This experiment describes a simple protocol for teaching acid–base titrations using potentiometry, conductivity, and/or photometry to determine end points without an added indicator. The chosen example examines the titratable acidity of a red wine with NaOH. Wines contain anthocyanins, the colors of which change with pH. Importantly, at the equivalence point, anthocyanins maintain their color, and this effect can be captured optically using a desktop scanner. RGB-based color values are obtained from the digitized images using a dedicated application and employed to generate plots of |v| versus titrant volume. The end point is the point at which the slope (d|v|/Δ/dV) of the titration curve is at a maximum; (d|v| is a vector expression of the color change). The photometric titrations were carried out using a plastic 96-well immunology plate and a flat-bed scanner, and RGB values were extracted simultaneously from all 96 wells in less than 5 min from the images using the ImageJ plugin “ReadPlate”. The wine was also titrated using conventional potentiometric and conductometric techniques, and methods were compared using F-test, t-test, and one-way ANOVA. The potentiometric titration yields pH versus titrant volume curves, and the equivalence point is where the slope, or the slope of the derivative curve (dpH/dV), is greatest. The conductometric titration yields conductivity versus titrant volume curves, and the end point is determined as the volume where the slope exhibits a marked change. All titration and derivative curves were plotted and analyzed using spreadsheet software.

Here, students determine aspirin (acetylsalicylic acid) mass in pharmaceutical tablets using a colorimetric method. Aspirin, salicylate, and salicylic acid do not absorb visible light. Thus, in alkaline medium, acetylsalicylic acid was hydrolyzed to salicylate; then, it was reacted with an acidic Fe(III) solution, and a violet complex was formed. Quantitative analysis was carried out using absorbance measured at 535 nm (standard method) and digital images obtained with a flatbed scanner (proposed method). Results obtained with both methods were compared using an F-test and a t-test; both methods had shown equivalent accuracy and precision at the 95% confidence level. In addition, one-way ANOVA showed that aspirin masses found by five student groups using both methods are equivalent at the 95% confidence level. In the proposed method, samples were placed in a 96 microwell plate, and RGB values were extracted automatically from all wells in less than 5 min using ImageJ's plugin "ReadPlate"; data obtained were organized using a spreadsheet to determine aspirin mass in pharmaceutical tablets, recovery, and percent error. Our goal was to design a portable, cost-effective, and user-friendly platform and to develop an experimental methodology that can easily be applied to any research and education laboratory using just a flatbed scanner.

Here, students determine the total phenolic content in beers using the Folin–Ciocalteu assay. The Folin–Ciocalteu reagent is a yellow complex, in an alkaline medium; it reacts with phenols and non-phenolic reducing substances to form a blue complex. Quantitative analysis was carried out using absorbance measured at 765 nm (standard method) and digital images (proposed method). In the proposed method, samples were placed in a 96 microwell plate; then, a plate image was obtained with a flatbed scanner. The ImageJ plugin ReadPlate extracts red (R)-values from all wells of the scanned image at the same time; then, R-values were exported to a spreadsheet that converted these values to absorbances. Using absorbances obtained with R-values, the spreadsheet also provides standard calibration plots, limits of detection (LODs), limits of quantification (LOQs), and beer sample total phenolic contents. The proposed method and the standard method were compared using two-tailed significance tests; students observed that both methods provide equivalent results, that both methods were linear in a 2–10 mg L–1 gallic acid concentration range, and that both methods had close LODs and LOQs, 0.55 and 1.70 gallic acid mg L–1, respectively. Beer sample total phenolic contents were compared using one-tailed significance tests. This laboratory experiment provides an effective instrumental alternative to spectrophotometric methods, which can be especially beneficial, where purchasing and maintaining a spectrophotometer is a challenge, and it also presents to the students one-tailed and two-tailed significant tests.