Making standard solutions to make a calibration curve.
This is a common analytical technique used to determine the concentration of an unknown solution.
In this case I was using UV-vis however you can make these for flame atomic spectroscopy and other techniques too.
You plot a graph of your absorption readings against your known concentrations, get a linear line of best fit. Measure the absorbance of your unknown and read off the graph to determine your concentration.
Further analysis would be the application of the Beer-Lambert law: Absorbance= concentration x path length x molar absorptivity