Determination of the Concentration of Phosphoric Acid in Colas Using Titration

Introduction

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.[footnoteRef:0] 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.

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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.[footnoteRef:1] 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.

[footnoteRef:2] 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. [1: ] [2: ]

To calculate the concentration of phosphoric acid, several steps are needed to find all the values needed.

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.

Materials

1. 5 brands of colas: Coca Cola, Coca Cola Zero, Local Cola (foreign language), Royal Crown Cola, Pepsi
2. 2 liters of NaOH solution (in excess)
3. 2 liters of distilled water (in excess)
4. 0.11 grams of oxalic acid
5. Phenolphthalein indicator

Apparatus

1. Titration stand
2. 2 x magnetic stirrer/heating plate
3. 2 x stirring magnets
4. 2 x pH probe
5. 2 x Datalogger
6. 2 x 100 ml beakers
7. 400 ml beakers
8. 250 ml graduated cylinder (± 2 ml at 20°C)
9. 2 x burette (±0.05cm)
10. Pipette (±0.06cm)

Safety Considerations

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. [3: ] [4: ]

Environmental Considerations

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.

Ethical Considerations

No ethical considerations apply to this experiment.

Variables

Independent Variable

Different brands of colas; Coca Cola, Coca Cola Zero, Local Cola, Royal Crown Cola, Pepsi.

The different brands of colas will contain different concentrations of phosphoric acid which are not dependent on the other variables of this experiment.

Dependent Variable

The volume of 0.014 sodium hydroxide (NaOH) required to neutralize the phosphoric acid of each cola.

The volume required is dependent on the differing concentrations of phosphoric acid in each brand.

Controlled Variables

The same two burets will be used for this experiment.

This will limit the variability that may induce errors caused by the usage of multiple burettes, the use of two rather than one will be further discussed in the evaluation.

Two point calibration of the pH probe.

Adjusting the meter at two pH values, pH 4 and pH 7. Being calibrated at two points allows for a greater accuracy of pH measurements.

Keeping the pH probe clean.

The pH probe is rinsed after every trial to avoid contamination from previous solutions with the one being tested.

Same solution of NaOH for this entire experiment.

This solution is prepared in excess so that all trials of this experiment are consistently standardized with the same solution.

Boil the gaseous drinks/colas for 25 min.

The gaseous drinks were boiled to remove the carbon dioxide found in gaseous drinks. Carbon dioxide can form carbonic acid, which also neutralizes NaOH, increasing the overall volume of NaOH needed for neutralization.

Standardization of the NaOH solution using oxalic acid.

When making the two liters of sodium hydroxide solution, it's presumed to have a concentration of 0.010 mol. However, due to the large volume, the estimated value is prone to error, so the standardization NaOH is required.

The 5 trials of each sample is carried out in one day. A total of 5 days were used.

The longer the solution is in contact with the air, the more atmospheric carbon dioxide is absorbed. So, performing the trials on the same day would ensure the amount of carbon dioxide in each trial is relatively close.

Preliminary trial

A preliminary experiment was carried out to standardize the sodium hydroxide solution by titrating sodium hydroxide against a known concentration of 0.044 mol of oxalic acid. Because sodium hydroxide absorbs water when in contact with the air, it's difficult to weigh a dry sample. So instead, it’ll be standardised with a primary standard, oxalic acid dihydrate. This will be more accurate because primary standards do not change its composition when in contact with the air, making it more ideal for producing a solution with a definite concentration. The main experiment uses the sodium hydroxide solution to standardize phosphoric acid, so an accurate measure of its concentration is necessary for later calculations opposed to an estimate. This will be an endpoint titration, and the indicator phenolphthalein is chosen because this is a weak acid strong base titration, the equivalence point at pH > 7 puts it within the range of phenolphthalein (8.3-10.0).

The equation for this reaction is: H2C2O4(aq) + 2 NaOH(aq) → Na2C2O4(aq) + 2H2O(l)

The mean concentration of NaOH is found to be 0.0133 moldm-3, with slight deviations. Increasing the confidence of the value for the concentration of NaOH, originally estimated to be 0.010 moldm-3 , but now with an precise experimental value.

