CEN/TC 260

Date: 2025-10

prEN 18322:2026

Secretariat: DIN

Inorganic fertilizers — Determination of the organic carbon content by dry combustion

Einführendes Element — Haupt-Element — Ergänzendes Element

Élément introductif — Élément central — Élément complémentaire

CCMC will prepare and attach the official title page.

Contents Page

European foreword 3

1 Scope 4

2 Normative references 4

3 Terms and definitions 4

4 Principle 5

4.1 Method A (indirect procedure) 5

4.2 Method B (direct procedure) 5

5 Interferences 5

6 Reagents 6

7 Equipment and consumables 7

8 Sampling and sample preparation 7

8.1 Sampling 7

8.2 Sample preparation 7

9 Procedure – Method A (indirect method) 8

9.1 Determination 8

9.2 Calibration 8

9.3 Control measurements 9

9.4 Calculation and expression of results 9

10 Procedure – Method B (direct method) 10

10.1 Determination 10

10.2 Calibration 11

10.3 Control measurements 11

10.4 Calculation and expression of results 12

11 Expression of results 12

12 Precision 12

12.1 Inter-laboratory study 12

12.2 Repeatability 13

12.3 Reproducibility 13

13 Test report 13

Annex A (informative) Results of the inter-laboratory study 14

Bibliography 16

European foreword

This document (prEN 18322:2026) has been prepared by Technical Committee CEN/TC 260 “Fertilizers and liming materials”, the secretariat of which is held by DIN.

This document is currently submitted to CEN Enquiry.

This document has been prepared under a standardization request addressed to CEN by the European Commission. The Standing Committee of the EFTA States subsequently approves these requests for its Member States.

This document specifies a method for the determination of total organic carbon content by elemental analysis using dry combustion. The method is applicable to inorganic fertilizers containing more than 0,1 % carbon expressed on dry mass.

NOTE This method can also be applied to other types of fertilizers, provided the user has verified the applicability.

This document is applicable to the fertilizing products blends where a blend is a mix of at least two of the following components: inorganic fertilizers, organic fertilizers, organo-mineral fertilizers, liming materials, soil improvers, growing media, inhibitors, plant biostimulants and where the following category: inorganic fertilizer is the highest % in the blend by mass or volume, or in the case of liquid form by dry mass. If inorganic fertilizer is not the highest % in the blend, the European Standard for the highest % of the blend applies. In case a fertilizing product blend is composed of components in equal quantity, the user decides which standard to apply. Variations in analytical methods for fertilizing product blends can lead to differing results as some components or matrix interactions can affect the outcome. Validation procedures have shown that developed standard methods are robust and reliable across diverse product compositions, but possible interferences and unexpected results when analysing fertilizing product blends are possible.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

EN 1482‑2, Fertilizers, liming materials and inhibitors — Sampling and sample preparation — Part 2: General sample preparation provisions

EN 12944‑1, Fertilizers, liming materials and inhibitors — Vocabulary — Part 1: General terms

EN 12944‑2, Fertilizers, liming materials and inhibitors — Vocabulary — Part 2: Terms relating to fertilizers

For the purposes of this document, the terms and definitions given in EN 12944‑1 and EN 12944‑2 and the following shall be applied.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

total organic carbon

TOC

quantity of carbon that is converted into carbon dioxide by combustion and not released as carbon dioxide by treatment with acid

Note 1 to entry: In agreement with Regulation (EU) 2019/1009 [1], carbon derived from urea and polymers containing urea are not considered as organic in this document.

3.2

total inorganic carbon

TIC

quantity of carbon that is released as carbon dioxide by acid treatment

Note 1 to entry: Typically, the TIC represents the carbonates present in a sample.

3.3

total carbon

TC

quantity of carbon present in the sample in the form of organic, inorganic and elemental carbon

In this procedure, the total organic carbon (TOC) content is obtained by the difference between the results of the measurements of TC and TIC.

The total carbon (TC) present in the sample is converted into carbon dioxide by combustion in an oxygen-containing gas flow free of carbon dioxide. To ensure complete combustion, catalysts and/or modifiers can be used.

The total inorganic carbon (TIC) is determined separately from another sub-sample by means of acidification and purging of the released carbon dioxide.

The released amount of carbon dioxide is measured e.g. by infrared spectrometry, thermal conductivity detection, or other suitable techniques.

In this procedure, the TIC present in the sample is previously removed by treating the sample with acid. The carbon dioxide released by the following combustion step is measured by one of the techniques mentioned in 4.1 and indicates the TOC directly.

