To find the molar absorptivity using a graph, you typically start with a plot of absorbance against concentration for a series of standard solutions. Here are the steps to determine the molar absorptivity:
- Prepare Standards: Prepare a series of standard solutions of known concentrations of the solute you are analyzing.
- Measure Absorbance: Use a spectrophotometer to measure the absorbance of each standard solution at the wavelength where the maximum absorbance occurs, known as the λmax.
- Plot the Data: Create a graph with concentration on the x-axis (in mol/L or M) and absorbance on the y-axis. Each point on the graph represents the absorbance of a corresponding concentration.
- Draw the Best Fit Line: Use the method of least squares to draw a straight line that best fits the data points. This line is typically linear, especially at low concentrations.
- Determine the Slope: The slope of the line (Δabsorbance/Δconcentration) represents the molar absorptivity (ε) multiplied by the path length of the cuvette (usually 1 cm for standard cuvettes). If the path length is 1 cm, then the slope directly equals the molar absorptivity.
In summary, after preparing a calibration curve by plotting absorbance against concentration and calculating the slope of the line, you can determine the molar absorptivity, which is crucial for quantifying the concentration of unknown samples using their absorbance values.