Accurate, Photoresistor-Based, Student-Built Photometer and Its Application to the Forensic Analysis of Dyes

Abstract

Student-built photometers are recognized as being useful tools in teaching laboratories that help students understand the concepts behind experimental measurements, and several devices have been reported that employ photoresistors as the detector of choice. We demonstrate here an improvement over these instruments and the analyses thereof, providing more rapid deployment in the laboratory and also considerable increases in both linearity and accuracy. This was confirmed by conducting quantitative absorption spectroscopy of known concentrations of potassium permanganate solution. A laboratory exercise utilizing this approach was employed in two postsecondary institutions to simulate the forensic analysis of unknown fabric dyes to determine the identity and concentration of an unknown dye solution.

Review of Student-Built Spectroscopy Instrumentation Projects

“…Many low-cost devices are constructed from cardstock or cardboard boxes, − ,,,− but other options include plastic ,,− or aluminum housings, ,, wood, − PVC pipe, or professional optical mounts. , A unique device using a graduated cylinder permitted careful investigation of the effect of path length on absorbance . More recently, LEGO blocks (Figure b) ,,,− and 3D printing (Figure c) ,,− have become popular.…”

“…Four main types of low-cost, solid state light detectors are available: LEDs, ,− ,,, photodiodes, ,,,,,− light dependent resistors (LDRs, also known as photocells or photoresistors), ,,,,,,,,,,,,,,− and phototransistors. , The first option, an LED, is commonly used as a light source in the emission mode; however, forward biased LEDs can be repurposed as detectors for light of slightly higher energy than the light they emit (Figure b). − When such light strikes the LED, a photocurrent is generated . This effect is useful to detect a narrow band of wavelengths since the LED responds only to wavelengths just below its emission maximum .…”

“…The third option, a LDR, decreases in resistance when photons excite electrons into the conduction band (Figure c). One advantage of LDRs is that their resistance is directly proportional to analyte concentration, which makes an intuitive readout for novice users; however, the response to light intensity is not linear over the entire working range, so correction may be needed for quantitative applications . Finally, a phototransistor functions similarly to a photodiode but generates intrinsic signal amplification.…”

“…Diawati et al’s student questionnaires and interviews showed that students found photometer construction to be challenging but interesting and were more confident in their understanding after the project . Similarly, second-year students in analytical chemistry reported that a photometer-building project enhanced their understanding of solution preparation, spectrophotometry, and Beer’s law . However, these studies are limited in that they include only students’ self-reported outcomes.…”

“…One advantage of LDRs is that their resistance is directly proportional to analyte concentration, which makes an intuitive readout for novice users; 74 however, the response to light intensity is not linear over the entire working range, so correction may be needed for quantitative applications. 71 Finally, a phototransistor functions similarly to a photodiode but generates intrinsic signal amplification. For even higher sensitivity, a photodarlington, which consists of a pair of transistors, may be used.…”