Lecture 7: Digestion of Samples

1. 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

2. 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.

3. 2. Heavy metals fractions in the soil as characterized by defined extraction solutions and shaking time according to Zeien and Brümmer (1989).

4. 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.

5. 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

6. 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