Replacing n,n-diethyl-p-phenylenediamine(DPD) with polyvinyl alcohol for the non-hazardous determination of trace oxidizing agents in water

doi:10.21203/rs.3.pex-2551/v1

Method Article

Replacing n,n-diethyl-p-phenylenediamine(DPD) with polyvinyl alcohol for the non-hazardous determination of trace oxidizing agents in water

https://doi.org/10.21203/rs.3.pex-2551/v1

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For decades, n,n-diethyl-p-phenylenediamine (DPD) has been used as a color indicator to detect strong oxidizing agents that can oxidize iodide in aqueous solutions. However, many consumer products that use this method are easily accessible and often used without proper waste disposal. In the present study, a novel method was developed based on the special optical properties of a red-colored complex formed by polyvinyl alcohol (PVA) and triiodide (I₃^-) generated from the oxidation of iodide. Using peracetic acid (PAA) as a test oxidizing agent, a liquid reagent containing 1% PVA and 0.1 g/L potassium iodide (KI) in 0.1 M pH 5.0 phosphate buffer showed strong linearity between absorption value at 485 nm and PAA concentration (≤ 5 mg/L). Test strips soaked in this liquid reagent and dried showed inconsistent color reaction with PAA, and the sensitivity was insufficient. In comparison, solid reagent in combination with effervescent powder performed with sufficient consistency and sensitivity in the reaction with PAA. The test in the form of effervescent powder has great potential to be developed into a green consumer product due to the non-hazardous properties of PVA. The health risks associated with KI can be largely reduced and should be warned for consumers.

chlorine test

halogens

peracetic acid

ozone

permanganate

The titrimetric and colorimetric method using n,n-diethyl-p-phenylenediamine(DPD) for the determination of free and combined chlorine in water was first invented by the chemist and bacteriologist Arthur Thomas Palin in 1957 . In this method, he reported that free chlorine reacts instantly with a DPD reagent (0.15% DPD sulfate, 0.2% sulfuric acid and 0.02% EDTA in water) and develops a pink color, while combined chlorine reacts with the DPD reagent only in the presence of potassium iodide (KI). For calibration standard, he suggested using a standard potassium permanganate solution or iodine solution. In addition, he noted the interference by oxidized manganese, implying potential compatibility of the DPD method for the determination of other oxidizing agents. Indeed, he adapted this method for the measurement of halogen compounds and ozone in 1975 . In the updated method, free halogens can be measured with the DPD reagent alone, while most combined halogens and ozone can be measured with supplementary KI. Since then, the DPD method has been used for the detection of other oxidizing agents/oxidation process (that can oxidize KI) in research. In practice, the DPD method has been developed into various commercial test products for detection of oxidizing disinfectants in water (CHEMetrics, Hach, LaMotte, etc.). Some of these test products, such as those for testing free and combined chlorine in swimming pools, are accessible to amateur consumers and lack proper waste disposal measures. According to EU Regulation (EC) No 1272/2008 and the Globally Harmonized System (GHS) of Classification and Labelling of Chemicals, DPD and its sulfate form are categorized into group GHS07 with hazards like acute oral toxicity, skin irritation and chronic aquatic toxicity. Liquid or solid DPD waste should not be disposed as regular municipal waste. Therefore, it is necessary to replace the DPD method with a green method, especially for the use by amateur consumers.

The most-accepted mechanism that DPD develops a pink color upon oxidation is the formation of intermediate semiquinoid cationic radical which belongs to the family of Wurster’s dye . The pink color of the DPD-Wurster’s dye in solution is, however, associated with the polymerization of the semiquinone radicals connected by adjacent N atoms instead of the monomeric form . Following this inspiration, a novel polymerization-based method was developed for the determination of aqueous oxidizing agents in the present study.

PVA

Molecular weight:

Low (9-10k), 80% hydrolysed (Sigma-Aldrich, Product No. 360627)

Medium (13-23k), 87-89% hydrolysed (Sigma-Aldrich, Product No. 363170)

High (89-98k), 99+% hydrolysed (Sigma-Aldrich, Product No. 341584)

Concentration: 0.25%, 0.5%, 1% and 2%

0.1 g/L, 0.2 g/L and 0.4 g/L

Phosphate buffer

0.1 M pH 5.0: 13.61 g/L KH₂PO₄

0.1 M pH 5.5: 13.12 g/L KH₂PO₄+ 1.29 g/L Na₂HPO₄

0.1 M pH 6.0: pH 5.5 buffer adjusted with 1 M KOH

Reagent: sample ratio

1:1 and 0.7:1

DR3900 Spectrophotometer (Hach Lange)

