., above, on a hot plate. 
 
Part 2, The calibration graph. 

The first step in analyzing the baking powder for sodium bicarbonate is to obtain the  data needed for the construction of a calibration graph. Weigh out 0.2 g of pure NaHCO₃. Record the mass to 0.01 gram. Transfer the NaHCO₃ completely to the filter flask shown in Figure 1. The flask must be dry. Fill the plastic vial to within about 1 cm of the top with vinegar and carefully place the vial upright in the flask without spilling any vinegar. This is best done with the forceps found in your drawer. Place the stopper tightly on the flask. Fill the plastic tub with water. Fill the calibrated volume (jar, in this case.) with water and place it in the water as shown. Insert the end of the tube coming from the flask into the jar. You will probably have to hold the jar to keep it from tipping. Swirl the flask to cause the vial to tip over, thereby causing the vinegar to react with the NaHCO₃ and releasing carbon dioxide. The CO₂ will displace water from the jar. Occasionally swirl the flask to speed up the reaction.
Continue the experiment until bubbles cease emerging from the tube. Remove the tube from the jar, and raise or lower the jar in the water until the water level inside the jar is the same as the level outside. Read the volume of the CO₂ gas on the mL scale on the side of the jar, and record it in your notebook (note that there are two scales on the jar, "mL" and "oz",use "mL"). Repeat the procedure with 0.3 g, 0.5 g, 0.7 g and 0.8 g of pure NaHCO3.

Part 3, Analysis of baking powder. 

Part 3a, Total CO₂ available: Repeat the calibration procedure using 1.9-2.0 g of baking powder in place of the pure NaHCO₃. Again, the flask must be completely dry before adding the baking powder. The volume of CO₂ produced is the total amount available. Do this experiment in duplicate.

Part 3b, CO₂ available at room temperature: Repeat the procedure in Part 3a, above, using exactly the same mass of baking powder except, this time, fill the vial with deionized water instead of vinegar.  The volume of CO₂ measured here is the amount available at room temperature. Again, do the experiment in duplicate.

Results

Include the following:
1. All observations.
2. The data for the calibration graph and the baking powder analysis.
3. The brand of the baking powder.
4.The calibration graph.
5. The complete calculation of: the percentage NaHCO₃ for each trial in Part 3a,  and the average percentage.

Conclusion

Answer each of the following questions.
1.  Explain each of the observations made in Part 1.     
2.  What is the average percentage of NaHCO₃  in the baking powder?
3.  What fraction of the total amount of CO₂  released is released at room temperature?

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This page was last modified February 11, 2004
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