Application Benefits

  • High Throughput – Rapid sample analysis up to 100 samples per hour 
  • Low Cost per Analysis – Less lab tech time needed to operate, very cheap consumables and reagents 
  • Smaller bench space – The single channel unit occupies less than one foot squared of table space and stands one foot tall

 Keywords

Sulfate, turbidity, EPA 9038, FIA, flow injection, robust

Introduction and Principle

The automated turbidimetric method for determination of sulfate has been in use by commercial agricultural and environmental laboratories for many years. The manual method, EPA 9038, was validated by the EPA in 1986 and since then the method has been automated and refined for high throughput operations. Principally, it operates based on the precipitation of sulfate as barium sulfate. The amount of barium sulfate is proportional to the concentration of sulfate in the sample and is calculated based on the turbidity of the Barium Sulfate solution. The measurement is taken at 420 nm with no reference wavelength [1].

The turbidimetric analysis method has been employed by laboratories with enthusiasm for many reasons. It has exceptional throughput while remaining robust. It outperforms IC in terms of throughput by a factor of 10 [2]. It is more robust than an ion selective electrode, which has significant phosphate interferences, especially with fertilizer samples. Although the AOAC Official Method for sulfur in fertilizers and plant tissues is the gravimetric barium sulfate method, like the IC, the AOAC method has a very slow throughput and is labor intensive [2]. The turbidimetric method works well at eliminating interferences. For these reasons, the turbidimetric method is the ideal choice for agriculture laboratories.

Experimental 

Experiments were carried out using the FIAlyzer-1000 Flow Injection Analyzer equipped with the Flame T-VIS-NIR SpectrometerExperiments were carried out using the FIAlyzer-1000 Flow Injection Analyzer equipped with the Flame T-VIS-NIR Spectrometer (pictured here).

Reagent Composition 

  • Carrier (C): Water 
  • Reagent 1 (R1): 0.01 M HCl with 10 ppm S-SO4 sulfate seed (1 L) 
  • Reagent 2 (R2): 0.3M HClO4 (1 L) 
  • Reagent 3 (R3): 0.2% PVA - 140 g/L BaCl2 x 2H2O solution (1 L) 
  • Wash Solution: 0.1 M DTPA - 2M KOH solution (1 L) 

Flow Rates

  • C: 0.9 mL/min 
  • R1: 1.8 mL/min
  • R2: 0.5 mL/min
  • R3: 0.4 mL/min
  • S: 1.6 mL/min

Sample Volumes

  • 280 μL

Detector Settings 

  • Primary Wavelength: 420nm
  • Reference Wavelength: 0nm

turb sulf schematic.jpg

Figure 1: A schematic view of the instrument setup.

Results

Turbidimetric Sulfate Application Note.jpggraph.jpg

Figure 2: Example calibration run

 Detection Limit  1 mg S/L - 0.03 mmol/L
 Range Upper Limit 20 mg S/L – 0.6 mmol/L 
 Sample throughput 100 samples/hr 
 Startup Time  5 minutes
 Shutdown Time  5 minutes

Table 1: Method Performance Parameters

Conclusions

The features and benefits of FIAlab’s single channel instrument make it an attractive option for measuring sulfate in agricultural samples including fertilizers, soils, and plant extracts. It has low consumable costs and can analyze samples as fast as 100 samples / hour. It is also quite robust, as it is based on the tried and true barium precipitation chemistry. The main weakness of this reagent chemistry, BaSO4 scale formation in the manifold, has been addressed by a judicious use of a syringe pump for washing purposes. This version of the assay is optimized in terms of sensitivity foragriculture analysis. For more information, contact FIAlab at 206-258-2290 or via email at This email address is being protected from spambots. You need JavaScript enabled to view it..

References

[1] “Method for Sulfate Determination on Soil Samples, Version 5.1”, FIAlab Instruments, Inc., 2018, p.
1 - 7
[2] Wall L. L., et al. “An automated turbidimetric method for total sulfur in plant tissue and sulfate
sulfur in soils”, Communications in Soil Science and Plant Analysis, 2008, 11:11, 1087-1103, DOI:
10.1080/00103628009367107