Spectrophotometry Basics

Spectrophotometry Overview

A spectrophotometer emits a light beam through a solution and measures the light that passes through. This function is used to determine how much light a chemical in the solution absorbed, ultimately estimating the chemical’s concentration with a standard curve. It is applied to determine nitrate concentration in stream water and chlorophyll $a$ in later labs.

Visible Light and Wavelengths

Visible light travels in waves, with different colors possessing different wavelengths. For example, red light has wavelengths ranging from approximately $650$ to $780$ nm.

Transmittance and Absorbance

A spectrophotometer can be set to emit a specific wavelength of light, optimal for absorption by the target chemical. Transmittance is the percentage of light that passes through the solution; for instance, if $3/4$ of light passes, transmittance is $75 \%$. Absorbance is the proportion of light absorbed by the solution. While transmittance is a percentage, absorbance is expressed as a proportion (e.g., $0.25$ for $25 \%$ absorbed light) and is the typically preferred measurement.