BS EN ISO 23470:2018 pdf download – Soil quality – Determination of effective cation exchange capacity (CEC) and exchangeable cations using a hexamminecobalt trichloride solution (ISO 23470:2018)
5.10 Stock solutions of Al, Ca, Fe, K, Mg, Mn, Na, β(Me) = 1,000 g/l.
These solutions are supplied with a certified composition from a reputable source and are checked on a regular basis.
5.11 Calcite saturated hexamminecobalt (III)chloride solution.
Fill up a 2 l beaker with hexamminecobalt(III)chloride solution (5.2). Add 2 g of fine-ground calcite. This mixture is placed in an ultrasonic bath for 30 min. To minimize temperature effects on the solubility of calcite, the solution is stirred for a further 30 min using a magnetic stirrer. After switching off the stirrer, undissolved calcite is allowed to settle overnight. The 2 l beaker is protected by a plastic foil on top during calcite saturation. The solution may be decanted or filtered, however, addition of un-reacted (settled) calcite to the soil sample should be avoided. Two litres (2 l) of exchange solution can be used for approximately 34 samples (34 × 50 ml) plus 2 blanks (2 × 50 ml).
NOTE Reagents 5.3 to 5.9 are prepared only in the case when determination of CEC is carried out by determination of ammonia nitrogen using distillation. They are not needed when spectrophotometric determination is used.
6.1 Analytical balance, the weighing uncertainty range of which will not exceed ±0,1 % of the test portion mass.
6.2 Containers for shaking, tightly stoppered, rigid or non-rigid wall, having a volume between75 ml and 100 ml.
6.3 50 ml dispenser, set at ±0,25 ml of the delivered volume.
6.4 End-over-end shaker, allowing the permanent suspension of the soil/extraction solution mixture, placed in ambient air at (20 ± 2) °C.
6.5 Centrifuge and relevant centrifuge vials.
Centrifugation time and speed depend on the type of centrifuge and are selected with a view of securing a clear supernatant. 4 000 r/min for 20 min are generally sufficient but special care shall be taken dueto the high content of fixed hexamminecobalt ions on colloidal particles.
6.6 Distillation apparatus, the different parts of the apparatus shall be assembled ensuring tightness in order to prevent any loss of ammonia or entrainment of sodium hydroxide.
6.8 Spectrophotometer, allowing measurements to be performed at wavelengths 380 nm and475 nm, equipped with a 10 mm path length measuring cell.
6.9 Inductively coupled plasma atomic emission spectrometer.
6.10 Ultrasonic bath.
6.11 Magnetic stirrer and magnetic stir bar.
7.1 Test portion
Pretreat soil in accordance with ISO 11464.
If Q is the number of centimoles of positive charges provided by a given volume of hexamminecobalt(III)chloride solution, the centimoles of positive charges carried by the test portion shall lie between Q/10 and Q/3 inclusive.
Table 1 gives examples of suitable test portions for a range of expected CEC values.
Weigh to within 0,1 % the selected mass of the test portion (see ISO 11464) and transfer to a container for shaking (6.2). Evaluation of plausible CEC and exchangeable cation values may be performed using the diagram type called Carbonate and Sulphate Field Model (CSF model); see Reference . Two different sample masses are used and plausibility is given when both resulting values (any exchangeable cation pair or the CECs) lie on the y = x line as shown in Annex D.
7.2 Exchange reaction
Add 50 ml of the hexamminecobalt(III)chloride solution (5.2) or calcite saturated hexamminecobalt(III) chloride solution (5.11) and shake for (60 ± 5) min by means of the shaker (6.4). Centrifuge by means of 6.5. Collect the clear filtrate or supernatant. Carry out the determinations no later than 24 h after extraction at the latest, otherwise, ascertain that the storage conditions do not influence the test result. Such storage may be done after diluting the samples in acidified solutions.
Produce an extraction “blank” under the same conditions, but without the test portion.
7.3 Determination of CEC
The measurement of hexamminecobalt concentration in the extract can be performed by determination of total ammonium nitrogen (according to 7.3.2), direct spectrophotometric measurement (according to 7.3.3) or total cobalt concentrations (according to 7.3.4).BS EN ISO 23470 pdf download.