# How can you find out the concentration of the given unknown solution using a spectrophotometer?

enginr | Certified Educator

A spectrophotometer is a device that can measure the intensity of light at different wavelengths. A sample holder called a spectrophotometric cell is filled with the solution of interest and placed inside the machine. A beam of light will be shined through the sample and the intensity of the light at a particular wavelength that passes through the sample is measured. For this reason, spectrophotometric cells are made of very clear glass or plastic. Care should be taken to avoid scratching cells or smudging them with fingerprints, as both can absorb some of the light, leading to an erroneous result. The more concentrated the solution, the less light will pass through (more absorbance).

If we are given a solution of unknown concentration, we will need a graph of absorption as a function of concentration for the solution in question. This graph might be available in a textbook or you can produce one yourself by preparing several known concentration solutions. Take a reading from each of the known concentration solutions using the spectrophotometer at the recommended wavelength. Plot your results using a logarithmic scale on the X-axis (concentration) and absorption on the y-axis. Then take a reading from the unknown solution and compare the measured absorbance to your graph to determine the concentration.