Difficulty: Introductory
Prerequisites: Basic understanding of electromagnetic radiation, wavelength, and the visible light spectrum. The column chromatography study notes (companion document) cover the separation technique used before spectroscopic analysis.
Once you have separated a mixture into its components, you need a way to identify what each component is. UV-Vis spectroscopy measures how much light a substance absorbs at each wavelength, producing a unique absorption spectrum that acts like a fingerprint. In this lab, you compared the absorption spectra of separated Kool-Aid fractions to known dye standards (Blue 1, Blue 2, Red 3, Red 40) to confirm which dyes were present. This technique is central to analytical chemistry, biochemistry, environmental monitoring, and clinical diagnostics.
A spectrophotometer shines light through a sample and measures how much is absorbed at each wavelength. Every dye has a characteristic peak wavelength (lambda-max). By comparing the absorption spectrum of an unknown fraction to spectra of known standards, you can identify the dye. The grape Kool-Aid blue fraction matched Blue 1 (lambda-max around 630 nm) and the red fractions matched Red 40 (lambda-max around 500 nm).
Absorbance (A)
A unitless measure of how much light a sample absorbs at a given wavelength. Higher absorbance means the sample absorbs more light at that wavelength. Absorbance cannot be negative under normal conditions; a negative reading indicates instrument error.
Think of it as: how much of the light beam the sample "swallows" instead of letting through.
Wavelength (nm)
The distance between successive peaks of a light wave, measured in nanometres (nm) for visible and UV light. Different wavelengths correspond to different colours: roughly 400 nm (violet) to 700 nm (red).
In simple terms, wavelength is the colour of the light, expressed as a number.
Absorption spectrum
A graph of absorbance versus wavelength for a given substance. The shape and peak positions of the spectrum are characteristic of the substance, like a fingerprint.
Think of it as the substance's unique light-absorption profile.
Lambda-max (also written as the Greek letter lambda with a subscript "max")
The wavelength at which a substance's absorbance is greatest. This is the single most useful number for identifying a compound by spectroscopy.
In simple terms, lambda-max is the colour of light the substance absorbs most strongly.
Spectrophotometer
An instrument that measures the intensity of light passing through a sample as a function of wavelength. It produces the absorbance data used to generate an absorption spectrum.
Think of it as a machine that shines a rainbow through your sample and records how much of each colour gets absorbed.
Calibration (zeroing, baseline correction)
The process of setting the spectrophotometer's baseline so that a blank sample (containing only solvent, no analyte) reads zero absorbance at all wavelengths. Incorrect calibration produces systematic errors, including negative absorbance values.
In simple terms, calibration is "telling the instrument what zero looks like" before you measure anything.
Visible spectrum
The portion of the electromagnetic spectrum visible to the human eye, roughly 380 to 750 nm. In this lab, all absorbance measurements were taken within this range (approximately 420 to 720 nm).
Think of it as the slice of light you can see, from violet through to red.
A beam of light passes through a sample in a cuvette.
The spectrophotometer scans across a range of wavelengths (in this lab, the visible range).
At each wavelength, the instrument measures how much light is transmitted versus absorbed.
The result is an absorption spectrum: a plot of absorbance (y-axis) vs. wavelength in nm (x-axis).
Each substance has a unique spectrum shape and lambda-max.
The peak of the curve is the lambda-max, the wavelength of maximum absorbance.
The height of the peak reflects the concentration of the substance (related through Beer-Lambert law).
The overall shape of the curve (broad vs. narrow, one peak vs. multiple peaks) helps distinguish between substances that might have similar lambda-max values.
A flat line near zero absorbance means the sample does not absorb at those wavelengths.
Four known dye standards were measured:
Blue 1 Standard (Figure 1): Lambda-max around 630 nm, in the orange-red region of absorption (the dye appears blue because it absorbs orange-red light).
Blue 2 Standard (Figure 2): Data was invalid due to spectrometer miscalibration. Negative absorbance values were recorded, which is physically impossible under normal conditions.
Red 3 Standard (Figure 3): Lambda-max around 525 nm.
Red 40 Standard (Figure 4): Lambda-max around 500 nm, absorbing in the blue-green region (the dye appears red because it absorbs blue-green light).
Three separated fractions from Kool-Aid were measured:
Well B, Clear fraction (Figure 5): Very low, flat absorbance across all wavelengths (max around 0.03 to 0.04). This confirms it contains no coloured dyes, only non-absorbing components like sugars and acids.
Well B4, Blue fraction (Figure 6): Lambda-max around 630 nm. This closely matches the Blue 1 Standard spectrum, confirming the blue dye in Kool-Aid is Blue 1.
Wells C6 and D6, Red fractions (Figures 7 and 8): Lambda-max around 500 nm. These closely match the Red 40 Standard spectrum, confirming the red dye in Kool-Aid is Red 40.
Match the lambda-max of the unknown fraction to the lambda-max of a known standard.
Compare the overall shape of the two spectra (peak width, secondary features).
If both lambda-max and shape align, the unknown is identified as that standard dye.
In this lab, Well B4 matched Blue 1 (not Blue 2), and Wells C6/D6 matched Red 40 (not Red 3).
The Blue 2 Standard produced negative absorbance values (Figure 2), with a trough reaching approximately negative 2.0 near 630 nm.
