By Julia Latterell
Introduction
For our research topic, we determined the amount of buffering that is in five different kinds of lemonade. In order to do this we found the pH values of Country Time,
Crystal Light, Old Country, Minute Made, and freshly hand squeezed lemonade and then titrated sodium hydroxide, NaOH, into each brand of lemonade. While doing this, we recorded the data with a CBL probe, which graphed the result of the addition of our buffer, citric acid, with NaOH. When completed, the graph showed a titration curve of the buffering effect. After studying the results, we were able to determine which brand of lemonade is the most acidic and which brand was most buffered.
Background Chemistry: Buffers
Buffers in lemonade come from the citric acid (structure seen below) and the citrate ion (C
6
H
5
O
7
3) and are “incorporated into various products to help control the effects of varying pH” (1). The buffer in lemonade, citric acid and the citrate ion control acidity.
The chemical formula for citric acid, H
3
C
6
H
5
O
7
, is shown by this structure
(2: Purdue Chemistry)
When citric acid and water reaction, the products of the reaction form the citrate ion
(C
6
H
5
O
7
3-
) and the hydronium ion (H
3
O
+
).
1. H
2
O (l) + H
3
C
6
H
5
O
7
<--------> H
3
O + + H
2
C
6
H
5
O
7
-
(base) (acid) (acid) (base)
2. H
2
O (l) + H
2
C
6
H
5
O
7
-
<--------> H
3
O
+
+ HC
6
H
5
O
7
2-
(base) (acid) (acid) (base)
3. H
2
O (l) + HC
6
H
5
O
7
2-
<--------> H
3
O
+
+ C
6
H
5
O
7
3-
(base) (acid) (acid) (base)
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So the final reaction of citric acid and water turns into this equation
3 H
2
O (l) + H
3
C
6
H
5
O
7
<--------> 3 H
3
O
+
+ C
6
H
5
O
7
3-
(base) (acid) (acid) (base)
The reaction of citric acid with water experiences a shift in equilibrium when more H
3
O
+
is added. The equilibrium shifts to the reactant side and the H
2
C
6
H
5
O
7 increases. When base is added, the equilibrium equation shifts to the product side and more citrate ion is created to balance the equilibrium. The Ka expression, which is shown in this equation, must stay at equilibrium, which occurs by the reaction shifting.
Ka = [H
3
O + ] [C
6
H
5
O
7
3]
[H
3
C
6
H
5
O
7
]
Background Chemistry: NaOH
For our project, we titrated the lemonades with NaOH. Adding NaOH to the five types of lemonade neutralized the citric acid. When NaOH was added to the lemonades, all three of the H
+
ions on the citric acid are neutralized with the OH
-
from the base to produce water, to produce neutral water.
H
3
C
6
H
5
O
7
+ 3 NaOH ----------> 3 HOH + Na
3
C
6
H5
5
O
7
Procedure
1.
We squeezed out at least 100.0 mL of real lemons by using a glass juicer. We stored it in a stock bottle labeled real lemons.
2.
We stored at least 100.0 mL of Minute Maid, Country Time, Old Country, and
Crystal Light lemonade into stock bottles labeled accordingly. Stored all stock bottles at room temperature. (We repeated some titrations two times).
3.
We made a 0.50 M solution of NaOH. a.
We weighed out 20.0 grams of NaOH. b.
We slowly mixed the NaOH into a 1.0 liter volumetric flask filled half ay with dionized, distilled water. c.
We used a stir plate to dissolve the NaOH faster. d.
Once the NaOH was dissolved, we put Parafilm over the opening and shook well. e.
Then we filled the flask to the etching with dionized, distilled water with a wash bottle. f.
We stored the NaOH in a stock bottle labeled 0.50 M NaOH. g.
We standardized the base by titrating the 0.50 M NaOH into 1.0 grams of oxalic acid. We added an indicator to the oxalic acid. After we reached endpoint, we calculated the molarity of the NaOH.
4.
We tested the neutralization reaction between NaOH and HCl.
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a.
We poured 50.0 mL of 1.0 M into an Erlenmeyer flask and added 100.0 mL of dionized, distilled water. b.
Then we filled the buret with 0.50 M NaOH to 0.00 mL. c.
We put the Erlenmeyer flask onto a stir plate and added an indicator to the
HCl. d.
Then we titrated slowly until the stoichometric point, which is signified when the indicator changes color. e.
We put the pH probe into the HCl. f.
Then we graphed the changes in pH during the titration by using a CBL and ChemBio program on the TI-83 graphing calculator. g.
Then we plugged the calculator into a laptop by using a computer link. h.
Afterwards, we uploaded the graphs into Logger Pro.
5.
We titrated 0.50 M NaOH into real squeezed lemonade, Minute Maid, Old
Country, Country Time, and Crystal Light. a.
