JBCS

INTRODUCTION

Soil organic matter (SOM) is a mixture of animal and plant residues in different stages of decomposition.^1-3 It plays an important role in agriculture and the environment due to the promotion and maintenance of soil health and global carbon storage.^1-8 The total organic carbon (TOC) content, or organic carbon (OC), allows the estimation of the amount of organic matter (OM) in soils, fertilizers, sewage sludge, sediments, and humic extracts.^9-13 Therefore, TOC is one of the main indicators of soil quality,⁸ being a parameter used in studies of carbon cycling and soil quality assessments.^4,14 The analytical methods for determining TOC include dry combustion, wet combustion, and chemical oxidation.¹⁵

Dry combustion methods involving an elemental analyzer (EA) are considered the most accurate for determining TOC, making them a reference for other methods.¹⁰ Dry combustion methods involve the thermal oxidation of OC in an oven and the determination of the released CO₂.^10,16,17 The results produced are reliable when the treatment of the sample to eliminate inorganic carbon (IC) precedes the analysis, which interferes with the results obtained.^10,16,18 However, the high costs of acquiring and maintaining an EA limit the application of the dry combustion methods. Thus, especially in field research, using an alternative method is more convenient, mainly because it can be performed in any soil analysis laboratory despite the need for equipment availability.

Wet combustion involves the determination of TOC by measuring the CO₂ produced in the oxidation of OC by K₂Cr₂O₇ in acidic conditions and heating after the previous removal of IC. CO₂ can be determined by infrared spectrometry,^19,20 thermal conductivity,²¹ turbidimetry,²² gravimetry,²³ ion chromatography²⁴ or acid/base titration.²⁵ In general, these methods are fast, accurate, and suitable for samples with high chloride content and rich in OM.¹⁰

Methods involving chemical oxidation are the most used in determining TOC.^15,26,27 As with wet combustion methods, they are based on the oxidation of OC with K₂Cr₂O₇ in an acidic medium and heating but involve measuring the excess oxidant rather than the CO₂ produced. The methods most used in research and analysis in routine laboratories are undoubtedly the Walkley-Black (WB) and Yeomans-Bremner (YB) methods because they are simple, fast, and inexpensive.^{10,13,18,26,28,29} In both methods, the OC is oxidized with a mixture of K₂Cr₂O₇ and concentrated H₂SO₄, and the excess oxidant is titrated with ferrous ammonium sulfate. Essentially, the main point of divergence between the WB and YB methods is the external heating source employed in the OC oxidation reaction. In the WB method, heating occurs only by the heat produced in the mixture of acid and water, although the literature reports variations in the method application that uses external heating sources to achieve temperatures above 135 °C.^12,28,30 In the YB method, heating is performed by an external source, leaving the OC to oxidize at 170 °C for 30 minutes.^13,26,28

Correction factors are applied in chemical oxidation methods as they do not promote the complete oxidation of OC.^{15,18,26,28,29} The correction factors can assume a wide range of variations depending on the analyzed soil. Pereira et al.²⁸ performed the determination of TOC in different types of soils using chemical oxidation methods (YB, modified YB, modified WB, and a method employed by EMBRAPA)³¹ and dry combustion (muffle furnace and EA). Taking the EA method as a reference, the correction factors ranged from 0.37 to 2.62. For the YB method, this variation was from 0.64 to 2.28.

The main drawback of chemical oxidation methods is the large amount of concentrated H₂SO₄ used, which generates hazardous waste.^32-34 Oxidation of OM with K₂Cr₂O₇, concentrated H₂SO₄, and heating exposes the analyst to several risks. In addition, H₂SO₄ purchase is controlled by security agencies in several countries, which makes its acquisition difficult.^35-38 For these reasons, some laboratories, such as the National Severe Storms Laboratory (NSSL), opted to discontinue the use of these methods and dedicated themselves to the search for alternative methods for the determination of TOC.³⁹

Potassium permanganate (KMnO₄) is a stronger oxidant than K₂Cr₂O₇.⁴⁰ The literature reports extensive use of KMnO₄ for the determination of only a fraction of OC, called permanganate oxidizable carbon or labile carbon.^41-53 Labile carbon quantifies the biologically active carbon in the soil and is used to assess the impacts of alternative management practices on soil quality.^46,51,52 However, up to date, there are no reports involving its use in the TOC determination, which suggests that any attempt to determine this parameter has not been successful.

This work proposes the use of KMnO₄ as an alternative oxidizing agent to K₂Cr₂O₇ for titrimetric determination of TOC to generate a more environmentally friendly waste with the advantage of reducing acid consumption. For this, the ideal reaction conditions (time, temperature, and acidity) for the oxidation with KMnO₄ were evaluated and identified through the oxidation undergone by potassium hydrogen phthalate (KHP). Potassium hydrogen phthalate is an important OM standard used to determine TOC.^54-56 The TOC of soil samples was determined by the proposed method and compared with the reference method of Yeomans and Bremner (YB).