Methodology

Titration of gaseous drinks containing phosphoric acid.

1. Measure 250 ml of the gaseous drink into a graduated cylinder.
2. Pour the 250 mlgaseous drink into the 400 ml beaker.
3. Boil the 250ml gaseous drink on a heating plate for 30 mins.
4. Pour the boiled 250 ml gaseous drink back into the graduated cylinder and add distilled water until it reaches 250 ml
5. Pipette 25mL of the cooled gaseous drink solution into 100 ml beaker.
6. Calibrate the pH probe, then submerge the pH-sensitive part of the probe into the 25ml of cooled gaseous drink solution.
7. Rinse the burette with the standardized 0.10 molNaOH solution and then fill the burette to 50ml.
8. Set up the experiment with the beaker containing the titrant placed underneath the burette containing the NaOH solution.
9. Proceed to titrate the NaOH solution against the cooled gaseous drink solution until a pH of 8 is reached.
10. A pH of 8 is chosen so that first equivalence point is reached on the titration curve of phosphoric acid and nearing the second equivalence point, only the first is needed.
11. Repeat steps 1-7 for each the remaining gaseous drinks, with 5 trials per drink to produce reliable data.
12. Determine the volume of NaOH required to reach the equivalence points and calculate the concentration of phosphoric acid for each gaseous drink.

Samples (Each trial uses 25 ml)

Volume of 0.0133 molNaOH solution

Concentration of Phosphoric Acid (moldm-3)

Mol of Phosphoric Acid

Beverage	Concentration (g/L)	Error (g/L)
Coca Cola	11.0	5.85 x 10^-3
Local Cola	14.4	7.66 x 10^-3
RC Cola	10.6	5.64 x 10^-3
Pepsi	13.7	7.29 x 10^-3
Coca Cola Zero	9.0	4.79 x 10^-3

Calculations

Preliminary Trial: Sodium Hydroxide against Oxalic Acid

Sample Calculation for trial 1

• Mol of oxalic acid.

n = m/Mr 0.11/90 = 0.0012

• Concentration of the oxalic acid solution.

c = n/v 0.0012/0.25 = 0.005

• Mol of oxalic acid used in the titration (25cm3).

n = c x v 0.005 x 0.025 = 0.000125

• Chemical equation ratio.

H2C2O4(aq) + 2 NaOH(aq) → Na2C2O4(aq) + 2 H2O(l)

1:2 0.000125 x 2 = 0.00025 mol of NaOH

• Concentration of sodium hydroxide solution.

c = n/v 0.00025/0.0188 = 0.0133

Calculation of Concentration Phosphoric Acid

Following a simplified method, the known concentration of the titrant, sodium hydroxide, is used to find the unknown concentration of the analyte, phosphoric acid.

The equation C1 x V1 = C2 x V2 is used.

C2 = = = Concentration of H3PO4

Sample calculation for the phosphoric acid found in Coca Cola.

= 5.85 x 10-3 moldm-3

Propagation of Error Calculations

Conclusion

Discussion

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+

All phosphoric acid molecules are

This experiment investigated the concentration of phosphoric acid in various gaseous drinks.

Forming a buffer, which resists a change in pH when small amounts of acids or base is added.

Strengths

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.

The methodology does account for errors associated with measuring volume. The 250 ml graduated cylinder has a relatively high uncertainty error, however, the measurements for the most important figures, volume of NaOH and volume of gaseous drink solution are measured with

Using a volumetric pipette to measure the volume of the gaseous drink solutions and the use of a burette greatly reduces instrumental errors

Another strength of this experiment is the use of two point calibration when calibrating the pH probe.

Buffer solutions of pH 4 and 7 were used to adjust the pH probe at two points. The benefit of a two point calibration opposed to a one point calibration is that its accurate at two points, pH 4 and 7, so any input points are rescaled with two reference points which helps correct offset and sloping errors.[footnoteRef:5] [5: ]

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.