Titrations of Powdered Drink Mixes

Introduction

Adding flavor to an otherwise flavorless drink is common for people of all ages by using drink mixes. Most powdered drink mixes contain Vitamin C and citric acid. Vitamin C is known chemically as ascorbic acid. Vitamin C is a vitamin in the body that is important for maintaining health. Citric acid acts as a reciprocal to the sugar added to drink mixes that balances out the sweetness with sour1. The ratio of Vitamin C to citric acid is 100:1 is drink mixes such as in this case, Kool-Aid.

This experiment demonstrates the Vitamin C to citric acid ratio as well as two different types of reactions. An acid-base reaction and a redox reaction will occur which determines the total amount of acid, citric, and ascorbic acids in a package of Kool-Aid mix. Sodium hydroxide and potassium iodate will be used as titrants within this titration. The concentration of the titrant is known and is used to determine the concentration of a reagent within a solution in a titration.

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An indicator must be added to the solution to indicate the endpoint of the reaction, when the color of solution changes. Once endpoint is reached, these equations will be used to determine amount of acid in solution:

In the first equation, there are 3 moles of NaOH to 1 mol of citric acid, this is used to determine amount of citric acid in the beverage sample. In the second equation, there is 1 mol of NaOH to 1 mole of ascorbic acid, so the amount of sodium hydroxide to ascorbic acid in the solution is proportional.

The second equation is a redox titration used to determine the amount of ascorbic acid in the solution. The following equation will be used to calculate the moles of ascorbic acid from iodine in the solution using stoichiometry.

A starch solution is used as the indicator for this reaction due to iodine reacting with the starch solution to create a starch complex. The goal of this experiment is to determine the amount of total acid, citric acid, and ascorbic acid or Vitamin C, using acid-base and redox titration techniques and stoichiometric properties.

Experimental Procedure

The materials for this experiment are powdered drink mix packets, 0.09818M NaOH, 0.00050M KIO3, 1.0M HCl, 0.5% starch solution, and solid KI. The equipment for this experiment is 50mL buret, 250mL Erlenmeyer flask, 10mL volumetric pipets, graduated cylinder, and a magnetic stir bar.

I. Preparation of powdered drink mix sample

First, the weight of the package of powdered drink mix was recorded. Then, a 250mL volumetric flask was filled halfway with distilled water and the powdered drink mix was added then swirled to dissolved. Then, using distilled water, the solution was diluted to the mark on the volumetric flask and fully mixed.

II. Preparing a buret

First, a buret was cleaned with soap thoroughly. The buret was then rinsed with distilled water followed by 2-3 mL of sodium hydroxide or potassium iodate, depending on which titration that would be done. This step was repeated 3 times so that the buret was fully prepared. The reagent was then filled to above the zero mark in the buret and drained between the zero to one mL marks, getting rids of any air bubbles inside the buret.

III. Determine the moles of total acid

150 mL of the 0.09818 M NaOH was added to a dry beaker. The initial volume was recorded. Then, 10 mL of the previously made powdered drink mix was added to an Erlenmeyer flask with 40 mL of distilled water, 10 drops of thymol blue indicator, and a magnetic stir bar. The flask was placed on a stirrer under the buret with the buret tip inside the mouth of the flask. The stirrer was turned on while the heat remained off. NaOH solution was added in increments of about 1 mL while continuously stirring until the color of the solution changes slightly but did not stay, followed by drop by drop increments until the new color stays for 30 seconds. The final volume was then recorded. The final volume was subtracted by the initial volume to determine the volume of NaOH that is necessary to reach completion. To determine the most accurate and precise results, the titration was repeated 3 times with each volume of NaOH being within 0.15 mL of each other.

IV. Determine the moles of ascorbic acid

The exact molarity of 0.00050 M potassium iodate was recorded. Then a buret was then filled with potassium iodate with the initial volume was recorded. 25 mL of the powdered drink mix solution was added to an Erlenmeyer flask with 20 mL of distilled water, 0.5 g of KI, 3 mL of 1.0 HCl, 3 mL of 0.5% starch solution, and a magnetic stir bar. The flask was then placed on a stirrer and was stirred with the tip of the buret being in the mouth of the flask. After each 1 mL the color of the solution was observed. Then, a color change occurred, lastly longer than 30 seconds which represented the endpoint of the titration was reached and the final volume was recorded. The final volume was subtracted by the initial volume to determine the required volume of potassium iodate to complete the titration. 10 mL was then subtracted from the volume calculated used in the trial titration. The titration was repeated three times with each volume being within 0.15 mL of each other to ensure that the titration was accurate and precise4.

Results

In table 1, the average volume of NaOH required to reach the endpoint of the titration was recorded. Calculation 1 consisted of the molarity along with this average volume was used to determine the moles of NaOH required to reach the endpoint. Then in calculation 2, the ratio of moles required to liters of solution was used to determine the moles of NaOH necessary for a 250 mL solution to reach the endpoint. Table 2 showed the average volume of potassium iodate that was recorded and then it was used to determine the moles of ascorbic acid in calculation 3. Calculation 2 was repeated with the moles of ascorbic acid to determine its molar presence in 250 mL solution.

