Iodoform Reaction-Based Turbidimetry for Analysis of Alcohols in Hand Sanitizers

Abstract

This work presents the first development of an analytical turbidimetric method for the determination of legal alcohols in alcohol-based hand sanitizer products. A typical iodoform reaction is exploited to form a yellow product in the form of precipitates. An iodoform test shows a positive result as yellow precipitates in the presence of ethanol and isopropanol; therefore, the test can only be used to distinguish between methanol and those legal alcohols. In the presence of molecular iodine (I2) and a strong alkaline solution, the legal alcohol is converted to the corresponding carbonyl compound (i.e., ethanol to acetaldehyde, isopropanol to acetone). The susceptibility of this intermediate towards the reaction with hydroxide ions (strong alkaline condition) results in formations of yellow precipitation of iodoform (CHI3) and a water-soluble carboxylate salt in the solution. Therefore, this change allows for the detection of legal alcohols through either naked-eye observation (as semi-quantitative analysis) or a common benchtop/portable photometer/spectrophotometer (as quantitative analysis) by means of turbidimetric analysis. In this work, turbidimetry is employed, which is a useful alternative detection method in analytical practice, especially with colored samples in hand sanitizing products. This is because they can employ wavelengths at which the colored solution does not absorb light. As a result of our developed method, the calibration plots are in the range of 30 to 100% (v/v) for both ethanol and isopropanol. The limit of detection (LOD) (3SD of y-intercept/slope) was found to be 7.4% (v/v) ethanol and 6.5% (v/v) isopropanol. Direct analysis of the non-pretreatment of the sample is achieved. The results indicate that our new proposed analytical method is fit for purpose and valid to detect the legal alcohols in alcohol-based hand sanitizing products for both international and Thai regulations (at least 70% (v/v)). Our quantitative results were also comparable to a standard analytical method, such as the use of a gas chromatography-flame ionization detector (GC-FID). Our developed method and analytical operation could potentially be developed into a practically portable analysis.