A spectrophotometer measures the amount of light that passes through a sample and can measure the concentration of a solution. The cost often ranges from hundreds to thousands of dollars depending on the sensitivity, but I built one for about $5.
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My team used a recycled candy tin as the case and drilled small holes in each of the short ends to stick the electrodes for the LED and photoresistor. We put foam insulation in each corner so there's a straight path from the LED to the photoresistor and a place for the sample to be held in place. On the long ends, we cut out a hole so the instrument can be outfitted with a tube to measure changes in concentrtion of a flowing sample. The inside was painted black to mitigate reflecting light.
Externally, a microcontroller is used to power each electrical component and record the voltage from the photoresistor.
molar extinction coeff
calibration
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