Question

Spectrophotometric Analysis of a Mixture: Caffeine and Benzoic Acid in a Soft Drink

Benzoic Acid

	230nm	270nm
2mg/mL	0.231	0.034
4mg/mL	0.390	0.051
6mg/mL	0.539	0.058
8mg/mL	0.685	0.070
10mg/mL	0.888	0.094

Caffeine

	230nm	270nm
4mg/mL	0.935	1.525
8mg/mL	0.285	0.442
12mg/mL	0.428	0.703
16mg/mL	0.624	1.066
20mg/mL	0.724	1.226

Soft Drink

	230nm	270nm
2mL	0.738	0.273
4mL	1.313	0.522

Mountain Dew Caffeine:54mg/12fl oz

Measure the ultraviolet absorption spectrum of the 2:100 and 4:100 dilutions of the soft drink. Find the concentrations of benzoic acid and the caffeine in the original soft drink. Report results from both dilute solutions.

If you need anymore information please let me know.

Answer 1

For this calculation, we need to plot absorbance vs concentration for all the 4 data and then we need to find the molar absorptivity of each at each wavelength. So, we get,

The graphs are plotted on excel.

In each graph, there is given an equation which is equation of line. It tells about the slope of the line. And the slope of the line is molar absorptivity of the compound at that wavelength.

For benzoic acid at 230 nm, slope = 0.0892. So, _230,benz = 0.0892

For benzoic acid at 270 nm, slope = 0.0096. So, _270,benz = 0.0096

For caffeine at 230 nm, slope = 0.0369. So, _230,caf = 0.0369

For caffeine at 270 nm, slope = 0.062. So, _270,caf = 0.062

The absorbance of soft drink:

0.738 at 230 nm for 2 mL

So, for 1 ml, it would be = 0.738/2 = 0.369

Let [benz] in soft drink = x M

[caf] = y M

Total absorbance = A₂₃₀ = ( _230,benz)[benz]+( _230,caf )[caf]

0.369 = 0.0892x + 0.0369y

Similarly, at 270 nm

A₂₇₀ = ( _270,benz)[benz]+( _270,caf )[caf]

0.137 = 0.0096x + 0.062y

These are two equation with two variables, so we can solve both the equations, we get:

x = 3.44

y = 1.67

So, concentration of benzoic acid in 1 ml = 3.44 mg. So, 2 ml would contain = 3.44*2 = 6.89 mg

concentration of caffeine in 1 ml = 1.67 mg. So, 2 ml would contain = 1.67*2 = 3.33 mg

Similarly, we can perform for 4 ml dilution:

The absorbance of soft drink:

1.313 at 230 nm for 4 mL

For 1 ml, it will be 1.313/4 = 0.33

Let [benz] in soft drink = x M

[caf] = y M

Total absorbance = A₂₃₀ = ( _230,benz)[benz]+( _230,caf )[caf]

0.33 = 0.0892x + 0.0369y

Similarly, at 270 nm

A₂₇₀ = ( _270,benz)[benz]+( _270,caf )[caf]

0.131 = 0.0096x + 0.062y

These are two equation with two variables, so we can solve both the equations, we get:

x = 3.01

y = 1.65

So, concentration of benzoic acid in 1 ml = 3.01 mg. So, 4 ml would contain = 3.01*4*2 = 12.04 mg

concentration of caffeine in 1 ml = 1.65 mg. So, 4 ml would contain = 1.65*4 = 6.60 mg

From both the dilutions, the answers are almost same for concentration per ml.