GB/T 44343-2024 Determination of total sulfur in fertilizers - High temperature combustion
1 Scope
This document describe a method to measure the total sulfur content in fertilizer and soil conditioner materials.
This method is applicable for measuring total sulfur concentration in solid and liquid fertilizers and its raw inputs in the range of 0.1% to 97%.
2 Normative references
This document is applicable to animal and vegetable fats and oils. If it is not desired to include soaps (particularly calcium soaps) or oxidized fatty acids in the insoluble impurities content, it is necessary to use a different solvent and procedure to make the determination meet relevant requirements.
ISO 8157 Fertilizers, soil conditioners and beneficial substances - Vocabulary)
Note: GB/T 6274-2016 Fertilizers and soil conditioners - Vocabulary (ISO 8157:1984, NEQ)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8157 apply.
4 Principle
This procedure involves conversion of sulfur (S) species from samples and standard substances into SO₂ through combustion at a temperature>1100℃ followed by measurement with thermal conductivity detection (TCD) or infrared (IR) detection reported as mass fraction percentage (%). In the case of thermal conductivity detection and where simultaneous measurements of additional elements, such as carbon (C), hydrogen (H), or nitrogen (N), are performed, an intermediate SO₂ separation by thermal adsorption/desorption is necessary.
5 Apparatus, material and reagents
5.1 General
CAUTION - Incorrect handling during the elemental analysis using combustion can lead to the risk of burns as certain instrument components are heated during the method. Even after switching off the instrument, some components stay hot for long periods of time. Serious burns can occur if working carelessly with the instrument. Follow the manufacturer's specific operating instructions to ensure safe handling of equipment.
Total sulfur measurements can be performed via variable apparatus types depending on detection method of choice.
5.2 Apparatus
5.2.1 Apparatus A: Combustion followed by thermal conductivity detection
For Apparatus A type instruments, shown in Figure 1, sulfur as SO, is determined by TCD with helium or argon carrier gas allowing for multi-element analysis. With this setup, the test portion should be introduced into the combustion zone in a way such that atmospheric contamination is removed. Oxygen is added over the test portion at a temperature >1100℃ converting all elements to their fully oxidized gaseous specie. A catalyst, such as tungsten (VI) oxide (WO₃), inside the combustion tube is used to aid oxidation. Following combustion, gases pass through a reducing environment and halogen scrubber in order that NOx species be converted to N₂ and removal of halogen contaminants, respectively. Other resulting combustion gas components CO₂, H₂O, and SO₂ are scrubbed or adsorbed on analyte-specific thermal adsorption/desorption columns. N₂ is not adsorbed and flows directly to the thermal conductivity detector. Each CO₂, H₂O, and SO₂ are desorbed sequentially following the previous elements complete measurement by the TCD allowing for clear separation of the analyte species. Scrubbing materials, such as chemical or physical absorbers, may be placed between the furnace and detector to remove CO₂ and/or H₂O if determination of either C and/or H is undesired. With the help of a calibration curve, software processing converts the SO₂ peak signal into a mass fraction percentage of S in the sample.
5.2.2 Apparatus B: Combustion followed by single-range infrared detection
For Apparatus B type instruments, shown in Figure 2, sulfur as SO₂ is determined by a sulfur-specific IR detector with oxygen carrier gas. The test portion is introduced into the combustion zone where oxygen in combination with a temperature>1100℃ converts S to SO₂. A catalyst, such as tungsten (VI) oxide (WO₃), inside the combustion tube is used to aid oxidation. The gas stream is dried before entering the detector. With the help of a calibration curve, software processing converts the SO₂ peak signal into a w/w percentage of S in the sample. For best results using this apparatus type, follow special instructions in Clause 6.
5.3 Materials, reagents and consumables
5.3.1 Materials
5.3.1.1 Analytical balance, resolution to at least 0.1 mg;
5.3.1.2 Test portions containers, typically tin foil or ceramic crucible;
5.3.1.3 Hand pellet press, for pelletizing powder materials;
5.3.1.4 Capsule sealing press, for making a gas-tight cold seal on tin capsule holding liquid materials.
5.3.2 Reagents
5.3.2.1 Helium or argon, minimum 99.995% purity.
5.3.2.2 Oxygen, minimum 99.5% purity.
5.3.2.3 Tungsten (VI) oxide (WO₃) granulate, grain size approximately 0.5 mm to 2 mm, minimum 99.7% purity - supplied by the instrument manufacturer.
5.3.2.4 Tungsten (VI) oxide (WO₃) powder as sample additive, minimum 99.7% purity - supplied by the instrument manufacturer.
5.3.2.5 Copper wires, approximately 0.5 mm length - supplied by the instrument manufacturer.
5.3.2.6 Copper oxide wires, approximately 0.5 mm length - supplied by the instrument manufacturer.
5.3.2.7 Pt catalyst, 5% on Al₂O3, pelletized - supplied by the instrument manufacturer.
5.3.2.8 Corundum balls (inert), 3 mm to 5 mm diameter - supplied by the instrument manufacturer.
5.3.2.9 Quartz wool (inert), fibre thickness of approximately 9 μm - supplied by the instrument manufacturer.
5.3.2.10 Silver wool, wire thickness of approximately 0.04 mm - supplied by the instrument manufacturer.
5.3.2.11 Desiccant, for example phosphorus pentoxide - supplied by the instrument manufacturer.
5.3.3 Consumables
See Figure 3.
6 Calibration curve and daily factor
Check calibration on the instrument, (5.2.1, 5.2.2), daily and perform as needed according to the manufacturer's recommendation. It is recommended that a non-hygroscopic pure chemical standard or set of standards be used for calibrating the instrument, such as sulfanilamide (≥99 %), ammonium or sodium sulfate (≥99 %), or sublimed sulfur (≥99 %). Use a minimum of five calibration points to generate the calibration curve and cover the absolute sulfur range encompassing that of the expected S concentration of unknowns. For apparatus A and B type instruments (5.2.1, 5.2.2), a single higher order calibration is sufficient. Any drift in the calibration curve can be observed and corrected for by daily use of an alternative non-hygroscopic pure chemical standard of known S concentration. Follow manufacturer's instructions for setting up and calculating drift corrections. If the drift correction or daily factor exceeds 0.9 or 1.1, perform the necessary maintenance and ensure reagents in the combustion or reduction tubes are not depleted.
