Optimization and feasibility assessment of the DPD colorimetric method with phosphate, iodide, and sulfuric acid for total residual oxidant monitoring in ballast water treatment systems

Jiyoon Cho; Taewan Kim; Mun Kyu Kim; Changha Lee

doi:10.4491/eer.2024.563

Environ Eng Res > Volume 30(3); 2025 > Article

Cho, Kim, Kim, and Lee: Optimization and feasibility assessment of the DPD colorimetric method with phosphate, iodide, and sulfuric acid for total residual oxidant monitoring in ballast water treatment systems

Research

Environmental Engineering Research 2025; 30(3): 240563.

Published online: October 8, 2024

DOI: https://doi.org/10.4491/eer.2024.563

Optimization and feasibility assessment of the DPD colorimetric method with phosphate, iodide, and sulfuric acid for total residual oxidant monitoring in ballast water treatment systems

Jiyoon Cho¹, Taewan Kim¹, Mun Kyu Kim², Changha Lee^1,^†

¹School of Chemical and Biological Engineering, Institute of Chemical Processes (ICP), Institute of Engineering Research, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea

²Engine Research Institute, HD Hyundai Heavy Industries, 1000 Bangeojinsunhwando-ro, Dong-gu, Ulsan 44032, Republic of Korea

^†Corresponding author: E-mail: leechangha@snu.ac.kr, Tel: +82-2-880-8630, Fax: +82-2-888-7295, ORCID: 0000-0002-0404-9405

Received September 23, 2024 Revised October 5, 2024 Accepted October 7, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The utilization of chlorine-based ballast water treatment systems is essential for preventing the spread of invasive species, making the continuous monitoring of total residual oxidant (TRO) levels imperative for compliance with environmental regulations. This study developed and optimized a phosphate buffer-sulfuric acid-based solution (PSS) for use in the N, N–diethyl–p–phenylenediamine (DPD) colorimetric method to determine TRO in ballast water. Batch experiments demonstrated that pH 6 is the optimal phosphate buffer condition. Furthermore, the addition of iodide ions promoted rapid and stable TRO measurements across various conditions including pH and ammonium concentrations, sodium hypochlorite (NaOCl) concentrations, temperature, suspended solids (SS), natural organic matter (NOM), and water types. The optimized PSS achieved TRO measurement accuracy within ±5% of reference values, notably surpassing the commercial citrate buffer-p–toluenesulfonic acid-based solution (CTS), especially under challenging conditions such as low NaOCl concentrations, higher temperatures, and the presence of SS. PSS exhibited an average error rate of 3.16%, compared to 4.44% for CTS. Field tests at a ballast water treatment facility further confirmed the reliability of PSS for real-time TRO monitoring. This optimized reagent provides a practical, cost-effective solution for ballast water treatment systems, supporting environmental protection and regulatory compliance.

Keywords: Ballast water treatment, DPD colorimetric method, Phosphate buffer-sulfuric acid-based solution (PSS), Total residual oxidants (TRO)

Graphical Abstract

Keywords: Ballast water treatment, DPD colorimetric method, Phosphate buffer-sulfuric acid-based solution (PSS), Total residual oxidants (TRO)

1. Introduction

Ballast water is vital for maintaining vessel stability during cargo operations [1]. However, it poses substantial environmental risks by facilitating the transport of living organisms, including pathogens, across regions, thus contributing to the spread of non-native or invasive species [1–5]. The introduction of these species can significantly disrupt marine ecosystems, resulting in ecological degradation and loss of biodiversity [4–6]. In response to these threats, the International Maritime Organization (IMO) established the International Convention for the Control and Management of Ship’s Ballast Water and Sediments (BWMC) in 2004 [7]. According to these regulations, ballast water must be treated and disinfected to limit the transfer of harmful organisms and pathogens.

