Researchers Frantisek Cacho, Jakub Masac and Andrej Kudry report in the open-access journal Results in Chemistry a flow-through coulometric titration method that determines iodides directly in water samples with very high chloride content, including seawater. The central challenge they tackled is one that plagues nearly every electroanalytical technique: interference. Chlorides, sulfates, bromides and fluorides coexist with iodide in real matrices and can distort or swamp the analytical signal. Seawater, with an average chloride concentration of nearly 20 grams per liter, represents perhaps the worst-case scenario for iodide determination. Conventional approaches, such as high-performance liquid chromatography or gas chromatography with mass spectrometry, typically require a time-consuming derivatization step in which iodine species are converted to detectable derivatives. Ion chromatography, while simpler, suffers from low sensitivity. Spectrophotometric methods based on the Sandell-Kolthoff reaction, in which iodine catalyzes the reduction of yellow cerium(IV) ions by arsenic, demand careful kinetic control. The new method sidesteps much of this complexity by exploiting a clever two-step electrochemical strategy inside a porous electrode.

The heart of the instrument is a compact flow cell equipped with a cylindrical working electrode made of porous glassy carbon, also known as reticulated vitreous carbon, with a porosity of 100 pores per inch, 4 millimeters long and 10 millimeters in diameter. A platinum auxiliary electrode and a saturated silver/silver chloride reference electrode complete the three-electrode arrangement. The measurement unfolds in two distinct phases. First, a 4-milliliter sample or standard is pumped through the system, filling the pores of the working electrode at a resting potential of 200 millivolts, where no interfering electrochemistry occurs. Then the potential is stepped up to 960 millivolts and held for 30 seconds, during which iodide ions are electrochemically oxidized to iodine within the electrode volume. In the second phase, a constant reduction current of minus 8 microamperes is applied, converting the generated iodine back to iodide while the instrument records the resulting signal.

Why bother with this oxidation-then-reduction round trip instead of simply oxidizing iodide directly? The answer lies in signal quality. The reduction of iodine proceeds at a more negative potential, where the signal-to-noise ratio is substantially higher and the detection limit correspondingly lower. The authors demonstrated that this two-step mechanism yields markedly better performance than direct oxidation. The signal itself is captured using a memory mapping technique, in which chronopotentiometric data are recorded not as a potential-versus-time wave but as a peak of channel counts plotted against potential. The integrated peak area is directly proportional to the chronopotentiometric transition time of the electrode process, and dividing that area by the sampling frequency of the analog-to-digital converter yields the transition time in seconds. This transition time is the quantity that scales linearly with iodide concentration, forming the basis of quantification.

One of the most compelling aspects of flow coulometry is that it can function as an absolute, calibration-free method. Under appropriate conditions, the analytical result is obtained by direct calculation from the measured signal using Faraday’s law, meaning the outcome depends only on physical constants and directly measured quantities rather than on reference materials. In the thin-layer configuration used here, operated galvanostatically, the relationship between transition time and analyte concentration is practically linear as long as the solution layer within the electrode does not exceed the thickness of the diffusion layer and the current density remains below roughly 1 microampere per square centimeter. At higher currents, diffusion limitations begin to distort the response. When the pore diameter of the electrode matches the diffusion layer thickness, the analyte concentration is essentially uniform throughout the electrode, and the potential directly reflects the concentration within its volume, cleanly marking the endpoint of electrolysis.

The team systematically optimized every operating parameter. The oxidation potential was scanned from 800 to 980 millivolts in 20-millivolt steps, with the maximum signal reached at 960 millivolts; beyond that, the electrode material itself began to oxidize, shortening its lifetime. The oxidation duration was varied from 10 to 60 seconds, and 30 seconds emerged as the sweet spot, providing near-maximal conversion to iodine without prolonging each measurement or stressing the electrode at the high applied potential. The reduction current was tested between minus 3 and minus 10 microamperes, with minus 8 microamperes giving the strongest response. Even the carrier electrolyte, a mixture of 0.1 molar sulfuric acid and 0.001 molar hydrochloric acid, was scrutinized: hydrochloric acid concentrations spanning six orders of magnitude were evaluated, and the optimum sat at 0.001 molar, with higher concentrations showing no effect on the signal.

