🌿 Organic carbon content of air-dry soil (< 2 mm fraction milled according to ISO 11464) — test portion of 20–500 mg depending on the expected carbon content (carbon mass ≤ 20 mg), oxidation with 5.0 ml of 0.27 mol/l potassium dichromate and 7.5 ml of concentrated sulfuric acid in calibrated 75 ml tubes in a heating block at (135 ± 2) °C for 30 min, dilution to 100 ml, settling for 1 h and centrifugation, absorbance of green Cr3+ at 585 nm in a 10 mm cell against a glucose standard series (0–20 mg C); result in g/kg on oven-dry basis (water content according to ISO 11465); not for soils with mineral-reducing compounds (Cl⁻ ≤ 2 mg in the test portion, Fe2+); organic matter = f × organic carbon, f 1.7–2.5; PN-ISO 14235:2003 (withdrawn 20.11.2024, no successor indicated)

Environment PN-ISO 14235

Overview

WHAT THE STANDARD COVERS. We read the text of the standard in the iTeh sample of ISO 14235:1998 (English text) — PN-ISO 14235:2003 adopts ISO 14235:1998 as identical (PKN record). The sample covers the whole text: foreword and clauses 1–10 up to page 5.

ACCORDING TO THE TEXT OF ISO 14235:1998. The standard was prepared by ISO/TC 190 "Soil quality", Subcommittee SC 3 "Chemical methods and soil characteristics". Scope (1): spectrometric determination of organic carbon content in soil by oxidation in a sulfochromic medium; applicable to all types of air-dry soil samples; not applicable to soils containing mineral-reducing compounds, e.g. Cl⁻ or Fe2+ — by convention the chloride content in the test portion shall not exceed 2 mg. References (2): ISO 3696, ISO 10694 (organic and total carbon after dry combustion), ISO 11464 (pretreatment of samples), ISO 11465 (dry matter and water content). Principle (3): organic carbon is oxidized with a mixture of potassium dichromate solution (in excess) and sulfuric acid at 135 °C; dichromate ions, colouring the solution orange-red, are reduced to Cr3+ ions, colouring it green; the intensity of the green colour is measured spectrophotometrically; it is assumed that oxidation of one carbon atom of the organic matter produces four electrons, so there is a direct relationship between the Cr3+ formed and the amount of organic carbon; the method is calibrated with glucose as a source of readily oxidizable carbon; note: 2 mg of chloride in the test sample corresponds to an apparent carbon content of about 0.7 mg. Reagents (4): water with specific conductivity not higher than 0.2 mS/m at 25 °C (grade 2 according to ISO 3696); concentrated sulfuric acid (1.84 g/cm³); potassium dichromate solution c(K2Cr2O7) = 0.27 mol/l — 80 g in a 1000 ml flask; anhydrous glucose; warning: the chromate ion in potassium dichromate is potentially toxic — avoid contact and ingestion, follow regulations, also for disposal of chromate solutions, which may damage the environment. Apparatus (5): standard glassware; analytical balance 0.1 mg; volumetric tubes 75.0 ± 0.2 ml fitting the block; heating block maintaining a uniform (135 ± 2) °C — holes at least (h + 10) mm deep, where h is the height of the solution in the tube, with at least 150 mm of the tube emerging from the block; temperature measured with a thermometer in holes with tubes containing oxidation solution; unused holes filled with tubes containing 15 ml of sulfuric acid; centrifuge; glass fibre filters; automatic pipette; spectrophotometer with a 10 mm cuvette set at 585 nm; water bath. Laboratory sample (6): the < 2 mm fraction of air-dry soil pretreated according to ISO 11464; part of the sample for water content according to ISO 11465; for carbon a representative subsample is milled to pass the sieve according to ISO 11464 (in the sample text: "250 mm aperture sieve", in ISO 11464 a 250 µm sieve). Procedure — general (7.1): oxidation efficiency depends on the mass of the test portion and the mass of carbon in it, even with dichromate in excess; the carbon mass in the test portion should not exceed 20 mg — the test portion follows Table 1: expected carbon 0–40 g/kg — 400–500 mg, 40–80 — 200–250 mg, 80–160 — 100–125 mg, 160–400 — 45–50 mg, above 400 g/kg — 20–25 mg. Oxidation (7.2): the test portion is transferred to a tube, first 5.0 ml dichromate and then 7.5 ml sulfuric acid are added by pipette, carefully homogenized; tubes go into the block preheated to 135 °C for 30 min, are removed and rapidly cooled in the water bath; about 50 ml of water is slowly added and they are cooled rapidly again; the content is transferred quantitatively to a 100 ml flask, made up with water and mixed; the suspension is left to settle for 1 h, part of the supernatant is decanted into a centrifuge tube and centrifuged for 10 min at 2000 g, and if solid particles remain — filtered; caution: adding water to sulfuric acid and working with dichromate are hazardous — gloves, mask or goggles and laboratory coat are essential. Standard series (7.3, Table 2): glucose is dissolved in 5.00 ml of dichromate solution, 7.5 ml acid added and digested as the sample; the solutions must be completely clear without centrifugation or filtration — otherwise digestion has failed and must be repeated; Table 2: tubes 1–5 with carbon content in 5 ml solution of 0, 5, 10, 15 and 20 mg (glucose mass column given in g: 0.00; 0.25; 0.50; 0.75; 1.00). Measurement (7.4): absorbance at 585 nm in the order — standards, blank, soil samples; without cross-contamination; the mass of organic carbon in the blank and samples is read from the standard curve. Calculation (8): w_oc = (a/m) × [(100 + w)/100], where w_oc — organic carbon content in g/kg on oven-dry basis, a — mass of organic carbon in the test portion in mg, m — mass of the test portion in g, w — water content in % on a dry-mass basis according to ISO 11465; note: comparison with ISO 10694 showed that in various soil samples 95 % of the total organic carbon is oxidized; organic matter content w_om = f × w_oc, where f is a conversion factor depending on the type of organic matter, 1.7–2.5 for agricultural soils. Repeatability (9, Table 3): two separate sequential measurements — up to 10 g/kg acceptable variation 1 g/kg absolute, above 10 g/kg — 6.5 %. Test report (10): reference to the standard, method, sample identification, organic carbon result in g/kg to one decimal place on oven-dry basis, conversion factor used (if expressed as organic matter), additional operations and factors affecting the result.

