Application Benefits

  • High Throughput – Rapid sample analysis up to 60 samples per hour
  • Low Cost per Analysis – Less lab tech time, very cheap consumables and reagents
  • Smaller bench space – The single-channel unit occupies less than one foot square off table space and stands one foot tall
  • Compliant with Standard Methods 4500 - avoid the use of toxic reagents

 

Keywords

  • Sulfate, methyl thymol blue, colorimetric, FIA, flow injection, automation, SM 4500-SO4(2-), throughput

Introduction and Principle

Many flow injection methods start with lightly-colored reagents mixing with a sample to yield a strongly-colored product. This is not the case with the sulfate MTB analysis. Initially, a solution of a stable dark blue complex of barium and methyl thymol blue (MTB) enters the FIA manifold. Once a sample containing sulfate mixes with this dark blue stream under ethanoic conditions, free sulfate ions are able to react with barium complexed MTB to yield BaSO4 and MTB. This newly freed MTB species has a gray color, which our unit’s spectrometer reads. Our flame spectrometer reads the signal
460 nm and for added quality control, uses a reference wavelength of 700 nm.

This is not the whole story. The barium MTB is also susceptible to reactions with other metallic cations such as calcium and magnesium [1]. To remove these and other cations that would interfere with the chemistry, our unit includes an ion exchange column, effectively removing these interferences. Since calcium is one of the main interferences, we recommend running a QC check with a mid-range sulfate standard in a matrix containing a typical level of CaCO3. If this standard yields a lesser response than the same standard in DI water, the IC column is ready for regeneration or replacement of the resin.

One thing to note is that the resin will release some of the cations in specific cases. For this reason, samples with a pH less than 2 must be neutralized [1]. Furthermore, highly ionic solutions will release the calcium from the cation exchange column. One study found a sample containing an excess of 2000 mg/l would scrub any accumulation of calcium from the column. Aside from highly ionic or acidic samples, there are a few select anions that interfere with the reaction. The good news is that these are seldom found in quantities that would produce a major interference in water samples. These include tannic acid, sulfide, sulfite and ortho-phosphate [2]. Compared to other methods, the automated sulfate analysis is superior in a few ways. First, its throughput is unrivaled by other techniques including ion chromatography and the gravimetric method. Furthermore, the consumables are much less expensive than either of the mentioned methods. Finally, it has been shown to have better precision and sensitivity than the gravimetric method [1].that would produce a major interference in water samples. These include tannic acid, sulfide, sulfite and ortho-phosphate [2].

Experimental 

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

Reagent Composition

  • Carrier (C): Water
  • Diluent Solution (D): 30 mg SO4/L
  • Reagent 1 (R1): 0.6mM Methylthymol Blue
  • Reagent 2 (R2): 0.18M NaOH Solution

Flow Rates

  • C: 0.8 mL/min
  • D: 0.7 mL/min
  • R1: 1.8 mL/min
  • R2: 0.5 mL/min

Sample Volumes

  • 420 μL

Detector Settings

  • Primary Wavelength: 460nm
  • Reference Wavelength: 700nm

 

Colorimetric Sulfate Application Note.jpg

 

Results

Results Sulfate.jpg

    Detection Limit         0.5 mg SO4(2-)/L    
Range Upper Limit 25 mg SO4(2-)/L 
    Sample Throughput     60 samples/hr

 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 wastewater, stormwater and other environmental samples. It has low consumable costs and can analyze samples as fast as 60 samples/hour [3]. Furthermore, the major interferences are seldom found in environmental samples and the rest are easily handled with ion exchange resins and sample pretreatment. 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] Adamski J.M., Villard S.P., “Application of the Methylthymol Blue Sulfate Method to Water and Wastewater Analysis”. Anal. Chem. 47 (1975) 1191-1194.


[2] Andrew D. Eaton et al., “Standard Methods for the Examination of Water and Wastewater.” Washington, DC: American Public Health Association, (1998) 4-193 – 4-194.

[3] “Method for Sulfate Determination by Barium Chloride – MTB Reagent, Version 1.1.” FIAlab Instruments,
Inc., 2018. 1 - 8.