Absorbance cannot be negative under correct operating conditions, because negative absorbance would mean the sample is emitting more light than the source, which does not happen with a passive dye solution.
The cause was spectrometer miscalibration: the baseline (blank) was set incorrectly, so all subsequent readings were offset downward.
This is an important practical lesson: always verify calibration before collecting data, and flag any negative absorbance as a sign of instrument error.
Beer-Lambert Law
A = \varepsilon \cdot b \cdot cWhere:
A = absorbance (unitless)
Epsilon = molar absorptivity (L mol^-1 cm^-1), a constant specific to each substance at a given wavelength
b = path length of the cuvette (cm), typically 1 cm
c = concentration of the absorbing species (mol/L)
This law tells you that absorbance is directly proportional to concentration. Double the concentration, double the absorbance (assuming you stay within the linear range of the instrument).
Absorbance from transmittance
A = -\log_{10}\left(\frac{I}{I_0}\right)Where I is the intensity of light after passing through the sample and I_0 is the intensity of the incident light. When I equals I_0 (no absorption), A = 0. When the sample absorbs all the light, I approaches 0 and A approaches infinity.
For this lab, you did not need to calculate absorbance manually; the spectrophotometer produced the values directly. However, you should understand these relationships for exam purposes.
UV-Vis spectroscopy is used in clinical labs to measure haemoglobin concentration in blood samples, in environmental science to detect pollutant levels in water, and in the food industry to verify that dye concentrations in products meet regulatory limits. The Beer-Lambert law is the basis for nearly all quantitative spectrophotometric assays.
Students often think a blue dye absorbs blue light. It does not. A blue dye absorbs the complementary colour (orange-red, around 600 to 640 nm) and transmits or reflects blue light, which is what your eye sees.
Students sometimes assume that a higher lambda-max means a more concentrated sample. Lambda-max is a property of the substance itself and does not change with concentration. Only the height (absorbance value) of the peak changes with concentration.
Students may think negative absorbance means the sample is fluorescent or emitting light. In a standard teaching lab, negative absorbance is almost always a calibration error, not a real physical phenomenon.
Students occasionally confuse absorbance with transmittance. They are inversely related: high absorbance means low transmittance, and vice versa.
Be prepared to identify an unknown substance by comparing its absorption spectrum to known standards. You need to match both lambda-max and spectrum shape.
Know the Beer-Lambert law and be able to explain what each variable represents. Expect a calculation where you are given three of the four variables and asked to solve for the fourth.
Understand the relationship between absorbed colour and observed colour (complementary colours).
Be ready to explain why negative absorbance readings indicate instrument error.
True or false: A substance that appears red absorbs red light. (False. It absorbs the complementary colour, which is blue-green, and reflects/transmits red.)
Fill in the blank: The wavelength at which a substance absorbs light most strongly is called ______. (Lambda-max.)
True or false: According to Beer-Lambert law, doubling the concentration of a solution doubles its absorbance. (True, within the linear range.)
Fill in the blank: Negative absorbance values in a lab setting typically indicate ______. (Spectrometer miscalibration.)
True or false: Two substances with the same lambda-max are necessarily the same compound. (False. The overall spectrum shape and other features must also match.)
Q: How do you identify an unknown dye using UV-Vis spectroscopy?
A: Measure the absorption spectrum of the unknown sample, then compare its lambda-max and overall spectrum shape to the spectra of known standards. If both match, the unknown is identified as that standard.
Q: The blue fraction from grape Kool-Aid has a lambda-max of approximately 630 nm. Which standard does it match, and how do you know?
A: It matches Blue 1 Standard, which also has a lambda-max around 630 nm with the same spectrum shape (a single strong peak in the 600 to 650 nm range). It does not match Blue 2, whose data was invalid due to calibration error.
Q: Why does the clear fraction (Well B) show nearly zero absorbance across the visible spectrum?
A: The clear fraction contains non-dye components (sugars, citric acid, flavourings) that do not absorb visible light. Only coloured compounds produce measurable peaks in the visible range.
Q: A student's spectrophotometer gives negative absorbance values for a dye standard. What went wrong?
A: The spectrophotometer was not properly calibrated (zeroed) before the measurement. The baseline was set too high, causing all readings to be offset into negative values. The student should recalibrate with a proper blank and remeasure.
Q: Using Beer-Lambert law, if a solution's absorbance at 500 nm is 0.80 and you dilute it to half the concentration, what will the new absorbance be?
A: 0.40. Beer-Lambert law states that absorbance is directly proportional to concentration. Halving the concentration halves the absorbance, assuming the path length and molar absorptivity remain the same.
Q: A dye appears red to the eye. In what region of the visible spectrum does it absorb most strongly?
A: It absorbs most strongly in the blue-green region (roughly 480 to 520 nm), because the observed colour is the complement of the absorbed colour.
This material connects to electromagnetic radiation and the nature of light (wave-particle duality, energy-wavelength relationship), which your course likely covers in the quantum mechanics or atomic structure unit. It also ties directly to the column chromatography companion notes: you cannot identify dyes by spectroscopy unless you first separate them, and the separation technique relies on polarity concepts from intermolecular forces. If your course later covers quantitative analysis or analytical techniques, Beer-Lambert law will reappear as the foundation for concentration measurements.
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