NOTE The quality of results of Method B is dependent on experience and practice, especially regarding the steps before the determination of TOC. Using automatic dispensing units regarding removal of TIC prior to determination of TOC improve the performance of Method B.

In the case of experience with samples containing high amounts of carbonates, the operator shall take care to correctly eliminate all the carbonates to make sure to produce reliable TOC results.

When present, carbon from urea is determined as organic carbon using the method described in this document. An interpretation of the measured value can therefore be problematic in cases where the sample contains relevant levels of urea. If needed, carbon from urea shall be determined separately by means of a suitable validated method (see EN 15705 [2]) and shall be subtracted and reported by the laboratory.

Depending on the detection method used, different interferences occur, for instance:

— the presence of cyanide interferes with the coulometric detection of TIC by modifying the pH value (dissolution of hydrogen cyanide (HCN));

— high content of halogenated compounds leads to an overestimation of TOC when coulometric detection is used; in some cases, the classical silver or copper trap is insufficient to absorb all halides.

Method B leads to incorrect results in the following cases:

— volatile organic substances are lost during sample preparation, especially during the acidification. If necessary, the carbon content resulting from volatile organic substances shall be determined separately;

— side reactions between the sample and the acid take place (e.g. decarboxylation, volatile reaction products).

When present, elemental carbon, carbides, cyanides, cyanates, isocyanates, isothiocyanates and thiocyanates are determined as organic carbon using the methods described in this document. An interpretation of the measured value is therefore problematic in cases where the sample contains relevant levels of the above-mentioned components. If needed, these components shall be determined separately by means of a suitable validated method and shall be subtracted and reported by the laboratory.

Use only reagents of recognized analytical grade, unless otherwise specified and distilled water or ultrapure water, with a specific conductivity not higher than 0,2 mS/m at 25 °C, free from the elements to be determined.

Hygroscopic substances shall be stored in a desiccator.

6.1 Calcium carbonate, CaCO₃.

6.2 Sodium carbonate, NaCO₃ anhydrous.

6.3 Tetrasodium ethylenediamine tetraacetate-tetra-hydrate, Na₄EDTA⋅4H₂O (C₁₀H₁₂N₂O₈Na₄⋅4H₂O), heated at 80 °C ± 1 °C for 2 h.

Other forms of Na₄-EDTA hydrates may be used if the water content is exactly known. In these cases, the composition of the control mixtures shall be recalculated accordingly (see also 6.10 and 6.11).

6.4 Potassium hydrogen phthalate, C₈H₅O₄K.

6.5 Acetanilide, C₈H₉NO.

6.6 Atropine, C₁₇H₂₃NO_3.

6.7 Spectrographic graphite powder, C.

6.8 Sodium salicylate, C₇H₅O₃Na.

6.9 Aluminium oxide, Al₂O₃, neutral, granular size < 200 μm, annealed at 600 °C ± 5 °C.

6.10 Control mixture A, prepared from sodium carbonate (6.2), Na₄-EDTA⋅4H₂O (6.3) and aluminium oxide (6.9) in a mass ratio of 2,34:1,00:7,28

The mixture shall be homogenized. It shall contain 2,5 % TIC and 2,5 % TOC (e.g. 22,06 g of sodium carbonate; 9,41 g Na₄-EDTA⋅4H₂O; 68,53 g of aluminium oxide).

6.11 Control mixture B, prepared from sodium salicylate (6.8), calcium carbonate (6.1), Na₄‑EDTA·4 H₂O (6.3) and aluminium oxide (6.9) in a mass ratio of 1,00:4,36:1,97:8,39

The mixture shall be homogenized. It shall contain 3,3 % TIC and 6,6 % TOC (e.g. 6,36 g of sodium salicylate; 27,78 g of calcium carbonate; 12,50 g of Na₄-EDTA·4H₂O; 53,36 g of aluminium oxide).

6.12 Inert absorbent material for determination on fluid samples.

6.13 Non-oxidizing mineral acid, used for carbon dioxide expulsion, e.g. phosphoric acid H₃PO₄ (w = 85 % [as a mass fraction]).

NOTE Due to potential corrosion by hydrochloric acid, phosphoric acid is preferred for TIC determination in Method A (9.1.3). Due to potential formation of P₄O₁₀ during combustion, hydrochloric acid HCl (w = 37 %) is preferred for removal of inorganic carbon in Method B (10.1.2).