First, we put 100.00 mL of lemonade into an Erlenmeyer flask and added an indicator. We put the flask on a stir plate. b.
We filled the buret up to the 0.00 mL line with NaOH. c.
We made sure to keep the conditions the same for all five lemonades. We kept them all at room temperature and titrated with the same buret. We kept the NaOH flow consistent by letting it flow at top speed for each titration. d.
We had to repeat a few titrations because the time we allowed on our calculator was no enough time for the lemonade to reach the stoichometric point. e.
We compared the graphs of the six titrations and distinguished the buffer patterns by the differences in the graphs. We put the pH probe into the lemonades and recorded the pH of each lemonade. We graphed by repeating steps f through h in part 4.
Results
Table 1: Solution vs Time at Stoichiometric Point (sec)
Solution
Country Time Lemonade
Time at Stoichiometric Point(sec)
54.4
Crystal Light Lemonade
Minute Maid Lemonade
48.1
41.6 fresh squeezed lemons > 249.6
Old Country Lemonade 35.7
Table 1 represents the time it took for each of the lemonades to reach its stoichiometric point. At 54.5 seconds Country Time Lemonade reached its stoichiometric point, at 48.1 seconds Crystal Light Lemonade reached its stoichiometric point, at 41.6 seconds Minute Maid Lemonade reached its stoichiometric point, Fresh
Squeezed Lemonade took the longest time to reach its stoichiometric point, which we did
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not end up reaching, and at 35.7 seconds Old Country Lemonade reached its stoichiometric point.
For the titration of NaOH and HCl and the titration of NaOH and fresh squeezed lemons, we used 0.5 second intervals and for the four other titrations, we used 0.2 second intervals. We used different intervals because we had to re-titrate HCl and fresh squeezed lemons over a longer period of time.
Figure 1: pH Level vs Time (sec) NaOH + HCl
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Figure 1 represents NaOH being titrated into HCl, with the stoichiometric point reached at 231.8 seconds at a pH of 1.8399.
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Figure 2: pH Level vs Time (sec) Country Time Lemonade
Figure 2 represents Country Time Lemonade reaching the stoichiometric point at
54.4 seconds at a pH of 7.0818.
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Figure 3: pH Level vs Time (sec) Crystal Light Lemonade
Figure 3 represents Crystal Light Lemonade reaching the stoichiometric point at
48.1 seconds at a pH of 7.0547.
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Figure 4: pH Level vs Time (sec) Minute Maid Lemonade
Figure 4 represents Minute Maid Lemonade reaching the stoichiometric point at
41.6 seconds at a pH of 6.9063.
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Figure 5: pH Level vs Time (sec) Fresh Squeezed Lemons
Figure 5 is representing Fresh Squeezed Lemons not successfully reaching their stoichiometric point within the allotted time.
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Figure 6: pH level vs Time (sec) Old Country Lemonade
Figure 6 is representing Old Country Lemonade reaching the stoichiometric point at 35.7 seconds at a pH of 6.9959.
Discussion
By looking at Table 1 and Figures 1-6, we concluded that real squeezed lemons had the most buffering compared to the four other types of lemonades. Even though the real lemons took the longest time to reach the stoichiometric point, we were able to successfully determine that with a few more seconds we think that it would have reached the stoichiometric point if we had used longer intervals. We also determined that real lemons had the highest pH, exceeding 6.5, because the higher the pH the longer it took to reach the stoichiometric point.
Country Time Lemonade reached the stoichiometric point at 54.4 seconds, Crystal
Light Lemonade reached the stoichiometric point at 48.1 seconds, Minute Maid
Lemonade reached the stoichiometric point at 41.6 seconds, and Old Country Lemonade reached the stoichiometric point at 35.7 seconds. By comparing these results to that of real squeezed lemons, we determined that real squeezed lemons had the most citric acid.
We were able to determine this because the longer the plateau the longer the buffering effect, NaOH, took to neutralize the acid.
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One way to possibly improve this project would be to titrate NaOH into actual citric acid instead of squeezed lemon juice. This would be helpful because we don’t know what else is in the lemons and titrating actual citric acid may be a better buffer to compare with the other lemonades. This reaction between citric acid and NaOH would look like this:
H
3
C
6
0
7
+ 3 NaOH ----------> 3 HOH + Na
3
C
6
H
5
O
7
Sources Cited
1. “The Buffer in Lemonade,” Chemistry with CBL, experiment 32.
2. Purdue Chemistry “Citric Acid,” (n.d.)
<http://www.chem.purdue.edu/gchelp/molecules/citric.html> Accessed 2004 April 28.
3. G. Hill, J. Holman, Chemistry in Context (Nelson, Surrey UK, 1989) pp.387-390.
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