LAB 8 – FREE IRON AND OM IN FOREST SOILS
RELATIONSHIP BETWEEN SOILS, FE AND OM
FREE IRON - AAS
Use atomic absorption spectrometry (AAS) in
order to analyze free iron in soil.
- Absorption of optical radiation by free
ions in gaseous state.
Beer-Lambert Law:
Linear relationship between light absorbance
and concentration of the compound
solution.
- if there is no absorption, all light will be
transmitted.
FREE IRON - EXTRACTIONS
Extractions needed to test free iron:
-
1 gram soil + 50 ml sodium tartrate acetate solution
Mechanically shaken ~15 min to disperse clay
+ 2 grams sodium dithionite
Hot water bath @ 40°C for 1 hour.
Filter solution.
Dilute into a flask to be used with AAS
Page 140 in soils manual
Sodium tartrate acetate: disperse aggregate soil
particles, solubilizing Fe
Sodium dithionate: extracts Fe from soil by reducing
Fe3+ to Fe2+
ATOMIC ABSORPTION SPECTROMETRY
Air-acetyle flame atomizer.
Selects specific wavelengths of light
absorbed by sample
Converts light signal into
electrical signal
proportional to light
intensity.
Ionized gas which
discharges radiation of
the element through
flame.
Takes the liquid solution and
creates a fine aerosol spray
into the flame to evaporate
it.
ATOMIC ABSORPTION SPECTROMETRY
1. Hollow cathode lamp
- Sealed lamp filled with
argon/neon with a metal
cathode with an element and
an anode.
- Ionized gas which discharges
radiation of the element
through flame.
2. Flame
3. Monochromator
4. Detector
ATOMIC ABSORPTION SPECTROMETRY
OM AND CARBON
Methods of OM Determination:
- LOI (Loss of Ignition)
- Muffle furnace burns away organic matter and the difference
in weight from before and after burning gives OM %.
- Subject to errors: volatilization of crystalline H20, salts,
inorganic carbons, etc...
- Determination of a constituent percentage of OM
% OM = % C x1.72
* Known relationship between carbon and organic matter
- Soil contains ~ 58% carbon (Walkley 1946) = 1.72
CNS 2000
LECO Induction Furnace
CNS 2000
- Measures Carbon, Nitrogen, and Sulphur %
- Dry Combustion Method
1. Screen for entering data / output of results
2. Opening to combustion chamber
CNS - SAMPLES
- Sample placed into a ceramic boat.
- Sample is pushed into combustion chamber
(1020°C). It’s combusted with oxygen.
- 97% to C02
- 3% to C0
- Water is vaporized and removed from oxygen
stream by magnesium perchlorate (anhydrone) to
stop it from interfering with C detector.
- Thermally resistant substances completely
oxidized.
CNS - ANALYSIS
- Gases pass through an oxidation catalyst (chromium
trioxide) -> C02, N2, N0x and water
- Gases then pass through reduction catalyst (metallic copper
@ ~650°C) which reduces the nitrogen to element
- C02 in gas is detected by a C02 detector.
- Light source (IR Infrared wavelength)
- C02 absorption causes loss of signal which is
proportional to concentration of C02
- Displayed as %C
LOI AND CARBON
35
LOI (loss of ignition) is an indirect measure of organic matter.
30
Samples are burned in a muffle furnace for 6 hours at 600°C,
then weighed after burning to see the difference in weight.
This shows how much OM (and other elements) have been
burned off. Mineral soil remains behind.
20
C%
25
15
y = 0.42x - 0.29
R² = 0.93
10
5
0
- Depending on ignition temperature LOI can cause loss of
salts, structural water, and inorganic carbons which effect
results.
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
LOI %
Close relationship between LOI and C%
(MF’s data from central and northwestern NB)
LOI AND CARBON