Complexometric Determination of Metal Ions by Microscopic Diffusional Titration

Abstract

Acid/base titrations of pico- and femtoliter microsamples have been performed previously using a diffusional microburet (DMB) for reagent delivery in a simple droplet-heptane system (Gratzl, M.; Yi, C. Anal. Chem. 1993, 65, 2085-2088). The lowest delivery rate achieved with a DMB was about 6 fmol/s, which would correspond to about a 1 microL/year volumetric flow rate with a hypothetical equivalent mechanical delivery scheme (Yi, C.; Gratzl, M. Anal. Chem. 1994, 66, 1976-1982). In this work, the feasibility of complexometric titrations in microscopic samples is explored. Stability of pH in the microdroplets required for different determinations and the effects of DMB shank geometry on titration characteristics are also studied. Diffusional microtitrations of Fe(III), Zn(II), and Cu(II) have been performed with EDTA. Xylenol orange and Eriochrome Black T provide clear color changes at the end point of the respective titrations, despite the microscopic size of the samples (between 16 and 1570 pL, corresponding to diameters between 30 and 144 microns). Random errors of the determinations relative to full scale were 6.6% for Fe(III), 5.8% for Cu(II), and 7.9% for Zn(II). The pH required for EDTA titrations of the individual metal ions stays stable in the acidic range. This makes the microscopic titration of a number of metal ions, such as Fe(III), Fe(II), Cu(II), and Pb(II), feasible in a simple droplet-heptane system without any modification. With a higher density of strongly alkaline buffer droplets (about 100 droplets/mm2) sprayed on the bottom of the Petri dish, or by flushing N2 above the heptane, the microscopic samples can also be kept alkaline despite ambient CO2 present. In this way, Zn(II) can also be titrated in microdroplets, requiring a pH around 10. This work renders it possible to perform a variety of complexometric titrations and other chemical manipulations in microdroplets even if they need to be kept alkaline. Similar titrations in single biological cells to assess intracellular buffer capacities of different metal ions, such as Ca(II) and Mg(II), are underway.

Microdroplet-Based Potentiometric Redox Measurements on Gold Nanoporous Electrodes

“…The fast mixing time and the ability of these electrodes to behave in a Nernstian fashion lends support to using these measurements in situations where time and limited sample volumes are of critical importance. Such examples may include microtitrations , and measurement of redox potentials of microscopic solutions such as blood and blood products . Future work is aimed in that direction.…”

“…Electrochemistry in ultrasmall volumes is valuable as it enables electrochemical measurements to be made when only minute amounts of liquid (e.g., from a biological sample) are available for testing. − It also allows for higher throughput sensing and multiplexing, single cell analysis, and ease of integration into microfluidic devices and platforms. − Small volume electrochemistry has been achieved using a number of strategies − that include microfabrication of ultrasmall vials with and without embedded electrodes as well as creating the electrochemical cell from a single drop of solution. ,,− ,,,, In the latter studies, the droplet is placed on a surface and the reference, counter, and working electrodes are inserted. , Alternatively, the droplet can be placed on the working electrode and the remaining two electrodes inserted in the solution . Also possible is the case when an array of oil droplets is placed on an electrode, which is then immersed in an aqueous (and thus immiscible) solution containing the reference and counter electrodes. , In this study, we report potentiometric redox measurements in subnanoliter droplets of solution using optically transparent metallic redox electrodes (e.g., nanoporous gold electrodes) fabricated on glass microscope coverslips.…”

Diffusional Microtitration of 20 μL Samples with Optical Indication

“…The interpretation of complexometric titration curves with similar dye indication has been previously discussed. 5 Since the calcium-EDTA complex has a much higher stability constant than that of the calcium-murexide complex (10 10.6 9 and 10 5.0 , 10 respectively), EDTA would first complex with free metal ions and then with those that are complexed to dye molecules. Thus, the color change, which depends on the state of the dye, would occur according to three phases: completely complexed, partially complexed, and completely free dye.…”

“…In diffusional microtitration, [1][2][3][4][5][6][7] visual end point determination of microliter sample droplets 1 and off-line optical determination based on gray scale changes of digitized images of pico-to femtoliter droplets [3][4][5] were performed earlier. For on-line instrumental indication, traditional 1,2 and microfabricated 6,7 potentiometric electrodes were tested.…”

A convenient chemical titration method to measure M(ii)/M(iii) ratios of layered double hydroxides

“…Fortunately, complexometric titration method has obvious advantages of being simple, cheap, and independent of instruments. 15,16 Meanwhile, it is also effective for measurements of a mixture containing two or more different metal ions. [17][18][19] Unfortunately, the publications of measurements of M(II)/M(III) ratio by chemical titration method are very limited so far.…”