🌱 Organic and total carbon in soil after dry combustion — PN-ISO 10694

Environment PN-ISO 10694

Overview

SCOPE FROM THE PKN CARD: "Podano metodę oznaczania całkowitej zawartości węgla w glebie po suchym spalaniu. Zawartość węgla organicznego oblicza się z całkowitej zawartości węgla po wprowadzeniu poprawki na węglany obecne w próbce. Jeżeli wcześniej węglany zostały usunięte, wówczas zawartość węgla organicznego mierzy się bezpośrednio. Metoda ma zastosowanie do wszystkich typów powietrznie suchych próbek gleby." [A method is given for determining the total carbon content of soil after dry combustion. The organic carbon content is calculated from the total carbon content after correction for the carbonates present in the sample. If the carbonates have been removed beforehand, the organic carbon content is measured directly. The method is applicable to all types of air-dried soil samples.]

PRINCIPLE (clause 3 of ISO 10694:1995). The carbon from the soil is oxidised to CO2 by heating the sample to not less than 900 °C in a stream of gas containing oxygen and free of CO2. The amount of CO2 formed is measured titrimetrically, gravimetrically, conductometrically, by gas chromatography or with an infrared detector — depending on the apparatus. At this temperature carbonates decompose completely, therefore: • TOTAL carbon is the result of combusting the sample without pretreatment; • ORGANIC carbon — INDIRECT ROUTE (8.2.1): the carbonate carbon is subtracted from the total carbon, wC,o = wC,t − 0.12 × wCaCO3, where the carbonate content (as CaCO3) is determined according to ISO 10693; • ORGANIC carbon — DIRECT ROUTE (7.3 and 8.2.2): the carbonates are removed before analysis with an excess of hydrochloric acid 4 mol/l, left for 4 h, dried for 16 h at 60–70 °C and only then combusted. Note 1 of the standard: at a pH-CaCl2 below 6.5 the presence of carbonates is unlikely — such a combination occurs only in freshly limed soils. One entry in the accreditation scopes cites the standard "z wyłączeniem punktu 8.2.1" [z wyłączeniem punktu = excluding clause] — this laboratory does not calculate organic carbon by the indirect route.

SAMPLE AND RESULT (clauses 6 and 8). The fraction below 2 mm of air-dried soil, prepared according to ISO 11464, is analysed; part of the sample is used to determine water according to ISO 11465 and, if necessary, carbonates according to ISO 10693. The result is given in grams per kilogram of oven-dried soil: wC,t = 1000 × (m2/m1) × 0.2727 × (100 + wH2O)/100, where m1 is the mass of the test portion, m2 — the mass of CO2 released, 0.2727 — the factor converting CO2 to C, and wH2O — the water content according to ISO 11465. ORGANIC MATTER (8.3): wom = f × wC,o. The factor f depends on the type of organic matter and for arable soils ranges from 1.7 to 2.0 (note 9). "Zawartość próchnicy (z obliczeń)" [humus content (calculated)] in an accreditation scope is therefore a number that depends on the f adopted — without stating f the result is not unambiguous.

REPEATABILITY (clause 9, Table 1) — the permissible difference between two successive determinations: • up to 2.5 g/kg — 0.25 g/kg absolute; • above 2.5 to 75 g/kg — 10 % relative; • above 75 g/kg — 7.5 g/kg absolute. INTERLABORATORY STUDY (Annex A, informative): in 1993 the Agricultural University in Wageningen organised a study of five soils in nine laboratories (eight sent in results). Soil No. 1 — a muck soil from POLAND — had on average 410.42 g/kg of organic carbon with a relative reproducibility standard deviation of 11.1 %; a sandy soil from Mali (2.47 g/kg) — as much as 62.92 %. At a low carbon content the reproducibility deteriorates sharply.

STATUS (state of the PKN card and the ISO website on 25.09.2026). PN-ISO 10694:2002 — Polish-language version, publication 28-01-2002, 12 pages, KT 191 Soil Chemistry, ICS 13.080.10; it implements ISO 10694:1995 [IDT]; the card has no "Withdrawn" header. ISO website: ISO 10694:1995 — published, last confirmed in 2021, but at the stage "International Standard to be revised [90.92]", with the note "Expected to be replaced by ISO/FDIS 15936 within the coming months.". The ISO 10694 page does not give the title and scope of the draft successor, so we do not quote them.

