Transcript Lecture 7. Digestion

The Islamic University of Gaza- Environmental Engineering Department
Environmental Measurements (EENV 4244)
Lecture 7: Digestion of Samples
Digestion of water samples with Kjeldhl method for determination of N
Destruction and extraction of soil and sludge for the determination of heavy metals
Destruction of plants and fish samples for the determination of heavy metals
Prepared by
Husam Al-Najar
1. Digestion of water samples with Kjeldhl method for determination of N
Kjeldahl nitrogen’’ is the sum of organic nitrogen and ammonia nitrogen.
Principle:
In the presence of H2SO4, potassium sulfate (K2SO4), and cupric sulfate (CuSO4) catalyst,
amino nitrogen of many organic materials is converted to ammonium. Free ammonia also is
converted to ammonium. After addition of base, the ammonia is distilled from an alkaline
medium and absorbed in boric or sulfuric acid
Storage of Samples
The most reliable results are obtained on fresh samples. If an immediate analysis is not
possible, preserve samples for kjeldahl digestion by acidifying to pH 1.5 to 2.0 with
concentrated H2SO4 and storing at 4°C. Do not use HgCl2 because it will interfere with
ammonia removal.
Interferences
During kjeldahl digestion, nitrate in excess of 10 mg/L can oxidize a portion of the ammonia
released from the digested organic nitrogen, producing N2O and resulting in a negative
interference. When sufficient organic matter in a low state of oxidation is present, nitrate can
be reduced to ammonia, resulting in a positive interference.
2. Heavy metals fractions in the soil as characterized by defined extraction solutions
and shaking time according to Zeien and Brümmer (1989).
Fraction
Extraction solution
Shaking
time
F1
Mobile
25 ml of 1 M NH4NO3
24 h.
F2
Easily exchangeable
50 ml of 1 M NH4OAc;
pH 6, adjusted with 50% acetic acid
24 h.
F3
Bound to Mn oxides
50 ml of 0.1 M NH2OH-HCl + 1M NH4OAc;
pH 6, adjusted with HCl
30 min.
F4
Bound to organic matter
50 ml of 0.025 M NH4-EDTA;
pH 4.6, adjusted with NH4 OH solution
90 min.
F5
Bound to amorphous and 50 ml of 0.2 M NH4-oxalate;
poorly crystalline Fe pH 3.25, adjusted with NH4 OH solution
oxides
F6
Bound to crystalline
Fe oxides
50 ml of 0.1 M ascorbic acid in 0.2 M NH4- oxalate; 30 min. in
pH 3.25, adjusted with NH4 OH solution
water
96oC
F7
Mainly bound to silicates
Aqua regia (3 parts HCL + 1 part HNO3)
4 h.
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3. Digestion of plant samples with H2SO4/Se/salicylic acid and H2O2
Field of application
• This digestion procedure can be applied for the determination of N-total (finally
measured as NH4), P, Na, K, Ca, Mg and Zn.
• The Ca content of the plant samples should not exceed 45 g Ca per kg dry weight.
Principle
• The large part of organic matter is oxidized by H2O2 at relatively low temperature.
After decomposition of the excess H2O2 and evaporation of water, the digestion is
completed by conc. H2O4 at approximately 300°C under the influence of Se as a
catalyst.
• Salicylic acid is used to form nitro-salicylic acid compounds in order to prevent loss of
free nitrate.
• Since CaSO4 may be formed when cooling after completing the digestion it is
necessary to wait 24 hours after the addition of water before Ca analysis. During this
period the CaSO4 will dissolve.
4. The destruction of plants and fish tissue for the determination of Cd, Cr, Cu,
Pb, Mn, Fe and Zn with the atomic absorption technique
Apparatus
a) Destruction-bloc with destruction tubes made of borosilicate glass
b) Nichiryo pipet model 3100 with removable tips
Reagents, all with a low metal content
a) Nitric acid, 65% HNO3
b) Hydrogen peroxide, 30% H2O2
c) Pumice
Glassware
• All rinsed with 1 + 1 HNO3
• Measuring cylinder, 50 mL
• Funnels with a diameter of 6 cm
• Volumetric flasks of 250 mL
5. The destruction of soil and sludge for the determination of Cd, Cr, Cu, Pb, Mn,
Fe and Zn with the atomic absorption technique
Apparatus
a) Destruction-bloc with destruction tubes made of borosilicate glass
b) Nichiryo pipet model 3100 with removable tips
Reagents
All reagents with a low percentage of heavy metals
a) Hydrochloric acid, 37% HCl
b) Nitric acid, 65% HNO3
c) Hydrogen peroxide, 30% H2O2
d) Pumice
Glassware
All rinsed with 1 + 1 HNO3
Measuring cylinder, 500 mL
Measuring cylinder, 50 mL
Funnels with a diameter of 6 cm
1 L flask for the acid-mixure, see note
Volumetric flasks of 250 mL