Ascorbic acid is proportional to the NaOH seen in given calculations in the introduction section, this was also the moles of sodium hydroxide present in the solution. Calculation 2, the moles of sodium hydroxide found, and calculation 5, the moles of NaOH required to neutralize 250 mL solution, were subtracted to determine the moles of NaOH necessary to neutralize the citric acid. The stoichiometric ratio of 3 moles of NaOH to 1 mole citric acid was used in calculation 7 to find the total moles of citric acid in the powdered drink mix solution. That value and the value of moles of ascorbic acid were converted from grams to moles in calculation 8. These values were used to determine the percent by mass of each acid in the powdered drink mix in calculation 9. Finally, the percent RDA of Vitamin C was determined and compared to the one listed on the package drink mix in calculation 10.

Table 1: Total Acid

Calculations:

1) Total moles of NaOH required to neutralize the acid in 10mL sample: (0.03488L NaOH)(0.09818M NaOH)= 0.003385 mol NaOH

2) Total moles of NaOH to neutralize 250mL sample

3) Moles of ascorbic acid in 25mL sample

4) Total moles of ascorbic acid in 250mL sample

5) Moles of NaOH required to neutralize the moles of ascorbic acid in the 250mL sample (1 mol NaOH:1 mol ascorbic acid)

6) Moles of NaOH required to neutralize the citric acid

7) Moles of citric acid present in the 250mL sample using step 6

8) Grams of citric acid and ascorbic acid in drink mix

9) Percent mass of citric acid and ascorbic acid in drink mix

10) Percent RDA of Vitamin C in each serving of powdered drink mix

Discussion

Sodium hydroxide neutralizes the citric acid and ascorbic acid in the first part of this experiment. The indicator changes color to indicate a change once the sodium hydroxide reacts with the acids. This color has to last for at least 30 seconds to represent the endpoint, meaning that all of the acids have been completely neutralized by the sodium hydroxide. Sodium hydroxide has a hydroxide base that reacts with the positive hydrogen of the citric acid. The acids are separated into an anion and also sodium into a cation leaving water in solution once the endpoint is reached2. The second part of the experiment consisted of a redox titration.

Potassium iodate is reacted with a starch solution and a powdered drink mix. The iodine loses an electron, meaning it is oxidized. This creates iodate and a starch complex that reduce the ascorbic acid. Similarly, to the acid base titration, the indicator changes color and when this color change lasts for at least 30 seconds then the reaction is complete, and the iodine has been oxidized. The results were precise due to each measured volume being within 0.15 mL of one another. The standard deviation of this experiment for the acid-base titration was 0.08 while for the redox titration was also 0.08. Since the standard deviation was low, then the results were accurate. Possible errors could occur from leftover sodium hydroxide or potassium iodate in the Erlenmeyer flask that could result in quicker completion.

Another possible error could be that the colors of the endpoint could not be the exact same for each titration. The percent error was 5.10% which means there is a little error possibly from the previous listed errors, but not significant enough to be ineffective. The measured amount of Vitamin C, or ascorbic acid, in the Kool-Aid packet was similar to the calculated RDA3. It was not exact so possible error could come from sodium hydroxide reacting with the salt and in the potassium iodide titration, there could have been a reaction with calcium phosphate. The acid-base titration generated iodine rather than titrating with an iodine solution because the potassium would have resulted in a different reaction with the iodine solution. The dissociation of the potassium iodate molecule generated iodine during the titration.

Conclusion

The main focus of this experiment was to determine the amounts of ascorbic and citric acids by percent mass in a powdered drink mix. The amount of NaOH needed to neutralize the citric acid and the amount of potassium iodate needed to neutralize the ascorbic acid were found through stoichiometric coefficients in the given equations. An acid-base and a redox titration were used to determine the amounts of acid of a solution of an unknown molarity.

References

1. Hu, Y.; Zhang, J.; He, L.; Hu, Y.; Zhong, L.; Dai, Z.; Zhou, D. Effects of Dietary Vitamin C on Growth, Antioxidant Activity, and Immunity in Ricefield Eel ( Monopterus Albus ). Journal of the World Aquaculture Society 2019, 51 (1), 159–170.
2. Max, J.-J.; Chapados, C. Infrared Titration of Aqueous NaOH by Aqueous HCl. Canadian Journal of Chemistry 2000, 78 (1), 64–72.
3. Sigmann, S. B.; Wheeler, D. E. Quantitative Determination of Citric and Ascorbic Acid in Powdered Drink Mixes: A High School or General Chemistry Experiment. Journal of Chemical Education 2004, 81 (10), 1479.
4. Spencer, B. C127 Environmental Chemistry: Titrations of Powdered Drink Mix Lab Handout. Presented at Indiana University, Bloomington, IN. 2019.