The concentration of salicylic acid in a solution can therefore be measured based on the intensity of the colour of the violet complex, using a spectrophotometer and the Beer-Lambert Law. The Beer-Lambert Law states that the absorbance of a substance is directly proportional with its concentration, with the equation, 𝐴 = 𝜀𝑙𝑐, where A is absorbance, ε is the molar extinction coefficient, l is the length of the light path, and c is concentration in mol/dm 3 ("The Beer-Lambert Law", 2023). The molar extinction coefficient is "a measure of how strongly a chemical species or substance absorbs light at a particular wavelength", and is different for every substance (What Is a Molar Extinction Coefficient? | AAT . The value of the molar extinction coefficient of salicylic acid can be found via a calibration curve drawn using known concentrations of salicylic acid, where the x axis represents concentration, and the y axis represents absorbance . According to the equation of the line, 𝑦 = 𝑚𝑥 + 𝑏, the coefficient of