A Colorimetric Test Strip for On-site Rapid Test of Boric Acid in Water-based Adhesive
Shan Li, Cong Wang, Xianyue Zhu*, Zeli Wang, Honghai Liu, Zhicai Wang, Le Zhao and Hongbo Wang** Author for corresponding; e-mail address: whbhn@126.com, zhuxianyue@ gslzcf.com
ORCID ID: https://orcid.org/ 0000-0001-6947-9468
Volume: Vol.53 No.4 (July 2026)
Research Article
DOI: https://doi.org/10.12982/CMJS.2026.077
Received: 30 September 2025, Revised: 28 June 2026, Accepted: 5 July 2026, Published: 24 July 2026
Citation: Li S., Wang C., Zhu X.y., Wang Z.l., Liu H.h., Wang Z.c., et al., A colorimetric test strip for on-site rapid test of boric acid in water-based adhesive. Chiang Mai Journal of Science, 2026; 53(4): e2026077. DOI 10.12982/CMJS.2026.077.
Graphical Abstract
Abstract
A colorimetric test strip has been developed for both qualitative and quantitative analysis of boric acids in water-based adhesive. To achieve optimal the analytical performance, key experimental parameters-including 0.5mmol 5-hydroxy flavones, 2.6mmol citric acid, a reaction temperature of 80C, and 40% nitric acid were systematically investigated. The test strip provides a rapid response within 6 min, exhibiting a distinct yellow coloration upon complexation between boric acid and 5-hydroxyflavoneunder acidic conditions. The color intensity showed a linear correlation with the concentration of boric acid ranging from 25 to 1000 mg/L, and the detection limit was determined to be 8 mg/L, the relative standard deviation (RSD) was less than 4.3%, and the spike recoveries ranged from 89.5% to 108.1%, indicating that the method possesses good precision and satisfactory accuracy. With its straightforward operation and rapid readout, the developed test strip represents a promising tool for on-site screening of boric acid in water-based adhesive sample.
1. INTRODUCTION
Water-based adhesive, commonly known as white latex, is a widely used thermoplastic adhesive. Polymerized from vinyl acetate monomers, it cures rapidly at room temperature, offering high bonding strength, toughness, and durability [1-4]. To enhance initial viscosity and operability, boric acid is frequently added during production. Boric acid utilizes its electron-deficient d-orbitals to bond with hydroxyl groups in the adhesive, forming complexes that improve mechanical and adhesion properties [5-9]. Additionally, boric acid adjusts pH, stabilizes the emulsion, and acts as a preservative.
Boric acid (H3BO3) is an inorganic compound that exists as an odorless white crystalline powder. As a monobasic, extremely weak acid, its acidity arises not from direct proton donation, but from its electron-deficient boron atom accepting hydroxide ions from water, thereby releasing protons. This electron-deficient nature also allows it to form stable complexes with polyhydroxy compounds like glycerol [10, 11]. Due to its cumulative toxicity, boric acid and borax have been declared unsafe as food additives by an FAO/WHO Expert Committee [12]. EFSA notes that boric acid can inhibit glutamate synthesis in the brain [13]. Consequently, China, the U.S. FDA, and the EU have prohibited its addition to food and restricted its use in food contact materials. Furthermore, water-based adhesives are widely used in cigarette production and participate in combustion. Therefore, due to potential inhalation health risks, regulating boric acid in adhesive formulations is essential.
Various analytical methods exist for determining boric acid [14-27], mainly including spectrophotometry [14-18], LC [19,20], GC-MS [21], ICP-AES [18, 22-24], ICP-MS [18, 24-26], and IC [28]. While ICP-AES and ICP-MS offer the highest sensitivity and lowest detection limits, they are costly, non-portable, and require complex equipment. Alternatively, Spectrophotometry provides a simpler, cost-effective alternative, and methylamine-H and curcumin are important chromogenic reagents for boric acid photometric determination. The methylimine-H spectrophotometric method while widely used for boron determination, has relatively low sensitivity and is susceptible to interference from temperature and light during color development [29]. Although this study adapts an AOAC-based curcumin spectrophotometric method [30, 31], but it has limitations such as a non-aqueous environment and a long color development reaction time. The boric acid test is highly specific to 5-hydroxyflavones and flavonols. According to Rangaswami and Seshadri, a positive reaction requires a 5-hydroxyl group along with three structural features: (a) a keto group, (b) an ortho-hydroxyl or methoxyl group, and (c) an exocyclic double bond conjugated with the C=O. Flavanones and simple aromatic ketones lacking these requirements fail to react [33]. This high structural specificity ensures minimal interference from other substances. Furthermore, the complexation occurs under mild conditions (80 °C, 6 minutes), which facilitates rapid on-site testing.
