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Soil · Air-dried soil, finely ground (typically < 0.5 mm)

Soil Organic Carbon by Walkley–Black Wet Oxidation

Estimates the organic carbon in soil by oxidising it with a known excess of potassium dichromate in concentrated sulphuric acid, then titrating the unused dichromate with standardised ferrous ammonium sulphate. A blank gives the total dichromate added.

  • Environmental & water
  • Dichromate wet oxidation with back titration by ferrous ammonium sulphate
  • Burette & glassware
  • Analytical balance
  • Fume hood

Reference: Based on Walkley and Black (1934) as described in standard soil-analysis manuals. Follow the reagent strengths, sample mass and factors given in your laboratory SOP.

PRINCIPLE

Why this method works

Organic carbon in soil is oxidised to CO₂ by acid dichromate. The heat released when concentrated sulphuric acid is mixed with the dichromate solution drives the reaction; no external heating is used. Dichromate (Cr₂O₇²⁻) is reduced to Cr³⁺. The excess dichromate is then titrated with ferrous ion (Fe²⁺), using diphenylamine or ferroin indicator. The difference between the blank and the sample titre gives the dichromate used by the carbon. One milliequivalent of dichromate oxidises 3 mg of carbon (12 ÷ 4, because carbon goes from 0 to +4). Because the heat of dilution oxidises only part of the organic carbon, a recovery factor is applied. The conventional factor is 1.3, which assumes about 77% recovery.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Potassium dichromate, 1 N (0.1667 M)Dissolve 49.04 g dried K₂Cr₂O₇ in water and make to 1 L
Sulphuric acidConcentrated, at least 96% (containing Ag₂SO₄ if chloride interference is expected)
Orthophosphoric acidAbout 85%, to sharpen the endpoint by complexing Fe³⁺ (used with diphenylamine)
Ferrous ammonium sulphate (FAS), about 0.5 NDissolve about 196 g Fe(NH₄)₂(SO₄)₂·6H₂O in water with 20 mL conc. H₂SO₄ and make to 1 L; standardise daily against the blank
IndicatorDiphenylamine in conc. H₂SO₄, or ferroin (o-phenanthroline ferrous complex)
Sodium fluoride (optional)About 0.2 g, with diphenylamine, to remove Fe³⁺ interference
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Grind

    Grind air-dried soil to pass a 0.5 mm sieve.

  2. 02

    Weigh

    Weigh 0.5–1.0 g into a 500 mL conical flask.

  3. 03

    Add dichromate

    Add exactly 10 mL of 1 N K₂Cr₂O₇ and swirl.

  4. 04

    Add acid

    Add 20 mL conc. H₂SO₄ quickly in a fume hood; swirl 1 minute.

  5. 05

    Stand

    Leave on an asbestos-free insulating sheet for 30 minutes.

  6. 06

    Dilute & titrate

    Add water, H₃PO₄ and indicator; titrate with FAS.

Procedure

  1. 1Grind a sub-sample of the < 2 mm soil in a mortar to pass a 0.5 mm sieve. Remove visible roots and plant debris.
  2. 2Weigh 1.000 g (use 0.5 g or less for dark, organic-rich soils) into a dry 500 mL conical flask.
  3. 3Prepare a blank flask with no soil and treat it exactly the same way.
  4. 4Pipette exactly 10 mL of 1 N potassium dichromate into each flask and swirl gently to wet the soil.
  5. 5In a fume hood, add 20 mL of concentrated sulphuric acid rapidly from a dispenser, directing it into the solution. Swirl gently for about 1 minute.
  6. 6Place the flask on a heat-insulating sheet and allow it to stand for 30 minutes.
  7. 7Add about 200 mL of water, then 10 mL of orthophosphoric acid (and NaF if used) and about 1 mL of diphenylamine indicator.
  8. 8Titrate with ferrous ammonium sulphate until the colour changes from violet-blue through to a bright green endpoint. Record the titre for the blank (B) and for each sample (S).
  9. 9If more than about 75% of the dichromate is consumed (sample titre less than about one quarter of the blank), repeat with a smaller sample mass.