STATUS (as of 03.10.2026). The PKN search engine (query "PN-ISO 14235", 03.10.2026) returns one result: PN-ISO 14235:2003 — Polish version, "Withdrawn", "Introduces: ISO 14235:1998 [IDT]". The record of the standard (read 03.10.2026): publication date 19-05-2003, withdrawal date 20-11-2024, 10 pages, KT 191 "Soil Chemistry"; the record does not indicate a replacing standard. We did not read the status of ISO 14235:1998 in the iTeh catalogue (since 03.10.2026 the catalogue pages require a browser with scripts). What the withdrawal changes for the result: the record indicates no successor, so there is no newer PN text to compare — both laboratories still cite the withdrawn edition.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, load up to 17.09.2026, read 03.10.2026). The number 14235 appears in 2 records at 2 laboratories: AB 869 and AB 1748. In the current PCA documents (read 03.10.2026; AB 1748 issue no. 11 of 18.02.2026, AB 869 issue no. 30 of 16.06.2026) the number appears at the same 2 laboratories; in each document the number of page markers equals the number of PDF pages, without duplicates; searching all PCA documents downloaded from the BIP on 03.10.2026 found no laboratories outside the database copy. Form at both: "PN-ISO 14235:2003". None of these items is suspended. The "Laboratories" tab (query "PN-ISO 14235") shows both laboratories — checked by a tab query on the labcoda.pl production server on 03.10.2026.