EXPERIMENTAL

Reagents and materials

The following reagents were used in its analytical grade: C₈H₅O₄K (KHP, 99.95%, Vetec, Duque de Caxias, Brazil), Na₂C₂O₄ (99%, Vetec, Duque de Caxias, Brazil), K₂Cr₂O₇ (99.9%, Merck, Duque de Caxias, Brazil), KMnO₄ (99%, Proquimios, Rio de Janeiro, Brazil), o-phenanthroline (Proquimios, Rio de Janeiro, Brazil), H₃PO₄ (85%, Sigma-Aldrich, Duque de Caxias, Brazil), H₂SO₄ (95 98%, Proquimios, Rio de Janeiro, Brazil), (NH₄)₂Fe(SO₄)₂·6H₂O (98.5 101%, Isofar, Duque de Caxias, Brazil) e FeSO₄.7H₂O (99%, Sigma-Aldrich, Duque de Caxias, Brazil). The C₈H₅O₄K was ground in a mortar and dried at 105 °C. All other reagents were used as they were commercially obtained. Solutions were prepared in distilled water. Soil samples were donated by the Soil Laboratory of the Agronomy Institute of the Universidade Federal Rural do Rio de Janeiro and are classified in the Brazilian Soil Classification System⁵⁷ as: latossolo (profile 88, horizon A, coordinates 7508038/602084 UTM, WGS84), chernossolo (profile 202, horizon A, coordinates 7506551/602447 UTM, WGS84), gleissolo (profile 59, horizon A, coordinates 7509474/604061 UTM, WGS84) and cambissolo (profile 74, horizon A, coordinates 7504384/603060 UTM, WGS84) equivalent to oxisols, molisols, entisols (Aqu-alf-and-ent-ept-) and inceptisols, respectively, in the Soil Taxonomy classification.⁵⁸

The OC oxidation procedures were carried out in a Quimis® dry block digester (model Q327-A242, Diadema, Brazil), preheated to the working temperature. Class A volumetric flasks and pipettes were used.

Preparation and standardization of standard solutions

The 0.1666 mol L^-1 K₂Cr₂O₇ e 0.07015 mol L^-1 Na₂C₂O₄ standard solutions were prepared by dissolving 49 g (± 0.0001 g) of K₂Cr₂O₇ and 9.4 g (± 0.0001 g) of Na₂C₂O₄, respectively, in water with posterior dilution (1000.0 mL) in a volumetric flask.

The 0.2 mol L^-1 KMnO₄ solution was prepared by dissolving 63.2 g de KMnO₄ in 2000 mL water. This solution was heated at 100 °C for 30 minutes, filtered through glass wool, stored in an amber bottle, protected from light, and kept at room temperature. This solution was standardized by titrating 50.00 mL of a 0.07015 mol L^-1 Na₂C₂O₄ solution with 40 mL of 3.0 mol L^-1 H₂SO₄ solution. The 0.02 mol L^-1 KMnO₄ solution was prepared by diluting the 0.2 mol L^-1 KMnO₄ solution and, later, standardizing it with 25.00 mL of 0.07015 mol L^-1 Na₂C₂O₄ standard solution mixed with 20 mL of 3.0 mol L^-1 H₂SO₄ solution.

The 0.2 mol L^-1 ferrous ammonium sulfate solution was produced by dissolving 156.8 g of (NH₄)₂Fe(SO₄)₂·6H₂O in 100 mL of concentrated H₂SO₄ and then diluting it in water until completing 2000.0 mL. Then, 10.00 mL of this solution was mixed with 20 mL of 3.0 mol L^-1 H₂SO₄ solution and subsequently titrated against 0.02 mol L^-1 KMnO₄ solution. Standardizations were performed in duplicate.

Proposed method

The proposed method basically consisted of oxidizing the OC with 0.2 mol L^-1 KMnO₄ (Reaction 1) and posteriorly reacting the excess of KMnO₄ (Reaction 2) and the MnO₂ produced (Reaction 3) with 0.2 mol L^-1 Fe²⁺. Finally, the excess Fe²⁺ was titrated with 0.02 mol L^-1 KMnO₄ (Reaction 2).

For the development and evaluation of this method, the percentage of oxidized carbon (%OC) was determined in a high purity KHP standard, under different conditions of acidity, temperature, and reaction time, as listed in Table 1. The influence of the KHP mass was also investigated (Table 1). The conditions that provided the best %OC in the KHP samples, closer to 100%, were used to analyze soil samples (Table 1).

The analysis comprised of weighing the sample, with an accuracy of ± 0.1 mg, in a 100 mL digestion tube, to which 10.00 mL of 0.2 mol L^-1 KMnO₄ solution and 10 mL of H₂SO₄ were added. The digestion tube was inserted into the digester block and kept under heating for the oxidation of OC (Reaction 1). After this time, the tube was set to cool for 15 minutes. The contents were transferred to an Erlenmeyer flask with the subsequent addition of 2.5 mL of 85% w/w H₃PO₄ to avoid Fe³⁺ interference in the visualization of the color change at the endpoint of titration (Reaction 4). With the aid of a burette, the standardized solution of 0.2 mol L^-1 ferrous ammonium sulfate was added until the solution was bleached (Reaction 2) and the brown solid was completely solubilized (Reaction 3). Excess Fe²⁺ was titrated with 0.02 mol L^-1 KMnO₄ solution until a slightly pink color persisted. The blank analysis did not use KHP or soil samples. The analyses were performed in triplicate.

