Determination of the Concentration of Phosphoric Acid in Colas Using Titration

Introduction and Background

As a basketball player, I tend to crave soft drinks after practice, and according to Advocate Aurora Health, it’s theorized that the body is seeking to replenish the calories lost when exercising, hence the sugar craving. My personal preference for soft drinks are colas, or specifically Coca-Cola, because of the unique taste found exclusively in colas. Part of this unique flavor is its tangy flavor created by the ingredient phosphoric acid (H3PO4). However, despite having a similar ingredients list among different Colas, some brands prove to be more popular than others, which means that they have different concentrations and ratios of ingredients, and so this investigation will find the different concentrations of phosphoric acid for 5 brands of cola.

Phosphoric acid, a triprotic acid, is not only responsible for the tangy taste found in colas but also doubles as a preservative used by manufacturers to prevent mold and bacterial growth common in sugary drinks. It's also commonly found naturally in foods, as well as being a food additive, which is sufficient for the recommended daily amount of around 700-1250 mg as it is essential for our body’s production of DNA and RNA and repair damaged muscle tissues among other functions.

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And seeing as beverages that contain phosphoric acid have around 500 mg of it, it can easily lead to overconsumption of phosphorus for certain individuals. The overconsumption of phosphorus is linked to a higher risk for osteoporosis and heart disease.

Methodology

First, perform a titration with the weak acid, phosphoric acid, against the strong base, sodium hydroxide (NaOH).

Then plotting the change of pH for every 0.05 ml, a titration curve can be plotted. Based on the titration curve, the equivalence point can be found. And from the volume of sodium hydroxide needed to reach the equivalence point, its mol can be calculated. Which by finding the number of moles of sodium hydroxide required to neutralize phosphoric acid, the concentration of phosphoric acid can be calculated through their 1:1 ratio in this chemical equation. H3PO4(aq) + NaOH(aq) → H2O(l) + Na3PO4(aq)

Hypothesis

Since this is a weak acid strong base titration, it can be predicted the titration curve will follow the trend of a small increase of pH at the beginning, followed by a slow change in pH as the weak acid acts as a buffer. And as enough base is added and moves out of the buffer zone, a significant increase in pH will be observed, signifying the first equivalence point has been reached. After the increase the graph is expected to plateau. The equivalence point is based on the volume of NaOH needed to fully neutralize phosphoric acid, colas with a higher concentration of phosphoric acid will therefore have a higher equivalence point. There is no clear factor to base a prediction for which colas will have more or less phosphoric acid.

Experimental Design

• Materials: Five cola brands, NaOH solution, distilled water, oxalic acid, phenolphthalein indicator.
• Apparatus: Titration setup, magnetic stirrer/heating plate, pH probes, dataloggers, beakers, graduated cylinders, burettes, pipettes.
• Safety Measures: Handling of hazardous chemicals like phosphoric and oxalic acids necessitates protective gear to mitigate risks of skin, eye, and respiratory tract damage.
• Environmental and Ethical Considerations: Ensures responsible disposal of chemical wastes and adherence to ethical standards.

Phosphoric acid is hazardous and causes damage to the skin and eyes, and respiratory tract when ingested. Similarly, oxalic acid is considered toxic, and causes damage to the skin and eyes on contact. Furthermore, ingestion of oxalic acid can cause “severe gastroenteritis and vomiting, shock and convulsions, ingesting 5 to 15 grams is fatal for humans.”[footnoteRef:3] Sodium hydroxide is corrosive and can cause severe irritation of the skin, eyes, mucous membranes, nose, and throat, as well as causing skin and eye burns when in contact.[footnoteRef:4] Appropriate clothing, lab goggles, gloves are recommended for protection when handling these chemicals. Heating/stirring plate is used to boil the gaseous drinks so it needs to be handled with caution to prevent burns.

Great care or preferably, heat resistant gloves should be used when moving hot objects, such as the beakers containing the boiled gaseous drink solutions. If any of the symptoms mentioned above appear, seek medical attention immediately, these symptoms require immediate medical attention. However, it should be noted the majority of the experiment uses diluted solutions of said chemicals, it is only during the preparation of solutions will use the chemicals in their solid form.

Disposal of laboratory wastes: solutions should be washed down the lab sink with excess water to dilute concentrated solutions that are potentially damaging to the pipes and the environment. None of the chemicals are on the red list, so disposal down the drain is acceptable.

Variables Identification

• Independent Variable: Cola brands (Coca Cola, Coca Cola Zero, Local Cola, Royal Crown Cola, Pepsi).
• Dependent Variable: Volume of NaOH required for neutralization.
• Controlled Variables: Consistency in the use of apparatus, calibration of pH probes, maintenance of solution volumes.