Electro-chlorination is an effective and widely adopted method for treating ballast water, known for its technical efficiency and cost-effectiveness in controlling marine organisms and pathogens [8–11]. This process generates chlorine compounds in ballast water, known collectively as total residual oxidants (TRO) [3,11]. According to IMO regulations, the allowable concentration of TRO in discharged ballast water should not surpass 0.1 mg/L as Cl₂, since concentrations above this threshold may pose environmental risks [12–14]. Continuous monitoring of TRO levels during the ballast water treatment and discharge process is essential to ensuring compliance with environmental safety standards.

The N, N–diethyl–p–phenylenediamine (DPD) colorimetric method is commonly used for TRO analysis in ballast water treatment systems [15–17]. This method offers simplicity and rapid oxidant measurement, requiring minimal analysis time [16–18]. In this method, DPD reacts with TRO and oxidizes into a pink-colored semi-quinoid cationic compound known as Würster dye (Fig. 1; [15–17,19]). The oxidized product exhibits absorption peaks around 515 nm and 551 nm [19,20]. For optimal sensitivity, absorption is typically measured at 515 nm, with alternative measurements at 551 nm, another peak, or at 530 nm, the saddle point of the spectrum [19,21–23].

Reagents used in TRO monitors for ballast water generally include a DPD indicator and a buffer [16]. Liquid DPD is susceptible to oxidization by atmospheric or dissolved oxygen depending on pH, necessitating an indicator component to lower the pH [19,24]. Buffers help adjust the final mixture pH to slightly acidic conditions, and additives like iodide ions (I⁻) or ethylenediaminetetraacetic acid (EDTA) are incorporated to improve reaction accuracy [16,19]. For slow-reacting chlorine compounds such as chloramines, I⁻ facilitates the formation of triiodide ions (I₃⁻), enabling rapid quantification (Eq. (1) and (2); [16,21]).

(1)

{NH}_{2} Cl + 3 I^{-} + H_{2} O + H^{+} \to {I_{3}}^{-} + {NH}_{4} OH + {Cl}^{-}

(2)

{NHCl}_{2} + 3 I^{-} + H_{2} O + 2 H^{+} \to {I_{3}}^{-} + {NH}_{4} OH + 2 {Cl}^{-}

Additionally, EDTA chelates trace metal ions, preventing oxidation and reducing measurement errors [19].

Typically, commercially available TRO monitor reagents use p–toluenesulfonic acid (pTSA) as a pH regulator for the DPD indicator, along with a buffer solution consisting of trisodium citrate, potassium iodide (KI), and EDTA [16]. In contrast, Standard Method 4500-Cl G utilizes sulfuric acid (H₂SO₄) as the pH regulator for the indicator and a buffer containing sodium phosphate and KI [25]. A concern with commercial formulations is the inclusion of EDTA, which has limited solubility in diluted acidic solutions and may affect reagent performance [19]. Given these concerns, exploring alternative reagent compositions based on H₂SO₄, sodium phosphate, and KI is worthwhile. Simplifying and optimizing the reagents using these components can offer additional benefits, such as cost reductions. However, limited research has been conducted to directly compare the effectiveness of H₂SO₄, sodium phosphate, and KI-based reagents with commercially available formulations. Moreover, the impact of various factors on TRO monitor reagents in seawater remains incompletely explored.

This study aims to address this gap by pursuing three primary objectives: (1) optimizing the constituent components of sodium phosphate, KI, and H₂SO₄-based reagents for TRO analysis, (2) evaluating the factors that influence the performance of these optimized reagents, and (3) comparing the performance and feasibility of optimized reagents with commercially available reagents.