General equation to TOC determination by the proposed method

Considering that millimoles of permanganate were added to the system, millimoles of permanganate reacted with the OC, and millimoles of permanganate reacted with the added Fe²⁺ ions, and we have the following mass balance:

If the sample solution contains millimoles of OC and millimoles of Fe²⁺ ions that react with the remaining permanganate, based on the stoichiometry of Reactions 1 and 2, Eq. 1 becomes:

For the Fe²⁺ ions added after permanganate oxidation, the mass balance is:

where: is the amount, in mmol, of Fe²⁺ ions added to the solution; is the amount, in mmol, of Fe²⁺ ions that react with the remaining permanganate from the OC oxidation; is the amount, in mmol, of Fe²⁺ ions that react with MnO₂; and is the amount, in mmol, of Fe²⁺ ions that react with the titrant (0.02 mol L^-1 KMnO₄). If millimoles of permanganate are needed to titrate Fe²⁺ and, considering the stoichiometry of Reactions 1, 2 and 3, Eq. 3 becomes:

Replacing Eq. 4 in Eq. 2, to eliminate the term results in Eq. 5:

The OC concentration, Fe²⁺ added, and titrant content in the blank analysis are different from the sample. Therefore, we have Eq. 6 for the blank, with the apostrophe (') being used to highlight the blank terms.

Equating Eq. 5 to Eq. 6, we have:

The term n_C − n_C´ corresponds to OC concentration in the sample, in mmol. In this case, we have Eq. 8:

where: m_C^sample is the carbon mass in the sample, in mg; and MM_C is the molar mass of carbon (12.0 g mol^-1). Replacing Eq. 8 in Eq. 7 and knowing that the quantity of Fe²⁺ ions and titrant spent can be calculated from the respective concentrations and volumes of the solutions used, the TOC in the sample can be determined by Eq. 9:

where: TOC is the total organic carbon content in the sample, in g kg^-1; and are the concentrations, in mol L^-1, of Fe²⁺ and MnO₄^- ions, respectively; and are the volumes, in mL, of ferrous ammonium sulfate solution spent in dissolving MnO₂ in the analysis of the blank and the sample, respectively; and are the volumes, in mL, of KMnO₄ solution spent in the titration of Fe²⁺ ions in the sample and blank analysis, respectively; m_sample is the sample mass, in g; and factor 3 is the combination of the molar mass of carbon (12.0 g mol^-1) with the stoichiometric term (4) of Eq. 7.

Computation of %OC

The percentage of oxidized carbon (%OC) by the permanganate in the KHP samples was calculated according to Eq. 10:

where: %OC is the percentage of oxidized carbon; TOC is the organic carbon content in the sample, in g kg^-1; MM_KHP is the molar mass of KHP, in g mol^-1; MM_C is the carbon molar mass, in g mol^-1; 8 is the stoichiometric factor referring to the amount of carbon in the KHP; 0.1 is the conversion factor for the percentage.

Reference method (Yeomans & Bremner)¹³

For the YB method, 500 mg (± 0.1 mg) of soil, 5.00 mL of 0.167 mol L^-1 K₂Cr₂O₇ solution, and 7.5 mL of concentrated H₂SO₄ were added to a 100 mL digestion tube. The tube was placed on the digester block and kept at 170 °C for 30 minutes for the OC oxidation (Reaction 5). After cooling and completing the volume with distilled water to make it 50 mL, 10 mL of 85% w/w H₃PO₄ and 5-8 drops of ferroin indicator solution were added to the tube. The mixture was titrated with 0.2 mol L^-1 ferrous ammonium sulfate solution (Reaction 6). The blank analysis did not comprise any soil sample addition. Additionally, blank analysis was performed without heating in a digester block to account for the amount of dichromate lost due to heating. Analyzes were performed in quintuplicate.

RESULTS AND DISCUSSION

The acidity effect on KHP oxidation

The effect of acidity on the determination of the %OC of KHP by KMnO₄ was evaluated by varying the concentration of H₂SO₄ from 5.0 × 10^-4 to 9.0 mol L^-1 at temperatures of 60 °C and 95 °C and 15 minutes of reaction (Table 1). The most promising H₂SO₄ concentration for KHP oxidation was 0.25 mol L^-1 (), which provided %OC values of 91% (± 4%) and 104.8% (± 1.6%) at 60 °C and 95 °C, respectively (Figure 1). However, adequate %OC values were also obtained at H₂SO₄ concentrations: of 0.125 mol L^-1, 95% (± 7%), at 60 °C; of 0.5 mol L^-1, 114% (± 9%) at 95 °C; and 1.8 mol L^-1, 91% (± 12%) at 95 °C.