The metrological performance is impressive for such a simple setup. The method exhibits a linear range from 0.1 to 40 milligrams per liter with a coefficient of determination of 0.9993. The limit of detection, calculated by the IUPAC-recommended ULA 2 procedure from the lower part of the calibration curve, is 0.03 milligrams per liter, and the limit of quantification is 0.09 milligrams per liter. Repeatability, assessed from ten consecutive measurements of a 5-milligram-per-liter iodide standard, came in at 5.00 plus or minus 0.03 milligrams per liter, a relative standard deviation of just 0.54 percent. Reproducibility, verified in an external laboratory, yielded 4.9 plus or minus 0.1 milligrams per liter with a relative standard deviation of 1.1 percent, confirming that the method travels well beyond the developing lab.

Interference testing was thorough and revealing. Twelve potential interferents, including chloride, bromide, fluoride, sulfate, sulfite, ammonium, potassium, calcium, magnesium, aluminum, boron and iron species, were challenged against iodide at ratios from 1:1 up to 1:100, and for chloride all the way to 1:5000, a range chosen to bracket the chloride level of seawater. Most ions caused no measurable interference. Bromide produced a slight positive bias only at a 1:100 ratio, where the iodide signal rose by 9 percent, a consequence of bromide being co-oxidized to bromine, which is a stronger oxidant than iodine and further drives iodide conversion; such bromide levels are highly unlikely in real samples. Iron(III) was the one significant offender, cutting the iodide signal by 22 percent at a 1:50 ratio and eventually eliminating it, because its reduction overlaps the iodine reduction signal. Iron(II) behaves identically after being oxidized to iron(III) in the first step. Fortunately, this interference can be suppressed simply by passing the sample through a cation exchanger in the sodium cycle.

The real-sample results showcase the method’s practical reach. Bottled Slovak mineral waters Vincentka, Saratica and Cigelka, thermal and sulfur spring waters from Chorvatsky Grob and Dudince, Mediterranean seawater collected at Lloret de Mar in Spain, and iodized table salt were all analyzed after simple pretreatment, typically just boiling to remove bicarbonate. Iodide concentrations determined by calibration curve agreed closely with standard-addition results, passing a Student t-test at the 95 percent confidence level. Vincentka contained 4.6 milligrams per liter, Cigelka 0.78, the Chorvatsky Grob spring 0.16, and seawater 0.13 plus or minus 0.03 milligrams per liter, while the table salt held 25.1 milligrams per kilogram. In the two samples falling below the quantification limit, standard additions were recovered at 101 to 110 percent. With its robust, inexpensive porous carbon electrode, minimal reagent consumption, on-line sample handling capability and green analytical chemistry credentials, the method could readily extend to continuous process monitoring of water quality, offering laboratories a fast, low-cost alternative to chromatographic and spectrometric techniques.

Subject of Research: Electroanalytical determination of iodide in high-chloride water samples by flow-through coulometric titration

Article Title: Determination of iodides in water samples with high chloride content by in-built-electrode coulometric titration using a porous carbon electrode

Article References: Cacho, F., Masac, J., & Kudry, A. (2026). Determination of iodides in water samples with high chloride content by in-built-electrode coulometric titration using a porous carbon electrode. Results in Chemistry, 31, Article 103938. https://doi.org/10.1016/j.rechem.2026.103938

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103938

Keywords: iodide determination, coulometric titration, porous glassy carbon electrode, flow-through analysis, seawater analysis, chloride interference, electrochemistry, iodine deficiency, green analytical chemistry, mineral water, detection limit, WHO iodine intake