2. Materials and Methods

2.1. Reagents

All chemicals used in this study were of reagent-grade quality and employed without further purification. The reagents utilized included perchloric acid (HClO₄), sodium hydroxide (NaOH), sodium phosphate monobasic (NaH₂PO₄), sodium phosphate dibasic (Na₂HPO₄), KI, DPD, H₂SO₄, ammonium chloride (NH₄Cl), sodium hypochlorite (NaOCl), silicon dioxide (SiO₂), humic acid (HA), sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), and methanesulfonic acid (MSA)(all sourced from Sigma-Aldrich). Additional reagents included total chlorine buffer solution (5 mM citrate buffer containing 270 μM EDTA and 6 mM KI when mixed with seawater samples), total chlorine indicator (5.8 mM pTSA when mixed with seawater samples), and DPD powder (1.2 mM when mixed with seawater samples; all sourced from HF Scientific). Solutions were prepared using deionized water from a Milli-Q Integral Water Purification System (Millipore) with a resistivity exceeding 18.2 MΩcm.

2.2. Water Samples and Water Quality Parameters

Seawater and freshwater samples were collected from Busan and Seochen, South Korea, respectively. Following collection, the samples were filtered using a 0.45 μm polytetrafluoroethylene (PTFE) membrane filter and stored at 4°C. Brackish water was obtained by mixing seawater and freshwater in a 1:1 ratio. Water quality parameters for the seawater and freshwater were analyzed (Table 1). Sample pH was measured using an Orion Star A326 pH meter (Thermo Fisher Scientific), and salinity was determined from conductivity readings obtained with a LAQUA F-74BW conductivity meter (Horiba). Total organic carbon (TOC) was quantified with a TOC analyzer (Sievers InnovOx, GE Analytical Instruments). Ion concentrations were measured by ion chromatography on a Dionex Aquion system (Thermo Fisher Scientific). Anionic separation was conducted on a 4 mm × 250 mm Dionex IonPac AS22 column (Thermo Fisher Scientific) with a 4.5 mM Na₂CO₃ and 1.4 mM NaHCO₃ mixed carbonate eluent. Cationic separation was carried out on a 4 mm ×250 mm Dionex IonPac CS12A column (Thermo Fisher Scientific) with a 20 mM MSA eluent.

2.3. Batch Experiment

Batch experiments were conducted at ambient temperature (25 ± 1°C) in a flask containing 100 mL of seawater, brackish water, or freshwater. The system, open to the atmosphere, was stirred vigorously at 600 rpm. The pH of the seawater was adjusted using HClO₄ or NaOH. To maintain the desired temperature, a chiller or a water bath heater was used. NH₄Cl, SiO₂, and HA were added as needed, with SiO₂and HA serving as surrogates for suspended solids (SS) and natural organic matter (NOM), respectively [26,27]. After all influencing factors were in place, TRO was generated by introducing NaOCl into the reactor. Subsequently, 2.5 mL samples were immediately drawn and mixed with phosphate buffer (5 mM in the final mixture; with or without KI) and H₂SO₄ solution (1 mM in the final mixture) containing DPD (1.2 mM in the final mixture) in a cuvette. The absorbance of this resultant mixture was then measured using a UV-Vis spectrophotometer (LAMBDA 465, PerkinElmer) at 515 nm, from 5 to 600 s. To evaluate the rapidity and stability of TRO concentration measurements, the percentage increase (% increase) in TRO concentrations between 5 and 600 s was calculated (Eq. (3)).

(3)

% i n c r e a s e = \frac{{[T R O]}_{600 s} - {[T R O]}_{5 s}}{{[T R O]}_{5 s}} \times 100

To confirm the formation of complexes between phosphate buffer and magnesium (Mg²⁺) or calcium (Ca²⁺) ions, phosphate buffer was added to seawater and allowed to react for 10 min. The reaction mixture was then filtered through a 0.45 ↓m PTFE membrane filter, and the concentrations of metal ions in the filtrate were analyzed. All experiments were conducted in at least duplicate, and results are reported as average values with standard deviations.