With the increasing public awareness of food safety, there has been a rapid proliferation of on‑site rapid testing methods in recent years [32-33]. The purpose of the current work was to conduct the qualitative and quantitative analysis of boric acids in water-based adhesive. The carbonyl group in flavonoids interacts with boric acid to form a yellow complex, a reaction enhanced and stabilized by the buffering capacity of citric acid [31]. Based on this reaction principle, a test strip was developed, optimized and applied as a simple, rapid and efficient assay for boric acid detection in water-based adhesive.
2. MATERIALS AND METHODS
2.1 Materials
5-hydroxy-flavanones and citric acid were purchased from Sigma-Aldrich (St. Louis, MO, USA). Nitric acid, phosphoric acid and hydrochloric acid were purchased from Merck (Merck KGaA, Darmstadt, Germany). All chemicals used in this study were of analytical grade. Qualitative filter paper (125 mm) was purchased from Whatman (manufactured in China). 1g/L boron acid standard solution (GBW(E)082782) was purchased from China National Institute of Metrology (Beijing, China).
An Agilent 7900 ICP-MS (Agilent, Milford, USA) was used for Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis. A blast drying oven was obtained from Espec Environmental Instruments Co., LTD (Shanghai, China). A KQ-300 ultrasonic generator was sourced from Kunshan Ultrasonic Instrument Co., LTD (Suzhou, China).
2.2 Fabrication of a Paper Strip for the Detection of Boron
To prepare the test strip solution, 0.1g 5-hydroxy-flavone and 0.5g citric acid were dissolved in 50mL ethanol via ultrasonication. Aliquots of 0.5 mL of the solution were dispensed onto the center of qualitative filter paper, a process repeated three times. After the solution had fully permeated the paper, the filter paper was dried at 80°C for about 10 min and then cut into 12mm test strips. These test strips were then utilized for the detection of boric acid.
2.3 Preparation of Water-based Adhesives Extracts for Boron Acid Detection
Four water-based adhesive samples (denatured starch adhesive (DSA), Polyvinyl alcohol (PVA), Polyvinyl acetate (PA)) were purchased from a local market in Zhengzhou, China. Two grams of each sample were mixed with 4 mL of 40% (V/V) HNO3 and homogenized using a vortex mixer for 2 min. The mixture was then centrifuged at a high-speed centrifuge (centrifuged at 5,000 rpm for 5 min), and the resulting supernatant was retained for subsequent boron acid analysis using the test strip.
2.4 Colorimetric Test Strips for Boron Acid Detection
A series of standard working solutions (0, 25, 50, 75,100, 200, 500, 1000 mg/L) was prepared by diluting a 1 g/L standard solution. Subsequently 0.25 mL of either the boron acid standard or the sample extract was applied to the test strip, and while the control strip received 0.25 mL of 40%(V/V) HNO3. The test strips were then heated at the temperature of 80°C for 6 min. Finally, the color intensity on the sample pad was visually compared with a standard color card (Figure 1) to determine the boric acid concentration.
3. RESULTS AND DISCUSSION
3.1 Detection of Boron Acids based on Test Strip
Flavonoids are an important class of natural pigment and key active component in traditional Chinese medicine. 5-hydroxyflavone undergoes a stoichiometric reaction with boric acid [B(OH)3] to form a 2 : 1 complex in the presence of inorganic acids or organic acids [31-32]. The resulting solution exhibits a distinct bright yellow color, as shown in Figure 2. This characteristic typically complex appears yellow with green fluorescence in the presence of oxalic acid, but displays only displays a yellow color without fluorescence when citrate acid is used. It is well known that this reaction (Figure 2) is primarily utilized for the identification of flavonoids.
A colorimetric test strip was developed based on the complexation reaction between boric acid and 5-hydroxyflavone. The method involved loading 5-hydroxyflavone and citric acid onto the test strip, followed by the addition of boric acid solution and a blank control sample (without boron acid) to initiate the color reaction. Visual observation revealed a distinct yellow color change, and the color intensity increased progressively with rising the concentration of boron acid, as shown in Figure 1.
3.2 Influence Factors on Boron Acid Detection
In general, the complexation reaction is primarily governed by three factors: acidity, temperature and reaction time. Acidity critically affects the reaction extent, while temperature and time exhibit an inverse relationship—higher temperatures shorten the required time by accelerating the reaction. All experiments in this study were conducted using a boric acid concentration of 50 ppm.
