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Microplate titration quantifies sodium hydroxide generated from formaldehyde reacting with excess sulfite in a 96-microwell plate. Phenolphthalein indicators change from red to colorless when all hydroxide ions react. Methodology optimized reagent concentrations, and reaction time and created a Calibration Chart for semi-quantitative determination. The chart shows formaldehyde concentration ranges corresponding to red well counts from 0 to 200 mM in 20 mM increments. Inter-operator repeatability demonstrates precision (3 replicates), correlating red wells with standard formaldehyde concentrations. This instrument-free technique uses readily available commercial plates, eliminating the need for specialized equipment and calibration. The methodology offers simplicity with its reliance on readily available commercial plates and minimal specialized equipment, hence it is cost-effective and easily transportable 96-microwell plates enhancing the methodology's portability, and efficient semi-quantitative analysis of formaldehyde. The analysis of twelve solutions from food samples agrees with the quantitative values using titration.

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This work describes the analysis of formaldehyde using a 96-well microplate as multiple headspaces for the separation of sulfur dioxide gas generated from the sulfite remaining after its reaction with the formaldehyde in the sample. The quantitation of the gas is by colorimetric detection of an indicator paper placed over the microplate. The samples are aqueous extracts of various foods that are possibly adulterated with formaldehyde. A known excess amount of sulfite is added to the extract solution aliquoted in the well. The remaining sulfite is acidified with hydrochloric acid to generate sulfur dioxide gas which diffuses through the headspace above the solution to be absorbed at the moist strip of the indicator paper placed over the mouth of the wells. Anthocyanins extracted from the butterfly pea flower is used as the pH indicator giving a color change from the increase of hydrogen ions by hydrolysis of the absorbed sulfur dioxide gas. The exposed paper strip is scanned, and the digital images of the colored region analyzed using ImageJ software. The optimized method has a linear range of 200-1000 mg L-1 formaldehyde with limit of detection ((2.57*SD of intercept)/(slope of calibration line)) of the aqueous extract of 40 mg L-1 and coefficient of determination (r2) > 0.9979. Samples of fresh produce, such as seafood, meat, and vegetables, and various processed food were analyzed for their possible formaldehyde content. The results obtained from the headspace paper-based colorimetric detection are not statistically different from the values obtained from the titration method by paired t-tests.

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This work presents a new method for minoxidil detection based on silver nanoparticle (AgNP) oxidation. Minoxidil, which is a pyrimidine N-oxide, can be reduced to its corresponding pyrimidine via a redox reaction. In this system, acetate buffer serves as a proton source. AgNPs act as electron donors that contribute electrons to the reaction, producing Ag+. Consequently, the sizes and numbers of AgNPs in the system decrease, which results in a decline in their resonance Rayleigh scattering (RRS). By monitoring the RRS intensity at 409 nm, a change in intensity was linearly related to the minoxidil concentration over a concentration range of 0.5 - 5.0 mM. The detection limit was 0.35 mM. This approach is simple and rapid. It is done by directly mixing the drug and AgNPs in an acidic buffer. The reaction was completed within 2 min. This proposed method was successfully utilized for quantification of minoxidil in topical hair-growth formulations.

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This work presents the use of a 96-well plate as headspaces for the determination of ascorbic acid in samples loaded in the 96-well plate. Ascorbic acid in the sample is oxidized to iodide by the addition of excess acidic iodate solution into the well. The iodide is further oxidized by the remaining iodate to molecular iodine. A single sheet of moist starch indicator paper is immediately placed over the 96-well plate after the addition of the iodate with the moisture forming a gas seal. The iodine gas in each well diffuses through the headspace to react with the starch paper producing circular areas of a colored starch-iodine complex. After 15 min the indicator paper is scanned, and the digital images of the complex are analyzed by using ImageJ software to obtain blue intensity values. The precision of the intensity values from 12 wells containing 20 µL of 2.84 mM standard ascorbic acid is <2% relative standard deviation. Optimal conditions for detection were investigated, including the starch concentration, the acidic iodate reagent, and the measurement time. The linear calibration range of ascorbic acid is 0.284-2.84 mM, based on the plot of concentration vs. -log(reflectance). The coefficient of determination (r2) is >0.998. Samples of fruit juice and dietary supplements were analyzed for their ascorbic acid contents. The results obtained from the headspace reflectance method are not statistically different from values obtained from the titration method using paired t-tests (α = 0.05).

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