HOW ACCREDITATION SCOPES CITE THE STANDARD. Measurement of 25.09.2026 on the database of PL accreditation scopes (1310 laboratories with entries): 28 entries at 17 laboratories. Forms: "PN-ISO 10694:2002" (15), "PN-ISO 10694" without a year (7), "PN-ISO 10694:2002 M" (3), "PN - ISO 10694:2002" (1) and "PN-EN 10694:2002" (2) — there is no standard with the designation PN-EN 10694 in the PKN catalogue (search "10694", 25.09.2026, returns only PN-ISO 10694:2002). Technique: high-temperature combustion with IR detection (15) or TCD (2). Methodologies: total carbon, total organic carbon (OWO/TOC), inorganic carbon, humus (calculated). AN OBJECT WIDER THAN THE STANDARD: next to "Gleba" [soil] (16) stand "Odpady przemysłowe, mineralne surowce i materiały budowlane, szlamy i osady ściekowe" [industrial waste, mineral raw materials and building materials, slurries and sewage sludge] (2), "Łupki i mułki przywęglowe" [coal-bearing shales and mudstones] (2), as well as ground, bottom sediments and waste. The standard applies to air-dried SOIL samples — for the other matrices this is an extension by the laboratory that requires its own validation. In three entries PN-EN 15936 stands next to it — a standard on the same technique for a wider range of matrices: the edition in force, PN-EN 15936:2022-07 (English-language version, KT 191), bears the title "Gleba, odpady, uzdatnione bioodpady oraz osady ściekowe — Oznaczanie całkowitej zawartości węgla organicznego (TOC) po suchym spalaniu" [Soil, waste, treated biowaste and sludge — Determination of total organic carbon (TOC) by dry combustion] and replaced the 2013-02 edition (PKN card, 25.09.2026). The declared ranges vary widely, from "(0,01 - 95,0)%" to "(1,0 – 400) g/kg" — some laboratories give percentages, some g/kg or mg/kg.

THE KEY OF THE "LABORATORIES" TAB: "PN-ISO 10694" — 14 laboratories and 25 entries, without any foreign entries. The tab will not show 3 laboratories (AB 313, AB 322, AB 646), which write "PN-EN 10694:2002" or "PN - ISO 10694:2002" with spaces.

HOW FAR THE TEXT WE USED EXTENDS: the text of ISO 10694:1995 made available by the publisher, clauses 1–10 and Annex A — the whole standard. The standard does not give the operating conditions of particular analysers: it refers to the manufacturer's instructions.

Method principle

A test portion of air-dried soil is combusted at not less than 900 °C in a stream of oxygen free of CO2; all the carbon — organic and carbonate — is converted to CO2, which is measured with a detector (most often infrared). Organic carbon is obtained by subtracting the carbon of separately determined carbonates (0.12 × CaCO3 content) or by combusting the sample after removing the carbonates with hydrochloric acid. The result is converted to oven-dried soil.

Applications

• Soils — total and organic carbon content, assessment of the organic matter supply
• Calculation of the organic matter (humus) content from organic carbon
• Testing of soils and ground in the assessment of contamination and remediation
• In accreditation scopes extended by laboratories to ground, sediments, waste and coal-bearing shales

Key parameters

Standard

Standard number

PN-ISO 10694:2002 (wersja polska; wprowadza ISO 10694:1995 [IDT])

Title (PL)

Jakość gleby — Oznaczanie zawartości węgla organicznego i całkowitej zawartości węgla po suchym spalaniu (analiza elementarna)

Title (EN)

Soil quality — Determination of organic and total carbon after dry combustion (elementary analysis)

Step-by-step procedure

1. Sample preparation

   Prepare air-dried soil according to ISO 11464, sieve through a 2 mm sieve; determine water according to ISO 11465 and, if necessary, carbonates according to ISO 10693.

2. Calibration of the apparatus

   According to the manufacturer's instructions, with one of the standard substances of 4.2.

3. Total carbon

   Combust a test portion (mass depending on the expected carbon content and the apparatus) at ≥ 900 °C and measure the CO2.

4. Organic carbon

   Either subtract the carbonate carbon (0.12 × wCaCO3) from the total carbon, or remove the carbonates with an excess of HCl 4 mol/l (4 h, then drying for 16 h at 60–70 °C) and combust the sample.

5. Calculation

   wC,t = 1000 × m2/m1 × 0.2727 × (100 + wH2O)/100 in g/kg of dry matter; for organic matter state the factor f adopted.

6. Report

   Result in g/kg of dry matter; for organic carbon state whether the carbonates were determined (indirect route) or removed before analysis (direct route) — a requirement of clause 10.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Combustion furnace at ≥ 900 °C — hot crucibles and furnace parts, gloves and tongs
• Hydrochloric acid 4 mol/l — gloves, goggles, work in a fume hood; crucibles large enough to carry samples with acid safely (note to 7.3)
• Oxygen cylinders — no grease on fittings, cylinders secured

Frequently asked questions