Stepwise classic DPD method

The classic DPD method with supplementary KI was used to analyze peracetic acid (PAA) samples at 1 mg/L.The analysis was carried out in separate forward and reverse steps, and each step was followed by a full wavelength scan using a DR 3900 spectrophotometer (Hach Lange, Düsseldorf, Germany). The forward steps included analysing the PAA samples alone, the PAA samples with KI reagent, and the PAA samples with both KI and DPD reagents. The reverse steps involved analysing the PAA samples alone, the PAA samples with DPD reagent, and the PAA samples with both DPD and KI reagents.

Starch as an indicator

The DPD reagent in the classic DPD method was replaced by 1% starch solution. PAA samples at 0.5-5 mg/L were first mixed with KI reagent and then with 1% starch. An endpoint full wavelength scan was performed.

Polyvinyl alcohol as an indicator

PAA samples at 0.5-5 mg/L were tested with a freshly prepared 0.1 M phosphate buffer containing polyvinyl alcohol (PVA) and KI. Various combinations of buffer pH, PVA concentration and KI concentration were tested (as shown in Reagents). An endpoint full wavelength scan was performed with each combination and the maximal absorption value and wavelength were recorded.

The potential of the PVA-KI reagent to be developed into consumer products was tested in the form of test strips and effervescent powder. The test strips were cut from standard lab filter paper, soaked in the PVA-KI phosphate buffer for 5 min and dried at 30°C. For measurement, the test strips were dipped in PAA samples for 1 sec and allowed for color development for 30 sec. The effervescent powder was the solid mixture of 150 mg PVA, 1.5 mg KI, 204 mg KH₂PO₄, 350 mg citric acid and 250 mg NaHCO₃ corresponding to 15-mL test volume. For measurement, the solid mixture was added directly to a 15-mL PAA sample and allowed for color development for 1 min.

Triiodide in classic DPD method

During the stepwise classic DPD method, it was observed that the mixture of 1 mg/L PAA sample with KI reagent instantly resulted in a pale-yellow color which had maximal absorption at 350 nm. This observation is consistent with , who determined that the yellow color was attributed to triiodide (I₃^-) formed from the oxidation of KI. In contrast, mixing 1 mg/L PAA sample with DPD reagent caused neither color change nor changes in absorption spectrum. Only the concurrent presence of KI and DPD reagents resulted in the typical pink color and maximal absorption at 550 nm. Noteworthy, I₃^- formed from PAA oxidation was unstable; the yellow color faded within a few minutes at room temperature. Furthermore, subsequent addition of DPD reagent resulted in lower absorption read than regular measurement. These observations indicate that I₃^-is a crucial intermediate product in classic DPD method and has great potential to be developed as a direct indicator for strong oxidizing agents.

Junglee et al. (2014)

Starch-triiodide complex

The changes in optical properties of iodine (I₂) in starch solution was first discovered in 1814 and since then, the classic blue color of starch-I₂complex has been developed into methods to detect iodine or starch . The structure of the starch-I₂ complex was investigated by x-ray diffraction; it was proposed that I₂molecules are filled in the channel of the helical starch chain . Later research revealed that the helical structure of the starch chain is affected by its degree of polymerization. Furthermore, it is rather polyiodide (formed by various combinations of I₃^-, I₂ and I^-) instead of I₂ alone that is filled in the channel of the starch helix and is responsible for the blue color . In the present study, however, neither blue coloration nor changes in absorption spectrum was observed after the reaction of 0.5-5 mg/L PAA with KI in 1% starch solution. It was speculated that the degree of polymerization of the tested starch solution was not high enough to form a stable starch-polyiodide complex with the theoretical low amount of I₃^-generated. Therefore, polyvinyl alcohol that has higher degree of polymerization than starch was test.

