Colorimetric method: ++ Cr3+ complex (green color, 600 or 625 nm) ++ method is suitable for automatization → segmented flow analysis or continuous flow analysis ++

Recent methods for determination oforganic and inorganic carbon

Background• In our environment, the present total carbon (TC) can be divided into total organic (TOC) and total inorganic carbon (TIC), depending on its chemical characteristics and main origin.

•Often both parameters - the organic (TOC) and the inorganic carbon (TIC) - are of interest, e.g. in many soil studies because of the supposed interaction between TIC amounts and the capacity to store TOC.

•The exact quantification, however, requires the separation of the named components and can be difficult due to the composition and variation in the relative proportions of the carbon fractions.

TOC + TIC = TC

SOMSOM

VOCVOCNVOCNVOC

OXCOXC

DOCDOC

PICPIC

DICDIC

POCPOC

DOMDOM

Objectiveto review the broad spectrum of currently used methods for carbon determination in solid and liquid samples.

Objectiveto review the broad spectrum of currently used methods for carbon determination in solid and liquid samples.

•depends on concentration and quantity of the used acid, on reaction temperature and time, sample volume or grain sizeHCl to estimate carbonate contents in soil, quantitative reaction with all carbonates except for siderite disadvantage: loss of SOMH2SO3 for soil treatment, except for samples containing dolomite or siderite, advantage: loss of SOM < 2% H3PO4 preferred in water analysisH2SO4 non-suitable, oxidizing acid

•addition of Fe2+ or Sn2+ to minimize loss of SOM

Thermal treatment, elimination of TOC

sandy soils 550°C for 4 harid zone soils 400°C for 8 hmineral soil 450°C for 12 hforest floor 450°C for 12 hdifferent loams 450°C for 4 hsediments 450 ± 10°C for 16 hsediments 475-500°C for 4-6 hparticulate carbon 500°C for 4 h

temperature and timeto remove TOCsamples

Fig.1: Combustion and decomposition temperaturesof different C materials

Conversion methods

Tab.1: Treatment of solid samples to removeTOC (according to LOI-Method)

Acid treatment, elimination of TICThe evolved CO2 can be used for TOC determination (Dual Temperature Method).

Chemical oxidation Combustion→ TOC determination, detection of the oxidant:K2Cr2O7 / H2SO4 (Walkley-Black-Method) ++ numerous modifications: e.g. with heating, Ag2SO4 as catalyst + + no separation ++ most widely used for soils, sediments, wastes ++ disadvantage: OXC = 75-85% of TOC + + quantification of TOC depends on soil type (no correlation to texture, pH, carbonate level) and on organic material (humus, lipids, proteins, elemental carbon are not completely oxidized) ++

K2S2O8 ++ with heat or UV ++ for DOC in water samples ++recovery 95%, in sea water 50-75% ++ Hg to complexe Cl-, ++ Ag as catalyst in acidic medium ++UV ++ for TOC in liquid samples ++ max. 1 g/l C ++ incomplete if POC and colloidal materials are present ++

→ TOC / TIC determination, detection of CO2 :Dry combustion ++ for solid or liquid samples ++ used catalysts and temperatures: WO3 (1000-1100°C), Cr2O3, Pt, CoO, CuO (950°C), CeO2 (850°C), alloys or oxids of Ir, Cu, Ni (680°C) ++ in the presence of O2 ++ additions to solid samples: V2O5, MnO2, CuO ++ quantitative oxidation of TOC (reference method) ++ high temperature combustion: decomposition of carbonates ++ problems in general: memory effects, gas flow rate for liquid samples (evaporation step!) ++Dual Temperature Combustion ++ TOC at 515°C, TIC at 925°C ++

Wet Combustion ++ Cr2O72- in acid mixture with heat ++ for

soils with high content of Cl- , rich in SOM ++ for soil extracts ++

Detection techniquesOxidant Determination CO2 determination

Titration of non-reduced Cr2O72- + + direct titration (more reliable) or

++ back titration (disadvantage: carbon-over estimation possible) ++Detection by ++ infrared spectrometry (high sensitivity and low detection limit) + + thermal conductivity (interferences by halids) ++ titration or gravimetry (not sensitive for low carbon contents) ++ manometric measurement (by volume or pressure, interferences by sulphur) ++ coulometric titration ++ flame ionazation ++ potentio-metry ++ turbidimetry ++ ion or gas chromatography ++

→ TOC determination, detection of CO2 :

Primary steps for analytical determination of organic and inorganic carbon:

→ TOC determination, detection of CO2 :

2. Conversion2. Conversionof TOC or TIC to CO2 by chemical oxidation,

combustion or decomposition

3. DetectionDetectionof the oxidant or

of the evolved CO2

by different techniques

1. Separation1. Separationof TOC and TIC by

acidification (TIC ) orthermal treatment (TOC )

Separation techniques

Car

bon

reco

vere

d [%

]

Temperature [°C]

300 400 500 600 700 800 900

0

20

40

60

80

100

120

CalciteDolomiteMagnesiteCellulose, WoodHumic Acid (peat)Humic acid (lignite)Coal (lignite)Coal (hard-coal)

The evolved CO2 can be used for TIC determination (Scheibler-Method, Stroehlein C-MAT 5500).

Routine Measurements & Analysis, Max-Planck-Institute for Biogeochemistry, Jena

Ines Hilke Birgit Fröhlich

Bisutti I, et al (2004) Determination of total organic carbon - an overview of current methods." Trac-Trends in Analytical Chemistry 23(10-11): 716-726. Bisutti I, et al (2007) A novel single-run dual temperature combustion (SRDTC) method for the determination of organic, in-organic and total carbon in soil samples. Talanta 71(2):521-528

Literature