Method 513 Determination of CEC in composts

Reagents.

• Barium acetate reagent, 0.5 M, pH 7.0 - dissolve 127.7 g barium acetate [Ba(CH3COO)2] in water and make up to 1 l. Adjust to pH 7.0 using acetic acid and/or barium hydroxide solution as appropriate.

• Thymol blue indicator (thymolsulphonphthalein) - triturate 1 g thymol blue

(acid form) in a clean glass mortar with 21.5 ml 0.1 M sodium hydroxide and dilute to 1 l with water.

Procedure. Place 0.2 g compost, milled to 0.75 mm (approximately 24 or 25 mesh) into a sintered glass filter funnel (Porosity 3, 16-40 pm) fitted with a rubber tube with a pinchcock. Using a measuring cylinder or dispenser, add 25 ml hydrochloric acid, 0.05 M, and stir intermittently for 20 min. Open the pinchcock and filter with suction. Repeat the acid treatment, refilter, and wash with successive additions of water (about 150 ml in total) until free of chloride, indicated by the absence of a white precipitate or cloudiness on addition of silver nitrate solution. With the pinchcock closed, add 25 ml 0.5 M barium acetate reagent solution and leave for 1 h. Filter and retain the filtrate, then repeat the treatment with barium acetate reagent, and retain the filtrate. Wash thoroughly with successive additions of water (about 150 ml in total), and combine these washings with the previous filtrates, which are then titrated with 0.05 M NaOH. This may be done potentiometrically to an inflexion point, or using thymol blue (changes from yellow to blue over pH 8.0-9.6). A blank extraction and titration is carried out using the same quantities of barium acetate reagent. The difference in titration figures is equivalent to the H+ displaced from the sample by the Ba2+, which gives the CEC.

Calculation. If the titre of 0.05 M NaOH is y ml, then:

no. mol = (y x 0.05) x 10-3 , which for monovalent Na+ is also the no.

molc.

(y x 0.05) x 10-3 molc = 5y x 10-5 molc per 0.2 g air-dry soil, which becomes

25y x 10-5 molc g-1 air-dry soil, or 25y x 10-2 molc kg-1 air-dry soil, which is

25y cmolc kg-1 air-dry soil (or 25y mEq 100g-1 air-dry soil).

If this is multiplied by 100/total organic matter %, it gives the CEC/TOM value expressed as cmolc kg-1 ash-free air-dry compost. Include any extra dilution factors, and, if required, convert to oven-dry soil using the appropriate factor, as in Method 5.2, Calculation (2).

Other tests adapted from Zucconi and Bertoldi (1987) for certain chemical parameters are listed below:

Ammoniacal nitrogen: distillation, or ion-selective electrode Calcium: extraction with acid followed by AAS or ICP

determination

Carbon: (organic matter ) x 0.5, or homogenize plus dichro-

mate oxidation (see Method 5.7b. Procedure: use 0.1 g sample), or elemental analysis C:N ratio: calculate from C and N values

Conductivity: conductivity meter

Heavy metals (total): Cd, Cu, Cr, Hg, Ni, Pb, Zn are extracted from the milled (^0.25 mm; 50 mesh Market Grade

Organic matter: pH:

Phosphorous: Potassium: Total solids:

Magnesium:

Moisture: Nitrate nitrogen:

Nitrogen:

or No. 60 testing sieve) oven-dry sample by acid (e.g. a mixture of HNO3 and H2O2, or HNO3 and HCl) and determined by AAS or ICP. extraction with acid followed by AAS or ICP determination by difference from value for total solids

(i) by reduction and distillation/titration; (ii) col-

orimetric determination, or (iii) selective ion electrode

(i) by Kjeldahl digest; (ii) elemental analyser, or (iii) dichromate oxidation. ignition at 450-600°C pH electrode extraction with acid followed by colorimetry extraction with acid followed by flame photometry dry at 105°C to constant weight.

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