Figure 1. Percentage of oxidized carbon (% CO) in KHP as a function of the negative logarithm of the sulfuric acid molar concentration (logC_H2SO4) at temperatures of 60 °C and 95 °C (N = 3)

Figure 1 shows that the oxidation of KHP should be carried out under moderate acidity conditions for greater accuracy. The low values of %OC obtained in a medium of lower acidity may be caused by the insufficient amount of H⁺ ions for Reaction 1. In a medium of greater acidity, low values of %OC are also found. In this case, it is necessary to consider the following discussion.

A gradual increase in MnO₂ formation was observed in the blank oxidation with an increase in acidity, suggesting that MnO₂ is also produced by a parallel reaction to the oxidation of KHP by KMnO₄. The increase in MnO₂ formation was visual and confirmed by the need to add a greater volume of ferrous ammonium sulfate solution in the system after oxidation with KMnO₄. Diluted solutions of KMnO₄ in H₂SO₄ are relatively stable. However, KMnO₄ reacts with water in more concentrated solutions, producing Mn²⁺ (Reaction 7).⁵⁹ Mn²⁺ ions, in turn, considerably increase the degradation of KMnO₄, producing MnO₂ (Reaction 8), which catalyzes the reaction of KMnO₄ with water,⁵⁹ forming more MnO₂ (Reaction 9). Hence, greater formation of MnO₂ was observed when using more concentrated solutions of H₂SO₄. Therefore, under conditions of higher acidity, these parallel reactions of KMnO₄ seem to be a priority in relation to its reaction with KHP in the investigated time (15 min), which decreases the %OC recovered (Figure 1).

The %OC values greater than 100% observed in Figure 1 may be due to the MnO₂ produced in the oxidation of OM (KHP) (Reaction 1), which catalyzes the reaction of KMnO₄ with water (Reaction 9).⁵⁹ The reaction of KMnO₄ with water in the blank analysis does not occur in the same proportion as in the sample analysis because of the absence of OM, which was confirmed by the low formation of MnO₂ observed in the blank. Consequently, the %OC became greater than 100%.

Optimization of reaction conditions for KHP oxidation

As the %OC values were satisfactory at H₂SO₄ concentrations of 0.125, 0.25, and 0.5 mol L^-1 (Figure 1), the effect of oxidation time (15 to 60 min) and temperature (60 at 95 °C) in the oxidation of KHP by KMnO₄ were assessed for these concentrations (Table 1, Figure 2).

Figure 2. Percentage of carbon oxidized by KMnO₄ in KHP, under different concentrations of H₂SO₄ (0.125, 0.25 and 0.5 mol L^-1), temperatures (60, 70, 80 and 95 °C) and reaction times (15, 30 and 60 min) (N = 3)

Since the desirable OC oxidation is 100%, the reaction conditions that provided relative errors (e) of up to 3.0% (|e|≤3.0%) for the KHP oxidation were: 0.125 mol L^-1 H₂SO₄, 70 °C and 30 min (1.9%); 0.25 mol L^-1 H₂SO₄, 70 °C and 15 min (2.6%); 0.25 mol L^-1 H₂SO₄, 70 °C and 30 min (2.0%); 0.5 mol L^-1 H₂SO₄, 70 °C and 30 min (3.0%); and 0.5 mol L^-1 H₂SO₄, 80 °C and 15 min (0.4%) (Figure 2). The coefficients of variation across the conditions investigated were less than 5.0%.

Figure 2 shows a trend of obtaining higher %OC with increasing time and temperature of OM oxidation. All %OC values at temperatures of 80 °C and 95 °C were statistically higher than 100% (Student's t-test; α = 0.05; one-tailed), except for oxidation with 0.5 mol L^-1 H₂SO₄ at 80 °C for 15 minutes. Such behavior seems to be related to the catalytic effect of MnO₂ on the reaction of water with KMnO₄ (Reaction 9). Hence, the oxidation of OM with KMnO₄ at 80 and 95 °C is not recommended.

KHP mass effect on %OC

The assessment of the KHP mass effect on the %OC value determined experimentally was motivated by the vast number of parallel reactions that can occur during the OM oxidation with permanganate. Other motivation factors were the different oxidation states that manganese could acquire when reduced and possible favoritism of a parallel reaction caused by some excess reagents or by the accumulation of some intermediate reaction.

The best results were obtained when using 10, 20 and 30 mg of KHP with 0.125 mol L^-1 H₂SO₄, for which the %OC values were 96% (± 3%), 96.9% (± 0.5%) and 94.8% (± 1.4%), respectively (Figure 3). Good values were also obtained with 0.25 mol L^-1 H₂SO₄ together with 20 mg of KHP, 94.0% (± 1.7%) and 0.5 mol L^-1 H₂SO₄ combined with 40 mg of KHP, 94% (± 2%) (Figure 3).

