Measuring Sunscreen UV Absorption

Hard

How well does your sunscreen actually block ultraviolet light? The SPF number on the bottle tells part of the story. Measuring the extinction coefficient (a value showing how strongly a substance absorbs light) reveals the rest. You dissolve a small amount of sunscreen in ethanol. Then you dilute that solution into six samples at different concentrations. A UV-visible spectrophotometer measures how much UV light each sample absorbs near 310 nanometers. Plot absorbance against concentration to create a Beer's Law graph. The slope of the line gives the extinction coefficient. A steeper slope means the sunscreen absorbs more UV light per gram. Compare extinction coefficients across several SPF ratings. Higher SPF products should show higher coefficients, confirming stronger UV absorption.

Hypothesis

The hypothesis is that the higher the SPF rating, the more effective the sunscreen will be at blocking UV light.

Science Concepts Learned

UV Absorption

Different materials soak up ultraviolet light at different strengths, and the extinction coefficient measures how strongly a substance absorbs light at a specific wavelength. You dissolve a small amount of sunscreen in ethanol, dilute it into six samples at different concentrations, then measure how much UV light each sample absorbs near 310 nanometers using a UV-visible spectrophotometer. Plotting absorbance against concentration gives a Beer's Law graph, and the slope of that line is the extinction coefficient — a steeper slope means the sunscreen absorbs more UV light per gram. Comparing extinction coefficients across several SPF ratings reveals whether higher SPF products actually show stronger UV absorption.

Beer's Law

Light gets weaker as it passes through more of a colored liquid — and that principle lets you measure how effectively sunscreen blocks UV rays. You dissolve a small amount of sunscreen in ethanol, then dilute that solution into six samples at different concentrations. A UV-visible spectrophotometer measures how much UV light each sample absorbs near 310 nanometers. Plotting absorbance against concentration produces a straight line, and its slope gives the extinction coefficient — a value showing how strongly the substance absorbs light. A steeper slope means more UV absorption per gram. Comparing coefficients across several SPF ratings shows whether higher SPF products actually absorb more, confirming the relationship the hypothesis predicts.

Method & Materials

You will work in groups of four to dissolve the sunscreens in ethanol, make a series of ethanol solutions with different concentrations, measure the absorbance of the different dilutions, and plot the absorbance of the sunscreen vs. concentration to calculate the extinction coefficient.

You will need a 50.00 mL volumetric flask, a 50 mL beaker, a 250 mL beaker, a 1.00 mL graduated pipet, a digital balance, a HP 8453 UV-Visible Spectrophotometer, 95% ethanol, and a sunscreen product.

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Results

The results of this experiment show that the higher the SPF rating, the more effective the sunscreen is at blocking UV light. This is an important finding, as it helps us understand how to better protect our skin from the sun's harmful rays.

Why do this project?

This science project is interesting and unique because it allows us to measure the effectiveness of sunscreen in a quantitative way.

Also Consider

Experiment variations to consider include testing different types of sunscreen, such as mineral-based or chemical-based, or testing different SPF ratings.