Procedure Summary:

1. NaOH Standardization: Using oxalic acid to determine the precise molarity of NaOH.
2. Titration: Gradually adding NaOH to cola samples while monitoring pH changes.
3. Data Analysis: Calculating phosphoric acid concentration from the NaOH volume at the equivalence point.

Experimental Results

Table 1: NaOH Standardization Results

Table 2: Phosphoric Acid Concentration in Colas

Conclusion

Deviations seen in after the equivalence point and after the second plateau near the second equivalence point, deviations at the end isn’t relevant for this experiment, only the first equivalence point is of concern.

The shape of the titration curve. Phosphoric acid, H3PO4, is a weak triprotic acid, but it can be titrated as a monoprotic or diprotic acid. In the case of this experiment it was titrated as a monoprotic acid, only reaching one equivalence point. The initial slope is expected to be steep, as rapid neutralization of phosphoric acid occurs. Because it is a weak acid it doesn’t fully dissociate with water as represented by the equation below.

H3PO4 → H2PO4- + H+

As phosphoric acid continues to be neutralized, its conjugate base dihydrogen phosphate ion, H2PO4-, is formed. This mixture of phosphoric acid and its conjugate base creates a buffer region where it resists a change in pH, which can be observed in all 5 charts, corresponding to the theoretical projections of the titration curve.

NaOH → OH- + Na+

The standardization of sodium hydroxide prior to the main experiment helped establish the known concentration. Five trials were performed, and each molar concentration of sodium hydroxide solution was calculated then averaged, achieving precise results. Rather than an estimate of 0.010 mol of NaOH, after standardization, the value is found to be 0.013 mol, a significant difference when calculating the concentration of phosphoric acid.

This experimental inquiry delineates the variances in phosphoric acid content among different cola brands, offering insights into their taste profiles and potential health impacts. The methodology affirms the hypothesized titration curve behavior, validating the acid's buffering action and the subsequent equivalence point marking complete neutralization. The findings underscore the importance of moderation in cola consumption to mitigate health risks associated with phosphoric acid overexposure.

Weaknesses and Improvements

While the use of the data logger to measure the pH is very accurate, the measurement for the change of pH for every 0.5 cm3 limits the precision of the five trials. This can be seen in charts 1,2,4, where the trials are very close, however, at the equivalence point, a much higher variation is present. This is because a miniscule amount of titrant is able to change the pH of the analyte, and at the equivalence point, even a difference of 0.1 ml will result in a drastic rise. An improvement in the method may implement a test run, find the approximate region of the equivalence point for each sample, and for the following trials. A change of pH per 0.1 ml rather than 0.5 ml will be used when nearing the region of the equivalence point, this implementation should plot a more precise graph.

Two burets were used in the experiment while only using one burette is optimal, due to time constraints, two were used simultaneously and yielded precise results, especially for Coca Cola, Local Cola, and Pepsi, seen in charts 1,2 and 4 respectively. So the use of one burette is desirable and easily achieved by allocating more time for the experiment, so titrations occur one at a time.

In the preliminary trial, the use of a color based indicator, is prone to incorrect judgement of color of the person performing the experiment, leading to potential errors. However, the results are fairly precise, so it doesn’t seem to be a problem.

When boiling the solutions of gaseous drinks to remove carbon dioxide, there were times when they required more than 30 minutes to remove all the visible carbon dioxide. It can be concluded that the time taken to remove carbon dioxide from 250 ml of each brand is different. Therefore, a weakness, as the effects of overheating, and time differences of heating are unknown on how they may impact the experimental results. As an improvement, perhaps a heating mantle can be used, as not only will the heat be distributed homogeneously, it's also thermostated. Allowing the temperature to be controlled, opposed to using a heating plate where the temperature can’t be controlled.

Gaseous drinks, such as the colas in this experiment contain carbon dioxide which would interfere with the results of the experiment. Hence the reason why the samples were boiled to remove the carbon dioxide from the samples. However, during the process of experimentation, the samples are put in beakers with direct contact to the air, allowing atmospheric carbon dioxide to diffuse into the samples. These changes couldn’t be accounted for in the experiment and remain as a source of error. The trials for each sample were therefore conducted on the same day to decrease the time, atmospheric carbon dioxide can diffuse into the solution. Additionally, a watch glass was used to cover the mouth of the beakers and graduated cylinders containing the gaseous drink solution when not in use.

References

1. Advocate Aurora Health. (n.d.). Sugar cravings post-exercise.
2. Akcil, A., & Koldas, S. (2006). Acid Mine Drainage (AMD): Causes, treatment and case studies. Journal of Cleaner Production, 14(12-13), 1139-1145.
3. Egiebor, N.O., & Oni, B. (2007). Analysis of the oxidation of chalcopyrite, chalcocite, galena, pyrrhotite, marcasite, and arsenopyrite. Canadian Metallurgical Quarterly, 46(4), 407-415.