Figure 3. Theoretical and experimental percentages of oxidized carbon in the reaction of KHP with KMnO₄ using different H₂SO₄ concentrations (0.125, 0.25 and 0.5 mol L^-1) and KHP masses (10, 20, 30, 40 and 50 mg) (N = 3)

The oxidation of OM by KMnO₄ using 0.5 mol L^-1 H₂SO₄ and 10, 20, and 30 mg of KHP was overestimated as the %OC varied from 120% to just over 210% (Figure 3), which may be related to the parallel KMnO₄ reaction with water (Reaction 9). Across the three acidity conditions tested, the %OC values were lower for the masses of 40 and 50 mg of KHP. This behavior is due to the amount of KMnO₄ used, which was insufficient to react completely with the OC of 40 and 50 mg of KHP, based on the stoichiometry of Reaction 1. The agreement between the theoretical and experimental %OC values under the conditions in that there was no substantial influence of parallel reactions (10 - 50 mg of KHP, in 0.125 and 0.25 mol L^-1 H₂SO₄ and 40 - 50 mg of KHP, in 0.5 mol L^-1 H₂SO₄, Figure 3) corroborate the proposal of this work, in which the primary reaction between OC and KMnO₄ involves the formation of MnO₂ (Reaction 1) confirming its feasibility as an oxidant to TOC determination.

Soil TOC determination

The oxidation of SOM with KMnO₄ was carried out at 70 °C for 30 min, using 0.125 mol L^-1 H₂SO₄ (Table 1), as these are the best conditions observed for the oxidation of KHP (Figure 2 and Figure 3). Although differences were observed between the proposed method and YB, for three types of soil (Table 2), these differences do not invalidate the method. Pereira et al.²⁸ also found a significant relative error ranging from 67% to 129% between TOC determined with EA and YB in cambissolo samples.

In fact, the nature of the matrix and environmental characteristics affect the reliability of different methods for measuring carbon content.²⁷ Different soils present diverse sorption properties (of inorganic and organic particles) as well as various OM protection mechanisms within the aggregates that contribute to the storage of OM and its lability.⁶⁰ The organic matter quality can be one of the contributors to the divergences observed in the methods.⁶¹ Hence, consistent measurements among different methods, matrices and applications can enable further trend comparisons.²⁷ Moreover, the differences among methods can be corrected by the use of correction factors (Table 2), calculated according to Eq. 11:

where: f is the correction factor; TOC_Ref and TOC are the organic carbon contents obtained by the reference and proposed methods (in g kg^-1), respectively.

Correction factors have been widely used in the routine analysis of soil samples,^26,28,62 as SOM can be highly complex, ranging from compounds such as simple sugars and amino acids to acids humic and fulvic. Thus, in some instances, the oxidation of OC may not be complete justifying the use of correction factors even in official methods that have been used for several decades, such as YB and WB.^26,28,62 Therefore, the differences observed in the results in Table 2 do not constitute a problem capable of disqualifying the proposed method since its potential was proven in the KHP analysis. In this case, the observed deviations for a specific type of soil can be corrected, as usual, using correction factors.

Concerning the precision of the methods (Table 2), it was observed that the proposed method was statistically similar to the reference method (YB) under the one-tailed F-test (α =0.05), except for the gleissolo, which was superior. These results display the similarity between the two methods, indicating that both can be applied interchangeably in the TOC determination.

Feasibility of the proposed method to be applied in carbon dynamics studies

Due to the importance of SOC to the sustainability of the ecosystems, its determination has become essential as a soil health indicator because it reflects soil organic matter dynamics.^4,8,14 In order to quantify and evaluate soil health and quality, adequate indicators are required.^8,53 Indicators suitable for soil health evaluation must be rapid and straightforward, cost-effective, sensitive to management, and present low analytical variability.^8,53

For the United States Department of Agriculture - Natural Resources Conservation Services, both dry combustion (EA) and wet oxidation are considered candidate indicator methods for OC. However, the EA is regarded as a more suitable option since it meets all criteria except for the minimal infrastructure and investment, which is partially met due to the high cost of acquiring a carbon analyzer. At the same time, the major drawbacks of the wet oxidation (YB) method are the hazardous waste and its disposal.⁸

The analysis of 100 soil samples through the method proposed in this study could reduce K₂Cr₂O₇ consumption by 49 g (Table 3). The Cr⁶⁺ has high carcinogenic, mutagenic, reproductive, dermal, and inhalation toxicity potential.⁶³ Therefore, although Cr⁶⁺ is reduced to Cr³⁺ (less toxic) at the end of the analysis, handling K₂Cr₂O₇ exposes the analyst to unnecessary risks that could be avoided by using a safer oxidant (e.g., KMnO₄).

Another benefit of the method is the reduction of H₂SO₄ used. The proposed method could consume 186 mL of concentrated H₂SO₄ for 100 soil samples analyzed compared to 950 mL of acid used by the YB method, meaning a reduction of 80.4% of H₂SO₄ consumption (Table 3). As security agencies in several countries control sulfuric acid purchases, reducing its demand can also optimize its use in a lab routine.^35-38 The advantages of the method enable an environmentally friendly method and the application in any analytical laboratory without demanding high economic resources as required by the EA. This context supports research settings where resources are limited by suggesting a method that is simple, fast, and inexpensive as the YB with fewer consumables and hazardous waste generation.

