A standard curve, also known as a calibration curve, is a general method used in analytical chemistry to determine the concentration of a substance in an unknown sample by comparing it to a set of standard samples of known concentration. It is a plot of how the instrumental response, the so-called analytical signal, changes with the concentration of the analyte (the substance to be measured) . Standard curves are generally used when you need quantitatively accurate results.

To create a standard curve, a series of known solutions with different concentrations of the analyte are prepared and measured using an instrument such as a spectrophotometer. The resulting data is used to plot a curve, with the concentration of the analyte on the x-axis and the instrument response (such as absorbance or transmittance) on the y-axis. The line that fits the points on the curve can then be used to determine the concentration of an unknown sample by measuring its instrument response and finding the corresponding point on the curve.

Standard curves are used to correct for the effects of instrument variability and to ensure that the instrument response is proportional to the concentration of the analyte. They are also useful when an instrument uses a sensor whose calibration varies from one sample to another, or changes with time or use.

In summary, a standard curve is a quantitative research tool used to determine the concentration of a substance in an unknown sample by comparing it to a set of standard samples of known concentration. It is a plot of how the instrumental response changes with the concentration of the analyte, and is used to correct for the effects of instrument variability and ensure that the instrument response is proportional to the concentration of the analyte.