You usually use a colorimeter where red light from the LED light source will pass through the solution in question and strike a photocell. A higher concentration of the solution absorbs more light (and transmits less) than a solution of lower concentration. The colorimeter monitors the light received by the photocell as either an absorbance or a percent transmittance value.
You usually prepare at least five solutions of a known concentration (standard solutions). Each is transferred to a small, rectangular cuvette that is placed into the colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. When a graph of absorbance versus concentration is plotted for ...
Full explanation with solution graph. 474 words.