Definition of a Calibration Curve
A calibration curve, also known as a standard curve, is a graph used in analytical chemistry and other scientific fields to relate a measured property (like absorbance, signal intensity, or peak area) to the concentration of a substance or the value of a physical property. It is constructed by measuring the property of several samples with precisely known concentrations or values (standards).
How a Calibration Curve is Created
To create a calibration curve, a series of 'standards' are prepared; these are solutions or samples with known, varying concentrations of the analyte. The chosen property is then measured for each standard. These data points are plotted on a graph, typically with the known concentrations on the x-axis and the measured responses on the y-axis, and a 'best-fit' line or curve is drawn through them.
Practical Example of its Use
For example, in a spectrophotometry experiment, a scientist might prepare five solutions of a specific colored compound at known concentrations (e.g., 0, 10, 20, 30, and 40 ppm). Each solution's absorbance is measured, and these absorbance values are plotted against their respective concentrations. This plot then forms the calibration curve, often appearing as a straight line for dilute solutions.
Importance and Applications
Once established, the calibration curve serves as a reference. Scientists can determine the concentration of an unknown sample by simply measuring its property (e.g., absorbance) and interpolating the corresponding concentration from the curve. This technique is fundamental for accurate quantitative analysis across diverse fields, from environmental testing and clinical diagnostics to quality control in manufacturing, ensuring reliable and precise experimental results.