Polyvinyl alcohol-triiodide complex

All tested combinations of PVA, KI and phosphate buffer instantly yielded a Bordeaux red color that maintained stable for hours after the reaction with 0.5-5 mg/L PAA. The endpoint solution showed a maximal absorption at 485-490 nm. The small variation in absorption spectrum was mainly affected by the PAA product variety. The best performing combination was 1% PVA (low molecular weight, 80% hydrolysed), 0.1 g/L KI in 0.1 M pH 5.0 phosphate buffer and a reagent : sample ratio of 0.7 : 1. This combination results in best linearity between PAA concentrations up to 5 mg/L and the absorption values at 485 nm (Figure 1). Similar red color and absorption spectrum was observed with a PVA-KI-based gel in response to radio irradiation . The authors proposed that KI was oxidized into I₃^- by free radicals generated from the radiolysis of water. The I₃^- subsequently formed a charge-transfer complex with partially saponified PVA and resulted in the red color. Unfortunately, the molecular structure of the PVA- complex hasn’t been revealed yet. Instead, intensive studies were conducted on the structure of PVA-I₂ complex. In his historical work, Zwick reported some of the earliest observations and findings about the PVA-I₂ complex. He proposed that I₂ and I^- form a polyiodide chain in the channel of the PVA helix, which has similar blue color and structure to the starch-iodine complex. Later researchers, however, proposed another model that the polyiodide chain is adsorbed on the aggregates of syndiotactic sequences in PVA (precipitation of multiple PVA chains). This model was recently confirmed with x-ray diffraction; the linear polyiodide chain is surrounded by 6 PVA chains . In the present study, the dominant iodic ions might be the mixture of I₃^- and I^- due to the oxidation of PAA. This may result in polyiodide chains that slightly differ from those in the PVA-I₂ complex and correspondingly, differ in color and absorption spectrum too.

Potential as consumer products

Both test strips and effervescent powder containing the mixture of PVA, KI and phosphate salt showed a more intensive red color in reaction with PAA at higher concentrations. The test strips, however, showed insufficient consistency and sensitivity for measurement. They were slowly oxidized by ambient oxygen and showed red edges prior to measurement after long-term storage. Short-term storage for a few days also resulted in differed color intensity at measurement. Color difference was visually distinguishable when the concentration difference of PAA was ≥ 1 mg/L. In contrast, the effervescent powder showed promising results. The reagents remained stable in solid form for their original shelf lives. They were mostly completed dissolved in the test solution after the 1-min effervescent period. The endpoint color was highly similar to the photometric method with all reagents in liquid form. Color difference was visually distinguishable when the concentration difference of PAA was as low as 0.25 mg/L, especially at low concentrations (Figure 2). Therefore, the effervescent powder has better potential to be developed as a consumer product. Mixed reagents for a single test could either be compressed into a single tablet or airtight sealed in a single mini-pack.

In terms of environmental and user safety, PVA is biodegradable and is not classified as hazardous according to EU Regulation (EC) No 1272/2008. The oral toxicity of PVA is very low, and it does not accumulate or cause mutagenic effect . KI is categorized as group GSH08 with chronic oral toxicity in human. The US Food and Drug Administration permits additive KI of maximal 225 µg/day in food for adult human consumption. The European Chemicals Agency reports a threshold of 10 µg/kg /day KI through oral intake to cause acute toxicity in humans; the predicted no effect concentration for aquatic organisms is 21.94 mg/L KI in sewage treatment plant . In the present study, the amount of KI in the effervescent powder for a single 15-mL test was 1.5 mg. Such a low amount is unlikely to cause hazard on aquatic organisms, but may cause acute damage to human in case of inadvertent ingestion. The amount of KI for a single test can be proportionally reduced with a smaller test volume, for instance, 0.5 mg for a 5-mL test. In this way, the health risk on consumers can be reduced. Nevertheless, warning sign should be easily recognizable on each individual package.

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The present study was kindly supported by funds of the German Government's Special Purpose Fund held at Landwirtschaftliche Rentenbank in Germany (Fund No. 887139).

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Replacing n,n-diethyl-p-phenylenediamine(DPD) with polyvinyl alcohol for the non-hazardous determination of trace oxidizing agents in water

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Version 1

Abstract

Figures

Introduction

Reagents

Equipment

Procedure

Stepwise classic DPD method

Starch as an indicator

Polyvinyl alcohol as an indicator

Anticipated Results

Triiodide in classic DPD method

Starch-triiodide complex

Polyvinyl alcohol-triiodide complex

Potential as consumer products

References

Acknowledgements

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Replacing n,n-diethyl-p-phenylenediamine(DPD) with polyvinyl alcohol for the non-hazardous determination of trace oxidizing agents in water

Status:

Version 1

Abstract

Figures

Introduction

Reagents

Equipment

Procedure

Stepwise classic DPD method

Starch as an indicator

Polyvinyl alcohol as an indicator

Anticipated Results

Triiodide in classic DPD method

Starch-triiodide complex

Polyvinyl alcohol-triiodide complex

Potential as consumer products

References

Acknowledgements

Status:

Version 1

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