6.14 Carrier gas, e.g. synthetic air, nitrogen, oxygen, helium or argon, free of carbon dioxide and organic impurities in accordance with the manufacturer's instructions.

Disposable equipment is acceptable in the same way as reusable glassware if the specifications are similar. Ordinary laboratory equipment, and particularly the following.

7.1 Analytical scale, capable of weighing to the nearest 0,000 1 g.

7.2 Equipment for determination of carbon in solid and liquid, with relevant accessories.

7.3 Purging unit for TIC determination, for Method A only.

7.4 Vessels or crucibles, made of e.g. ceramic, silica, quartz, silver or platinum.

NOTE Tin and nickel vessels are not acid resistant. Tin vessels are suitable only for Method A.

7.5 Pasteur pipettes.

7.6 Drying system, thermostatically controlled and capable of maintaining temperature of (105 ± 2)°C.

7.7 Desiccator, with an active drying agent such as silica gel.

7.8 Muffle furnace, thermostatically controlled.

Sampling is not part of the method specified in this document. Recommended sampling method are given in EN 1482‑1 [6] and EN 1482‑3 [7].

It is important that the laboratory receives a sample that is representative of both the product under consideration and the given analysis. The sample should not have been damaged or changed during transport or storage.

Sample preparation shall be carried out in accordance with EN 1482‑2.

The mass of the test portion shall be as large as possible and shall be chosen so that the liberated quantity of carbon dioxide lies within the working range of the equipment/calibration.

To minimize carbon blank values the vessel shall be pre-treated by heating (in a muffle furnace or the TC apparatus itself). The sample prepared according to Clause 8 is weighed into a suitable vessel (7.4).

The sample is combusted or decomposed in a flow of carrier gas containing oxygen (6.14). The combustion temperature shall be high enough to convert all carbon completely to carbon dioxide.

In case, samples contain carbonates, which are difficult to decompose, e.g. barium carbonate, the temperature shall be increased or modifiers, e.g. tin, copper shall be used for the best release of the carbon dioxide.

The temperature range of commercially available instruments is between 900 °C and 1500 °C. During the combustion of reactive samples, explosion or fuming shall be prevented by covering the sample with inert material e.g. silica sand. The amount of carbon dioxide released during the combustion is measured e.g. by infrared spectrometry, thermal conductivity detection, or other suitable techniques, and is expressed as total carbon (TC).

The sample prepared according to Clause 8 is weighed into the purging unit (7.3) or in the sample vessel (7.4).

The system is closed gas-tight and flushed with carrier gas until no more carbon dioxide from ambient air is present. Then acid (6.12) is added and the carbon dioxide is stripped by purging or stirring and/or heating. The released carbon dioxide is transferred to the detector by the carrier gas.

The addition of wetting agents, e.g. surfactants, can improve wetting of the surface of the sample.

The addition of anti-foaming agents, e.g. silicone oil, can be helpful in the case of strongly foaming samples.

The amount of carbon dioxide released during the gas evolution is measured e.g. by infrared spectrometry, thermal conductivity detection, or other suitable techniques and is expressed as total inorganic carbon (TIC).

Samples containing persistent carbonates (e.g. concrete, cement) require treatment with hot acid for a complete release of carbon dioxide according to manufacturers' instructions.

If a relative method is used for detection, e.g. infrared detection, calibration is necessary.

Calibration shall be performed according to the manufacturer's instruction.

Examples of calibration substances suitable for TC are calcium carbonate (6.1), potassium hydrogen phthalate (6.4), acetanilide (6.5), atropine (6.6), spectrographic graphite powder (6.7).

The following procedure shall be applied for calibration:

— Establish the preliminary working range;

— Measure a minimum of five standard samples. Typically, different sample weights of one calibration substance are used to cover the calibration range. The absolute amount of carbon of these standard samples shall be distributed evenly over the working range;

— Carry out a linear regression analysis and test the linearity of the calibration function (see ISO 8466‑1 [5]);

— Use the calibration function for calculating the mean values of the recovery of each standard sample.

The function shall be linear. Otherwise, the working range shall be restricted to the linear range.

If an absolute method is used for detection, e.g. coulometry, only control measurements according to 9.3 shall be carried out.

This calibration shall be carried out for initial validation purposes or after major changes of the equipment.

Control measurements shall be carried out using control mixture A (6.10) for the procedures according to 9.1.2 (TC) and 9.1.3 (TIC). Analysis of one concentration from the middle of the respective working range, possibly repeated two or three times, is sufficient. For the TC and TIC the mean recovery shall be between 80 % and 120 % with a coefficient of variation ≤ 5 %.

