Rattikan Chantiwas a, Prapin Wilairat a, Papawarin Putthasa b, Benjamapohn Cheotchinda b, Saowapak Teerasong b c, Wiboon Praditweangkum b
a Department of Chemistry and Center
of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol
University, Rama VI Rd., Bangkok 10400, Thailand
b Department of Chemistry and Applied
Analytical Chemistry Research Unit, School of Science, King Mongkut’s Institute
of Technology Ladkrabang, Bangkok 10520, Thailand
c Flow Innovation-Research for Science
and Technology Laboratories (FIRST Labs), Thailand
* Corresponding authors. E-mail
addresses: rattikan.cha@mahidol.ac.th (R. Chantiwas), wiboon.pra@kmitl.ac.th
(W. Praditweangkum).
Abstract:
A miniaturized,
chromate-free methodology for chloride determination in high-salt food matrices
is presented in accordance with the principles of Green Analytical Chemistry.
Classical argentometric precipitation is adapted to a microvolume,
instrument-light assay by confining silver chloride formation within inverted
microcentrifuge tubes and directly measuring the height of the compact
precipitate layer using a simple ruler. Because the cylindrical tube region has
a constant cross-sectional area, precipitate height is directly proportional to
chloride concentration, enabling straightforward linear calibration without
optical detection, signal transduction, or complex data processing. Controlled
tube inversion and low-speed centrifugation ensure that precipitation occurs
exclusively within this cylindrical zone, producing a sharp, reproducible
solid–liquid interface. Under optimized conditions (150 µL sample volume and
180 µL of AgNO₃ in nitric acid; centrifugation at 600 rpm for 3 min followed by
5 min settling), the method provides a linear working range of 50–500 mM
chloride (r² = 0.9988), a detection limit of 14 mM, and relative standard
deviations of 3–4 % at 200–300 mM. Validation of 12 commercial Thai fish sauce
samples showed no statistically significant differences compared to AOAC
potentiometric titration at the 95% confidence level, with recoveries of
100–103 % and %RSD values of 1–4.2 %. The primary advantage of this method is
its strictly linear height–concentration relationship, enabled by
geometry-controlled confinement of the precipitate. Unlike other miniaturized
or paper-based assays, it eliminates the need for image analysis or device
fabrication. From a green chemistry perspective, the assay eliminates the use
of potassium chromate indicators and disposable paper-based devices, operates
at the microliter scale using reusable plastic microtubes, and requires only
low-speed centrifugation. Analytical Greenness (AGREE), Analytical Eco-scale,
Green Analytical Procedure Index (GAPI), and White Analytical Chemistry (WAC)
evaluations (AGREE score: 0.76; Eco-scale: 83; WAC score: 88.9) classify the
method as an excellent example of green analysis. The proposed approach offers
a robust and sustainable screening tool for chloride determination in high-salt
foods.
KEYWORDS
Chloride; Fish sauce; Miniaturized
assay; Chromate-free; Green analytical chemistry; Precipitate height; Microcentrifuge
tubes
Reference:
Chantiwas, R.; Wilairat, P.; Putthasa, P.; Cheotchinda, B.; Teerasong, S.; Praditweangkum, W. Miniaturized chromate-free chloride assay in fish sauce based on linear height calibration of AgCl precipitate in inverted microtubes. Green Analytical Chemistry 2026, 17, 100343. doi: 10.1016/j.greeac.2026.100343