2.4. TRO Monitor Experiment

The CLX-XT TRO & Chlorine Monitor (HF Science) was employed to compare and evaluate the accuracy of TRO concentration measurements using both commercially available and optimized reagents. Samples of seawater, brackish water, or freshwater were prepared in glass bottles for continuous injection into the monitor. The pH of the seawater was adjusted with NaOH, and NH₄Cl, SiO₂, HA, and NaOCl were added according to the experimental conditions. A chiller or water bath heater was used to maintain the desired temperature. Optimized reagents comprised a phosphate buffer (5 mM in the final mixture) with KI (300 μM in the final mixture) and an H₂SO₄ solution (1 mM in the final mixture) containing DPD (1.2 mM in the final mixture), or commercially available reagents from HF Scientific, were utilized, and TRO concentrations were measured according to the instrument manual. The Standard Method 4500-Cl G served as the reference method, with absorbance measurements taken using a UV-Vis spectrophotometer at 515 nm. To quantitatively assess the measurement accuracy and reliability between different reagents or methods, the measurement error rate between the X and Y reagents or methods (X/Y) was calculated using Eq. (4), which represents the percentage difference in TRO concentration measurements between the two methods.

(4)

E r r o r (%) = \frac{{[T R O]}_{X} - {[T R O]}_{Y}}{{[T R O]}_{Y}} \times 100

Field evaluations were conducted at the ballast water treatment facility of the Korea Institute of Ocean Science and Technology (KIOST) in Geoje, South Korea, where ballast water underwent treatment via electro-chlorination. All experiments were replicated at least five times, and the results are reported as average values with standard deviations.

3. Results and Discussion

3.1. Optimization of the Reagent Composition

Batch experiments were conducted to optimize the reagent composition. The necessity of incorporating I⁻ into the buffer reagent was evaluated under chloramine-generating conditions, which involved adding ammonium ions (NH₄⁺) (Fig. 2a). In the absence of I⁻, it took more than 120 s to measure the actual TRO concentration. However, the TRO concentration stabilized from 5 to 600 s with the addition of I⁻, confirming the importance of I⁻ for rapid and accurate TRO concentration measurements. To further optimize the I⁻ concentration, TRO was measured across various I⁻ concentrations (0–100 μM in the final mixture) (Fig. 2b). Without I⁻, the % increase was 5%, but this reduced to 0% with just 15 μM (approximately half of the TRO concentration) added. Reliable measurements were attained with any quantity above this level. As a result, the I⁻ concentration in the final mixture was set at or above the TRO concentration.

In TRO concentration analysis, the reaction of DPD is significantly influenced by the pH of the final mixture. The impact of phosphate buffer in adjusting this pH was evaluated at pH values of 5, 6, and 7, both with and without I⁻ (Fig. 2c and 2d). In the absence of I⁻, the % increase exceeded ±5% under all pH conditions, indicating low measurement accuracy (Fig. 2c). At pH 7, TRO decomposition was observed at approximately 30%. When I⁻ was added, the % increase at pH 5 was −12%, suggesting that this condition is unsuitable for the reagent (Fig. 2d). However, at pH 6 and 7, the % increase was 0% and −1%, respectively, demonstrating that rapid and accurate measurements were possible. The results, both with and without I⁻, showed that pH 6 provided the most stable measurements, aligning with previous studies that suggest the reaction is optimized at pH 6 [25,28].

Concerns arise regarding the potential precipitation of phosphate buffer due to interactions with Mg²⁺ and Ca²⁺ in water samples, possibly affecting its buffering capacity [19,29]. To investigate the formation of complexes with Mg²⁺ and Ca²⁺, the phosphate buffer was reacted with seawater, and the concentrations of Mg²⁺ and Ca²⁺ in seawater were measured (Fig. S1 of the Supplementary Information). When phosphate buffer was reacted with seawater at a concentration five times higher (25 mM) than that used in the final mixture (5 mM), the ion concentrations were maintained, indicating that the impact of precipitation was negligible.

