Abstract

In recent years, there has been renewed global interest in accurate environmental monitoring, particularly in water quality assessment. This growing demand has driven the development of various tools, techniques, and platforms for the detection and quantification of environmental pollutants. Although many of these systems provide reliable analytical performance, they remain limited by high cost, lack of suitability for point-of-use deployment, dependence on technical expertise, and reliance on complex instrumentation. Tablet-based sensors have emerged as a promising alternative, offering a portable, low-cost, and user-friendly platform capable of addressing many of the limitations associated with conventional methods. Considering this, this research will focus on: (1) the development of pullulan tablet-based sensor for simplified water hardness measurement; and (2) the tuning of pullulan molecular weight using microwave assisted acid degradation towards improving the response time of tablet-based sensors.
The pullulan-based tablet sensor that utilises a reverse titration approach was developed for a faster and point-of-use measurement of water hardness. This system addressed key limitations of classical EDTA titration. It is made up of two distinct pullulan-encapsulated reagent tablets: one tablet contains Eriochrome Black T (EBT) and N-cyclohexyl-3-aminopropanesulfonic acid (CAPS) buffer, and the other contains disodium Ethylenediaminetetraacetic Acid (Na₂EDTA). A three-step detection approach was utilised to detect and quantify water hardness level. First, an immediate color change produced by the EBT tablet provides a quick qualitative detection of hardness. Second, dissolving both tablets and making up the sample volume to a reference mark gives a simple semi-quantitative estimate. Third, a full quantitative measurement is obtained through reverse titration, in which water is added gradually until the characteristic red-wine endpoint appears. This method improves tolerance to interference from competing metal ions and increases accuracy compared with traditional titration. A comparison test using real water sample between the tablet sensor and classical EDTA titration showed an excellent agreement. The tablets remain stable for over seven months, and the system requires no trained personnel, laboratory glassware, or bulky instruments. Overall, this low-cost, user-friendly, and interference-resistant platform provides a practical solution for fast and reliable water hardness analysis directly at the point of use.
During the evaluation of water hardness using the developed tablet-based sensor, it was observed that complete dissolution required significant agitation, which may limit applicability in time-sensitive or low-resource settings. To address this limitation, a microwave-assisted degradation strategy was employed to tailor the molecular weight of pullulan and enhance dissolution.
A structured Design of Experiments (DoE) approach was implemented, and the degradation process was modeled using Minitab software (Version 22.4.0). The resulting predictive model enables estimation of intrinsic viscosity as a function of temperature, hold time, and acid concentration, thereby providing a controllable framework for tuning polymer properties.
The practical relevance of the modified pullulan was demonstrated through controlled release studies using cobalt (II) chloride as a model compound. The degraded pullulan exhibited significantly faster release compared to the undegraded polymer, confirming its suitability for time-sensitive applications without the need for external agitation equipment.
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