WHAT THE LABORATORIES TEST (PCA items). AB 1748: soil — organic carbon content (1–400) g/kg and organic matter content (calculated), spectrophotometric method. AB 869: soil — organic carbon content (1–350) g/kg, spectrophotometric method. Separate Knowledge Base entries exist, among others, for PN-ISO 10694 (organic and total carbon after dry combustion) and PN-EN 15936 (total organic carbon in soil, waste and sludge).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. Chlorides and Fe2+: reducing compounds consume dichromate — 2 mg of chloride in the test portion gives an apparent carbon of about 0.7 mg, hence chloride in the test portion must not exceed 2 mg (1, 3). Test portion: carbon in the test portion ≤ 20 mg, mass according to Table 1 — with too large a portion oxidation is incomplete despite excess dichromate (7.1). Block: uniform (135 ± 2) °C measured in tubes containing solution, empty holes filled with tubes of acid (5.4). Standards: a turbid standard solution means failed digestion to be repeated (7.3); in Table 2 of the sample the glucose mass is given in grams while the carbon content is 5–20 mg — it is worth checking this table in the PN text before preparing standards. Result: conversion to an oven-dry basis requires water content according to ISO 11465 (6, 8). Comparisons: the method oxidizes about 95 % of the organic carbon determined by dry combustion according to ISO 10694 (8, note). Organic matter: the factor f depends on the type of organic matter (1.7–2.5 for agricultural soils) and must be stated in the report (8, 10) — AB 1748 reports organic matter as calculated. Upper end: above 400 g/kg the test portion is only 20–25 mg (Table 1) — the range of AB 1748 ends at 400 g/kg. Status: both laboratories cite a standard withdrawn on 20.11.2024.

WHAT WE DO NOT GIVE. We do not summarize ISO 10694, ISO 11464 or ISO 11465 in this entry. We do not give legal requirements for organic carbon content in soils.

Method principle

Soil organic matter is oxidized by an excess of potassium dichromate in concentrated sulfuric acid at 135 °C. The orange-red dichromate ions are converted to green Cr3+ ions, and since oxidation of one carbon atom yields four electrons, the amount of Cr3+ is directly related to the amount of organic carbon. The intensity of the green colour is measured at 585 nm, and the carbon mass is read from a standard curve prepared with glucose.

Applications

• All types of air-dry soil samples (1)
• Organic matter content of soil with a conversion factor f (1.7–2.5 for agricultural soils) (8)
• Soil — organic carbon and organic matter content — 2 accreditation scope records of 2 laboratories in PCA

Key parameters

Standard

Standard number

PN-ISO 14235:2003

Title (PL)

Jakość gleby — Oznaczanie zawartości węgla organicznego przez utlenianie dwuchromianem(VI) w środowisku kwasu siarkowego(VI)

Title (EN)

Soil quality — Determination of organic carbon by sulfochromic oxidation

Step-by-step procedure

1. Sample

   < 2 mm fraction according to ISO 11464, milled subsample; water content according to ISO 11465 (6). PN-ISO 14235:2003 — withdrawn 20.11.2024, no successor indicated (PKN record).

2. Test portion

   According to Table 1, so that carbon is ≤ 20 mg (7.1).

3. Oxidation

   5.0 ml dichromate, 7.5 ml H2SO4, 30 min at 135 °C, rapid cooling, about 50 ml water (7.2).

4. Clarification

   To 100 ml, 1 h settling, centrifugation 10 min at 2000 g, filtration if necessary (7.2).

5. Standards and measurement

   Glucose in dichromate as the sample; 585 nm: standards, blank, samples (7.3, 7.4).

6. Result

   w_oc = (a/m) × (100 + w)/100 in g/kg; optionally organic matter f × w_oc; repeatability according to Table 3 (8, 9).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The chromate ion in potassium dichromate is potentially toxic — avoid contact and ingestion; regulations also cover disposal of chromate solutions, which are harmful to the environment (4.3, warning)
• Adding water to sulfuric acid and working with dichromate are hazardous — gloves, mask or goggles, laboratory coat (7.2, caution)

Frequently asked questions