Blank values shall be considered, if necessary.

If the required recoveries are not achieved, the following measures are helpful.

For TC analysis:

— checking the homogeneity of the control mixture;

— checking the calibration;

— increasing the temperature during release of carbon dioxide;

— reducing the flow of the carrier gas;

— encouraging a turbulent flow in the combustion tube;

— use of modifiers;

— use of catalysts for post-oxidation of combustion gases.

For TIC analysis:

— optimizing the stirring speed and/or the gas flow in the purging vessel;

— improving the gas exchange in the purging vessel;

— avoiding condensation in the system.

The TC and TIC mass contents are calculated from:

— calibration function and sample mass if relative detection methods are used;

— specific constants and sample mass if absolute detection methods are used.

The calculation of TOC is achieved from the difference of the mean values of TC and TIC according to Formula (1):

(1)

where

w_TOC is the TOC content as carbon in the sample in relation to the dry mass, expressed in %;

w_TC is the mean value of the TC content as carbon in the sample in relation to the dry mass, prepared according to Clause 8, expressed in %;

w_TIC is the mean value of the TIC content as carbon in the sample in relation to the dry mass, prepared according to Clause 8, expressed in %.

In case of mixing the sample with aluminium oxide (see Clause 8) a dilution factor, F, following Formula (2) shall be used:

(2)

where

F is the dilution factor resulting from the sample preparation of the sample according to Clause 8;

m_s is the mass of the sample, expressed in g;

m_a is the mass of aluminium oxide, expressed in g.

TC and TIC determination shall be performed at least twice. The respective difference of the two values shall be ≤ 10 % of the mean.

If this is not the case, at least one further determination is necessary; then the relative repeatability standard deviation of the three results shall be ≤ 10 %. If this is not the case, the relative repeatability standard deviation of the three results shall be reported together with the result or all results of the different determination shall be reported.

The mass of the test portion shall be as large as possible and shall be chosen so that the liberated quantity of carbon dioxide lies within the working range of the equipment/calibration.

The sample prepared according to Clause 8 is weighed into a suitable vessel (7.4). The vessel may be prepared by suitable thermal treatment (in a muffle furnace or the combustion apparatus itself) to minimize carbon blank values.

In order to remove the inorganic carbon prior to the determination of TOC, the sample is carefully treated with a small volume of non-oxidizing mineral acid (6.13). Add the acid slowly (dropwise) to avoid foaming and splashing of the sample. Add as little acid as possible but enough to soak the entire sample and to remove the inorganic carbon completely.

Allow at least 4 h for the complete removal of the carbon dioxide. Mixing the sample can reduce the time needed for decomposition.

If moistening with the acid is difficult, the sample may be dampened beforehand with as little water as possible.

The moisture may be partly removed before combustion. The temperature during this sample treatment is not allowed to exceed 40 °C.

Combust the sample in the carrier gas containing oxygen (6.14).

The combustion temperature shall be high enough to convert the organic carbon completely to carbon dioxide. The use of modifiers, e.g. tin, copper can increase the recovery.

The temperature range of commercially available instruments is between 900 °C and 1500 °C.

During the combustion of reactive samples, explosion or fuming can be prevented by covering the sample with inert material e.g. silica sand after removal of the inorganic carbon.

The amount of carbon dioxide released during the combustion is measured e.g. by infrared spectrometry, thermal conductivity detection, or other suitable techniques, and is expressed as total organic carbon (TOC).

NOTE Corrosion of the combustion device can occur as a result of the acid remaining in the sample. This effect can be reduced by additional drying time. Salt deposits can contaminate the system.

The calibration for TOC shall be done in accordance with the calibration for determination of TC (9.2).

The same calibration substances can be used.

Control measurements shall be carried out using control mixture B (6.11) for the procedure according to 10.1.2. Analysis of one concentration from the middle of the respective working range, possibly repeated two or three times, is sufficient. For the TOC the mean recovery shall be between 80 % and 120 % with a coefficient of variation ≤ 5 %.

Blank values shall be considered if necessary.

If the required recoveries are not achieved, the following measures can be helpful.

For TOC analysis:

— checking the homogeneity of the control mixture;

— checking the calibration;

— increasing the combustion temperature;

— reducing the flow of the carrier gas;

— encouraging a turbulent flow in the combustion tube;

— use of modifiers;

— use of catalysts for post-oxidation of the combustion gases.

Removal of TIC:

— decreasing the drying temperature of the acidified sample;

— decreasing the drying time of the acidified sample;

— omitting the drying step.

