Tennessee Wastewater Operator Practice Exam

In a calibration curve for a spectrophotometer, what is plotted on the horizontal axis?

1. Wavelength

2. Absorbance

3. Concentration

4. Time

The correct answer is: Concentration

In a calibration curve for a spectrophotometer, the horizontal axis is used to represent concentration. This relationship allows operators to determine the concentration of an unknown sample based on its absorbance value. The underlying principle is that absorbance is directly proportional to the concentration of the absorbing species in a solution, as described by Beer-Lambert law. When a series of known standards with varying concentrations are measured for their absorbance, the resulting data points can be plotted to create a calibration curve. By plotting concentration against absorbance, you establish a reference graph, which can then be used to interpret the absorbance of unknown samples, leading to a calculation of their respective concentrations. Other aspects that might be plotted, such as wavelength or time, are not relevant to the calibration curve itself. Wavelength helps identify which light is being absorbed, while time may indicate how quickly a reaction occurs, but neither is involved in determining a direct relationship for concentration against absorbance on the calibration curve.