Standard preparation

  1. 1The FAS is standardised by the blank titration each day: N(FAS) = (10 mL × 1 N) ÷ blank titre (mL).
  2. 2Run a reference soil with a known organic-carbon value in each batch.

Notes for analysts

  • The 1.3 factor is a convention. Actual recovery varies with soil type; some laboratories derive their own factor against dry combustion.
  • Walkley–Black does not measure charcoal or carbonate carbon well; it estimates easily oxidisable organic carbon.
  • Potassium dichromate is toxic and a carcinogen (Cr VI). Collect the waste for hazardous-waste disposal; never pour it down the sink.
  • Adding concentrated acid generates strong heat. Use a fume hood, face shield and acid-resistant gloves.
TEST CONDITIONS

Method parameters (typical)

Sample mass1.0 g (0.1–0.5 g for organic-rich soils)
Dichromate10.0 mL of 1 N K₂Cr₂O₇ (pipetted)
Acid20 mL concentrated H₂SO₄
Reaction time30 minutes, heat of dilution only
TitrantFerrous ammonium sulphate, about 0.5 N
IndicatorDiphenylamine (with H₃PO₄) or ferroin
Recovery factor1.3 (conventional; assumes about 77% oxidation)
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Blank titreConsistent from day to day for the same FAS batch (drift shows FAS oxidation)
Dichromate consumedNot more than about 75% of that added; otherwise reduce sample mass
Reference soilWithin control-chart limits
DuplicatesAgree within the relative difference set in your SOP

Typical sequence

  1. 1Blank × 2
  2. 2Reference soil
  3. 3Samples (with one duplicate per ten)
  4. 4Blank at end if FAS stability is in doubt
CALCULATION

Calculation with a worked example

Formula
Organic carbon (%) = [(B − S) × N × 0.003 × 100 ÷ W] × 1.3

Where

B
FAS titre for the blank (mL)
S
FAS titre for the sample (mL)
N
Normality of FAS = (10 × 1) ÷ B
0.003
g of carbon per milliequivalent of dichromate (12 ÷ 4000)
W
Mass of soil taken (g)
1.3
Conventional recovery factor (100 ÷ 77)

Given

Dichromate added
10 mL of 1 N
B (blank titre)
20.4 mL
S (sample titre)
12.6 mL
W
1.000 g

Working

  1. 1
    N(FAS) = (10 × 1) ÷ 20.4 = 0.4902 N
  2. 2

    Dichromate used by the soil = (20.4 − 12.6) × 0.4902 = 7.8 × 0.4902 = 3.824 meq

  3. 3
    Check: 7.8 ÷ 20.4 = 38% of the dichromate consumed — within the working range (below about 75%)
  4. 4

    Carbon oxidised = 3.824 × 0.003 g = 0.01147 g; per 1.000 g soil × 100 = 1.147%

  5. 5

    Corrected for incomplete oxidation: 1.147 × 1.3 = 1.491%

  6. 6

    If organic matter is required: 1.491 × 1.724 (conventional van Bemmelen factor) = 2.57%

Organic carbon = 1.49% (organic matter about 2.57%)

ACCEPTANCE CRITERIA

What result passes

  • Organic carbon is reported and interpreted rather than passed or failed.
  • Many Indian soil-testing programmes use rating classes of roughly below 0.5% (low), 0.5–0.75% (medium) and above 0.75% (high); check the classes adopted by your programme.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
TruenessComparison with dry-combustion (CHN/TOC) results on reference soils; this also confirms the local recovery factor
PrecisionDuplicates and reference-soil control chart
RangeAdjust sample mass so 25–75% of dichromate is consumed
InterferencesChloride, Fe²⁺ and Mn oxides can bias results; assess for the soils you test
In LimsPulse · Sample Management

Run this method in LimsPulse: specifications and STPs under version control, results calculated and checked against limits, and a signed Certificate of Analysis.

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