3.2. Effects of Influencing Factors on the Optimized Reagent

The effects of various influencing factors, including pH, NH₄⁺, initial NaOCl dose, temperature, SS, NOM, and water type, were evaluated. The impact of the presence or absence of I⁻ was also assessed. Chloramine species, which influence the speed of TRO measurement, are generated more rapidly under certain pH and NH₄⁺ conditions [30, 31]. TRO concentrations were measured using the optimized reagent in the presence of 60 μM NH₄⁺ across pH values from 7.5 to 8.4, the typical pH range of seawater (Fig. 3a and 3b) [32,33]. In the absence of I⁻, increasing pH levels accelerated the formation of chloramine species, resulting in lower TRO concentrations measured at 5 s compared to 600 s (Fig. 3a). However, with I⁻ added, consistent TRO concentrations were observed across all pH levels and measurement times, demonstrating that the optimized reagent maintains excellent performance even in the presence of chloramines (Fig. 3b).

The performance of TRO measurements was further assessed across varying initial NaOCl doses (Fig. 3c and 3d). In the absence of I⁻, measurement accuracy diminished at both low (0.1 mg/L) or too high (15 mg/L) NaOCl concentrations (Fig. 3c). However, with the addition of I⁻, accurate measurements were achieved within 5 s, regardless of the concentration ranges (Fig. 3d).

The temperature dependence of TRO measurement performance was also evaluated over a range from 0°C to 40°C (Fig. 3e and 3f). At 0°C, without I⁻, the TRO concentrations measured at 5 s were lower than those at 600 s, reflecting the reduced reaction rate at lower temperatures (Fig. 3e). With I⁻, the 5 s measurement at 0°C was significantly higher than those without I⁻, although it remained lower than the 600 s value (Fig. 3f). At 40°C, irrespective of the presence of I⁻, the accelerated reaction rate due to higher temperature allowed for rapid TRO measurement (Fig. 3e and 3f). Nevertheless, TRO degradation was also accelerated, resulting in lower TRO concentrations at 600 s compared to those at 0°C and 25°C.

SS can influence TRO measurements due to their turbidity. The effect of SS was assessed by adding SiO₂, a standard turbidity reference, at concentrations of 10, 50, and 100 mg/L (Fig. 4a and 4b) [26]. In the absence of I⁻, the TRO concentrations measured at 5 s were generally lower than those at 600 s (Fig. 4a). However, with the addition of I⁻, TRO measurements were rapid and precise under all conditions; however, as the SS concentration increased, a clear trend of TRO degradation was observed (Fig. 4b).

The impact of NOM was evaluated using HA as a surrogate, within the typical concentration range found in seawater (3 to 12 mg/L) (Fig. 4c and 4d) [27,34]. Without I⁻, NOM interference caused the TRO concentration measured at 5 s to be lower than that at 600 s. However, with I⁻ introduced, TRO concentrations were consistently accurate and prompt, irrespective of the NOM concentration.

The accuracy of TRO measurements was compared across freshwater, brackish water, and seawater (Fig. 4e and 4f). In the absence of I⁻, measurements at 5 s were generally lower than those at 600 s across all conditions, with the most significant difference observed in freshwater due to elevated NH₄⁺ concentrations (Fig. 4e). With I⁻ present, no variations were noted between the 5 and 600 s measurements under any conditions (Fig. 4f).

The effects of various factors, based on the presence or absence of I⁻, were summarized as a% increase (Fig. 5). Without I⁻, most conditions exhibited a% increase of over 4%, with values reaching nearly 150% in the presence of NH₄⁺. Additionally, marked increases were noted when the initial NaOCl dose was either very low or very high, as well as at lower temperatures. However, with I⁻ added, most conditions showed a% increase of less than 1%, suggesting that precise and rapid TRO measurements are achievable under varying conditions. The sole exception occurred at 0°C, where the reaction between I⁻ and TRO slowed, leading to a% increase of 5%.

