Simplified determination of complex stoichiometry for colorimetric metal indicators by inkjet printing

This paper presents the determination of complex binding stoichiometry for colorimetric metal indicators according to the Job plot method by means of an office inkjet printer in combination with digital colour analysis. In contrast to the conventional solution-based analysis involving manual preparation of many mixed solutions, the elaborated inkjet-based approach allows convenient automated mixing of metal salt and indicator solutions in various ratios on a paper substrate. Inkjet-deposited liquid volumes and mixing ratios were controlled by simple colour value setting on a PC. For this purpose, a custom printing colour profile has been elaborated to achieve a linear relationship between the software set CMYK (Cyan, Magenta, Yellow and Key plate (Black)) colour values and the amount of deposited ink. The inkjet-based Job plot analysis was validated against the solution-based conventional approach for binding stoichiometry determination of five known metal ion-indicator systems. Simple colour scanning and colour analysis of printed spots on filter paper gave accurate results for five different stoichiometries and colorimetric response patterns. This inkjet printing approach using a customized printing colour profile allows rapid, flexible and precise deposition and mixing of reagent solutions, and thus, is expected to be useful for wide application purposes, which require the controlled deposition of reagents on a substrate.