CONCLUSIONS

This study shows that KMnO₄, when reduced to MnO₂, has a similar potential to K₂Cr₂O₇ for the determination of TOC. The advantage of this new methodology involves a lower consumption of H₂SO₄ and a low temperature to oxidize the OM. Changing the oxidant and reaction conditions brings less risk to health and the environment. For the method to reach a broader application, it is recommended to analyze a larger number of soil types and determine the conversion factors by comparing the results obtained with those of the reference method (EA).

ACKNOWLEDGEMENTS

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES) - Finance Code 001.

REFERENCES

1. Mishra, G.; Sarkar, A.; J. Environ. Manage. 2020, 257, 110002. [Crossref]

2. Meliyo, J. L.; Msanya, B. M.; Kimaro, D. N.; Massawe, B. H. J.; Hieronimo, P.; Mulungu, L.; Deckers, J.; Gulinck, H.; J. Soil Sci. Environ. Manage. 2016, 7, 123. [Crossref]

3. Ivezić, V.; Kraljević, D.; Lončarić, Z.; Engler, M.; Kerovec, D.; Zebec, V.; Jović, J.; Presented in part at 51^st Croatian and 11^th International Symposium on Agriculture, Opatija, February, 2016.

4. Liang, C.; Amelung, W.; Lehmann, J.; Kästner, M.; Glob. Change Biol. 2019, 25, 3578. [Crossref]

5. Scharlemann, J. P. W.; Tanner, E. V. J.; Hiederer, R.; Kapos, V.; Carbon Manage. 2014, 5, 81. [Crossref]

6. Stockmann, U.; Adams, M. A.; Crawford, J. W.; Field, D. J.; Henakaarchchi, N.; Jenkins, M.; Minasny, B.; McBratney, A. B.; Courcelles, V. R.; Singh, K.; Wheeler, I.; Abbott, L.; Angers, D. A.; Baldock, J.; Bird, M.; Bookes, P. C.; Chenu, C.; Jastrow, J. D.; Lal, R.; Lehmann, J.; O'Donnell, A. G.; Parton, W. J.; Whitehead, D.; Zimmermann, M.; Agric., Ecosyst. Environ. 2013, 164, 80. [Crossref]

7. Ciais, P.; Sabine, C.; Bala, G.; Bopp, L.; Brovkin, V.; Canadell, J.; Chhabra, A.; DeFries, R.; Galloway, J.; Heimann, M.; Jones, C.; Le Quéré, C.; Myneni, R. B.; Piao, S.; Thornton, P. In Climate change 2013: The physical science basis. Contribution of working group I to the fifth assessment report of the Intergovernmental Panel on Climate Change; Stocker, T. F.; Qin, D.; Plattner, G.-K.; Tignor, M.; Allen, S. K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V.; Midgley, P. M., eds.; Cambridge University Press: Cambridge, 2013, ch. 6.

8. Stott, D. E. In Recommended Soil Health Indicators and Associated Laboratory Procedures. Soil Health Technical Note No. 450-03; USDA-NRCS: Washington, 2019, available at https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=44475.wba, accessed in October 2022.

9. Serbanescu, A.; Barbu, M.; Nicolescu, I.; Bucur, E.; Presented in part at International Symposium "The Environment and the Industry", Bucharest, September, 2017. [Crossref]

10. Bisutti, I.; Hilke, I.; Raessler, M.; TrAC, Trends Anal. Chem. 2004, 23, 716. [Crossref]

11. Tam, N. F. Y.; Yao, M. W. Y.; Int. J. Environ. Anal. Chem. 1998, 72, 137. [Crossref]

12. Ciavatta, C.; Govi, M.; Antisari, L. V.; Sequi, P.; Commun. Soil Sci. Plant Anal. 1991, 22, 795. [Crossref]

13. Yeomans, J. C.; Bremner, J. M.; Commun. Soil Sci. Plant. Anal. 1988, 19, 1467. [Crossref]

14. Deluz, C.; Nussbaum, M. O.; Gondret, K.; Boivin, P.; Front. Environ. Sci. 2020, 8, 113. [Crossref]

15. Barra, C. M.; Sousa, E. B.; Netto-Ferreira, J. B.; Lã, O. R.; Rocha Junior, J. G.; J. Eng. Res. 2022, 2, 1. [Crossref]

16. Nelson, D. W.; Sommers, L. E. In Methods of Soil Analysis. Part 3. Chemical Methods; Sparks, D.; Page, A.; Helmke, P.; Loeppert, R.; Soltanpour, P. N.; Tabatabai, M. A.; Johnston, C. T.; Sumner, M. E., eds.; Soil Science Society of America and American Society of Agronomy: Madison, 1996, ch. 34. [Crossref]

17. Grewal, K. S.; Buchan, G. D.; Sherlock, R. R.; Eur. J. Soil Sci. 1991, 42, 251. [Crossref]

18. Nayak, A. K.; Rahman, M. M.; Naidu, R.; Dhal, B.; Swain, C. K.; Nayak, A. D.; Tripathi, R.; Shahid, M.; Islam, M. R.; Pathak, H.; Sci. Total Environ. 2019, 665, 890. [Crossref]

19. APHA-AWWA: Standard Methods for the Examination of Water and Wastewater, 19^th Edition; American Public Health Association, Washington, 1995.