3.2.1 Effects of acid type on boron acid detection
The type and concentration of acid are two key factors affecting the complexation reaction. Figure 3 presents a comparative study of the colorimetric responses obtains with hydrochloric acid, phosphoric acid and nitric acid at different concentrations (volume ratio) on the test strips. Both phosphoric acid and nitric acid promote the formation a yellow complex. In the nitric acid system, the color intensity increases notably with rising acid concentration. However, the phosphoric acid system yields only a weak color change, which remains largely unaffected by concentration variations. Most importantly, hydrochloric acid fails to produce any observable color reaction.
3.2.2 Effects of temperature on boron acid detection
Temperature played a critical role in the complexation reaction, as elevated temperatures typically shorten the required reaction time. To evaluated this effect, we examined the temperature range of 40-100 °C. The results (Figure 4) showed that the color development time decreases progressively with the increase of temperature, confirming that heating promoted the color reaction between boric acid and 5-hydroxyflavone. Based on these findings, 80 °C was selected as the optimal condition, achieving complete the color development within 6 min while maintaining test strip stability and color integrity for at least 20 minutes.
3.3 Sample Preparation
Boric acid serves as cross-linking agent for PVA blend, forming a PVA-B(OH)3 complex that has diverse structure-sensitive applications. Unlike covalent chemical bonds, the interaction is predominantly coordinative, leading to the formation of polymetaboric chains. In general, it is well known that nucleophilic coordination compounds of boric acid tend to undergo open-chain cleavage in strong acidic media. Accordingly, we chose 40% nitric acid as the extraction reagent for releasing boric acid, in line with the colorimetric assay. Through high-strength vortex extraction, the cross-linked boric acid present in the adhesive was successfully converted to its free form. Applying this procedure to an adhesive sample containing 150 mg/kg boric acid yielded a preparation solution with a concentration of 300 mg/L, as evidenced by the test strip’s yellow coloration matching the 300 mg/L standard on the color chart. These results demonstrate that 40% nitric acid extraction is highly effective in breaking the cross-linking between boric acid and the adhesive matrix.
3.4 Analytical Performance of the Test Strip
For quantitative analysis of boric acid in water-based adhesive, a standard colorimetric card was prepared. As shown in Fig.1, the yellow color developed on the test strip intensified progressively with the increasing boric acid concentration from 25 to 1000 ppm, demonstrating on a linear correlation.
The sensitivity of the assay was assessed by stepwise dilution of the 25 ppm standard solution. Each diluted was applied to a test strip and compared with a blank control until the color difference became visually indistinguishable, thereby establishing the detection limit. Under these conditions, the detection limit was found to be 8 ppm. Six samples were subjected to repeatability tests as well as spike recovery experiments at low, medium, and high levels. The corresponding results are summarized in Table 1, which demonstrate that the method offers good repeatability (0.8%~4.3%) and favorable recovery rates (89.5~108.1%). For actual sample detection, a preliminary screening was conducted to estimate the concentration level, followed by measurement using a more finely graduated colorimetric card for improved accuracy. The results obtained for several adhesive samples were summarized Table 1, alongside those from ICP-MS analysis [28]. The comparison showed that the proposed method is basically consistent with the ICP-MS reference method.
In addition, the storage stability of the test strips was examined, and it was found the test strip could be preserved for at least 60 days in a sealed dry environment.
However, a notable limitation of this method is that it yields only semi-quantitative concentration range rather than an accurate numeric result, as the readout depends visual color comparison. Consequently, this method is best suited for rapid screening applications.
4. CONCLUSIONS
This study developed a simple, rapid, and sensitive test strip for detecting boric acid in water-based adhesive. The method requires minimal reagents (4 mL), reducing waste, and the results correlated well with ICP-MS in both qualitative and semi-quantitative analysis. Ultimately, this cost-effective strip is a promising on-site screening tool for the quality control of water-based adhesives.
ACKNOWLEDGEMENTS
The author sincerely thanks the staff of the Technology Center of Gansu Tobacco Industry Co., Ltd. for their technical assistance and the experimental equipment they provided. We are also very grateful to our colleagues for the beneficial discussions they provided during the preparation of this manuscript.
AUTHOR CONTRIBUTIONS
Hongbo Wang: Conceptualization, Methodology, Resources, Writing- Reviewing and Editing. Shan Li: Resources, Data curation, Writing - Original draft preparation. Cong Wang: Visualization, Data analysis. Xianyue Zhu: Validation, Investigation. Zeli Wang: Supervision, Anlaysis. Honghai Liu: Software, Validation. Zhicai Wang: Formal analysis. Le Zhao: Methodology.
CONFLICT OF INTEREST STATEMENT
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
FUNDING
This project is funded by the Science and Technology Department of Gansu Tobacco Industry Co., LTD.: Contract Number KJXMM-2025-04.
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