DETERMINATION OF PHOSPHATE IN COLA SOFT DRINKS.docx

IMRAN R. LOURENҪO

812002025

16.09.13

DETERMINATION OF PHOSPHATE IN COLA SOFT DRINKS DEMONSTRATOR: Pooja

AIM: to determine the concentration of phosphates in cola soft drinks and to find the molar absorptivity of the phospho-molybdate complex using UV-VIS spectroscopy.

INTRODUCTION: Phosphorus is an element which is essential to the life of many organisms. It can be analyzed in many ways. UV-VIS spectroscopy is one such way in which the concentration of phosphorus can be quantified. The procedure utilized in this experiment uses the reaction between ascorbic acid and antimony-phospho-molybdate complex to find the concentration of phosphate. Phosphate is the ion in which phosphorus exists in solution. The ascorbic acid, reduces this complex, which is yellow, to a molecule which is blue in colour. The shade of blue obtained is directly proportional to the [P] in solution. All spectroscopic methods are based on Beer-Lambert’s Law which is:

A=εcl Where

A = absorbance ε = molar absorptivity (M-1cm-1) c = concentration (mol/L) l = cell length (cm)

When the calibration curve is forced through the origin and the cell length is 1cm, the value of ε is equivalent to the slope of the calibration curve. The blue colour of the solution formed can only be measured at wavelengths of 650nm or 850nm in the visible spectrum.

PROCEDURE: Followed as seen in Chem 2670 Advanced Chemistry Laboratory Manual 1 with the exception of Part B, where the dilution factor of the standard solutions was changed to x 200.

IMRAN R. LOURENҪO

812002025

16.09.13

FURTHER QUESTIONS: 1. The value doesn’t fit as close as 0.9900. It lies at 0.9712, so it does have a good fit, and is therefore a good calibration curve. 2. The values for the [P] mg/L for the original Coca-Cola samples are similar for the 0.5mL and 1.0mL volumes. However they all vary by a certain degree, which in this case is a large discrepancy. The first sample differed by 1.354326 mg, the second by 1.1957721mg and the third sample by 2.45513006 mg. They should have no differences however, as what is contained in 0.5mL should be doubled in 1.0mL. Here we see that they do double yes, but also go over the double factor by a small amount. 3. I believe the most error was introduced while making up the sample solutions and the standard solutions. I believe this was so as the dilutions had to redone multiple times before the correct dilution factor was attained, introducing many chances for error to be carried down the dilution line. This would involve measuring volumes using the burette, pipette and measuring cylinder, and also making up solutions to the mark in volumetric flasks. These errors were also seen as problematic in the previous lab carried out on the 09.09.13. To prevent this from occurring in the following week, practice on proper use of the laboratory equipment and how to read the volumes and measure them accurately should be done.

DISCUSSION: From the data attained, the concentration of phosphates in Coca-Cola soft drink was determined to be 21.94976467mg/L ± 0.563099347 in the 0.5mL original samples and 20.28135528mg/L ± 0.290145034 in the 1.0mL original samples. These values cannot be compared to the actual value in the Coca-Cola sample as they do not indicate how much phosphoric acid was added to the soft drink.

CONCLUSION: The molar absorptivity of the antimony-phospho-molybdate complex, was determined to be 652693 M-1cm-1. The concentration of phosphates in the Coca-Cola samples (original) were deduced to be 21.94976467mg/L ± 0.563099347 in the 0.5mL and 20.28135528mg/L ± 0.290145034 in the 1.0mL samples.

REFERENCES: Harris, D.C. 2011 Quantitative Chemical Analysis. 8e WH Freeman and Company, New York. Section 4-8 & 2-11.