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 The Islamic University of Gaza- Environmental Engineering Department Environmental Measurements (EENV 4244)

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 H 2 SO 4, potassium sulfate (K 2 SO 4 ), and cupric sulfate (CuSO 4 ) 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 H 2 SO 4 and storing at 4°C. Do not use HgCl 2 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 N 2 O 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.

FractionExtraction solutionShaking time F1Mobile25 ml of 1 M NH 4 NO 3 24 h. F2Easily exchangeable50 ml of 1 M NH 4 OAc; pH 6, adjusted with 50% acetic acid 24 h. F3Bound to Mn oxides50 ml of 0.1 M NH 2 OH-HCl + 1M NH 4 OAc; pH 6, adjusted with HCl 30 min. F4Bound to organic matter50 ml of M NH 4 -EDTA; pH 4.6, adjusted with NH 4 OH solution 90 min. F5Bound to amorphous and poorly crystalline Fe oxides 50 ml of 0.2 M NH 4 -oxalate; pH 3.25, adjusted with NH 4 OH solution 4 h. F6Bound to crystalline Fe oxides 50 ml of 0.1 M ascorbic acid in 0.2 M NH 4 - oxalate; pH 3.25, adjusted with NH 4 OH solution 30 min. in water 96 o C F7Mainly bound to silicatesAqua regia (3 parts HCL + 1 part HNO 3 )- 2. Heavy metals fractions in the soil as characterized by defined extraction solutions and shaking time according to Zeien and Brümmer (1989).

3. Digestion of plant samples with H 2 SO 4 /Se/salicylic acid and H 2 O 2 Field of application This digestion procedure can be applied for the determination of N-total (finally measured as NH 4 ), 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 H 2 O 2 at relatively low temperature. After decomposition of the excess H2O2 and evaporation of water, the digestion is completed by conc. H 2 O 4 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 CaSO 4 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 CaSO 4 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% HNO 3 b) Hydrogen peroxide, 30% H 2 O 2 c) Pumice Glassware All rinsed with HNO 3 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% HNO 3 c) Hydrogen peroxide, 30% H 2 O 2 d) Pumice Glassware All rinsed with HNO 3 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