20. Thomas, O.; El Khorassani, H.; Touraud, E.; Bitar, H. L. E.; Talanta 1999, 50, 743. [Crossref]

21. Gács, I.; Payer, K.; Anal. Chim. Acta 1989, 220, 1. [Crossref]

22. Paniz, J. N. G.; Flores, E. M. M.; Dressler, V. L.; Martins, A. F.; Anal. Chim. Acta 2001, 445, 139. [Crossref]

23. Allinson, L. E.; Bollen, W. B.; Moodie, C. D.; Methods of Soil Analysis, Part 2; American Society of Agronomy: Madison, 1986.

24. Fung, Y. S.; Wu, Z.; Dao, K. L.; Anal. Chem. 1996, 68, 2186. [Crossref]

25. Nelson, D. W.; Sommers, L. E.; Methods of Soil Analysis - Part 2; American Society of Agronomy, Inc.: Madison, 1986.

26. Gatto, A.; de Barros, N. F.; Novais, R. F.; Silva, I. R.; Mendonça, E. S.; Villa, E. M. A.; Rev. Bras. Ciênc. Solo 2009, 33, 735. [Crossref]

27. Conyers, M. K.; Poile, G. J.; Oates, A. A.; Waters, D.; Chan, K. Y.; Soil Res. 2011, 49, 27. [Crossref]

28. Pereira, M. G.; Valladares, G. S.; dos Anjos, L. H. C. D.; Benites, V. D. M.; Espíndula Jr, A.; Ebeling, A. G.; Sci. Agric. (Piracicaba, Braz.) 2006, 63, 187. [Crossref]

29. Walkley, A.; Black, I. A.; Soil Sci. 1934, 37, 29. [Crossref]

30. Dell'Abate, M. T.; Canali, S.; Trinchera, A.; Benedetti, A.; Sequi, P.; Nutr. Cycl. Agroecosys. 1988, 51, 217. [Crossref]

31. Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA), Centro Nacional de Pesquisa de Solos; Manual de métodos de análises de solos, 2^nd ed.; EMBRAPA, CNPS: Rio de Janeiro, 1997.

32. Wang, L. K.; Wang, M. S. In Evolutionary Progress in Science, Technology, Engineering, Arts, and Mathematics (STEAM); Tsao, H., ed.; Lenox Institute Press: Massachusetts, 2022, available at https://hcommons.org/deposits/item/hc:44605, accessed in October 2022.

33. Sato, J. H.; de Figueiredo, C. C.; Marchão, R. L.; Madari, B. E.; Benedito, L. E. C.; Busato, J. G.; de Souza, D. M.; Sci. Agric. (Piracicaba, Braz.) 2014, 71, 302. [Crossref]

34. Crouse, K. K.; Hardy, D. H.; Heckendorn, S.; Huluka, G.; Joines, D. K.; Kissel, D. E.; Miller, R.; Mitchell, C. C.; Moore, K. P.; Mylavarapu, R.; Oldham, J. L.; Provin, T.; Savoy, H. J.; Sikora, F. J.; Sonon, L.; Wang, J. J.; Warncke, D. D.; Zhang, H. In Soil Test Methods from the Southeastern United States; Mylavarapu, R., ed.; Southern Cooperative Series Bulletin No. 419, 2014, ch. 5.4.

35. Chile; Decreto Supremo N° 1.358, 17 de abril de 2007, Establece normas que regulan las medidas de control de precursores y sustancias químicas esenciales; Ministerio del Interior, Santiago, Chile, 2007, available at https://www.bcn.cl/leychile/navegar?idNorma=260089, accessed in October 2022.

36. Commission of the European Communities; 2019/1148/EU: Commission Decision of 11 July 2019; Official Journal of the European Union, 2019, L l86, p.1, available at https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=OJ:L:2019:186:TOC, accessed in October 2022.

37. Ministério da Justiça e Segurança Pública Gabinete do Ministro; Portaria N° 240, de 12 de março de 2019. Estabelece procedimentos para o controle e a fiscalização de produtos químicos e define os produtos químicos sujeitos a controle pela Polícia Federal, Diário Oficial da união (DOU), Brasília, Brazil, 2019, https://pesquisa.in.gov.br/imprensa/jsp/visualiza/index.jsp?data=14/03/2019&jornal=515&pagina=41, accessed in October 2022.

38. Ministerio de Seguridad; Decreto Nacional N° 593/2019. Resolución N° 1122/2019 de 27 de agosto de 2019. Manual de Procedimientos Administrativos. Reglamentación de la Ley Nro 26.045 "del Registro Nacional de Precursores Químicos", Buenos Aires, Argentina, 2019, available at https://www.argentina.gob.ar/sites/default/files/renpre-manual-procedimientos-administrativos-resolucion-1122-19.pdf, accessed in October 2022.