The TOC mass contents of the samples prepared according to Clause 8 are calculated from:

— calibration function and sample mass if relative detection methods are used;

— specific constants and sample mass if absolute detection methods are used.

The TOC determination shall be measured at least twice. The respective difference of the two values shall be ≤ 10 % of the mean.

If this is not the case, at least one further determination is necessary; then the relative repeatability standard deviation shall be ≤ 10 %. If this is not the case, the relevant relative repeatability standard deviation shall be reported together with the result or all results of the different determination shall be reported.

The results are expressed as organic carbon in % (C_org). Results shall be reported to a maximum of two significant figures.

EXAMPLE

organic carbon (C_org) 18 % C;

organic carbon (C_org) 1,8 % C;

organic carbon (C_org) 0,18 % C.

The statistical evaluation was conducted for the combined results from method A and method B, as the two procedures are considered equivalent.

This approach is also justified from a statistical point of view, as the subsamples for each method were prepared independently.

This equivalence is supported by the fact that the results obtained from method A are consistent with those from method B.

Therefore, the performance parameters do not differentiate between the two methods, but instead apply equally to the results obtained from both method A and method B.

Details of inter-laboratory study on the precision are summarized in Annex A.

Repeatability and reproducibility were calculated according to ISO 5725‑2 [3] and ISO 5725‑3 [4].

It is possible that the values derived from this study are not applicable to concentration ranges and matrices other than those given.

The absolute difference between two independent single test results, obtained using the same method on identical test material in the same laboratory by the same operator using the same equipment within a short interval of time, will in no more than 5 % of the cases be greater than the repeatability limit r given in Table 1.

The absolute difference between two independent single test results, obtained using the same method on identical test material in different laboratories with different operators using different equipment, will in no more than 5 % of the cases be greater than the reproducibility limit R given in Table 1.

Table 1 — Mean values, repeatability and reproducibility limits

The test report shall contain at least the following information:

a) the test method used, together with a reference to this document including its year of publication;

b) complete identification of the sample;

c) the applied method (Method A or Method B);

d) information on applied pre-treatment procedure, if relevant;

e) expression of results, according to 9.4 or 10.4 respectively;

f) any deviation from this method and report any circumstances that could have affected the results;

g) the date of the measurement.

1. 
   (informative)
   
   Results of the inter-laboratory study
   1. Inter-laboratory study

The precision of the method has been determined in the year 2025 in an inter-laboratory study with 7 laboratories participating for method A and method B and carried out on 4 samples (Table A.1) of inorganic fertilizers, commercially available, the same samples for method A and method B.

Table A.1 — Materials tested in the inter-laboratory study (Method A and Method B)

Samples were shaken well and divided into aliquots, labelled with the appropriate anonymous code and were maintained protected from light at +4°C ± 1 °C until shipment.

The procedure was carried out in accordance with Clause 9 and Clause 10.

Specific instructions are sent to laboratories.

The statistical results are given in Table A.2.

The statistical evaluation was conducted for the combined results from method A and method B, as the two procedures are considered equivalent.

This approach is also justified from a statistical point of view, as the subsamples for each method were prepared independently.

This equivalence is supported by the fact that the results obtained from method A are consistent with those from method B.

Therefore, the performance parameters do not differentiate between the two methods, but instead apply equally to the results obtained from both method A and method B.

• 1. Statistical results for the determination of organic carbon by method A and B

Table A.2 — Statistical results for determination of organic carbon by method A and B

Bibliography

[1] Regulation (EU) 2019/1009 of the European Parliament and of the Council of 5 June 2019 laying down rules on the making available on the market of EU fertilising products and amending Regulations (EC) No 1069/2009 and (EC) No 1107/2009 and repealing Regulation (EC) No 2003/2003

[2] EN 15705, Inorganic fertilizers - Determination of methylen-urea oligomers using high-performance liquid chromatography (HPLC)

[3] ISO 5725‑2, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method

[4] ISO 5725‑3, Accuracy (trueness and precision) of measurement methods and results – Part 3: Intermediate precision and alternative designs for collaborative studies

[5] ISO 8466‑1, Water quality - Calibration and evaluation of analytical methods - Part 1: Linear calibration function

[6] EN 1482‑1, Fertilizers, liming materials and inhibitors - Sampling and sample preparation - Part 1: General sampling provisions

[7] EN 1482‑3, Fertilizers, liming materials and inhibitors - Sampling and sample preparation - Part 3: Sampling of static heaps