Determination of Concentration using an External Calibration Curve

By: Pailin Muchan

If we want to know the concentration of our sample solution, how can we do it?

In analytical chemistry, a calibration curve, also known as a standard curve, is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration. If the concentration of the sample responds to the instrumental signal, the concentration of your samples can be quantified. It can be said that the calibration curve provides data on an empirical relationship.

Here is the simple way to determine the sample concentration using the external calibration curve.

First, you must know what your sample is to prepare the standard solution.

Step 1: Prepare different concentrations of standard solutions.

Step 2: measure or analyze all standard samples and unknown samples by corresponding instrument (ie. UV/VIS spectrometer, Gas or liquid chromatography, etc.)

Step 3: Plot the graph between instrumental responses as the y-axis (i.e. peak area, peak height, intensity, etc.) and the concentrations of the standard solution as the x-axis, then create the equation of the line. If the calibration obtained is a straight line, the equation will be y = mx + c (m = slope of the graph and c = interception). In addition, the precision of the  curve is reflected by the R-square (expected R2 is 1.000) of the equation.

Step 4: Calculate the concentration of the unknown using the calibration curve equation. Put the response of the unknown as the “y” value and solve the equation to get the “x” value that represents the concentration of the unknown one.

The accuracy of the results will depend on the accuracy of the instrument and the precision of the prepared standard solution.