3.3. Feasibility of the Optimized Reagent on TRO Monitor

Based on batch experiment results, the buffer reagent was optimized to a 5 mM phosphate buffer containing 300 μM KI, with the indicator reagent optimized to a 1 mM H₂SO₄ solution containing 1.2 mM DPD when mixed with seawater samples. This optimized reagent offers a cost-saving advantage due to its simpler composition and lower KI concentration compared to commercial reagents. The accuracy of TRO concentration measurements using the optimized reagent was compared with that of a commercial reagent, both evaluated using a TRO monitor. The optimized reagent was designated as the phosphate buffer-sulfuric acid-based solution (PSS), while the commercial reagent was referred to as the citrate buffer-pTSA-based solution (CTS).

As pH levels and NH₄⁺ concentrations increased, chloramine species could form, inhibiting the rapid oxidation of DPD. However, even when the pH increased from 8.0 to 8.4 and 60 μM NH₄⁺ was added, both PSS and CTS produced results comparable to the reference value of approximately 1.91 mg/L (Fig. 6a). The presence of I⁻ in both PSS and CTS buffers enabled immediate and accurate TRO concentration measurements by the TRO sensor.

TRO concentrations were measured across an initial NaOCl dose range of 0.1 to 8 mg/L (Fig. 6b). PSS provided measurements closely aligned with the reference values, whereas CTS tended to underestimate TRO concentrations, especially at the 0.1 mg/L and 8 mg/L conditions. Notably, CTS’s error bar was larger than PSS’s at the low concentration of 0.1 mg/L.

The effect of temperature was explored over a range from 0°C to 45°C. At 45°C, TRO decomposition accelerated, causing a decrease in concentration to approximately 1 mg/L (Fig. 6c) [31]. Under most conditions, measurements from PSS, CTS, and the reference were similar. However, at 45°C, PSS recorded higher TRO concentrations compared to CTS and the reference.

TRO concentrations were also measured in the presence of SS using both PSS and CTS (Fig. 6d). Both reagents consistently showed lower TRO measurements than the reference, confirming that SS interferes with TRO measurements in the TRO monitor. The discrepancy was more pronounced with CTS, indicating that CTS faced greater challenges in measuring TRO accurately in the presence of SS.

The impact of NOM on TRO measurements was assessed using HA (Fig. 6e). As NOM was introduced, TRO levels decreased due to reactions with the organic matter [31]. Despite this, PSS measurements remained relatively consistent with the reference values. Whereas CTS consistently underestimated TRO concentrations compared to both the reference and PSS.

Regarding the impact of water type, a reduction in TRO concentration was noted in brackish water (Fig. 6f), which, however, did not substantially impact the accuracy of TRO measurements. TRO concentrations were measured with reasonable accuracy in both freshwater and seawater, suggesting that both PSS and CTS can provide reliable measurements across various water types.

Measurement error rates of reagents and their distribution were detailed (Fig. 7). The measurement error rates between PSS and the reference generally remained within ±5%, except in conditions involving 0.1 mg/L NaOCl and a high temperature of 45°C, where the error rate exceeded 10% (Fig. 7a). The measurement error rates between CTS and the reference showed that CTS often resulted in underestimation. Notably, in the presence of SS, error rates reached about −10% to −20%, indicating a significant reduction in measurement accuracy under these conditions. When comparing the error rates between PSS and CTS, a notable error rate of around 15% was observed under conditions with 0.1 mg/L TRO and 10 mg/L SS. Nevertheless, the error rates mostly remained within ±5% and given that the large error in SS conditions was due to CTS underestimation, PSS was validated as a feasible alternative reagent. The standard deviation of the measurement error rates for PSS and CTS compared to the reference was represented using a box and violin plot (Fig. 7b). On average, PSS showed an error rate of 3.16% compared to the reference, whereas CTS recorded an error rate of 4.44%, suggesting that PSS could serve as a substitute reagent for the TRO monitor.

