This X-ray fluorescence method is based on techniques similar to those described for total P (Method 9A1). There are no recent inter-laboratory proficiency data from ASPAC for this test. According to Vempati et al. (1996), quantitative analysis by XRF could have uncertainty of 10% relative to the measured value: see Appendix 2. Note that Method 17A1 provides an alternative multi-elemental XRF option that uses modern instrumentation. Both total S and total P could be included in the multi-element suite.
Table 10.2. Quantities and volumes required for preparation of pellets of known S content.
Wt of silica (g) |
Gypsum (g) |
Vol. of S standards (mL) |
Equivalent soil concentration (%S) |
3.6574 |
1.3425 |
– |
5.0 |
4.3287 |
0.6713 |
– |
2.5 |
4.7315 |
0.2685 |
– |
1.0 |
4.8657 |
0.1343 |
– |
0.5 |
4.9329 |
0.0671 |
– |
0.25 |
4.9732 |
0.0268 |
– |
0.1 |
High strength |
|||
4.9588 |
– |
0.5 |
0.2 |
4.9691 |
– |
0.375 |
0.15 |
4.9794 |
– |
0.25 |
0.10 |
4.9897 |
– |
0.125 |
0.05 |
Low strength |
|||
4.9948 |
– |
0.125 |
0.025 |
4.9979 |
– |
0.050 |
0.01 |
Boric Acid
As for Method 9A1.
Silica
As for Method 9A1.
Sulfur Standard - High Strength
1 mL contains 20 mg of S.
Dissolve 8.2430 g ammonium sulfate [(NH4)2SO4; previously dried at 105°C for 4 h] in deionised water and dilute to 100 mL in a volumetric flask.
Sulfur Standard – Low Strength
1 mL contains 10 mg of S.
Prepare by diluting 50 mL of S Standard – High Strength to 100 mL using deionised water and a 100 mL volumetric flask.
Gypsum
Analytical-grade CaSO4.2H2O.
Prepare solid standards of known S content by adding gypsum or volumes of either High or Low S Standard to weighed quantities of silica, as detailed in Table 10.2. See Method 9A1 for more details of the preparation technique. These standard pellets have an extended life.
Proceed in a manner similar to that described in Method 9A1: see Note 1.
Follow the XRF manufacturer’s recommendations to set up and operate the instrument. This will typically be a sequential/simultaneous XRF spectrometer, probably with fixed channel monochromators optimised for total S. When feasible, make the net count (i.e. peak-background) equal to the concentration of the standard pellet (or related to it by some power of 10), so that elemental concentration may be recorded directly from the display.
Report total S (%): an oven-dry basis can be assumed.
1. Soil sample residues in the grinding head of the ‘shatterbox’ can be a serious source of contamination, especially when components become worn. For routine applications, these residues can be overcome between samples by scrubbing and washing with water then drying. Alternatively, the residues can be minimised by grinding a small quantity of acid-washed silica sand between each unknown sample.