39. Ranks, C. D.; Kimble, J. M.; Samson-Liebig, S. E.; Sobecki, T. M. In Assessment Methods for Soil Carbon; Lal, R.; Kimble, J. M.; Follett, R. F.; Stewart, B. A., eds.; Lewis Publisher: New York, 2000, ch. 8.

40. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. In Fundamentals of Analytical Chemistry, Cengage Learning: Andover, 2013.

41. Culman, S. W.; Hurisso, T. T.; Wade, J. In Soil Health Series: Volume 2 Laboratory Methods for Soil Health Analysis; Karlen, D. L.; Stott, D. E.; Mikha, M. M., eds.; Soil Science Society of America and John Wiley & Sons: Madison, 2021.

42. Si, L.; Mi, W.; Sun, Y.; Tao, W.; Zhang, J.; Su, L.; Agronomy 2022, 12, 29. [Crossref]

43. Singh, P.; Benbi, D. K.; Arch. Agron. Soil Sci. 2018, 64, 1879. [Crossref]

44. de Souza, G. P.; de Figueiredo, C. C.; de Sousa, D. M. G.; Sci. Agric. (Piracicaba, Braz.) 2016, 73, 535. [Crossref]

45. Wang, G.; Guan, D.; Zhang, Q.; Peart, M. R.; Chen, Y.; Peng, Y.; J. Soils Sediments 2015, 15, 2199. [Crossref]

46. Fraga, I.; Bajouco, R.; Pinto, R.; Brito, L. M.; Bezerra, R.; Coutinho, J.; Comemoração do Ano Internacional do Solo, Simpósio O Solo na Investigação Científica em Portugal, Instituto de Agronomia, Universidade de Lisboa, Lisboa, Portugal, September, 2015.

47. Verma, B. C.; Datta, S. P.; Rattan, R. K.; Singh, A. K.; J. Environ. Biol. 2013, 34, 1069.

48. Mandal, U. K.; Yadav, S. K.; Sharma, K. L.; Ramesh, V.; Venkanna, K.; Indian J. Agric. Sci. 2011, 81, 927.

49. Kalisz, B.; Lachacz, A.; Glazewski, R.; Klasa, A.; Arch. Environ. Prot. 2012, 38, 87. [Crossref]

50. Chan, K. Y.; Bowman, A.; Oates, A.; Soil Sci. 2001, 166, 61. [Crossref]

51. Blair, G. J.; Lefroy, R. D. B.; Lise, L.; Aust. J. Agric. Res. 1995, 46, 1459. [https://doi.org/10.1071/AR9951459]

52. Hurisso, T. T.; Culman, S. W.; Horwath, W. R.; Wade, J.; Cass, D.; Beniston, J. W.; Bowles, T. M.; Grandy, A. S.; Franzluebbers, A. J.; Schipanski, M. E.; Lucas, S. T.; Ugarte, C. M.; Soil Sci. Soc. Am. J. 2016, 80, 1352. [Crossref]

53. Hurisso, T. T.; Culman, S. W.; Zhao, K.; Soil Sci. Soc. Am. J. 2018, 82, 939. [Crossref]

54. Kim, C.; Ji, T.; Eom, J. B.; Meas. Sci. Technol. 2018, 29, 045802. [Crossref]

55. Aiken, G.; Kaplan, L. A.; Weishaar, J.; J. Environ. Monit. 2002, 4, 70. [Crossref]

56. Walinga, I.; Kithome, M.; Novozamsky, I.; Houba, V. J. G.; Van der Lee, J. J.; Commun. Soil Sci. Plant Anal. 1992, 23, 1935. [Crossref]

57. dos Santos, H. G.; Jacomine, P. K. T.; dos Anjos, L. H. C.; de Oliveira, V. Á.; Lumbreras, J. F.; Coelho, M. R.; de Almeida, J. A.; de Araújo Filho, J. C.; de Oliveira, J. B.; Cunha, T. J. F. In Brazilian Soil Classification System; Embrapa Solos - Livro técnico (INFOTECA-E), Brasília, Brazil, 2018.

58. United States Department of Agriculture (USDA) - Natural Resources Conservation Service (NRC); Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys; USDA - NRC, Washington, 1999.

59. Burgot, J. L.; Ionic Equilibria in Analytical Chemistry; Springer Science & Business Media: Germany, 2012.

60. Kögel-Knabner, I.; Amelung, W.; Geoderma 2021, 384, 114785. [Crossref]

61. Yoon, G.; Park, S. M.; Yang, H.; Tsang, D. C.; Alessi, D. S.; Baek, K.; Chemosphere 2018, 199, 453. [Crossref]

62. Kumar, S.; Ghotekar, Y. S.; Dadhwal, V. K.; J. Earth Syst. Sci. 2019, 128, 1. [Crossref]

63. Associação Brasileira de Normas Técnicas (ABNT); ABNT NBR 14725-2, Produtos químicos - Informações sobre segurança, saúde e meio ambiente Parte 2: Sistema de classificação de perigo, Rio de Janeiro, 2009.