Field applicability of TRO measurements using PSS and CTS was assessed at a ballast water treatment facility that uses electro-chlorination (Fig. S2 of the Supplementary Information). The average TRO concentrations measured were 4.93 mg/L with PSS and 4.83 mg/L with CTS, indicating that PSS has measurement accuracy close to that of CTS in practical applications, and thus holds potential for field application as a reagent.

4. Conclusions

This study developed and tested an optimized reagent composition for TRO measurement in ballast water treatment systems. The inclusion of I⁻ proved crucial for achieving rapid and stable TRO measurements under diverse conditions, including variations in pH, NH₄⁺, SS, NOM, temperature, and different water types. The optimized PSS demonstrated accuracy comparable to the commercial CTS, with the added benefit of reduced costs due to its simplified composition. PSS consistently delivered precise results under varied conditions, while CTS exhibited higher error rates, particularly at low NaOCl concentrations, elevated temperatures, and in the presence of SS. Field testing at a ballast water treatment facility confirmed PSS as a reliable reagent for real-time TRO monitoring. Given these findings, PSS offers a viable alternative to commercial reagents, especially in scenarios where rapid and economical TRO measurements are required. Future research should further explore the long-term stability of PSS in various operational environments.

Supplementary Information

eer-2024-563-supplementary.pdf

Acknowledgments

This work was supported by HD Hyundai Heavy Industries Co., Ltd.

Notes

Conflict-of-Interest Statement

The authors declare that they have no conflicts of interest.

Author Contributions

J.C. (Ph.D. student) conceived and designed the study, conducted the experiments, and drafted and revised the manuscript. T.K. (Ph.D.) conceived and designed the study and conducted the experiments. M.K.K. (Ph.D.) helped conceptualize the study and reviewed the manuscript. C.L. (Professor) supervised the project and reviewed and revised the manuscript.

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Fig. 1

The oxidation reaction of DPD.

Fig. 2

Effects of (a) I⁻, (b) I⁻ dose, (c) phosphate buffer pH without I⁻, and (d) phosphate buffer pH with I⁻ on the measurement of TRO concentration. ([NaOCl]₀ = 2 mg/L, [NH₄⁺]₀ = 60 μM for (a), [I⁻]₀ = 100 μM for (a) and 50 μM for (d))

Fig. 3

Effects of (a, b) pH and NH₄⁺, (c, d) initial NaOCl dose, and (e, f) temperature on TRO concentration measurements without (a, c, e) and with I⁻ (b, d, f). ([NaOCl]₀ = 2 mg/L as Cl₂ for (a, b, e, f), [NH₄⁺]₀ = 60 μM for (a, b), [I⁻]₀ = 100 μM for (b), 60 μM for (d), and 30 μM for (f)

Fig. 4

Effects of (a, b) SS, (c, d) NOM, and (e, f) water type on TRO concentration measurements without (a, c, e) and with I⁻ (b, d, f). ([NaOCl]₀ = 2 mg/L as Cl₂, [I⁻]₀ = 30 μM for (b, d, f))

Fig. 5

Effects of various influencing factors in the presence or absence of I⁻.

Fig. 6

Effects of (a) pH and NH₄⁺, (b) initial NaOCl dose, (c) temperature, (d) SS, (e) NOM, and (f) water type on the measurement of TRO concentration in seawater using a TRO sensor. ([NaOCl]₀ = 2 mg/L as Cl₂, [NH₄⁺]₀ = 60 μM for (a))

Fig. 7

(a) Measurement error rates of reagents and (b) distribution of error rates.

Table 1

Water quality parameters of Busan seawater and Seocheon freshwater

Parameter		Seawater	Freshwater
pH		8.0	7.8
Salinity (‰)		34.80	0.06
TOC (mg/L)		1.0	3.1

Anion (mg/L)	Cl⁻	19,243	35
Anion (mg/L)	Br⁻	47	2

Cation (mg/L)	Na⁺	10,159	21
	K⁺	527	6
	Mg²⁺	1,221	6
	Ca²⁺	679	24
	NH₄⁺	N.D*	3

N.D: Not detected